diff --git a/backend/src/config.ts b/backend/src/config.ts
index c3c00a10..4a4946e4 100644
--- a/backend/src/config.ts
+++ b/backend/src/config.ts
@@ -5,6 +5,7 @@ import {
   type AuthenticationConfigProvider,
   type AuthMode,
 } from "./auth/config.js";
+import { createProjectedAuthenticationConfigProvider } from "./auth/runtime-projection.js";
 import type { WorkspaceRegistryConfig } from "./workspaces/types.js";
 
 export interface AppConfig {
@@ -176,13 +177,31 @@ function positiveDimension(value: string | undefined, fallback: number): number
 }
 
 export function loadConfig(env: Record<string, string | undefined>): AppConfig {
-  const authConfigFile = absoluteAuthPath(env.THT_AUTH_CONFIG_FILE ?? "/run/thothii-auth/auth.yaml", "file");
+  const defaultAuthConfigFile = "/run/thothii-auth/auth.yaml";
+  const authConfigFile = absoluteAuthPath(env.THT_AUTH_CONFIG_FILE ?? defaultAuthConfigFile, "file");
   const authStateRoot = absoluteAuthPath(env.THT_AUTH_STATE_ROOT ?? "/data/auth", "state root");
-  const hasAuthenticationConfig = authConfigFileExists(authConfigFile);
+  const runtimeProjectionRoot = env.THT_AUTH_RUNTIME_PROJECTION_ROOT;
+  let hasAuthenticationConfig = false;
+  let authentication: AuthenticationConfigProvider | undefined;
+  if (runtimeProjectionRoot !== undefined) {
+    let projectionRoot: string;
+    try {
+      projectionRoot = absoluteAuthPath(runtimeProjectionRoot, "runtime projection root");
+    } catch {
+      throw new Error("authentication configuration is invalid");
+    }
+    if (env.THT_AUTH_CONFIG_FILE !== undefined && env.THT_AUTH_CONFIG_FILE !== defaultAuthConfigFile) {
+      throw new Error("authentication configuration is invalid");
+    }
+    authentication = createProjectedAuthenticationConfigProvider(projectionRoot);
+    hasAuthenticationConfig = true;
+  } else {
+    hasAuthenticationConfig = authConfigFileExists(authConfigFile);
+    authentication = hasAuthenticationConfig ? createAuthenticationConfigProvider(authConfigFile) : undefined;
+  }
   if (hasAuthenticationConfig && env.AUTH_MODE !== undefined) {
     throw new Error("authentication configuration and AUTH_MODE cannot both be set");
   }
-  const authentication = hasAuthenticationConfig ? createAuthenticationConfigProvider(authConfigFile) : undefined;
   let authMode: AuthMode;
   if (authentication) {
     authMode = authentication.current().value.mode;
diff --git a/backend/test/auth-runtime-projection.test.ts b/backend/test/auth-runtime-projection.test.ts
index b6bccaa6..5a9a58f2 100644
--- a/backend/test/auth-runtime-projection.test.ts
+++ b/backend/test/auth-runtime-projection.test.ts
@@ -20,6 +20,7 @@ import { stringify } from "yaml";
 import { afterEach, expect, test, vi } from "vitest";
 import { createProjectedAuthenticationConfigProvider } from "../src/auth/runtime-projection.js";
 import { createCurrentLocalUserRegistryResolver } from "../src/auth/local-registry.js";
+import { loadConfig } from "../src/config.js";
 
 const fsHook = vi.hoisted(() => ({
   path: undefined as string | undefined,
@@ -308,6 +309,35 @@ test("loads a complete OIDC projection without a users snapshot", () => {
   });
 });
 
+test("loadConfig selects an immutable projected local provider and its in-memory registry", async () => {
+  const root = projectionRoot();
+  writeReadyProjection(root, localProjectionFixture("projected-user", passwordHash));
+
+  const config = loadConfig({
+    THT_AUTH_RUNTIME_PROJECTION_ROOT: root,
+    THT_AUTH_STATE_ROOT: "/state/auth",
+  });
+  const loaded = config.authentication?.current();
+  expect(loaded).toMatchObject({ value: { mode: "local" }, runtimeProjection: expect.any(Object) });
+  const registry = createCurrentLocalUserRegistryResolver().resolve(loaded!);
+  expect(await registry?.findByUsername("PROJECTED-USER")).toMatchObject({ username: "projected-user" });
+});
+
+test("loadConfig selects an immutable projected OIDC provider without direct-file fallback", () => {
+  const root = projectionRoot();
+  writeReadyOidcProjection(root);
+
+  const config = loadConfig({
+    THT_AUTH_RUNTIME_PROJECTION_ROOT: root,
+    THT_AUTH_CONFIG_FILE: "/run/thothii-auth/auth.yaml",
+    THT_AUTH_STATE_ROOT: "/state/auth",
+  });
+  expect(config.authentication?.current()).toMatchObject({
+    value: { mode: "oidc" },
+    runtimeProjection: expect.any(Object),
+  });
+});
+
 test("rejects a trailing-slash runtime root", () => {
   const root = projectionRoot();
   writeReadyProjection(
diff --git a/backend/test/config.test.ts b/backend/test/config.test.ts
index fc4920b1..68ba2742 100644
--- a/backend/test/config.test.ts
+++ b/backend/test/config.test.ts
@@ -117,6 +117,31 @@ test("loadConfig makes an existing auth.yaml authoritative and rejects AUTH_MODE
   }
 });
 
+test.each([
+  ["relative root", { THT_AUTH_RUNTIME_PROJECTION_ROOT: "relative" }],
+  ["conflicting direct file", {
+    THT_AUTH_RUNTIME_PROJECTION_ROOT: "/run/thothii-auth",
+    THT_AUTH_CONFIG_FILE: "/different/auth.yaml",
+  }],
+])("rejects projected authentication configuration: %s", (_name, env) => {
+  expect(() => loadConfig(env)).toThrow("authentication configuration is invalid");
+});
+
+test("keeps the direct auth-file provider when the runtime projection environment is absent", () => {
+  const { directory, file } = authFile(oidcAuthConfig());
+  try {
+    const loaded = loadConfig({ THT_AUTH_CONFIG_FILE: file, THT_AUTH_STATE_ROOT: "/state/auth" });
+		const current = loaded.authentication?.current();
+		expect(current).toMatchObject({
+			sourcePath: file,
+			value: { mode: "oidc" },
+		});
+		expect(current?.runtimeProjection).toBeUndefined();
+  } finally {
+    rmSync(directory, { recursive: true, force: true });
+  }
+});
+
 test("loadConfig rejects an auth config path that exists but is not a regular file", () => {
   const directory = mkdtempSync(join(realpathSync(tmpdir()), "thothii-app-auth-config-directory-"));
   try {
diff --git a/deploy/compose.auth-runtime-projection.yaml b/deploy/compose.auth-runtime-projection.yaml
new file mode 100644
index 00000000..aa3f88fd
--- /dev/null
+++ b/deploy/compose.auth-runtime-projection.yaml
@@ -0,0 +1,9 @@
+services:
+  core:
+    environment:
+      THT_AUTH_RUNTIME_PROJECTION_ROOT: /run/thothii-auth
+    volumes:
+      - type: bind
+        source: ${THT_AUTH_RUNTIME_ROOT:?set THT_AUTH_RUNTIME_ROOT}
+        target: /run/thothii-auth
+        read_only: true
diff --git a/scripts/test-auth-runtime-projection-compose.sh b/scripts/test-auth-runtime-projection-compose.sh
new file mode 100755
index 00000000..56a21e98
--- /dev/null
+++ b/scripts/test-auth-runtime-projection-compose.sh
@@ -0,0 +1,117 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+root="$(cd "$(dirname "$0")/.." && pwd -P)"
+tmp_base="${TMPDIR:-/tmp}"
+tmp="$(mktemp -d "${tmp_base%/}/thoth-auth-runtime-compose.XXXXXX")"
+trap 'rm -rf "$tmp"' EXIT HUP INT TERM
+
+canonical="$tmp/canonical-auth"
+runtime="$tmp/runtime-auth"
+mkdir -p "$canonical" "$runtime" "$tmp/data" "$tmp/pi-state" "$tmp/workspace-registry"
+chmod 0700 "$canonical" "$runtime"
+printf '%s\n' '{}' >"$tmp/pi-auth.json"
+printf '%s\n' 'fixture-secret-sentinel' >"$tmp/thothii.secrets"
+chmod 0600 "$tmp/pi-auth.json" "$tmp/thothii.secrets"
+
+write_env() {
+  local path="$1"
+  {
+    printf '%s\n' \
+      'THT_WORKSPACE_GIT_REMOTE=https://git.example.invalid/platform/workspaces.git' \
+      "PI_AUTH_FILE=$tmp/pi-auth.json" \
+      "THT_SECRETS_FILE=$tmp/thothii.secrets" \
+      "THT_AUTH_CONFIG_ROOT=$canonical" \
+      "THT_DATA_ROOT=$tmp/data" \
+      "THT_PI_STATE_ROOT=$tmp/pi-state" \
+      "THT_WORKSPACE_REGISTRY_ROOT=$tmp/workspace-registry"
+  } >"$path"
+}
+
+write_env "$tmp/nonprojected.env"
+cp "$tmp/nonprojected.env" "$tmp/projected.env"
+printf 'THT_AUTH_RUNTIME_ROOT=%s\n' "$runtime" >>"$tmp/projected.env"
+
+cat >"$tmp/operator.yaml" <<'YAML'
+services:
+  core:
+    environment:
+      THT_AUTH_RUNTIME_PROJECTION_ROOT: operator-marker
+YAML
+
+cat >"$tmp/current-image.yaml" <<'YAML'
+services:
+  core:
+    environment:
+      THT_AUTH_RUNTIME_PROJECTION_ROOT: current-marker
+YAML
+
+render() {
+  local output="$1"
+  local env_file="$2"
+  shift 2
+  local -a files=(-f "$root/compose.yaml" -f "$root/deploy/compose.server.yaml")
+  local file
+  for file in "$@"; do
+    files+=(-f "$file")
+  done
+  docker compose --project-directory "$root" --env-file "$env_file" "${files[@]}" \
+    config --format json >"$output"
+}
+
+render "$tmp/nonprojected.json" "$tmp/nonprojected.env"
+render "$tmp/projected.json" "$tmp/projected.env" \
+  "$tmp/operator.yaml" "$root/deploy/compose.auth-runtime-projection.yaml"
+render "$tmp/current.json" "$tmp/projected.env" \
+  "$tmp/operator.yaml" "$root/deploy/compose.auth-runtime-projection.yaml" \
+  "$tmp/current-image.yaml"
+
+for mode in nonprojected projected current; do
+  node - "$tmp/$mode.json" "$mode" "$canonical" "$runtime" <<'NODE'
+const fs = require("fs");
+const [path, mode, canonical, runtime] = process.argv.slice(2);
+const config = JSON.parse(fs.readFileSync(path, "utf8"));
+const core = config.services?.core;
+if (!core) throw new Error(`${mode}: missing core service`);
+
+const mounts = (core.volumes || []).filter((mount) => mount.target === "/run/thothii-auth");
+if (mounts.length !== 1 || mounts[0].type !== "bind" || !mounts[0].read_only) {
+  throw new Error(`${mode}: expected exactly one read-only auth bind`);
+}
+if (mode === "nonprojected") {
+  if (mounts[0].source !== canonical) throw new Error("nonprojected: canonical auth source changed");
+  if (Object.hasOwn(core.environment || {}, "THT_AUTH_RUNTIME_PROJECTION_ROOT")) {
+    throw new Error("nonprojected: projected environment unexpectedly present");
+  }
+} else {
+  if (mounts[0].source !== runtime) throw new Error(`${mode}: runtime source did not replace canonical source`);
+  if ((core.volumes || []).some((mount) => mount.source === canonical)) {
+    throw new Error(`${mode}: canonical source is still mounted`);
+  }
+  const expected = mode === "projected" ? "/run/thothii-auth" : "current-marker";
+  if (core.environment?.THT_AUTH_RUNTIME_PROJECTION_ROOT !== expected) {
+    throw new Error(`${mode}: override ordering failed`);
+  }
+}
+
+for (const [name, service] of Object.entries(config.services || {})) {
+  if (name !== "core" && Object.hasOwn(service.environment || {}, "THT_AUTH_RUNTIME_PROJECTION_ROOT")) {
+    throw new Error(`${mode}: ${name} received projected auth environment`);
+  }
+}
+const maintenance = config.services?.["workspace-maintenance"];
+if ((maintenance?.volumes || []).some(
+  (mount) => mount.target === "/run/thothii-auth" || mount.target === "/data/auth",
+)) {
+  throw new Error(`${mode}: workspace-maintenance received auth mount`);
+}
+if (Object.keys(maintenance?.environment || {}).some((key) => key.startsWith("THT_AUTH_"))) {
+  throw new Error(`${mode}: workspace-maintenance received auth environment`);
+}
+if (JSON.stringify(config).includes("fixture-secret-sentinel")) {
+  throw new Error(`${mode}: rendered Compose leaked a secret sentinel`);
+}
+NODE
+done
+
+echo "runtime auth projection Compose contract passed."
diff --git a/scripts/test-canonical-install-compose.sh b/scripts/test-canonical-install-compose.sh
index f9ccfa23..05e9b3f5 100755
--- a/scripts/test-canonical-install-compose.sh
+++ b/scripts/test-canonical-install-compose.sh
@@ -79,6 +79,9 @@ const authConfig = config.services.core.volumes?.filter((mount) => mount.target
 if (authConfig.length !== 1 || authConfig[0].type !== "bind" || !authConfig[0].read_only) {
   throw new Error(profile + ": core must receive one read-only authentication config bind");
 }
+if (Object.hasOwn(config.services.core.environment || {}, "THT_AUTH_RUNTIME_PROJECTION_ROOT")) {
+  throw new Error(profile + ": non-projected fixture unexpectedly selected runtime projection");
+}
 if (profile === "local") {
   const authState = config.services.core.volumes?.filter((mount) => mount.target === "/data/auth") || [];
   if (authState.length !== 1 || authState[0].type !== "volume" || authState[0].source !== "auth-state") {
diff --git a/tools/tht/cmd/tht/main.go b/tools/tht/cmd/tht/main.go
index fa79b1ff..bd6bfc4d 100644
--- a/tools/tht/cmd/tht/main.go
+++ b/tools/tht/cmd/tht/main.go
@@ -32,6 +32,8 @@ import (
 	"github.com/aritmolab/thothii/tools/tht/internal/workspaceops"
 )
 
+var requireRuntimeAuthProjectionReady = authconfig.RequireRuntimeAuthProjectionReady
+
 const usage = `Usage: tht [--installation <absolute-path>/thothii-installation.yaml] <command>
 
 When --installation is omitted, tht uses THOTHII_INSTALLATION or discovers one valid
@@ -192,6 +194,9 @@ func run(ctx context.Context, args []string, stdout, stderr io.Writer) int {
 		if len(commandArgs) != 1 || commandArgs[0] != "--check-only" {
 			return commandUsageError(stderr, "update currently requires --check-only")
 		}
+		if err := requireRuntimeAuthProjectionReady(installation); err != nil {
+			return lifecycleFailure(stderr, errors.New("runtime authentication projection is unavailable"), secretValues)
+		}
 		result, err = runner.Run(ctx, installation.ComposeArgs("config", "--quiet"), nil)
 	case "backup":
 		return backupCommand(ctx, installation, commandArgs, stdout, stderr)
diff --git a/tools/tht/cmd/tht/main_test.go b/tools/tht/cmd/tht/main_test.go
index 8be1bfe1..6ebcb978 100644
--- a/tools/tht/cmd/tht/main_test.go
+++ b/tools/tht/cmd/tht/main_test.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -1049,6 +1050,31 @@ func TestRunPreservesChildExitCodes(t *testing.T) {
 	}
 }
 
