diff --git a/backend/src/app.ts b/backend/src/app.ts
index 0fea37f7..6a26e9e2 100644
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -14,6 +14,7 @@ import type { LoadedAuthConfig } from "./auth/types.js";
 import { createCurrentLocalUserRegistryResolver, type LocalUserRegistry } from "./auth/local-registry.js";
 import { AuthSessionOperationalError, createFileAuthSessionStore, type AuthSessionStore, type AuthSessionValidity } from "./auth/session-store.js";
 import { registerAuthRoutes } from "./auth/routes.js";
+import { createOidcProtocol, type OidcProtocol } from "./auth/oidc-client.js";
 import { sessionRoutes } from "./routes/sessions.js";
 import { sqlRoutes } from "./routes/sql.js";
 import { metaRoutes, type ListModelsFn } from "./routes/meta.js";
@@ -48,6 +49,7 @@ export interface BuildAppDeps {
   piManagement?: PiManagementService;
   localUserRegistry?: LocalUserRegistry;
   authSessionStore?: AuthSessionStore;
+  oidcProtocol?: OidcProtocol;
 }
 
 export interface AppWithAuthSessionStore extends FastifyInstance {
@@ -192,6 +194,26 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
     currentAuthConfigRevision: () => loaded.revision,
     currentLocalUser: (subject) => localUserForSnapshot(loaded, subject),
   });
+  const resolveOidcProtocol = (loaded: LoadedAuthConfig): OidcProtocol | undefined => {
+    if (deps?.oidcProtocol) return deps.oidcProtocol;
+    if (loaded.value.mode !== "oidc") return undefined;
+    const clientSecret = process.env.THT_OIDC_CLIENT_SECRET;
+    if (typeof clientSecret !== "string" || clientSecret.length === 0 || clientSecret.length > 4096 || /\p{Cc}/u.test(clientSecret)) {
+      return undefined;
+    }
+    try {
+      return createOidcProtocol({
+        issuer: loaded.value.oidc.issuer,
+        clientId: loaded.value.oidc.clientId,
+        clientSecret,
+        callbackUrl: new URL("/api/auth/oidc/callback", loaded.value.publicUrl).href,
+        scopes: loaded.value.oidc.scopes,
+        groupsClaim: loaded.value.oidc.groupsClaim,
+      });
+    } catch {
+      return undefined;
+    }
+  };
   const authSessionStore = deps?.authSessionStore ?? (config.authMode === "local" || config.authMode === "oidc"
     ? createFileAuthSessionStore(config.authStateRoot, {
       currentAuthConfigRevision: () => {
@@ -251,6 +273,7 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
     sessionStore: authSessionStore,
     localUserRegistry: deps?.localUserRegistry,
     resolveLocalUserRegistry,
+    resolveOidcProtocol,
   });
   sessionRoutes(app, {
     mgr, tht: tht as ThtRunner, hub, getSettings, readiness, listModels, workspaceRegistry,
diff --git a/backend/src/auth/oidc-client.ts b/backend/src/auth/oidc-client.ts
new file mode 100644
index 00000000..c039919f
--- /dev/null
+++ b/backend/src/auth/oidc-client.ts
@@ -0,0 +1,294 @@
+import {
+  authorizationCodeGrant,
+  buildAuthorizationUrl,
+  calculatePKCECodeChallenge,
+  customFetch,
+  discovery,
+  type Configuration,
+} from "openid-client";
+import { constants, createPublicKey, verify as verifySignature } from "node:crypto";
+
+export interface OidcIdentity {
+  issuer: string;
+  subject: string;
+  displayName?: string;
+  groups: readonly string[];
+  tokenExpiresAt: Date;
+}
+
+export interface OidcProtocol {
+  authorizationUrl(input: { state: string; nonce: string; codeVerifier: string }): Promise<URL>;
+  callback(input: { currentUrl: URL; state: string; nonce: string; codeVerifier: string }): Promise<OidcIdentity>;
+  diagnose(signal: AbortSignal): Promise<void>;
+  verifyDeviceFlow?(signal: AbortSignal, present: (uri: string, code: string) => void): Promise<OidcIdentity>;
+}
+
+export class OidcProtocolError extends Error {
+  constructor() {
+    super("oidc_protocol_invalid");
+  }
+}
+
+export interface OidcProtocolOptions {
+  issuer: string;
+  clientId: string;
+  clientSecret: string;
+  callbackUrl: string;
+  scopes: readonly string[];
+  groupsClaim: string;
+  fetch?: typeof globalThis.fetch;
+}
+
+const MAX_GROUPS = 128;
+const MAX_GROUP_LENGTH = 256;
+const MAX_ID_TOKEN_LENGTH = 16 * 1024;
+const MAX_JWKS_BYTES = 1024 * 1024;
+const text = (value: unknown, maximum = 2048): value is string =>
+  typeof value === "string" && value.length > 0 && value.length <= maximum && !/\p{Cc}/u.test(value);
+
+function configuredUrl(value: string): URL {
+  let url: URL;
+  try {
+    url = new URL(value);
+  } catch {
+    throw new OidcProtocolError();
+  }
+  if (url.protocol !== "https:" || url.username || url.password || url.search || url.hash) throw new OidcProtocolError();