+func TestRunUpdateCheckOnlyRefusesProjectedAuthenticationBeforeCompose(t *testing.T) {
+	for _, state := range []string{"missing", "blocked", "divergent"} {
+		t.Run(state, func(t *testing.T) {
+			fixture := projectedUpdateFixture(t)
+			previous := requireRuntimeAuthProjectionReady
+			requireRuntimeAuthProjectionReady = func(installation config.Installation) error {
+				if !installation.HasRuntimeAuthProjection() {
+					t.Fatal("update readiness gate received an unprojected installation")
+				}
+				return errors.New("synthetic " + state + " projection")
+			}
+			t.Cleanup(func() { requireRuntimeAuthProjectionReady = previous })
+
+			var stdout, stderr bytes.Buffer
+			if code := run(context.Background(), []string{"--installation", fixture.installationPath, "update", "--check-only"}, &stdout, &stderr); code == 0 {
+				t.Fatalf("update --check-only accepted %s projection", state)
+			}
+			assertDockerNotInvoked(t, fixture)
+			if strings.Contains(stdout.String()+stderr.String(), "synthetic "+state+" projection") {
+				t.Fatalf("update leaked readiness detail: stdout=%q stderr=%q", stdout.String(), stderr.String())
+			}
+		})
+	}
+}
+
 func TestRunPiStatusUsesImageBundledPi(t *testing.T) {
 	fixture := newCLIFixture(t, "THT_LLM_URL=https://llm.example.invalid\n")
 	fixture.setEnvironment(t)
@@ -1575,6 +1601,25 @@ func (f cliFixture) setProfile(t *testing.T, profile string) {
 	}
 }
 
+func projectedUpdateFixture(t *testing.T) cliFixture {
+	t.Helper()
+	fixture := newCLIFixture(t, "SAFE_VALUE=1\n")
+	fixture.setProfile(t, "server")
+	runtimeRoot := filepath.Join(fixture.root, "runtime-auth")
+	if err := os.Mkdir(runtimeRoot, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(fixture.projectDirectory, "deploy", "compose.auth-runtime-projection.yaml"), []byte("services: {}\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	descriptor := "profile: server\nprojectDirectory: " + fixture.projectDirectory + "\nenvFile: " + fixture.envFile + "\nauthentication:\n  configDirectory: " + filepath.Join(fixture.root, "auth") + "\n  runtimeProjection:\n    directory: " + runtimeRoot + "\n    uid: 10001\n    gid: 10001\n"
+	if err := os.WriteFile(fixture.installationPath, []byte(descriptor), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	fixture.setEnvContents(t, "SAFE_VALUE=1\nTHT_AUTH_RUNTIME_ROOT="+strconv.Quote(runtimeRoot)+"\n")
+	return fixture
+}
+
 func (f cliFixture) invocations(t *testing.T) [][]string {
 	t.Helper()
 	contents, err := os.ReadFile(f.argsFile)
diff --git a/tools/tht/internal/authconfig/commands.go b/tools/tht/internal/authconfig/commands.go
index 7d6e9d48..fe4a5e7d 100644
--- a/tools/tht/internal/authconfig/commands.go
+++ b/tools/tht/internal/authconfig/commands.go
@@ -22,6 +22,7 @@ import (
 	"unicode/utf16"
 	"unicode/utf8"
 
+	"github.com/aritmolab/thothii/tools/tht/internal/authprojection"
 	"github.com/aritmolab/thothii/tools/tht/internal/compose"
 	"github.com/aritmolab/thothii/tools/tht/internal/config"
 	"github.com/aritmolab/thothii/tools/tht/internal/output"
@@ -46,6 +47,8 @@ var errCommandRefused = errors.New("authentication command refused")
 
 var writeNewAuthFile = safeio.WriteCanonicalNewFile
 var removeAuthFile = safeio.RemoveCanonicalPrivateRegular
+var runProjectedAuthMutation = RunProjectedMutation
+var publishProjectedAuthCanonical = PublishProjectedCanonical
 
 // Run implements the host-only authentication operator surface. It accepts password bytes only
 // from an echo-free terminal or a bounded private file, and never writes them to either stream.
@@ -62,27 +65,165 @@ func RunWithRunner(ctx context.Context, installation config.Installation, args [
 	directory := installation.AuthenticationDirectory()
 	switch args[0] {
 	case "configure":
-		if err := configure(directory, args[1:], stdin, stderr); err != nil {
+		if err := runInstallationAuthMutation(ctx, installation, func() error {
+			return configure(directory, args[1:], stdin, stderr)
+		}); err != nil {
+			return authFailure(stderr, authMessage(err))
+		}
+		return 0
+	case "publish":
+		if err := publishInstallationAuthentication(ctx, installation, args[1:]); err != nil {
 			return authFailure(stderr, authMessage(err))
 		}
 		return 0
 	case "status":
+		if installation.HasRuntimeAuthProjection() {
+			if err := projectedStatus(installation, args[1:], stdout); err != nil {
+				return authFailure(stderr, authMessage(err))
+			}
+			return 0
+		}
 		if err := status(directory, args[1:], stdout); err != nil {
 			return authFailure(stderr, authMessage(err))
 		}
 		return 0
 	case "user":
-		if err := user(directory, args[1:], stdin, stdout, stderr); err != nil {
+		if err := runInstallationUserMutation(ctx, installation, args[1:], func() error {
+			return user(directory, args[1:], stdin, stdout, stderr)
+		}); err != nil {
 			return authFailure(stderr, authMessage(err))
 		}
 		return 0
 	case "check":
+		if err := RequireRuntimeAuthProjectionReady(installation); err != nil {
+			return authFailure(stderr, authMessage(err))
+		}
 		return checkCommand(ctx, installation, args[1:], stdout, stderr, runner)
 	default:
 		return authFailure(stderr, "unknown auth subcommand")
 	}
 }
 
+// RuntimeAuthProjectionStatus reads only public runtime-projection metadata and compares it with
+// a detached, validated snapshot of the canonical authentication store. It never publishes or
+// repairs the projection.
+func RuntimeAuthProjectionStatus(installation config.Installation) (ProjectionStatus, error) {
+	projection := installation.RuntimeAuthProjection()
+	if projection == nil {
+		return ProjectionStatus{}, errCommandRefused
+	}
+	canonical, err := loadSnapshotBytes(installation.AuthenticationDirectory())
+	if err != nil {
+		return ProjectionStatus{}, errCommandRefused
+	}
+	published, err := authprojection.Inspect(authprojection.Spec{
+		RuntimeRoot: projection.Directory,
+		UID:         projection.UID,
+		GID:         projection.GID,
+	})
+	if errors.Is(err, authprojection.ErrBlocked) {
+		return ProjectionStatus{
+			State:             "blocked",
+			CanonicalRevision: canonical.CanonicalRevision,
+			Equal:             false,
+		}, nil
+	}
+	if err != nil {
+		return ProjectionStatus{}, errCommandRefused
+	}
+	return ProjectionStatus{
+		State:             published.Selector.State,
+		Generation:        published.Snapshot.Generation,
+		CanonicalRevision: canonical.CanonicalRevision,
+		Equal:             equalProjection(published, canonical),
+	}, nil
+}
+
+// RequireRuntimeAuthProjectionReady is the shared fail-closed pre-admission check. It is a no-op
+// for an installation that does not declare a runtime projection.
+func RequireRuntimeAuthProjectionReady(installation config.Installation) error {
+	if !installation.HasRuntimeAuthProjection() {
+		return nil
+	}
+	status, err := RuntimeAuthProjectionStatus(installation)
+	if err != nil || status.State != "ready" || !status.Equal {
+		return errCommandRefused
+	}
+	return nil
+}
+
+func runInstallationAuthMutation(ctx context.Context, installation config.Installation, mutate func() error) error {
+	projection := installation.RuntimeAuthProjection()
+	if projection == nil {
+		return mutate()
+	}
+	if err := requireProjectedAuthMutationPrivilege(); err != nil {
+		return errCommandRefused
+	}
+	return runProjectedAuthMutation(ctx, installation.AuthenticationDirectory(), ProjectionSpec{
+		RuntimeRoot: projection.Directory,
+		UID:         projection.UID,
+		GID:         projection.GID,
+	}, mutate)
+}
+
+func runInstallationUserMutation(ctx context.Context, installation config.Installation, args []string, mutate func() error) error {
+	if len(args) == 0 || args[0] == "list" || !installation.HasRuntimeAuthProjection() {
+		return mutate()
+	}
+	switch args[0] {
+	case "add", "set-password", "enable", "disable", "grant", "revoke", "logout-all":
+		return runInstallationAuthMutation(ctx, installation, mutate)
+	default:
+		return mutate()
+	}
+}
+
+func publishInstallationAuthentication(ctx context.Context, installation config.Installation, args []string) error {
+	if len(args) != 0 || !installation.HasRuntimeAuthProjection() {
+		return errCommandRefused
+	}
+	if err := requireProjectedAuthMutationPrivilege(); err != nil {
+		return errCommandRefused
+	}
+	projection := installation.RuntimeAuthProjection()
+	status, err := publishProjectedAuthCanonical(ctx, installation.AuthenticationDirectory(), ProjectionSpec{
+		RuntimeRoot: projection.Directory,
+		UID:         projection.UID,
+		GID:         projection.GID,
+	})
+	if err != nil || status.State != "ready" || !status.Equal {
+		return errCommandRefused
+	}
+	return nil
+}
+
+func projectedStatus(installation config.Installation, args []string, stdout io.Writer) error {
+	jsonMode := len(args) == 1 && args[0] == "--json"
+	if len(args) != 0 && !jsonMode {
+		return errCommandRefused
+	}
+	status, err := RuntimeAuthProjectionStatus(installation)
+	if err != nil {
+		return errCommandRefused
+	}
+	if jsonMode {
+		return json.NewEncoder(stdout).Encode(struct {
+			State             string `json:"state"`
+			Generation        string `json:"generation"`
+			CanonicalRevision string `json:"canonicalRevision"`
+			Equal             bool   `json:"equal"`
+		}{
+			State:             status.State,
+			Generation:        status.Generation,
+			CanonicalRevision: status.CanonicalRevision,
+			Equal:             status.Equal,
+		})
+	}
+	_, err = fmt.Fprintf(stdout, "State: %s\nGeneration: %s\nCanonical revision: %s\nEqual: %t\n", status.State, status.Generation, status.CanonicalRevision, status.Equal)
+	return err
+}
+
 // AuthDiagnostic is the closed JSON contract emitted by the backend diagnostic command.
 type AuthDiagnostic struct {
 	Level   string  `json:"level"`
diff --git a/tools/tht/internal/authconfig/commands_linux_test.go b/tools/tht/internal/authconfig/commands_linux_test.go
new file mode 100644
index 00000000..a2478db3
--- /dev/null
+++ b/tools/tht/internal/authconfig/commands_linux_test.go
@@ -0,0 +1,372 @@
+//go:build linux
+
+package authconfig
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/aritmolab/thothii/tools/tht/internal/authprojection"
+	"github.com/aritmolab/thothii/tools/tht/internal/compose"
+	"github.com/aritmolab/thothii/tools/tht/internal/config"
+	"github.com/aritmolab/thothii/tools/tht/internal/safeio"
+)
+
+func TestProjectedAuthMutatorsBlockBeforeCanonicalWriteAndPublishOnlyEqualSnapshots(t *testing.T) {
+	installation, spec := projectedAuthInstallation(t)
+	adminPassword := writePasswordFile(t, "initial projected administrator password\n")
+	userPassword := writePasswordFile(t, "projected ordinary user password\n")
+	previous := runProjectedAuthMutation
+	blockedObservations := 0
+	runProjectedAuthMutation = func(ctx context.Context, canonicalRoot string, projection ProjectionSpec, mutate func() error) error {
+		return previous(ctx, canonicalRoot, projection, func() error {
+			status, err := authprojection.Inspect(authprojection.Spec{RuntimeRoot: projection.RuntimeRoot, UID: projection.UID, GID: projection.GID})
+			if !errors.Is(err, authprojection.ErrBlocked) || status.Selector.State != "blocked" {
+				t.Fatalf("projection before canonical mutation = %#v, %v; want blocked", status, err)
+			}
+			blockedObservations++
+			return mutate()
+		})
+	}
+	t.Cleanup(func() { runProjectedAuthMutation = previous })
+
+	for _, args := range [][]string{
+		{"configure", "--mode", "local", "--public-url", "http://127.0.0.1:8080", "--admin-user", "admin", "--password-file", adminPassword},
+		{"user", "add", "operator", "--role", "user", "--password-file", userPassword},
+		{"user", "set-password", "operator", "--password-file", adminPassword},
+		{"user", "disable", "operator"},
+		{"user", "enable", "operator"},
+		{"user", "grant", "operator", "--role", "admin"},
+		{"user", "revoke", "operator", "--role", "admin"},
+		{"user", "logout-all", "operator", "--yes"},
+	} {
+		var stdout, stderr bytes.Buffer
+		if code := Run(context.Background(), installation, args, strings.NewReader(""), &stdout, &stderr); code != 0 {
+			t.Fatalf("%v = %d, stdout=%q stderr=%q", args, code, stdout.String(), stderr.String())
+		}
+		assertProjectedCanonicalReadyAndEqual(t, installation.AuthenticationDirectory(), spec)
+		if strings.Contains(stdout.String()+stderr.String(), "projected administrator password") || strings.Contains(stdout.String()+stderr.String(), "$argon2id$") {
+			t.Fatalf("%v leaked secret material", args)
+		}
+	}
+	if blockedObservations != 8 {
+		t.Fatalf("blocked observations = %d, want 8", blockedObservations)
+	}
+}
+
+func TestProjectedAuthMutationLeavesBlockedAfterChangedCanonicalFailure(t *testing.T) {
+	installation, spec := projectedAuthInstallation(t)
+	adminPassword := writePasswordFile(t, "initial projected administrator password\n")
+	if code := Run(context.Background(), installation, []string{"configure", "--mode", "local", "--public-url", "http://127.0.0.1:8080", "--admin-user", "admin", "--password-file", adminPassword}, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code != 0 {
+		t.Fatalf("configure = %d", code)
+	}
+	previous := runProjectedAuthMutation
+	runProjectedAuthMutation = func(ctx context.Context, canonicalRoot string, projection ProjectionSpec, mutate func() error) error {
+		return previous(ctx, canonicalRoot, projection, func() error {
+			if err := mutate(); err != nil {
+				return err
+			}
+			return errors.New("synthetic-password-sentinel")
+		})
+	}
+	t.Cleanup(func() { runProjectedAuthMutation = previous })
+
+	var stdout, stderr bytes.Buffer
+	code := Run(context.Background(), installation, []string{"user", "logout-all", "admin", "--yes"}, strings.NewReader(""), &stdout, &stderr)
+	if code == 0 || strings.Contains(stdout.String()+stderr.String(), "synthetic-password-sentinel") {
+		t.Fatalf("changed mutation failure was accepted or leaked: code=%d stdout=%q stderr=%q", code, stdout.String(), stderr.String())
+	}
+	_, err := authprojection.Inspect(authprojection.Spec{RuntimeRoot: spec.RuntimeRoot, UID: spec.UID, GID: spec.GID})
+	if !errors.Is(err, authprojection.ErrBlocked) {
+		t.Fatalf("Inspect() error = %v, want blocked runtime projection", err)
+	}
+}
+
+func TestProjectedAuthMutatorFailuresRestoreOnlyUnchangedCanonicalState(t *testing.T) {
+	mutators := []string{"configure", "user add", "user set-password", "user enable", "user disable", "user grant", "user revoke", "user logout-all"}
+	for _, mutator := range mutators {
+		t.Run(mutator+" restores ready before callback", func(t *testing.T) {
+			installation, spec, args := preparedProjectedMutation(t, mutator)
+			previous := runProjectedAuthMutation
+			runProjectedAuthMutation = func(ctx context.Context, canonicalRoot string, projection ProjectionSpec, mutate func() error) error {
+				return previous(ctx, canonicalRoot, projection, func() error {
+					return errors.New("synthetic-password-sentinel")
+				})
+			}
+			t.Cleanup(func() { runProjectedAuthMutation = previous })
+
+			var stdout, stderr bytes.Buffer
+			if code := Run(context.Background(), installation, args, strings.NewReader(""), &stdout, &stderr); code == 0 {
+				t.Fatalf("%s accepted injected pre-mutation failure", mutator)
+			}
+			if strings.Contains(stdout.String()+stderr.String(), "synthetic-password-sentinel") {
+				t.Fatalf("%s leaked injected failure: stdout=%q stderr=%q", mutator, stdout.String(), stderr.String())
+			}
+			assertProjectedCanonicalReadyAndEqual(t, installation.AuthenticationDirectory(), spec)
+		})
+
+		t.Run(mutator+" leaves blocked after changed canonical error", func(t *testing.T) {
+			var installation config.Installation
+			var spec ProjectionSpec
+			var args []string
+			if mutator == "configure" {
+				installation, spec = projectedAuthInstallation(t)
+				passwordFile := writePasswordFile(t, "fresh projected bootstrap password\n")
+				args = []string{"configure", "--mode", "local", "--public-url", "http://127.0.0.1:8080", "--admin-user", "admin", "--password-file", passwordFile}
+			} else {
+				installation, spec, args = preparedProjectedMutation(t, mutator)
+			}
+			previous := runProjectedAuthMutation
+			runProjectedAuthMutation = func(ctx context.Context, canonicalRoot string, projection ProjectionSpec, mutate func() error) error {
+				return previous(ctx, canonicalRoot, projection, func() error {
+					if err := mutate(); err != nil {
+						return err
+					}
+					return errors.New("synthetic-password-sentinel")
+				})
+			}
+			t.Cleanup(func() { runProjectedAuthMutation = previous })
+
+			var stdout, stderr bytes.Buffer
+			if code := Run(context.Background(), installation, args, strings.NewReader(""), &stdout, &stderr); code == 0 {
+				t.Fatalf("%s accepted changed-canonical injected failure", mutator)
+			}
+			if strings.Contains(stdout.String()+stderr.String(), "synthetic-password-sentinel") {
+				t.Fatalf("%s leaked injected failure: stdout=%q stderr=%q", mutator, stdout.String(), stderr.String())
+			}
+			_, err := authprojection.Inspect(authprojection.Spec{RuntimeRoot: spec.RuntimeRoot, UID: spec.UID, GID: spec.GID})
+			if !errors.Is(err, authprojection.ErrBlocked) {
+				t.Fatalf("%s Inspect() error = %v, want blocked projection", mutator, err)
+			}
+		})
+	}
+}
+
+func TestProjectedAuthPublishStatusAndCheckFailClosed(t *testing.T) {
+	installation, spec := projectedAuthInstallation(t)
+	passwordFile := writePasswordFile(t, "projected authentication password\n")
+	if code := Run(context.Background(), installation, []string{"configure", "--mode", "local", "--public-url", "http://127.0.0.1:8080", "--admin-user", "admin", "--password-file", passwordFile}, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code != 0 {
+		t.Fatalf("configure = %d", code)
+	}
+	var stdout, stderr bytes.Buffer
+	if code := Run(context.Background(), installation, []string{"publish"}, strings.NewReader(""), &stdout, &stderr); code != 0 {
+		t.Fatalf("publish = %d, stdout=%q stderr=%q", code, stdout.String(), stderr.String())
+	}
+	if code := Run(context.Background(), installation, []string{"publish", "secret"}, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code == 0 {
+		t.Fatal("publish accepted content arguments")
+	}
+	stdout.Reset()
+	if code := Run(context.Background(), installation, []string{"status", "--json"}, strings.NewReader(""), &stdout, &stderr); code != 0 {
+		t.Fatalf("status = %d, stderr=%q", code, stderr.String())
+	}
+	var raw map[string]json.RawMessage
+	if err := json.Unmarshal(stdout.Bytes(), &raw); err != nil {
+		t.Fatalf("status JSON = %q, %v", stdout.String(), err)
+	}
+	if len(raw) != 4 {
+		t.Fatalf("status keys = %#v, want exactly projection public keys", raw)
+	}
+	for _, key := range []string{"state", "generation", "canonicalRevision", "equal"} {
+		if _, ok := raw[key]; !ok {
+			t.Fatalf("status keys = %#v, missing %q", raw, key)
+		}
+	}
+	var status struct {
+		State             string `json:"state"`
+		Generation        string `json:"generation"`
+		CanonicalRevision string `json:"canonicalRevision"`
+		Equal             bool   `json:"equal"`
+	}
+	if err := json.Unmarshal(stdout.Bytes(), &status); err != nil || status.State != "ready" || !status.Equal || status.Generation == "" || status.CanonicalRevision == "" {
+		t.Fatalf("status = %q, %#v, %v", stdout.String(), status, err)
+	}
+	if strings.Contains(stdout.String(), "password") || strings.Contains(stdout.String(), "$argon2id$") {
+		t.Fatalf("status exposed secret material: %q", stdout.String())
+	}
+
+	transaction, err := BeginExternalProjectionTransaction(context.Background(), installation.AuthenticationDirectory(), spec)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer transaction.Close()
+	canonical, err := loadSnapshotBytes(installation.AuthenticationDirectory())
+	if err != nil {
+		t.Fatal(err)
+	}
+	stdout.Reset()
+	stderr.Reset()
+	if code := Run(context.Background(), installation, []string{"status", "--json"}, strings.NewReader(""), &stdout, &stderr); code != 0 {
+		t.Fatalf("blocked status JSON = %d, stderr=%q", code, stderr.String())
+	}
+	raw = nil
+	if err := json.Unmarshal(stdout.Bytes(), &raw); err != nil {
+		t.Fatalf("blocked status JSON = %q, %v", stdout.String(), err)
+	}
+	if len(raw) != 4 {
+		t.Fatalf("blocked status keys = %#v, want exactly projection public keys", raw)
+	}
+	for _, key := range []string{"state", "generation", "canonicalRevision", "equal"} {
+		if _, ok := raw[key]; !ok {
+			t.Fatalf("blocked status keys = %#v, missing %q", raw, key)
+		}
+	}
+	status = struct {
+		State             string `json:"state"`
+		Generation        string `json:"generation"`
+		CanonicalRevision string `json:"canonicalRevision"`
+		Equal             bool   `json:"equal"`
+	}{}
+	if err := json.Unmarshal(stdout.Bytes(), &status); err != nil || status.State != "blocked" || status.Generation != "" || status.CanonicalRevision != canonical.CanonicalRevision || status.Equal {
+		t.Fatalf("blocked status = %q, %#v, %v", stdout.String(), status, err)
+	}
+	stdout.Reset()
+	stderr.Reset()
+	if code := Run(context.Background(), installation, []string{"status"}, strings.NewReader(""), &stdout, &stderr); code != 0 {
+		t.Fatalf("blocked status text = %d, stderr=%q", code, stderr.String())
+	}
+	wantText := "State: blocked\nGeneration: \nCanonical revision: " + canonical.CanonicalRevision + "\nEqual: false\n"
+	if stdout.String() != wantText {
+		t.Fatalf("blocked status text = %q, want %q", stdout.String(), wantText)
+	}
+
+	calls := 0
+	runner := runnerFunc(func(_ context.Context, _ []string, _ io.Reader) (compose.Result, error) {
+		calls++
+		return compose.Result{}, errors.New("backend diagnostic must not run")
+	})
+	stdout.Reset()
+	stderr.Reset()
+	if code := RunWithRunner(context.Background(), installation, []string{"check", "--json"}, strings.NewReader(""), &stdout, &stderr, runner); code == 0 || calls != 0 {
+		t.Fatalf("check admitted blocked projection: code=%d calls=%d stdout=%q stderr=%q", code, calls, stdout.String(), stderr.String())
+	}
+}
+
+func TestRequireRuntimeAuthProjectionReadyRejectsMissingBlockedAndDivergentStates(t *testing.T) {
+	for _, state := range []string{"missing", "blocked", "divergent"} {
+		t.Run(state, func(t *testing.T) {
+			installation, spec := projectedAuthInstallation(t)
+			passwordFile := writePasswordFile(t, "projected readiness password\n")
+			if code := Run(context.Background(), installation, []string{"configure", "--mode", "local", "--public-url", "http://127.0.0.1:8080", "--admin-user", "admin", "--password-file", passwordFile}, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code != 0 {
+				t.Fatalf("configure = %d", code)
+			}
+			if err := RequireRuntimeAuthProjectionReady(installation); err != nil {
+				t.Fatalf("ready projection rejected: %v", err)
+			}
+			switch state {
+			case "missing":
+				if err := os.RemoveAll(spec.RuntimeRoot); err != nil {
+					t.Fatal(err)
+				}
+			case "blocked":
+				transaction, err := BeginExternalProjectionTransaction(context.Background(), installation.AuthenticationDirectory(), spec)
+				if err != nil {
+					t.Fatal(err)
+				}
+				t.Cleanup(func() { _ = transaction.Close() })
+			case "divergent":
+				if err := MutateUsers(installation.AuthenticationDirectory(), func(registry *Registry) error {
+					registry.Users[0].AuthRevision++
+					return nil
+				}); err != nil {
+					t.Fatal(err)
+				}
+			}
+			if err := RequireRuntimeAuthProjectionReady(installation); err == nil {
+				t.Fatalf("RequireRuntimeAuthProjectionReady accepted %s projection", state)
+			}
+		})
+	}
+}
+
+func TestProjectedAuthCommandsRefuseBeforeMutationWhenNotRoot(t *testing.T) {
+	installation, _ := projectedAuthInstallation(t)
+	passwordFile := writePasswordFile(t, "projected authentication password\n")
+	previous := authProjectionEffectiveUID
+	authProjectionEffectiveUID = func() int { return 1000 }
+	t.Cleanup(func() { authProjectionEffectiveUID = previous })
+	if code := Run(context.Background(), installation, []string{"configure", "--mode", "local", "--public-url", "http://127.0.0.1:8080", "--admin-user", "admin", "--password-file", passwordFile}, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code == 0 {
+		t.Fatal("non-root projected configure succeeded")
+	}
+	if _, err := os.Lstat(filepath.Join(installation.AuthenticationDirectory(), authFileName)); !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("canonical auth was written after non-root refusal: %v", err)
+	}
+}
+
+func projectedAuthInstallation(t *testing.T) (config.Installation, ProjectionSpec) {
+	t.Helper()
+	root := t.TempDir()
+	canonicalRoot, runtimeRoot := filepath.Join(root, "canonical-auth"), filepath.Join(root, "runtime-auth")
+	for _, directory := range []string{canonicalRoot, runtimeRoot} {
+		if err := safeio.EnsurePrivateDirectory(directory); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := os.WriteFile(filepath.Join(root, "operator.env"), []byte("SAFE_VALUE=1\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	uid, gid := uint32(os.Geteuid()), uint32(os.Getegid())
+	installation := config.Installation{Profile: "server", EnvFile: filepath.Join(root, "operator.env"), Authentication: config.Authentication{
+		ConfigDirectory:   canonicalRoot,
+		RuntimeProjection: &config.RuntimeProjection{Directory: runtimeRoot, UID: uid, GID: gid},
+	}}
+	return installation, ProjectionSpec{RuntimeRoot: runtimeRoot, UID: uid, GID: gid}
+}
+
+func assertProjectedCanonicalReadyAndEqual(t *testing.T, canonicalRoot string, spec ProjectionSpec) {
+	t.Helper()
+	status, err := authprojection.Inspect(authprojection.Spec{RuntimeRoot: spec.RuntimeRoot, UID: spec.UID, GID: spec.GID})
+	if err != nil || status.Selector.State != "ready" {
+		t.Fatalf("Inspect() = %#v, %v; want ready", status, err)
+	}
+	snapshot, err := loadSnapshotBytes(canonicalRoot)
+	if err != nil || status.Snapshot.Generation != snapshot.Generation || status.Snapshot.CanonicalRevision != snapshot.CanonicalRevision {
+		t.Fatalf("projection = %#v, canonical = %#v, %v; want equality", status.Snapshot, snapshot, err)
+	}
+}
+
+func preparedProjectedMutation(t *testing.T, mutator string) (config.Installation, ProjectionSpec, []string) {
+	t.Helper()
+	installation, spec := projectedAuthInstallation(t)
+	adminPassword := writePasswordFile(t, "prepared projected administrator password\n")
+	userPassword := writePasswordFile(t, "prepared projected user password\n")
+	configure := []string{"configure", "--mode", "local", "--public-url", "http://127.0.0.1:8080", "--admin-user", "admin", "--password-file", adminPassword}
+	if code := Run(context.Background(), installation, configure, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code != 0 {
+		t.Fatalf("prepare %s configure = %d", mutator, code)
+	}
+	if mutator == "configure" {
+		return installation, spec, configure
+	}
+	if code := Run(context.Background(), installation, []string{"user", "add", "operator", "--role", "user", "--password-file", userPassword}, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code != 0 {
+		t.Fatalf("prepare %s add = %d", mutator, code)
+	}
+	if mutator == "user enable" {
+		if code := Run(context.Background(), installation, []string{"user", "disable", "operator"}, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code != 0 {
+			t.Fatalf("prepare %s disable = %d", mutator, code)
+		}
+	}
+	if mutator == "user revoke" {
+		if code := Run(context.Background(), installation, []string{"user", "grant", "operator", "--role", "admin"}, strings.NewReader(""), ioDiscard{}, ioDiscard{}); code != 0 {
+			t.Fatalf("prepare %s grant = %d", mutator, code)
+		}
+	}
+	args := map[string][]string{
+		"user add":          {"user", "add", "second", "--role", "user", "--password-file", userPassword},
+		"user set-password": {"user", "set-password", "operator", "--password-file", adminPassword},
+		"user enable":       {"user", "enable", "operator"},
+		"user disable":      {"user", "disable", "operator"},
+		"user grant":        {"user", "grant", "operator", "--role", "admin"},
+		"user revoke":       {"user", "revoke", "operator", "--role", "admin"},
+		"user logout-all":   {"user", "logout-all", "operator", "--yes"},
+	}[mutator]
+	if args == nil {
+		t.Fatalf("unknown projected mutator %q", mutator)
+	}
+	return installation, spec, args
+}
diff --git a/tools/tht/internal/authconfig/commands_projection_linux.go b/tools/tht/internal/authconfig/commands_projection_linux.go
new file mode 100644
index 00000000..dce19ecb
--- /dev/null
+++ b/tools/tht/internal/authconfig/commands_projection_linux.go
@@ -0,0 +1,14 @@
+//go:build linux
+
+package authconfig
+
+import "os"
+
+var authProjectionEffectiveUID = os.Geteuid
+
+func requireProjectedAuthMutationPrivilege() error {
+	if authProjectionEffectiveUID() != 0 {
+		return errCommandRefused
+	}
+	return nil
+}
diff --git a/tools/tht/internal/authconfig/commands_projection_unsupported.go b/tools/tht/internal/authconfig/commands_projection_unsupported.go
new file mode 100644
index 00000000..6c09fcc7
--- /dev/null
+++ b/tools/tht/internal/authconfig/commands_projection_unsupported.go
@@ -0,0 +1,5 @@
+//go:build !linux
+
+package authconfig
+
+func requireProjectedAuthMutationPrivilege() error { return errCommandRefused }
diff --git a/tools/tht/internal/backup/create_test.go b/tools/tht/internal/backup/create_test.go
index 991a7126..0a1b8d5c 100644
--- a/tools/tht/internal/backup/create_test.go
+++ b/tools/tht/internal/backup/create_test.go
@@ -175,6 +175,23 @@ func TestCreateReferencesAuthFilesByDefaultAndArchivesThemOnlyWithSecretCustody(
 	}
 	prepareBackupFixturePrivatePaths(t, []string{authDirectory}, []string{authPath, usersPath})
 	fixture.installation.Authentication.ConfigDirectory = authDirectory
+	runtimeRoot := filepath.Join(filepath.Dir(fixture.installation.Path), "auth-runtime")
+	if err := os.Mkdir(runtimeRoot, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	runtimeSentinel := []byte("runtime-projection-must-not-be-archived")
+	runtimeFiles := []string{
+		filepath.Join(runtimeRoot, "CURRENT"), filepath.Join(runtimeRoot, "manifest.json"),
+		filepath.Join(runtimeRoot, ".stage-test"), filepath.Join(runtimeRoot, ".current-test.tmp"),
+		filepath.Join(runtimeRoot, ".auth-transaction.lock"), filepath.Join(runtimeRoot, ".auth.lock"),
+	}
+	for _, path := range runtimeFiles {
+		if err := os.WriteFile(path, runtimeSentinel, 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	prepareBackupFixturePrivatePaths(t, []string{runtimeRoot}, runtimeFiles)
+	fixture.installation.Authentication.RuntimeProjection = &config.RuntimeProjection{Directory: runtimeRoot, UID: 10001, GID: 10001}
 