+  return url;
+}
+
+function httpsEndpoint(value: unknown): URL {
+  if (!text(value, 2048)) throw new OidcProtocolError();
+  let url: URL;
+  try {
+    url = new URL(value);
+  } catch {
+    throw new OidcProtocolError();
+  }
+  if (url.protocol !== "https:" || url.username || url.password || url.hash) throw new OidcProtocolError();
+  return url;
+}
+
+function groupsFromClaims(claims: Record<string, unknown>, name: string): string[] {
+  const indirect = claims._claim_names;
+  if ((indirect && typeof indirect === "object" && !Array.isArray(indirect)
+    && Object.prototype.hasOwnProperty.call(indirect, name))
+    || claims.hasgroups === true) throw new OidcProtocolError();
+  const raw = claims[name];
+  if (!Array.isArray(raw) || raw.length > MAX_GROUPS) throw new OidcProtocolError();
+  const groups: string[] = [];
+  const unique = new Set<string>();
+  for (const group of raw) {
+    if (!text(group, MAX_GROUP_LENGTH) || group.trim().length === 0 || unique.has(group)) throw new OidcProtocolError();
+    unique.add(group);
+    groups.push(group);
+  }
+  return groups;
+}
+
+function identityFromClaims(claims: Record<string, unknown>, options: OidcProtocolOptions): OidcIdentity {
+  if (claims.iss !== options.issuer || !text(claims.sub, 512)) throw new OidcProtocolError();
+  const audience = claims.aud;
+  if (!(audience === options.clientId || (Array.isArray(audience) && audience.includes(options.clientId)))) {
+    throw new OidcProtocolError();
+  }
+  if (typeof claims.exp !== "number" || !Number.isSafeInteger(claims.exp) || claims.exp * 1000 <= Date.now()) {
+    throw new OidcProtocolError();
+  }
+  const tokenExpiresAt = new Date(claims.exp * 1000);
+  if (Number.isNaN(tokenExpiresAt.getTime())) throw new OidcProtocolError();
+  return {
+    issuer: options.issuer,
+    subject: claims.sub,
+    ...(text(claims.name, 256) ? { displayName: claims.name } : {}),
+    groups: groupsFromClaims(claims, options.groupsClaim),
+    tokenExpiresAt,
+  };
+}
+
+function jsonPart(part: string): Record<string, unknown> {
+  if (!/^[A-Za-z0-9_-]+$/.test(part) || part.length > MAX_ID_TOKEN_LENGTH) throw new OidcProtocolError();
+  try {
+    const value = JSON.parse(new TextDecoder("utf-8", { fatal: true }).decode(Buffer.from(part, "base64url")));
+    if (!value || typeof value !== "object" || Array.isArray(value)) throw new OidcProtocolError();
+    return value as Record<string, unknown>;
+  } catch {
+    throw new OidcProtocolError();
+  }
+}
+
+function signatureAlgorithm(algorithm: string): {
+  digest: string | null;
+  pss?: boolean;
+  saltLength?: number;
+  ecdsaPartLength?: number;
+} {
+  switch (algorithm) {
+    case "RS256": return { digest: "RSA-SHA256" };
+    case "RS384": return { digest: "RSA-SHA384" };
+    case "RS512": return { digest: "RSA-SHA512" };
+    case "PS256": return { digest: "sha256", pss: true, saltLength: 32 };
+    case "PS384": return { digest: "sha384", pss: true, saltLength: 48 };
+    case "PS512": return { digest: "sha512", pss: true, saltLength: 64 };
+    case "ES256": return { digest: "sha256", ecdsaPartLength: 32 };
+    case "ES384": return { digest: "sha384", ecdsaPartLength: 48 };
+    case "ES512": return { digest: "sha512", ecdsaPartLength: 66 };
+    case "EdDSA": return { digest: null };
+    default: throw new OidcProtocolError();
+  }
+}
+
+function derLength(length: number): Buffer {
+  if (length < 128) return Buffer.from([length]);
+  if (length < 256) return Buffer.from([0x81, length]);
+  throw new OidcProtocolError();
+}
+
+function derInteger(raw: Buffer): Buffer {
+  let start = 0;
+  while (start < raw.length - 1 && raw[start] === 0) start += 1;
+  let value = raw.subarray(start);
+  if ((value[0] & 0x80) !== 0) value = Buffer.concat([Buffer.from([0]), value]);
+  return Buffer.concat([Buffer.from([0x02]), derLength(value.length), value]);
+}
+
+function joseEcdsaSignatureToDer(signature: Buffer, partLength: number): Buffer {
+  if (signature.length !== partLength * 2) throw new OidcProtocolError();
+  const sequence = Buffer.concat([
+    derInteger(signature.subarray(0, partLength)),
+    derInteger(signature.subarray(partLength)),
+  ]);
+  return Buffer.concat([Buffer.from([0x30]), derLength(sequence.length), sequence]);
+}
+
+async function verifyIdTokenSignature(
+  idToken: unknown,
+  config: Configuration,
+  options: OidcProtocolOptions,
+): Promise<void> {
+  if (!text(idToken, MAX_ID_TOKEN_LENGTH)) throw new OidcProtocolError();