 	defaultOutput := filepath.Join(t.TempDir(), "default.zip")
 	defaultResult, err := createWithDependencies(context.Background(), fixture.installation, CreateRequest{Output: defaultOutput}, testDependencies(t, newBackupRunner(fixture.installation, false)))
@@ -204,6 +221,16 @@ func TestCreateReferencesAuthFilesByDefaultAndArchivesThemOnlyWithSecretCustody(
 		t.Fatalf("secret backup warning = %q, want custody guidance", secretResult.Warning)
 	}
 	secretArchive := readFixtureArchive(t, secretOutput)
+	for path, contents := range secretArchive.files {
+		if bytes.Contains(contents, runtimeSentinel) {
+			t.Fatalf("backup payload includes runtime projection data at %q", path)
+		}
+	}
+	for _, entry := range secretArchive.manifest.Entries {
+		if strings.HasPrefix(entry.SourcePath, runtimeRoot+string(filepath.Separator)) || strings.Contains(entry.Path, "auth-runtime") {
+			t.Fatalf("backup manifest references runtime projection entry %#v", entry)
+		}
+	}
 	for _, path := range []string{authPath, usersPath} {
 		if !manifestHasArchivedSecret(secretArchive.manifest, path) {
 			t.Fatalf("secret backup did not archive authentication file %q", path)
diff --git a/tools/tht/internal/backup/restore.go b/tools/tht/internal/backup/restore.go
index a4f339fb..db639f84 100644
--- a/tools/tht/internal/backup/restore.go
+++ b/tools/tht/internal/backup/restore.go
@@ -8,6 +8,7 @@ import (
 	"io"
 	"time"
 
+	"github.com/aritmolab/thothii/tools/tht/internal/authconfig"
 	"github.com/aritmolab/thothii/tools/tht/internal/config"
 	"github.com/aritmolab/thothii/tools/tht/internal/lifecycle"
 )
@@ -32,13 +33,20 @@ type RestoreResult struct {
 
 type restoreVerify func(context.Context, config.Installation, archiveRunner) error
 
+type authProjectionRestoreTransaction interface {
+	PublishCanonical() (authconfig.ProjectionStatus, error)
+	RestorePriorIfCanonicalUnchanged() error
+	Close() error
+}
+
 type restoreDependencies struct {
 	preflight func(context.Context, config.Installation, PreflightRequest) (PreflightResult, error)
 	// checkpoint requires the opaque capability created by lifecycle acquisition. It must not call
 	// public Create, which would re-acquire the non-reentrant lock and deadlock the transaction.
 	checkpoint               func(context.Context, *lifecycle.Transaction, config.Installation, CreateRequest) (Result, error)
 	prepareRecovery          func(context.Context, config.Installation, string) (PreflightResult, error)
-	recover                  func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error
+	recover                  func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error
+	beginAuthProjection      func(context.Context, config.Installation) (authProjectionRestoreTransaction, error)
 	cleanupCheckpoint        func(string) error
 	acquireTransaction       func(config.Installation) (*lifecycle.Transaction, error)
 	runner                   archiveRunner
@@ -107,6 +115,12 @@ func restoreWithDependencies(ctx context.Context, installation config.Installati
 		return RestoreResult{}, err
 	}
 	defer preflight.CloseArchive()
+	authRestoreRequired := installation.HasRuntimeAuthProjection() && manifestArchivesAuthentication(preflight.Manifest)
+	if authRestoreRequired {
+		if err := requireAuthProjectionRestorePrivilege(); err != nil {
+			return result, err
+		}
+	}
 
 	checkpoint, err := deps.checkpoint(ctx, transaction, installation, CreateRequest{IncludeSecrets: true, Confirm: true})
 	if err != nil {
@@ -150,11 +164,24 @@ func restoreWithDependencies(ctx context.Context, installation config.Installati
 		return result, errors.Join(err, cleanupErr)
 	}
 
+	var authTransaction authProjectionRestoreTransaction
+	if authRestoreRequired {
+		if deps.beginAuthProjection == nil {
+			cleanupErr := deps.cleanupCheckpoint(checkpoint.Path)
+			return result, errors.Join(errors.New("restore authentication projection dependency is unavailable"), cleanupErr)
+		}
+		authTransaction, err = deps.beginAuthProjection(ctx, installation)
+		if err != nil {
+			cleanupErr := deps.cleanupCheckpoint(checkpoint.Path)
+			return result, errors.Join(errors.New("restore authentication projection could not be blocked"), cleanupErr)
+		}
+	}
+
 	state := restoreTransactionState{}
 	defer func() {
 		if state.recoveryRequired(resultErr) {
 			recoveryContext, cancel := boundedCleanupContext()
-			recoveryErr := deps.recover(recoveryContext, installation, recovery, recoveryStage, state.wasRunning)
+			recoveryErr := deps.recover(recoveryContext, installation, recovery, recoveryStage, state.wasRunning, authTransaction)
 			cancel()
 			if recoveryErr != nil {
 				resultErr = errors.Join(resultErr, fmt.Errorf("restore recovery checkpoint: %w", recoveryErr))
@@ -174,6 +201,20 @@ func restoreWithDependencies(ctx context.Context, installation config.Installati
 			checkpointCleanupSucceeded = false
 			cleanupErr = errors.Join(cleanupErr, fmt.Errorf("destroy recovery checkpoint: %w", checkpointErr))
 		}
+		authCleanupSucceeded := true
+		if authTransaction != nil {
+			if resultErr != nil && !state.mutated {
+				if restoreErr := authTransaction.RestorePriorIfCanonicalUnchanged(); restoreErr != nil {
+					authCleanupSucceeded = false
+					cleanupErr = errors.Join(cleanupErr, errors.New("restore authentication projection could not restore its prior selector"))
+				}
+			}
+			if closeErr := authTransaction.Close(); closeErr != nil {
+				authCleanupSucceeded = false
+				cleanupErr = errors.Join(cleanupErr, errors.New("restore authentication projection transaction could not be closed"))
+			}
+			authTransaction = nil
+		}
 		if !state.maintenanceAttempted {
 			if cleanupErr != nil {
 				result = RestoreResult{}
@@ -204,7 +245,7 @@ func restoreWithDependencies(ctx context.Context, installation config.Installati
 		}
 		// A failed checkpoint recovery deliberately leaves admissions blocked. Starting or
 		// deactivating at that point would expose an unverified, possibly partial restore.
-		if state.mayDeactivateMaintenance() && restartCompleted {
+		if state.mayDeactivateMaintenance() && restartCompleted && authCleanupSucceeded {
 			deactivateErr, deactivated := retryBoundedCleanup(func(cleanupContext context.Context) error {
 				return maintenance(cleanupContext, installation, deps.runner, false)
 			})
@@ -250,6 +291,11 @@ func restoreWithDependencies(ctx context.Context, installation config.Installati
 	if err := deps.resetAuthenticationState(ctx, installation, deps.runner); err != nil {
 		return result, fmt.Errorf("reset authentication state: %w", err)
 	}
+	if authTransaction != nil {
+		if err := publishRestoredAuthentication(authTransaction); err != nil {
+			return result, err
+		}
+	}
 	if state.wasRunning {
 		if err := composeStartAndVerify(ctx, installation, deps.runner); err != nil {
 			return result, err
@@ -265,6 +311,26 @@ func restoreWithDependencies(ctx context.Context, installation config.Installati
 	return result, nil
 }
 
+func manifestArchivesAuthentication(manifest Manifest) bool {
+	for _, entry := range manifest.Entries {
+		if entry.Archived && entry.Kind == EntryExternalSecret && entry.Owner == "authentication-configuration" {
+			return true
+		}
+	}
+	return false
+}
+
+func publishRestoredAuthentication(transaction authProjectionRestoreTransaction) error {
+	status, err := transaction.PublishCanonical()
+	if err != nil || status.State != "ready" || !status.Equal {
+		if err != nil {
+			return fmt.Errorf("publish restored authentication projection: %w", err)
+		}
+		return errors.New("publish restored authentication projection")
+	}
+	return nil
+}
+
 func ensureCombinedRestoreCapacity(candidate, recovery PreflightResult) error {
 	if candidate.freeBytes == nil || candidate.stagingRoot == "" || candidate.stagingRoot != recovery.stagingRoot {
 		return errors.New("candidate and recovery archives do not share controlled restore staging")
diff --git a/tools/tht/internal/backup/restore_host.go b/tools/tht/internal/backup/restore_host.go
index 68edab64..98436209 100644
--- a/tools/tht/internal/backup/restore_host.go
+++ b/tools/tht/internal/backup/restore_host.go
@@ -51,9 +51,20 @@ func productionRestoreDependencies(installation config.Installation) restoreDepe
 		},
 		cleanupCheckpoint:  cleanupRecoveryCheckpoint,
 		acquireTransaction: lifecycle.AcquireTransaction,
-		runner:             runner,
-		sleep:              time.Sleep,
-		restoreFile:        restoreFilePayload,
+		beginAuthProjection: func(ctx context.Context, target config.Installation) (authProjectionRestoreTransaction, error) {
+			projection := target.RuntimeAuthProjection()
+			if projection == nil {
+				return nil, errors.New("runtime authentication projection is unavailable")
+			}
+			return authconfig.BeginExternalProjectionTransaction(ctx, target.AuthenticationDirectory(), authconfig.ProjectionSpec{
+				RuntimeRoot: projection.Directory,
+				UID:         projection.UID,
+				GID:         projection.GID,
+			})
+		},
+		runner:      runner,
+		sleep:       time.Sleep,
+		restoreFile: restoreFilePayload,
 		restoreVolume: func(ctx context.Context, _ config.Installation, volume VolumeMetadata, input io.Reader) error {
 			result, err := runner.Stream(ctx, volumeRestoreCommand(volume.Name), input, io.Discard)
 			if err != nil || result.ExitCode != 0 {
@@ -75,8 +86,8 @@ func productionRestoreDependencies(installation config.Installation) restoreDepe
 	deps.prepareRecovery = func(ctx context.Context, target config.Installation, path string) (PreflightResult, error) {
 		return deps.preflight(ctx, target, PreflightRequest{Archive: path, Confirm: true, AllowExternalSecrets: true})
 	}
-	deps.recover = func(ctx context.Context, target config.Installation, recovery PreflightResult, staged *stagedArchive, wasRunning bool) error {
-		return recoverRestoreTransaction(ctx, target, recovery, staged, wasRunning, deps)
+	deps.recover = func(ctx context.Context, target config.Installation, recovery PreflightResult, staged *stagedArchive, wasRunning bool, transaction authProjectionRestoreTransaction) error {
+		return recoverRestoreTransaction(ctx, target, recovery, staged, wasRunning, deps, transaction)
 	}
 	return deps
 }
@@ -88,7 +99,7 @@ func cleanupRecoveryCheckpoint(path string) error {
 	return nil
 }
 
-func recoverRestoreTransaction(ctx context.Context, installation config.Installation, recovery PreflightResult, staged *stagedArchive, wasRunning bool, deps restoreDependencies) (resultErr error) {
+func recoverRestoreTransaction(ctx context.Context, installation config.Installation, recovery PreflightResult, staged *stagedArchive, wasRunning bool, deps restoreDependencies, transaction authProjectionRestoreTransaction) (resultErr error) {
 	if staged == nil || staged.file == nil {
 		return errors.New("recovery checkpoint was not staged before restore mutation")
 	}
@@ -109,6 +120,11 @@ func recoverRestoreTransaction(ctx context.Context, installation config.Installa
 	if err := deps.resetAuthenticationState(ctx, installation, deps.runner); err != nil {
 		return errors.Join(resultErr, err)
 	}
+	if transaction != nil {
+		if err := publishRestoredAuthentication(transaction); err != nil {
+			return errors.Join(resultErr, err)
+		}
+	}
 	if wasRunning {
 		if err := composeStartAndVerify(ctx, installation, deps.runner); err != nil {
 			resultErr = errors.Join(resultErr, err)
diff --git a/tools/tht/internal/backup/restore_projection_linux.go b/tools/tht/internal/backup/restore_projection_linux.go
new file mode 100644
index 00000000..a646e359
--- /dev/null
+++ b/tools/tht/internal/backup/restore_projection_linux.go
@@ -0,0 +1,17 @@
+//go:build linux
+
+package backup
+
+import (
+	"errors"
+	"os"
+)
+
+var restoreProjectionEffectiveUID = os.Geteuid
+
+func requireAuthProjectionRestorePrivilege() error {
+	if restoreProjectionEffectiveUID() != 0 {
+		return errors.New("projected authentication restore requires root")
+	}
+	return nil
+}
diff --git a/tools/tht/internal/backup/restore_projection_linux_test.go b/tools/tht/internal/backup/restore_projection_linux_test.go
new file mode 100644
index 00000000..cc0a5a32
--- /dev/null
+++ b/tools/tht/internal/backup/restore_projection_linux_test.go
@@ -0,0 +1,42 @@
+//go:build linux
+
+package backup
+
+import (
+	"context"
+	"io"
+	"testing"
+
+	"github.com/aritmolab/thothii/tools/tht/internal/config"
+	"github.com/aritmolab/thothii/tools/tht/internal/lifecycle"
+)
+
+func TestRestoreAuthBearingArchiveRefusesNonRootBeforeTransactionOrWrite(t *testing.T) {
+	installation, archive := projectedRestoreFixture(t)
+	deps := restoreTestDependencies(t, newBackupRunner(installation, false))
+	checkpointCalled := false
+	beginCalled := false
+	writeCalled := false
+	deps.checkpoint = func(context.Context, *lifecycle.Transaction, config.Installation, CreateRequest) (Result, error) {
+		checkpointCalled = true
+		return Result{}, nil
+	}
+	deps.beginAuthProjection = func(context.Context, config.Installation) (authProjectionRestoreTransaction, error) {
+		beginCalled = true
+		return nil, nil
+	}
+	deps.restoreFile = func(context.Context, config.Installation, ArchiveEntryMetadata, io.Reader) error {
+		writeCalled = true
+		return nil
+	}
+	previous := restoreProjectionEffectiveUID
+	restoreProjectionEffectiveUID = func() int { return 1000 }
+	t.Cleanup(func() { restoreProjectionEffectiveUID = previous })
+
+	if _, err := restoreWithDependencies(context.Background(), installation, RestoreRequest{Archive: archive, Confirm: true}, deps); err == nil {
+		t.Fatal("projected authentication restore unexpectedly accepted non-root execution")
+	}
+	if checkpointCalled || beginCalled || writeCalled {
+		t.Fatalf("non-root restore crossed mutation boundary: checkpoint=%t begin=%t write=%t", checkpointCalled, beginCalled, writeCalled)
+	}
+}
diff --git a/tools/tht/internal/backup/restore_projection_unsupported.go b/tools/tht/internal/backup/restore_projection_unsupported.go
new file mode 100644
index 00000000..cf9253a5
--- /dev/null
+++ b/tools/tht/internal/backup/restore_projection_unsupported.go
@@ -0,0 +1,9 @@
+//go:build !linux
+
+package backup
+
+import "errors"
+
+func requireAuthProjectionRestorePrivilege() error {
+	return errors.New("projected authentication restore is unsupported")
+}
diff --git a/tools/tht/internal/backup/restore_test.go b/tools/tht/internal/backup/restore_test.go
index 3d3b919f..8fd7e4eb 100644
--- a/tools/tht/internal/backup/restore_test.go
+++ b/tools/tht/internal/backup/restore_test.go
@@ -13,6 +13,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/aritmolab/thothii/tools/tht/internal/authconfig"
 	"github.com/aritmolab/thothii/tools/tht/internal/compose"
 	"github.com/aritmolab/thothii/tools/tht/internal/config"
 	"github.com/aritmolab/thothii/tools/tht/internal/lifecycle"
@@ -624,7 +625,7 @@ func TestRestoreLifecycleLockExcludesCompetingTransactionsUntilTerminalCleanup(t
 					}
 					return targetFailure
 				}
-				deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+				deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 					if err := gate("recovery"); err != nil {
 						return err
 					}
@@ -645,7 +646,7 @@ func TestRestoreLifecycleLockExcludesCompetingTransactionsUntilTerminalCleanup(t
 					}
 					return targetFailure
 				}
-				deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+				deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 					if err := gate("recovery-failure"); err != nil {
 						return err
 					}
@@ -666,7 +667,7 @@ func TestRestoreLifecycleLockExcludesCompetingTransactionsUntilTerminalCleanup(t
 					cancel()
 					return context.Canceled
 				}
-				deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+				deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 					if err := gate("recovery"); err != nil {
 						return err
 					}
@@ -892,7 +893,7 @@ func TestRestoreCannotApplyAStaleCheckpointOverAnInterleavedRestore(t *testing.T
 	firstDeps.restoreFile = func(context.Context, config.Installation, ArchiveEntryMetadata, io.Reader) error {
 		return errors.New("first target mutation failed before changing state")
 	}
-	firstDeps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+	firstDeps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 		recoveryObserved = targetState
 		targetState = checkpointState
 		firstRunner.running, firstRunner.coreRunning = true, true
@@ -1095,7 +1096,7 @@ func TestRestoreFileFailureRollsBackSecretAwareCheckpointBeforeCleanup(t *testin
 		return Result{Path: "/tmp/recovery.zip"}, nil
 	}
 	var events []string
-	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 		events = append(events, "recover")
 		return nil
 	}
@@ -1135,7 +1136,7 @@ func TestRestoreFailureAfterAuthenticationMutationRollsBackAndClearsRuntimeState
 		events = append(events, "auth-runtime-reset-failed")
 		return resetErr
 	}
-	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 		events = append(events, "secret-aware-recovery-and-reauth-reset")
 		return nil
 	}
@@ -1161,7 +1162,7 @@ func TestRestoreCleanupFailureDoesNotSuppressRollback(t *testing.T) {
 	cleanupErr := errors.New("checkpoint cleanup failure")
 	recovered := false
 	deps.restoreFile = func(context.Context, config.Installation, ArchiveEntryMetadata, io.Reader) error { return mutationErr }
-	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 		recovered = true
 		return nil
 	}
@@ -1223,7 +1224,7 @@ func TestRestoreStartFailureRecoversPreviouslyRunningTarget(t *testing.T) {
 	backingRunner := newBackupRunner(installation, true)
 	deps := restoreTestDependencies(t, failStartRestoreRunner{fakeBackupRunner: backingRunner})
 	recovered := false
-	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 		recovered = true
 		backingRunner.running = true
 		return nil
@@ -1246,7 +1247,7 @@ func TestRestoreVerificationFailureRecoversPreviouslyRunningTarget(t *testing.T)
 	verificationErr := errors.New("Pi is unavailable")
 	deps.verify["pi"] = func(context.Context, config.Installation, archiveRunner) error { return verificationErr }
 	recovered := false
-	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 		recovered = true
 		return nil
 	}
@@ -1352,7 +1353,7 @@ func TestRestoreRecoversBehindBarrierForEveryVerificationFailure(t *testing.T) {
 			}
 			var recoveryBarrierActive bool
 			var recoveryContext cleanupContextObservation
-			deps.recover = func(ctx context.Context, _ config.Installation, _ PreflightResult, _ *stagedArchive, _ bool) error {
+			deps.recover = func(ctx context.Context, _ config.Installation, _ PreflightResult, _ *stagedArchive, _ bool, _ authProjectionRestoreTransaction) error {
 				recoveryContext = observeCleanupContext(ctx)
 				recoveryBarrierActive = runner.maintenance
 				runner.running, runner.coreRunning = true, true
@@ -1392,7 +1393,7 @@ func TestRestoreDoesNotRollbackAfterFinalDeactivationResponseLoss(t *testing.T)
 	}
 	deps := restoreTestDependencies(t, runner)
 	recoveryCalls := 0
-	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 		recoveryCalls++
 		return nil
 	}
@@ -1450,7 +1451,7 @@ func TestRestoreUsesBoundedRecoveryContextAfterPostMutationCancellation(t *testi
 	}
 	var recoveryContext cleanupContextObservation
 	var recoveryBarrierActive bool
-	deps.recover = func(ctx context.Context, _ config.Installation, _ PreflightResult, _ *stagedArchive, _ bool) error {
+	deps.recover = func(ctx context.Context, _ config.Installation, _ PreflightResult, _ *stagedArchive, _ bool, _ authProjectionRestoreTransaction) error {
 		recoveryContext = observeCleanupContext(ctx)
 		recoveryBarrierActive = runner.maintenance
 		runner.running, runner.coreRunning = true, true
@@ -1536,7 +1537,7 @@ func TestRecoverRestoreTransactionVerifiesRecoveredStateBeforeReturning(t *testi
 	}
 
 	staged := stageRecoveryForTest(t, installation, recovery)
-	if err := recoverRestoreTransaction(context.Background(), installation, recovery, staged, true, deps); err != nil {
+	if err := recoverRestoreTransaction(context.Background(), installation, recovery, staged, true, deps, nil); err != nil {
 		t.Fatal(err)
 	}
 	if got, want := checks, []string{"health", "doctor", "pi", "workspace"}; !equalStrings(got, want) {
@@ -1566,7 +1567,7 @@ func TestRecoverRestoreTransactionFailsClosedForEveryVerification(t *testing.T)
 			}
 
 			staged := stageRecoveryForTest(t, installation, recovery)
-			err = recoverRestoreTransaction(context.Background(), installation, recovery, staged, true, deps)
+			err = recoverRestoreTransaction(context.Background(), installation, recovery, staged, true, deps, nil)
 			if !errors.Is(err, verificationErr) {
 				t.Fatalf("recoverRestoreTransaction() error = %v, want %v", err, verificationErr)
 			}
@@ -1646,8 +1647,8 @@ func TestRestoreReleasesBarrierOnlyAfterVerifiedRecoveryFromLostResponse(t *test
 				}
 				return nil
 			}
-			deps.recover = func(ctx context.Context, target config.Installation, checkpoint PreflightResult, staged *stagedArchive, wasRunning bool) error {
-				return recoverRestoreTransaction(ctx, target, checkpoint, staged, wasRunning, deps)
+			deps.recover = func(ctx context.Context, target config.Installation, checkpoint PreflightResult, staged *stagedArchive, wasRunning bool, transaction authProjectionRestoreTransaction) error {
+				return recoverRestoreTransaction(ctx, target, checkpoint, staged, wasRunning, deps, transaction)
 			}
 
 			_, err = restoreWithDependencies(context.Background(), installation, RestoreRequest{Archive: archive, Confirm: true}, deps)
@@ -1744,7 +1745,7 @@ func TestRestoreCleansMaintenanceAfterMutationAndRollbackFailures(t *testing.T)
 			deps.restoreFile = func(context.Context, config.Installation, ArchiveEntryMetadata, io.Reader) error {
 				return mutationErr
 			}
-			deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error {
+			deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
 				if test.recoveryErr == nil {
 					backing.running, backing.coreRunning = true, true
 				}
@@ -2005,7 +2006,9 @@ func restoreTestDependencies(t *testing.T, runner archiveRunner) restoreDependen
 		prepareRecovery: func(ctx context.Context, installation config.Installation, _ string) (PreflightResult, error) {
 			return Preflight(ctx, installation, PreflightRequest{Archive: recoveryArchive, Confirm: true, AllowExternalSecrets: true}, permissivePreflightDependencies())
 		},
-		recover:            func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool) error { return nil },
+		recover: func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
+			return nil
+		},
 		cleanupCheckpoint:  func(string) error { return nil },
 		acquireTransaction: lifecycle.AcquireTransaction,
 		runner:             runner,
@@ -2025,3 +2028,235 @@ func restoreTestDependencies(t *testing.T, runner archiveRunner) restoreDependen
 		},
 	}
 }
+
+type projectionRestoreStub struct {
+	events     *[]string
+	blocked    bool
+	publishErr error
+	restoreErr error
+	closeErr   error
+}
+
+func (stub *projectionRestoreStub) PublishCanonical() (authconfig.ProjectionStatus, error) {
+	*stub.events = append(*stub.events, "publish")
+	if stub.publishErr != nil {
+		return authconfig.ProjectionStatus{}, stub.publishErr
+	}
+	stub.blocked = false
+	return authconfig.ProjectionStatus{State: "ready", Generation: "g", CanonicalRevision: "sha256:g", Equal: true}, nil
+}
+
+func (stub *projectionRestoreStub) RestorePriorIfCanonicalUnchanged() error {
+	*stub.events = append(*stub.events, "restore-prior")
+	if stub.restoreErr != nil {
+		return stub.restoreErr
+	}
+	stub.blocked = false
+	return nil
+}
+
+func (stub *projectionRestoreStub) Close() error {
+	*stub.events = append(*stub.events, "close")
+	return stub.closeErr
+}
+
+type restartEventRunner struct {
+	archiveRunner
+	events *[]string
+}
+
+func (runner restartEventRunner) Run(ctx context.Context, args []string, input io.Reader) (compose.Result, error) {
+	if strings.HasSuffix(strings.Join(args, " "), " start") {
+		*runner.events = append(*runner.events, "restart")
+	}
+	return runner.archiveRunner.Run(ctx, args, input)
+}
+
+func (runner restartEventRunner) Stream(ctx context.Context, args []string, input io.Reader, output io.Writer) (compose.Result, error) {
+	return runner.archiveRunner.Stream(ctx, args, input, output)
+}
+
+func (runner restartEventRunner) SessionInventoryScope() string {
+	return runner.archiveRunner.SessionInventoryScope()
+}
+
+func projectedRestoreFixture(t *testing.T) (config.Installation, string) {
+	t.Helper()
+	installation := preflightTestInstallation(t)
+	authRoot := filepath.Join(t.TempDir(), "canonical-auth")
+	if err := os.Mkdir(authRoot, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	installation.Authentication.ConfigDirectory = authRoot
+	installation.Authentication.RuntimeProjection = &config.RuntimeProjection{Directory: filepath.Join(t.TempDir(), "runtime-auth"), UID: 10001, GID: 10001}
+	archive := filepath.Join(t.TempDir(), "auth-restore.zip")
+	writePreflightArchive(t, archive, preflightArchiveSpec{includeSecrets: true, entries: []preflightArchiveEntry{{
+		path: "authentication-secrets/000-auth.yaml", body: []byte("candidate auth\n"), kind: EntryExternalSecret,
+		sensitive: true, owner: "authentication-configuration", sourcePath: filepath.Join(authRoot, "auth.yaml"),
+	}}})
+	return installation, archive
+}
+
+func assertOrderedEvents(t *testing.T, events []string, wants ...string) {
+	t.Helper()
+	at := 0
+	for _, want := range wants {
+		for at < len(events) && events[at] != want {
+			at++
+		}
+		if at == len(events) {
+			t.Fatalf("events = %v, want ordered subsequence %v", events, wants)
+		}
+		at++
+	}
+}
+
+func TestRestoreAuthBearingArchiveBlocksBeforeWritePublishesBeforeRestart(t *testing.T) {
+	installation, archive := projectedRestoreFixture(t)
+	var events []string
+	backing := newBackupRunner(installation, true)
+	runner := restartEventRunner{archiveRunner: backing, events: &events}
+	deps := restoreTestDependencies(t, runner)
+	transaction := &projectionRestoreStub{events: &events}
+	deps.beginAuthProjection = func(context.Context, config.Installation) (authProjectionRestoreTransaction, error) {
+		transaction.blocked = true
+		events = append(events, "begin")
+		return transaction, nil
+	}
+	deps.restoreFile = func(_ context.Context, _ config.Installation, entry ArchiveEntryMetadata, _ io.Reader) error {
+		if entry.Owner == "authentication-configuration" {
+			if !transaction.blocked {
+				t.Fatal("auth destination write began without blocked projection")
+			}
+			events = append(events, "restore-auth")
+		}
+		return nil
+	}
+	if _, err := restoreWithDependencies(context.Background(), installation, RestoreRequest{Archive: archive, Confirm: true}, deps); err != nil {
+		t.Fatal(err)
+	}
+	assertOrderedEvents(t, events, "begin", "restore-auth", "publish", "restart", "close")
+	if transaction.blocked {
+		t.Fatalf("successful restore closed while projection remained blocked: %v", events)
+	}
+}
+
+func TestRestoreAuthCandidatePublicationFailureRecoversThenStaysBlockedWithoutRestart(t *testing.T) {
+	installation, archive := projectedRestoreFixture(t)
+	var events []string
+	backing := newBackupRunner(installation, true)
+	runner := restartEventRunner{archiveRunner: backing, events: &events}
+	deps := restoreTestDependencies(t, runner)
+	publicationErr := errors.New("synthetic publication failure")
+	transaction := &projectionRestoreStub{events: &events, publishErr: publicationErr}
+	deps.beginAuthProjection = func(context.Context, config.Installation) (authProjectionRestoreTransaction, error) {
+		transaction.blocked = true
+		return transaction, nil
+	}
+	deps.recover = func(_ context.Context, _ config.Installation, _ PreflightResult, _ *stagedArchive, _ bool, transaction authProjectionRestoreTransaction) error {
+		events = append(events, "restore-checkpoint")
+		_, err := transaction.PublishCanonical()
+		return err
+	}
+	if _, err := restoreWithDependencies(context.Background(), installation, RestoreRequest{Archive: archive, Confirm: true}, deps); !errors.Is(err, publicationErr) {
+		t.Fatalf("restore error = %v, want publication failure", err)
+	}
+	assertOrderedEvents(t, events, "publish", "restore-checkpoint", "close")
+	if !transaction.blocked {
+		t.Fatal("publication failure reopened projection")
+	}
+	if backing.startCount != 0 {
+		t.Fatalf("restart after failed candidate/recovery publication = %d", backing.startCount)
+	}
+	if !backing.maintenance {
+		t.Fatal("admissions reopened after failed recovery publication")
+	}
+}
+
+func TestRestoreAuthVerificationFailuresRepublishCheckpointBeforeRecoveryRestart(t *testing.T) {
+	for _, failed := range []string{"health", "doctor", "pi", "workspace"} {
+		t.Run(failed, func(t *testing.T) {
+			installation, archive := projectedRestoreFixture(t)
+			var events []string
+			backing := newBackupRunner(installation, true)
+			runner := restartEventRunner{archiveRunner: backing, events: &events}
+			deps := restoreTestDependencies(t, runner)
+			transaction := &projectionRestoreStub{events: &events}
+			deps.beginAuthProjection = func(context.Context, config.Installation) (authProjectionRestoreTransaction, error) {
+				transaction.blocked = true
+				return transaction, nil
+			}
+			for _, name := range []string{"health", "doctor", "pi", "workspace"} {
+				name := name
+				deps.verify[name] = func(context.Context, config.Installation, archiveRunner) error {
+					events = append(events, "candidate-"+name)
+					if name == failed {
+						return errors.New("synthetic " + name + " failure")
+					}
+					return nil
+				}
+			}
+			deps.recover = func(ctx context.Context, target config.Installation, _ PreflightResult, _ *stagedArchive, wasRunning bool, transaction authProjectionRestoreTransaction) error {
+				events = append(events, "restore-checkpoint")
+				if _, err := transaction.PublishCanonical(); err != nil {
+					return err
+				}
+				events = append(events, "verify-checkpoint")
+				if wasRunning {
+					return composeStartAndVerify(ctx, target, runner)
+				}
+				return nil
+			}
+			if _, err := restoreWithDependencies(context.Background(), installation, RestoreRequest{Archive: archive, Confirm: true}, deps); err == nil {
+				t.Fatalf("%s failure accepted", failed)
+			}
+			assertOrderedEvents(t, events, "publish", "candidate-"+failed, "restore-checkpoint", "publish", "verify-checkpoint", "restart", "close")
+			if transaction.blocked {
+				t.Fatalf("verified checkpoint recovery remained blocked: %v", events)
+			}
+		})
+	}
+}
+
+func TestRestoreAuthPreMutationFailureRestoresPriorReadySelector(t *testing.T) {
+	installation, archive := projectedRestoreFixture(t)
+	var events []string
+	backing := newBackupRunner(installation, false)
+	runner := &commandFailureRunner{fakeBackupRunner: backing, failures: []*commandFailure{{
+		match: func(command string) bool { return strings.Contains(command, " ps --all --format json") },
+		err:   errors.New("synthetic pre-mutation failure"), remaining: 1,
+	}}}
+	deps := restoreTestDependencies(t, runner)
+	transaction := &projectionRestoreStub{events: &events}
+	deps.beginAuthProjection = func(context.Context, config.Installation) (authProjectionRestoreTransaction, error) {
+		transaction.blocked = true
+		return transaction, nil
+	}
+	deps.recover = func(context.Context, config.Installation, PreflightResult, *stagedArchive, bool, authProjectionRestoreTransaction) error {
+		t.Fatal("recovery ran before any destination mutation")
+		return nil
+	}
+	if _, err := restoreWithDependencies(context.Background(), installation, RestoreRequest{Archive: archive, Confirm: true}, deps); err == nil {
+		t.Fatal("pre-mutation failure accepted")
+	}
+	assertOrderedEvents(t, events, "restore-prior", "close")
+	if transaction.blocked {
+		t.Fatal("unchanged canonical pre-write failure did not restore ready selector")
+	}
+}
+
+func TestRestoreNonAuthArchiveNeverBeginsProjection(t *testing.T) {
+	installation := preflightTestInstallation(t)
+	deps := restoreTestDependencies(t, newBackupRunner(installation, false))
+	called := false
+	deps.beginAuthProjection = func(context.Context, config.Installation) (authProjectionRestoreTransaction, error) {
+		called = true
+		return nil, errors.New("must not begin")
+	}
+	if _, err := restoreWithDependencies(context.Background(), installation, RestoreRequest{Archive: restoreArchive(t), Confirm: true}, deps); err != nil {
+		t.Fatal(err)
+	}
+	if called {
+		t.Fatal("non-auth archive began projection transaction")
+	}
+}
diff --git a/tools/tht/internal/config/installation.go b/tools/tht/internal/config/installation.go
index 9632ba40..e66598b0 100644
--- a/tools/tht/internal/config/installation.go
+++ b/tools/tht/internal/config/installation.go
@@ -39,7 +39,14 @@ type descriptor struct {
 }
 
 type authenticationDescriptor struct {
-	ConfigDirectory string `yaml:"configDirectory"`
+	ConfigDirectory   string                       `yaml:"configDirectory"`
+	RuntimeProjection *runtimeProjectionDescriptor `yaml:"runtimeProjection"`
+}
+
+type runtimeProjectionDescriptor struct {
+	Directory string `yaml:"directory"`
+	UID       uint32 `yaml:"uid"`
+	GID       uint32 `yaml:"gid"`
 }
 
 type workspaceRepositoryDescriptor struct {
@@ -55,9 +62,17 @@ type WorkspaceRepository struct {
 	Access string
 }
 
+// RuntimeProjection is the non-secret runtime root and numeric container ownership contract.
+type RuntimeProjection struct {
+	Directory string
+	UID       uint32
+	GID       uint32
+}
+
 // Authentication is the non-secret filesystem location for the installation auth configuration.
 type Authentication struct {
-	ConfigDirectory string
+	ConfigDirectory   string
+	RuntimeProjection *RuntimeProjection
 }
 
 // Installation is a validated local Compose installation. It intentionally contains paths, not
@@ -114,6 +129,14 @@ func Load(path string) (Installation, error) {
 		return Installation{}, errors.New("authentication.configDirectory must be an absolute canonical path")
 	}
 
+	authentication := Authentication{ConfigDirectory: raw.Authentication.ConfigDirectory}
+	if raw.Authentication.RuntimeProjection != nil {
+		authentication.RuntimeProjection = &RuntimeProjection{
+			Directory: raw.Authentication.RuntimeProjection.Directory,
+			UID:       raw.Authentication.RuntimeProjection.UID,
+			GID:       raw.Authentication.RuntimeProjection.GID,
+		}
+	}
 	installation := Installation{
 		Path:             path,
 		Profile:          raw.Profile,
@@ -124,7 +147,7 @@ func Load(path string) (Installation, error) {
 			Branch: raw.WorkspaceRepository.Branch,
 			Access: raw.WorkspaceRepository.Access,
 		},
-		Authentication: Authentication{ConfigDirectory: raw.Authentication.ConfigDirectory},
+		Authentication: authentication,
 		Overrides:      make([]string, 0, len(raw.Overrides)),
 	}
 	values, err := installation.environmentValues()
@@ -140,6 +163,14 @@ func Load(path string) (Installation, error) {
 		}
 		installation.Overrides = append(installation.Overrides, filepath.Clean(override))
 	}
+	if err := installation.validateRuntimeAuthProjection(values); err != nil {
+		return Installation{}, err
+	}
+	if installation.HasRuntimeAuthProjection() {
+		if err := requireRegularFile(installation.runtimeAuthProjectionComposePath(), "runtime authentication Compose override"); err != nil {
+			return Installation{}, err
+		}
+	}
 	for _, composeFile := range installation.ComposeFiles()[:2] {
 		if err := requireRegularFile(composeFile, "Compose file"); err != nil {
 			return Installation{}, err
@@ -161,6 +192,49 @@ func Load(path string) (Installation, error) {
 // AuthenticationDirectory returns the descriptor-owned, non-secret authentication root.
 func (i Installation) AuthenticationDirectory() string { return i.Authentication.ConfigDirectory }
 
+// RuntimeAuthProjection returns an independent descriptor copy when this installation uses the
+// Linux-only runtime auth publication contract.
+func (i Installation) RuntimeAuthProjection() *RuntimeProjection {
+	if i.Authentication.RuntimeProjection == nil {
+		return nil
+	}
+	projection := *i.Authentication.RuntimeProjection
+	return &projection
+}
+
+// HasRuntimeAuthProjection reports whether the descriptor selects the runtime auth projection.
+func (i Installation) HasRuntimeAuthProjection() bool {
+	return i.Authentication.RuntimeProjection != nil
+}
+
+func (i Installation) validateRuntimeAuthProjection(values map[string]string) error {
+	projection := i.RuntimeAuthProjection()
+	if projection == nil {
+		if values["THT_AUTH_RUNTIME_ROOT"] != "" {
+			return errors.New("THT_AUTH_RUNTIME_ROOT requires authentication.runtimeProjection")
+		}
+		return nil
+	}
+	if i.Profile != "server" {
+		return errors.New("authentication.runtimeProjection requires the server profile")
+	}
+	if err := safeio.ValidateCanonicalPath(projection.Directory); err != nil || projection.Directory == i.AuthenticationDirectory() {
+		return errors.New("authentication.runtimeProjection.directory must be a distinct absolute canonical path")
+	}
+	if projection.UID != 10001 || projection.GID != 10001 {
+		return errors.New("authentication.runtimeProjection requires uid and gid 10001")
+	}
+	if values["THT_AUTH_RUNTIME_ROOT"] != projection.Directory {
+		return errors.New("authentication.runtimeProjection.directory must match THT_AUTH_RUNTIME_ROOT")
+	}
+	for _, override := range i.Overrides {
+		if filepath.Clean(override) == i.runtimeAuthProjectionComposePath() {
+			return errors.New("runtime authentication Compose override is automatic and must not be declared")
+		}
+	}
+	return nil
+}
+
 var safeGitBranch = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9._/-]*$`)
 var scpSSHRemote = regexp.MustCompile(`^git@[^:/\s]+:[^\s]+$`)
 
@@ -250,6 +324,9 @@ func (i Installation) ComposeFiles() []string {
 		filepath.Join(i.ProjectDirectory, "deploy", "compose."+i.Profile+".yaml"),
 	}
 	files = append(files, i.Overrides...)
+	if i.HasRuntimeAuthProjection() {
+		files = append(files, i.runtimeAuthProjectionComposePath())
+	}
 	currentImage := i.CurrentImageOverridePath()
 	if info, err := os.Lstat(currentImage); err == nil && info.Mode().IsRegular() {
 		files = append(files, currentImage)
@@ -257,6 +334,10 @@ func (i Installation) ComposeFiles() []string {
 	return files
 }
 
+func (i Installation) runtimeAuthProjectionComposePath() string {
+	return filepath.Join(i.ProjectDirectory, "deploy", "compose.auth-runtime-projection.yaml")
+}
+
 // ControlDirectory contains state that is private to one installation descriptor, even when
 // multiple installations intentionally share one source checkout.
 func (i Installation) ControlDirectory() string {
diff --git a/tools/tht/internal/config/installation_test.go b/tools/tht/internal/config/installation_test.go
index c6524b0f..13223393 100644
--- a/tools/tht/internal/config/installation_test.go
+++ b/tools/tht/internal/config/installation_test.go
@@ -52,6 +52,104 @@ func TestLoadSelectsServerComposeFiles(t *testing.T) {
 	assertStringsEqual(t, installation.ComposeFiles(), want)
 }
 
+func TestLoadAcceptsServerRuntimeProjectionAndPlacesAutomaticOverrideBeforeCurrentImage(t *testing.T) {
+	installationPath, projectDirectory, envFile, override := writeInstallation(t, "server")
+	root := filepath.Dir(installationPath)
+	authDirectory := filepath.Join(root, "canonical-auth")
+	runtimeDirectory := filepath.Join(root, "runtime-auth")
+	automaticOverride := filepath.Join(projectDirectory, "deploy", "compose.auth-runtime-projection.yaml")
+	if err := os.WriteFile(automaticOverride, []byte("services: {}\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	candidate := Installation{Path: installationPath, ProjectDirectory: projectDirectory}
+	currentImage := candidate.CurrentImageOverridePath()
+	if err := os.MkdirAll(filepath.Dir(currentImage), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(currentImage, []byte("services: {}\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	writeRuntimeProjectionFixture(t, installationPath, envFile, "server", authDirectory, runtimeDirectory, runtimeDirectory, 10001, 10001, []string{override})
+
+	installation, err := Load(installationPath)
+	if err != nil {
+		t.Fatalf("Load() error = %v", err)
+	}
+	if !installation.HasRuntimeAuthProjection() {
+		t.Fatal("HasRuntimeAuthProjection() = false, want true")
+	}
+	projection := installation.RuntimeAuthProjection()
+	if projection == nil || projection.Directory != runtimeDirectory || projection.UID != 10001 || projection.GID != 10001 {
+		t.Fatalf("RuntimeAuthProjection() = %#v", projection)
+	}
+	projection.Directory = "mutated"
+	if got := installation.RuntimeAuthProjection(); got == nil || got.Directory != runtimeDirectory {
+		t.Fatalf("RuntimeAuthProjection() did not return an independent copy: %#v", got)
+	}
+
+	want := []string{
+		filepath.Join(projectDirectory, "compose.yaml"),
+		filepath.Join(projectDirectory, "deploy", "compose.server.yaml"),
+		override,
+		automaticOverride,
+		currentImage,
+	}
+	assertStringsEqual(t, installation.ComposeFiles(), want)
+}
+
+func TestLoadRejectsInvalidRuntimeProjection(t *testing.T) {
+	for _, test := range []struct {
+		name             string
+		profile          string
+		configDirectory  func(root string) string
+		runtimeDirectory func(root string) string
+		environmentRoot  func(root string) string
+		uid, gid         uint32
+		manualOverride   bool
+	}{
+		{name: "local profile", profile: "local", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(root string) string { return filepath.Join(root, "runtime-auth") }, environmentRoot: func(root string) string { return filepath.Join(root, "runtime-auth") }, uid: 10001, gid: 10001},
+		{name: "relative runtime directory", profile: "server", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(string) string { return "relative-runtime-auth" }, environmentRoot: func(string) string { return "relative-runtime-auth" }, uid: 10001, gid: 10001},
+		{name: "noncanonical runtime directory", profile: "server", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(root string) string { return root + "/runtime-auth/../runtime-auth" }, environmentRoot: func(root string) string { return filepath.Join(root, "runtime-auth") }, uid: 10001, gid: 10001},
+		{name: "equal canonical and runtime directories", profile: "server", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, environmentRoot: func(root string) string { return filepath.Join(root, "canonical-auth") }, uid: 10001, gid: 10001},
+		{name: "uid mismatch", profile: "server", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(root string) string { return filepath.Join(root, "runtime-auth") }, environmentRoot: func(root string) string { return filepath.Join(root, "runtime-auth") }, uid: 10000, gid: 10001},
+		{name: "gid mismatch", profile: "server", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(root string) string { return filepath.Join(root, "runtime-auth") }, environmentRoot: func(root string) string { return filepath.Join(root, "runtime-auth") }, uid: 10001, gid: 10000},
+		{name: "missing runtime environment", profile: "server", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(root string) string { return filepath.Join(root, "runtime-auth") }, environmentRoot: func(root string) string { return "" }, uid: 10001, gid: 10001},
+		{name: "mismatched runtime environment", profile: "server", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(root string) string { return filepath.Join(root, "runtime-auth") }, environmentRoot: func(root string) string { return filepath.Join(root, "different-runtime-auth") }, uid: 10001, gid: 10001},
+		{name: "manual automatic override", profile: "server", configDirectory: func(root string) string { return filepath.Join(root, "canonical-auth") }, runtimeDirectory: func(root string) string { return filepath.Join(root, "runtime-auth") }, environmentRoot: func(root string) string { return filepath.Join(root, "runtime-auth") }, uid: 10001, gid: 10001, manualOverride: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			installationPath, projectDirectory, envFile, override := writeInstallation(t, test.profile)
+			root := filepath.Dir(installationPath)
+			automaticOverride := filepath.Join(projectDirectory, "deploy", "compose.auth-runtime-projection.yaml")
+			if err := os.WriteFile(automaticOverride, []byte("services: {}\n"), 0o600); err != nil {
+				t.Fatal(err)
+			}
+			overrides := []string{override}
+			if test.manualOverride {
+				overrides = append(overrides, automaticOverride)
+			}
+			writeRuntimeProjectionFixture(t, installationPath, envFile, test.profile, test.configDirectory(root), test.runtimeDirectory(root), test.environmentRoot(root), test.uid, test.gid, overrides)
+
+			if _, err := Load(installationPath); err == nil {
+				t.Fatal("Load() unexpectedly accepted an invalid runtime authentication projection")
+			}
+		})
+	}
+}
+
+func TestLoadRejectsRuntimeProjectionEnvironmentWithoutDescriptor(t *testing.T) {
+	installationPath, _, envFile, _ := writeInstallation(t, "server")
+	authDirectory := filepath.Join(filepath.Dir(installationPath), "auth")
+	runtimeDirectory := filepath.Join(filepath.Dir(installationPath), "runtime-auth")
+	contents := "THT_AUTH_CONFIG_ROOT=" + strconv.Quote(authDirectory) + "\nTHT_AUTH_RUNTIME_ROOT=" + strconv.Quote(runtimeDirectory) + "\n"
+	if err := os.WriteFile(envFile, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := Load(installationPath); err == nil {
+		t.Fatal("Load() unexpectedly accepted THT_AUTH_RUNTIME_ROOT without runtimeProjection")
+	}
+}
+
 func TestLoadRequiresAndReturnsTypedWorkspaceRepositoryForGitInstallations(t *testing.T) {
 	installationPath, projectDirectory, envFile, _ := writeInstallation(t, "local")
 	gitOverride := filepath.Join(projectDirectory, "deploy", "compose.git-ssh.yaml")
@@ -280,6 +378,25 @@ func TestParseAuthenticationDirectoryEnvironment(t *testing.T) {
 	}
 }
 
+func writeRuntimeProjectionFixture(t *testing.T, installationPath, envFile, profile, configDirectory, runtimeDirectory, environmentRoot string, uid, gid uint32, overrides []string) {
+	t.Helper()
+	lines := []string{"THT_AUTH_CONFIG_ROOT=" + strconv.Quote(configDirectory)}
+	if environmentRoot != "" {
+		lines = append(lines, "THT_AUTH_RUNTIME_ROOT="+strconv.Quote(environmentRoot))
+	}
+	if err := os.WriteFile(envFile, []byte(strings.Join(lines, "\n")+"\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	projectDirectory := filepath.Join(filepath.Dir(installationPath), "project directory with spaces")
+	contents := "profile: " + profile + "\nprojectDirectory: " + projectDirectory + "\nenvFile: " + envFile + "\nauthentication:\n  configDirectory: " + configDirectory + "\n  runtimeProjection:\n    directory: " + runtimeDirectory + "\n    uid: " + strconv.FormatUint(uint64(uid), 10) + "\n    gid: " + strconv.FormatUint(uint64(gid), 10) + "\noverrides:\n"
+	for _, override := range overrides {
+		contents += "  - " + override + "\n"
+	}
+	if err := os.WriteFile(installationPath, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func writeInstallation(t *testing.T, profile string) (string, string, string, string) {
 	t.Helper()
 
diff --git a/tools/tht/internal/doctor/report.go b/tools/tht/internal/doctor/report.go
index 0f8d0843..1cd9e4f8 100644
--- a/tools/tht/internal/doctor/report.go
+++ b/tools/tht/internal/doctor/report.go
@@ -28,6 +28,8 @@ const (
 
 const probeTimeout = 5 * time.Second
 
+var requireRuntimeAuthProjectionReady = authconfig.RequireRuntimeAuthProjectionReady
+
 const registryValidationProgram = `const fs=require("node:fs");const path="/data/workspace-registry/state/active.json";const s=JSON.parse(fs.readFileSync(path,"utf8"));const hex=/^[0-9a-f]{40}$/;if(!hex.test(s.head)||!Array.isArray(s.revisions)||s.revisions.some((r)=>!r||typeof r.id!=="string"||!r.id||!hex.test(r.commit)||!hex.test(r.blob))){process.exit(1)}for(const r of s.revisions){fs.accessSync("/data/workspace-registry/snapshots/"+r.commit+"/"+r.id+".yaml",fs.constants.R_OK)}`
 
 // Check is one named, redacted diagnostic outcome.
@@ -118,6 +120,13 @@ func RunWithProbe(ctx context.Context, installation config.Installation, runner
 	} else {
 		add("files", StatusPassed, "declared host files have safe permissions")
 	}
+	if installation.HasRuntimeAuthProjection() {
+		if err := requireRuntimeAuthProjectionReady(installation); err != nil {
+			add("auth-projection", StatusFailed, "runtime authentication projection is unavailable")
+			return finalize(report), nil
+		}
+		add("auth-projection", StatusPassed, "runtime authentication projection is ready and equal to canonical authentication")
+	}
 	if secretErr != nil {
 		add("docker", StatusSkipped, "declared secret files are unavailable")
 		add("compose", StatusSkipped, "declared secret files are unavailable")
diff --git a/tools/tht/internal/doctor/report_test.go b/tools/tht/internal/doctor/report_test.go
index 695954ad..ac0bad33 100644
--- a/tools/tht/internal/doctor/report_test.go
+++ b/tools/tht/internal/doctor/report_test.go
@@ -27,6 +27,56 @@ func TestRunReportsUnavailableDockerWithoutReturningAnExecutionError(t *testing.
 	}
 }
 
+func TestRunReportsOneSanitizedRuntimeAuthProjectionFailureBeforeCompose(t *testing.T) {
+	installation := doctorInstallation(t, "")
+	installation.Profile = "server"
+	installation.Authentication.RuntimeProjection = &config.RuntimeProjection{
+		Directory: "/runtime-auth", UID: 10001, GID: 10001,
+	}
+	previous := requireRuntimeAuthProjectionReady
+	requireRuntimeAuthProjectionReady = func(config.Installation) error {
+		return errors.New("synthetic-runtime-projection-secret")
+	}
+	t.Cleanup(func() { requireRuntimeAuthProjectionReady = previous })
+
+	runner := &doctorRunner{services: healthyServices}
+	report, err := Run(context.Background(), installation, runner)
+	if err != nil {
+		t.Fatal(err)
+	}
+	failures := 0
+	for _, check := range report.Checks {
+		if check.Name != "auth-projection" {
+			continue
+		}
+		failures++
+		if check.Status != StatusFailed || check.Detail != "runtime authentication projection is unavailable" {
+			t.Fatalf("auth-projection check = %#v", check)
+		}
+	}
+	if failures != 1 {
+		t.Fatalf("auth-projection failures = %d, report = %#v", failures, report)
+	}
+	if strings.Contains(reportText(report), "synthetic-runtime-projection-secret") {
+		t.Fatalf("runtime projection report leaked internal detail: %#v", report)
+	}
+	if len(runner.calls) != 0 {
+		t.Fatalf("doctor reached Compose diagnostics after failed projection gate: %v", runner.calls)
+	}
+}
+
+func TestRunLeavesUnprojectedDoctorChecklistUnchanged(t *testing.T) {
+	report, err := Run(context.Background(), doctorInstallation(t, ""), &doctorRunner{services: healthyServices})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, check := range report.Checks {
+		if check.Name == "auth-projection" {
+			t.Fatalf("unprojected report unexpectedly contains auth-projection: %#v", report)
+		}
+	}
+}
+
 func TestValidateVolumesRequiresAuthState(t *testing.T) {
 	legacy := `{"volumes":{"settings":{},"pi-state":{},"workspace-registry":{},"workspace-secrets":{},"sessions":{},"qdrant-data":{},"embedding-models":{}}}`
 	if err := ValidateVolumes(legacy); err == nil || !strings.Contains(err.Error(), "auth-state") {
diff --git a/tools/tht/internal/service/service.go b/tools/tht/internal/service/service.go
index a1d9d8c9..d067ba9f 100644
--- a/tools/tht/internal/service/service.go
+++ b/tools/tht/internal/service/service.go
@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/aritmolab/thothii/tools/tht/internal/authconfig"
 	"github.com/aritmolab/thothii/tools/tht/internal/compose"
 	"github.com/aritmolab/thothii/tools/tht/internal/config"
 )
@@ -17,6 +18,7 @@ import (
 const healthTimeout = 5 * time.Minute
 
 var healthPollInterval = time.Second
+var requireRuntimeAuthProjectionReady = authconfig.RequireRuntimeAuthProjectionReady
 
 // HealthFailure identifies the last non-ready service after a bounded health wait.
 type HealthFailure struct {
@@ -41,6 +43,9 @@ func Start(ctx context.Context, installation config.Installation, runner compose
 	if runner == nil {
 		return errors.New("start requires a Docker command runner")
 	}
+	if err := requireRuntimeAuthProjectionReady(installation); err != nil {
+		return errors.New("runtime authentication projection is unavailable")
+	}
 	if build {
 		if err := runCompose(ctx, installation, runner, "build"); err != nil {
 			return fmt.Errorf("image build: %w", err)
diff --git a/tools/tht/internal/service/service_test.go b/tools/tht/internal/service/service_test.go
index c428091b..a3917456 100644
--- a/tools/tht/internal/service/service_test.go
+++ b/tools/tht/internal/service/service_test.go
@@ -2,6 +2,7 @@ package service
 
 import (
 	"context"
+	"errors"
 	"io"
 	"strings"
 	"testing"
@@ -36,6 +37,30 @@ func TestStartSkipsBuildUnlessRequested(t *testing.T) {
 	}
 }
 
+func TestStartRefusesProjectedAuthenticationBeforeComposeWhenNotReady(t *testing.T) {
+	for _, state := range []string{"missing", "blocked", "divergent"} {
+		t.Run(state, func(t *testing.T) {
+			previous := requireRuntimeAuthProjectionReady
+			requireRuntimeAuthProjectionReady = func(installation config.Installation) error {
+				if !installation.HasRuntimeAuthProjection() {
+					t.Fatal("readiness gate received an unprojected installation")
+				}
+				return errors.New("synthetic " + state + " projection")
+			}
+			t.Cleanup(func() { requireRuntimeAuthProjectionReady = previous })
+
+			runner := &recordingRunner{}
+			err := Start(context.Background(), projectedTestInstallation(), runner, true)
+			if err == nil {
+				t.Fatalf("Start() accepted %s runtime projection", state)
+			}
+			if len(runner.stages) != 0 {
+				t.Fatalf("Start() reached Compose for %s projection: %v", state, runner.stages)
+			}
+		})
+	}
+}
+
 type recordingRunner struct{ stages []string }
 
 func (r *recordingRunner) Run(_ context.Context, args []string, _ io.Reader) (compose.Result, error) {
@@ -61,6 +86,15 @@ func testInstallation() config.Installation {
 	}
 }
 
+func projectedTestInstallation() config.Installation {
+	installation := testInstallation()
+	installation.Profile = "server"
+	installation.Authentication.RuntimeProjection = &config.RuntimeProjection{
+		Directory: "/runtime-auth", UID: 10001, GID: 10001,
+	}
+	return installation
+}
+
 const healthyServices = `[
   {"Service":"core","State":"running","Health":"healthy"},
   {"Service":"frontend","State":"running","Health":"healthy"},
diff --git a/tools/tht/internal/setup/files.go b/tools/tht/internal/setup/files.go
index 26b481be..60200383 100644
--- a/tools/tht/internal/setup/files.go
+++ b/tools/tht/internal/setup/files.go
@@ -30,6 +30,10 @@ var installationIDPattern = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9_-]*$`)
 // file and leaves no final target until all content is synced.
 var atomicWriteNewFile = writeNewFileAtomically
 
+// effectiveUID is a package-private seam so projected server setup can prove its root gate
+// happens before any filesystem mutation.
+var effectiveUID = currentEffectiveUID
+
 type answers struct {
 	installationID, profile          string
 	workspaceRemote, workspaceBranch string
@@ -51,7 +55,12 @@ type generatedDescriptor struct {
 		Access string `yaml:"access"`
 	} `yaml:"workspaceRepository"`
 	Authentication struct {
-		ConfigDirectory string `yaml:"configDirectory"`
+		ConfigDirectory   string `yaml:"configDirectory"`
+		RuntimeProjection *struct {
+			Directory string `yaml:"directory"`
+			UID       uint32 `yaml:"uid"`
+			GID       uint32 `yaml:"gid"`
+		} `yaml:"runtimeProjection,omitempty"`
 	} `yaml:"authentication"`
 	Overrides []string `yaml:"overrides"`
 }
@@ -70,6 +79,9 @@ func EnsureFiles(request Request, input io.Reader, output io.Writer) (FilesResul
 	if err := validateAnswers(values); err != nil {
 		return FilesResult{}, err
 	}
+	if values.profile == "server" && effectiveUID() != 0 {
+		return FilesResult{}, errors.New("projected server setup requires root")
+	}
 
 	directory, err := installationDirectory(root, values.installationID)
 	if err != nil {
@@ -77,7 +89,11 @@ func EnsureFiles(request Request, input io.Reader, output io.Writer) (FilesResul
 	}
 	descriptorPath := filepath.Join(directory, descriptorName)
 	environmentPath := filepath.Join(directory, environmentName)
-	if err := safeio.EnsurePrivateDirectory(filepath.Join(directory, "auth")); err != nil {
+	if values.profile == "server" {
+		if err := ensureProjectedAuthDirectories(filepath.Join(directory, "auth"), filepath.Join(directory, "auth-runtime")); err != nil {
+			return FilesResult{}, errors.New("projected authentication directories are unavailable or unsafe")
+		}
+	} else if err := safeio.EnsurePrivateDirectory(filepath.Join(directory, "auth")); err != nil {
 		return FilesResult{}, errors.New("authentication directory is unavailable or unsafe")
 	}
 	result := FilesResult{DescriptorPath: descriptorPath, EnvironmentPath: environmentPath}
@@ -305,6 +321,17 @@ func render(root, descriptorPath string, value answers) ([]byte, []byte, error)
 	descriptor := generatedDescriptor{Profile: value.profile, ProjectDirectory: root, EnvFile: filepath.Join(filepath.Dir(descriptorPath), environmentName)}
 	descriptor.Workspace.Remote, descriptor.Workspace.Branch, descriptor.Workspace.Access = value.workspaceRemote, value.workspaceBranch, value.workspaceAccess
 	descriptor.Authentication.ConfigDirectory = filepath.Join(filepath.Dir(descriptorPath), "auth")
+	if value.profile == "server" {
+		descriptor.Authentication.RuntimeProjection = &struct {
+			Directory string `yaml:"directory"`
+			UID       uint32 `yaml:"uid"`
+			GID       uint32 `yaml:"gid"`
+		}{
+			Directory: filepath.Join(filepath.Dir(descriptorPath), "auth-runtime"),
+			UID:       10001,
+			GID:       10001,
+		}
+	}
 	descriptor.Overrides = []string{filepath.Join(root, "deploy", "compose.git-"+value.workspaceAccess+".yaml")}
 	descriptorBytes, err := yaml.Marshal(descriptor)
 	if err != nil {
@@ -334,6 +361,7 @@ func render(root, descriptorPath string, value answers) ([]byte, []byte, error)
 	if value.profile == "server" {
 		installationDirectory := filepath.Dir(descriptorPath)
 		lines = append(lines,
+			"THT_AUTH_RUNTIME_ROOT="+dotenvValue(descriptor.Authentication.RuntimeProjection.Directory),
 			"THT_DATA_ROOT="+dotenvValue(filepath.Join(installationDirectory, "data")),
 			"THT_PI_STATE_ROOT="+dotenvValue(filepath.Join(installationDirectory, "pi-state")),
 			"THT_WORKSPACE_REGISTRY_ROOT="+dotenvValue(filepath.Join(installationDirectory, "workspace-registry")),
diff --git a/tools/tht/internal/setup/files_linux.go b/tools/tht/internal/setup/files_linux.go
new file mode 100644
index 00000000..02d2c1a1
--- /dev/null
+++ b/tools/tht/internal/setup/files_linux.go
@@ -0,0 +1,74 @@
+//go:build linux
+
+package setup
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/sys/unix"
+)
+
+func currentEffectiveUID() int { return os.Geteuid() }
+
+func ensureProjectedAuthDirectories(canonicalRoot, runtimeRoot string) error {
+	parent := filepath.Dir(canonicalRoot)
+	canonicalName, runtimeName := filepath.Base(canonicalRoot), filepath.Base(runtimeRoot)
+	if parent != filepath.Dir(runtimeRoot) || canonicalName == runtimeName || canonicalName == "." || runtimeName == "." {
+		return errors.New("projected authentication directory layout is invalid")
+	}
+	parentFD, err := unix.Open(parent, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+	if err != nil {
+		return errors.New("projected authentication parent is unavailable")
+	}
+	defer unix.Close(parentFD)
+	if err := requireProjectedDirectoryMetadata(parentFD, 0, 0); err != nil {
+		return err
+	}
+	if err := ensureProjectedAuthDirectoryAt(parentFD, canonicalName, 0, 0); err != nil {
+		return err
+	}
+	if err := ensureProjectedAuthDirectoryAt(parentFD, runtimeName, 10001, 10001); err != nil {
+		return err
+	}
+	if err := unix.Fsync(parentFD); err != nil {
+		return errors.New("projected authentication parent could not be synchronized")
+	}
+	return nil
+}
+
+func ensureProjectedAuthDirectoryAt(parentFD int, name string, uid, gid int) error {
+	fd, err := unix.Openat(parentFD, name, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+	if err != nil && !errors.Is(err, unix.ENOENT) {
+		return errors.New("projected authentication directory is unavailable")
+	}
+	if errors.Is(err, unix.ENOENT) {
+		if err := unix.Mkdirat(parentFD, name, 0o700); err != nil && !errors.Is(err, unix.EEXIST) {
+			return errors.New("projected authentication directory could not be created")
+		}
+		fd, err = unix.Openat(parentFD, name, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
+		if err != nil {
+			return errors.New("projected authentication directory is unavailable")
+		}
+	}
+	defer unix.Close(fd)
+	if err := unix.Fchown(fd, uid, gid); err != nil {
+		return errors.New("projected authentication directory ownership could not be set")
+	}
+	if err := unix.Fchmod(fd, 0o700); err != nil {
+		return errors.New("projected authentication directory mode could not be set")
+	}
+	if err := unix.Fsync(fd); err != nil {
+		return errors.New("projected authentication directory could not be synchronized")
+	}
+	return requireProjectedDirectoryMetadata(fd, uint32(uid), uint32(gid))
+}
+
+func requireProjectedDirectoryMetadata(fd int, uid, gid uint32) error {
+	var metadata unix.Stat_t
+	if err := unix.Fstat(fd, &metadata); err != nil || metadata.Mode&unix.S_IFMT != unix.S_IFDIR || metadata.Mode&0o777 != 0o700 || metadata.Uid != uid || metadata.Gid != gid {
+		return errors.New("projected authentication directory metadata is invalid")
+	}
+	return nil
+}
diff --git a/tools/tht/internal/setup/files_linux_test.go b/tools/tht/internal/setup/files_linux_test.go
new file mode 100644
index 00000000..40e2b23d
--- /dev/null
+++ b/tools/tht/internal/setup/files_linux_test.go
@@ -0,0 +1,56 @@
+//go:build linux
+
+package setup
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+
+	"github.com/aritmolab/thothii/tools/tht/internal/config"
+)
+
+func TestEnsureFilesProjectedServerCreatesExactNumericAuthenticationRoots(t *testing.T) {
+	if os.Geteuid() != 0 {
+		t.Skip("requires root to verify numeric projected ownership")
+	}
+	root := newProject(t, "projected server root")
+	for _, name := range []string{"compose.server.yaml", "compose.auth-runtime-projection.yaml"} {
+		if err := os.WriteFile(filepath.Join(root, "deploy", name), []byte("services: {}\n"), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	setNonInteractiveAnswers(t, newExternalSecrets(t, root))
+	result, err := EnsureFiles(Request{
+		ProjectRoot: root, InstallationID: "server", Profile: "server", NonInteractive: true,
+	}, nil, ioDiscard{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	installation, err := config.Load(result.DescriptorPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	projection := installation.RuntimeAuthProjection()
+	if projection == nil {
+		t.Fatal("generated server descriptor lacks runtime auth projection")
+	}
+	assertNumericDirectoryMetadata(t, installation.AuthenticationDirectory(), 0, 0, 0o700)
+	assertNumericDirectoryMetadata(t, projection.Directory, 10001, 10001, 0o700)
+}
+
+func assertNumericDirectoryMetadata(t *testing.T, path string, uid, gid uint32, mode os.FileMode) {
+	t.Helper()
+	info, err := os.Stat(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	metadata, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		t.Fatalf("stat metadata for %s = %T", path, info.Sys())
+	}
+	if metadata.Uid != uid || metadata.Gid != gid || info.Mode().Perm() != mode {
+		t.Fatalf("metadata for %s = uid=%d gid=%d mode=%o, want uid=%d gid=%d mode=%o", path, metadata.Uid, metadata.Gid, info.Mode().Perm(), uid, gid, mode)
+	}
+}
diff --git a/tools/tht/internal/setup/files_test.go b/tools/tht/internal/setup/files_test.go
index 6d1c43f4..ca024837 100644
--- a/tools/tht/internal/setup/files_test.go
+++ b/tools/tht/internal/setup/files_test.go
@@ -225,6 +225,24 @@ func TestEnsureFilesIncludesServerStorageLocations(t *testing.T) {
 	}
 }
 
+func TestEnsureFilesProjectedServerRefusesBeforeAnyWriteWhenNotRoot(t *testing.T) {
+	root := newProject(t, "projected server non-root")
+	setNonInteractiveAnswers(t, newExternalSecrets(t, root))
+	previous := effectiveUID
+	effectiveUID = func() int { return 1000 }
+	t.Cleanup(func() { effectiveUID = previous })
+
+	_, err := EnsureFiles(Request{
+		ProjectRoot: root, InstallationID: "server", Profile: "server", NonInteractive: true,
+	}, strings.NewReader(""), ioDiscard{})
+	if err == nil || !strings.Contains(err.Error(), "root") {
+		t.Fatalf("EnsureFiles() error = %v, want root refusal", err)
+	}
+	if _, statErr := os.Lstat(filepath.Join(root, "deploy", "server")); !errors.Is(statErr, os.ErrNotExist) {
+		t.Fatalf("projected server path was created before root refusal: %v", statErr)
+	}
+}
+
 func TestEnsureFilesRejectsUnsafeServiceEndpointsBeforeWritingConfiguration(t *testing.T) {
 	for _, test := range []struct {
 		name, environment, value string
diff --git a/tools/tht/internal/setup/files_unsupported.go b/tools/tht/internal/setup/files_unsupported.go
new file mode 100644
index 00000000..30c7ad49
--- /dev/null
+++ b/tools/tht/internal/setup/files_unsupported.go
@@ -0,0 +1,11 @@
+//go:build !linux
+
+package setup
+
+import "errors"
+
+func currentEffectiveUID() int { return -1 }
+
+func ensureProjectedAuthDirectories(_, _ string) error {
+	return errors.New("runtime authentication projection setup is unsupported")
+}
diff --git a/tools/tht/internal/setup/run.go b/tools/tht/internal/setup/run.go
index f5f1921e..7c3cb202 100644
--- a/tools/tht/internal/setup/run.go
+++ b/tools/tht/internal/setup/run.go
@@ -19,6 +19,9 @@ import (
 	"github.com/aritmolab/thothii/tools/tht/internal/service"
 )
 
+var publishProjectedCanonical = authconfig.PublishProjectedCanonical
+var requireRuntimeAuthProjectionReady = authconfig.RequireRuntimeAuthProjectionReady
+
 // Result records the completed setup phases. DescriptorPath always identifies the descriptor
 // selected by this invocation, including an idempotent rerun.
 type Result struct {
@@ -85,7 +88,7 @@ func Run(ctx context.Context, runner compose.Runner, request Request, input io.R
 func configureAuthentication(ctx context.Context, installation config.Installation, request Request, input io.Reader, output io.Writer) error {
 	directory := installation.AuthenticationDirectory()
 	if _, _, err := authconfig.Load(directory); err == nil {
-		return nil
+		return publishConfiguredAuthentication(ctx, installation)
 	}
 	if _, err := os.Lstat(filepath.Join(directory, "auth.yaml")); !errors.Is(err, os.ErrNotExist) {
 		return errors.New("setup authentication configuration is invalid")
@@ -100,6 +103,25 @@ func configureAuthentication(ctx context.Context, installation config.Installati
 	if _, _, err := authconfig.Load(directory); err != nil {
 		return errors.New("setup authentication configuration is invalid")
 	}
+	return publishConfiguredAuthentication(ctx, installation)
+}
+
+func publishConfiguredAuthentication(ctx context.Context, installation config.Installation) error {
+	projection := installation.RuntimeAuthProjection()
+	if projection == nil {
+		return nil
+	}
+	status, err := publishProjectedCanonical(ctx, installation.AuthenticationDirectory(), authconfig.ProjectionSpec{
+		RuntimeRoot: projection.Directory,
+		UID:         projection.UID,
+		GID:         projection.GID,
+	})
+	if err != nil || status.State != "ready" || !status.Equal {
+		return errors.New("setup authentication runtime projection could not be published")
+	}
+	if err := requireRuntimeAuthProjectionReady(installation); err != nil {
+		return errors.New("setup authentication runtime projection could not be verified")
+	}
 	return nil
 }
 
diff --git a/tools/tht/internal/setup/run_test.go b/tools/tht/internal/setup/run_test.go
index bd99fa83..f7e083fa 100644
--- a/tools/tht/internal/setup/run_test.go
+++ b/tools/tht/internal/setup/run_test.go
@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/aritmolab/thothii/tools/tht/internal/authconfig"
+	"github.com/aritmolab/thothii/tools/tht/internal/authprojection"
 	"github.com/aritmolab/thothii/tools/tht/internal/compose"
 	"github.com/aritmolab/thothii/tools/tht/internal/config"
 	"github.com/aritmolab/thothii/tools/tht/internal/doctor"
@@ -99,6 +100,102 @@ func TestRunConfiguresAndStaticallyValidatesLocalAuthBeforeComposeRender(t *test
 	assertSetupStages(t, runner, "docker engine", "docker compose", "architecture", "compose config")
 }
 
+func TestRunConfigureOnlyPublishesInitialProjectedServerAuthentication(t *testing.T) {
+	projectRoot, request := setupRunFixture(t, true)
+	request.Profile = "server"
+	if err := os.WriteFile(filepath.Join(projectRoot, "deploy", "compose.auth-runtime-projection.yaml"), []byte("services: {}\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := Run(context.Background(), &setupRunner{}, request, strings.NewReader(""), io.Discard); err != nil {
+		t.Fatal(err)
+	}
+	installation, err := config.Load(filepath.Join(projectRoot, "deploy", "ci", "thothii-installation.yaml"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	projection := installation.RuntimeAuthProjection()
+	if projection == nil {
+		t.Fatal("generated server installation has no runtime auth projection")
+	}
+	status, err := authprojection.Inspect(authprojection.Spec{RuntimeRoot: projection.Directory, UID: projection.UID, GID: projection.GID})
+	if err != nil || status.Selector.State != "ready" {
+		t.Fatalf("initial runtime auth projection = %#v, %v; want ready", status, err)
+	}
+}
+
+func TestRunConfigureOnlyLeavesProjectedAuthenticationBlockedWhenInitialPublicationFails(t *testing.T) {
+	projectRoot, request := setupRunFixture(t, true)
+	request.Profile = "server"
+	if err := os.WriteFile(filepath.Join(projectRoot, "deploy", "compose.auth-runtime-projection.yaml"), []byte("services: {}\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	previous := publishProjectedCanonical
+	publishProjectedCanonical = func(ctx context.Context, canonicalRoot string, spec authconfig.ProjectionSpec) (authconfig.ProjectionStatus, error) {
+		transaction, err := authconfig.BeginExternalProjectionTransaction(ctx, canonicalRoot, spec)
+		if err != nil {
+			return authconfig.ProjectionStatus{}, err
+		}
+		if err := transaction.Close(); err != nil {
+			return authconfig.ProjectionStatus{}, err
+		}
+		return authconfig.ProjectionStatus{}, errors.New("synthetic-password-sentinel")
+	}
+	t.Cleanup(func() { publishProjectedCanonical = previous })
+
+	_, err := Run(context.Background(), &setupRunner{}, request, strings.NewReader(""), io.Discard)
+	if err == nil || strings.Contains(err.Error(), "synthetic-password-sentinel") {
+		t.Fatalf("Run() error = %v, want sanitized publication failure", err)
+	}
+	installation, loadErr := config.Load(filepath.Join(projectRoot, "deploy", "ci", "thothii-installation.yaml"))
+	if loadErr != nil {
+		t.Fatal(loadErr)
+	}
+	projection := installation.RuntimeAuthProjection()
+	if projection == nil {
+		t.Fatal("generated server installation has no runtime auth projection")
+	}
+	_, inspectErr := authprojection.Inspect(authprojection.Spec{RuntimeRoot: projection.Directory, UID: projection.UID, GID: projection.GID})
+	if !errors.Is(inspectErr, authprojection.ErrBlocked) {
+		t.Fatalf("Inspect() error = %v, want blocked projection", inspectErr)
+	}
+}
+
+func TestRunConfigureOnlyVerifiesProjectedAuthenticationAfterPublication(t *testing.T) {
+	projectRoot, request := setupRunFixture(t, true)
+	request.Profile = "server"
+	if err := os.WriteFile(filepath.Join(projectRoot, "deploy", "compose.auth-runtime-projection.yaml"), []byte("services: {}\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	previous := requireRuntimeAuthProjectionReady
+	calls := 0
+	requireRuntimeAuthProjectionReady = func(installation config.Installation) error {
+		calls++
+		projection := installation.RuntimeAuthProjection()
+		if projection == nil {
+			t.Fatal("post-publication readiness received an unprojected installation")
+		}
+		status, err := authprojection.Inspect(authprojection.Spec{
+			RuntimeRoot: projection.Directory,
+			UID:         projection.UID,
+			GID:         projection.GID,
+		})
+		if err != nil || status.Selector.State != "ready" {
+			t.Fatalf("post-publication projection = %#v, %v; want ready", status, err)
+		}
+		return errors.New("synthetic-readiness-secret")
+	}
+	t.Cleanup(func() { requireRuntimeAuthProjectionReady = previous })
+
+	_, err := Run(context.Background(), &setupRunner{}, request, strings.NewReader(""), io.Discard)
+	if err == nil || strings.Contains(err.Error(), "synthetic-readiness-secret") {
+		t.Fatalf("Run() error = %v, want sanitized post-publication readiness failure", err)
+	}
+	if calls != 1 {
+		t.Fatalf("post-publication readiness calls = %d, want 1", calls)
+	}
+}
+
 func TestRunRejectsIncompleteNonInteractiveLocalAuthenticationBeforeComposeRender(t *testing.T) {
 	_, request := setupRunFixture(t, true)
 	request.NonInteractive = true