+  const [protectedPart, payloadPart, signaturePart, extra] = idToken.split(".");
+  if (!protectedPart || !payloadPart || !signaturePart || extra) throw new OidcProtocolError();
+  const header = jsonPart(protectedPart);
+  if (!text(header.alg, 16) || !text(header.kid, 256)) throw new OidcProtocolError();
+  const metadata = config.serverMetadata();
+  if (!Array.isArray(metadata.id_token_signing_alg_values_supported)
+    || !metadata.id_token_signing_alg_values_supported.includes(header.alg)
+    || !text(metadata.jwks_uri, 2048)) throw new OidcProtocolError();
+  const jwksUrl = httpsEndpoint(metadata.jwks_uri);
+  let response: Response;
+  try {
+    response = await (options.fetch ?? globalThis.fetch)(jwksUrl, {
+      headers: { accept: "application/json" }, redirect: "error",
+    });
+    if (!response.ok) throw new OidcProtocolError();
+    const body = await response.text();
+    if (Buffer.byteLength(body, "utf8") > MAX_JWKS_BYTES) throw new OidcProtocolError();
+    const parsed = JSON.parse(body) as { keys?: unknown };
+    if (!Array.isArray(parsed.keys) || parsed.keys.length === 0 || parsed.keys.length > 16) throw new OidcProtocolError();
+    const matching = parsed.keys.filter((key): key is Record<string, unknown> =>
+      Boolean(key) && typeof key === "object" && !Array.isArray(key) && key.kid === header.kid);
+    if (matching.length !== 1) throw new OidcProtocolError();
+    const key = matching[0];
+    if (key.use !== undefined && key.use !== "sig") throw new OidcProtocolError();
+    if (key.alg !== undefined && key.alg !== header.alg) throw new OidcProtocolError();
+    const algorithm = signatureAlgorithm(header.alg);
+    if (!/^[A-Za-z0-9_-]+$/.test(signaturePart)) throw new OidcProtocolError();
+    const signature = Buffer.from(signaturePart, "base64url");
+    if (signature.length === 0) throw new OidcProtocolError();
+    const publicKey = createPublicKey({ key: key as never, format: "jwk" });
+    const normalizedSignature = algorithm.ecdsaPartLength
+      ? joseEcdsaSignatureToDer(signature, algorithm.ecdsaPartLength)
+      : signature;
+    const verified = algorithm.pss
+      ? verifySignature(algorithm.digest, Buffer.from(`${protectedPart}.${payloadPart}`), {
+        key: publicKey, padding: constants.RSA_PKCS1_PSS_PADDING, saltLength: algorithm.saltLength,
+      }, normalizedSignature)
+      : verifySignature(algorithm.digest, Buffer.from(`${protectedPart}.${payloadPart}`), publicKey, normalizedSignature);
+    if (!verified) throw new OidcProtocolError();
+  } catch (error) {
+    if (error instanceof OidcProtocolError) throw error;
+    throw new OidcProtocolError();
+  }
+}
+
+export function createOidcProtocol(options: OidcProtocolOptions): OidcProtocol {
+  const issuerUrl = configuredUrl(options.issuer);
+  const callbackUrl = configuredUrl(options.callbackUrl);
+  if (!text(options.clientId, 512) || !text(options.clientSecret, 4096)
+    || !text(options.groupsClaim, 128) || options.scopes.length === 0 || options.scopes.length > 16
+    || options.scopes.some((scope) => !text(scope, 128))) throw new OidcProtocolError();
+
+  let discovered: Promise<Configuration> | undefined;
+  const configuration = async (): Promise<Configuration> => {
+    if (!discovered) {
+      discovered = (async () => {
+        try {
+          const config = await discovery(
+            issuerUrl,
+            options.clientId,
+            { client_secret: options.clientSecret, redirect_uris: [callbackUrl.href], response_types: ["code"] },
+            undefined,
+            options.fetch ? { [customFetch]: options.fetch as never } : undefined,
+          );
+          const metadata = config.serverMetadata();
+          if (metadata.issuer !== options.issuer) throw new OidcProtocolError();
+          httpsEndpoint(metadata.authorization_endpoint);
+          httpsEndpoint(metadata.token_endpoint);
+          httpsEndpoint(metadata.jwks_uri);
+          return config;
+        } catch (error) {
+          if (error instanceof OidcProtocolError) throw error;
+          throw new OidcProtocolError();
+        }
+      })();
+    }
+    return await discovered;
+  };
+
+  return {
+    async authorizationUrl(input) {
+      try {
+        const challenge = await calculatePKCECodeChallenge(input.codeVerifier);
+        return buildAuthorizationUrl(await configuration(), {
+          response_type: "code",
+          redirect_uri: callbackUrl.href,
+          scope: options.scopes.join(" "),
+          state: input.state,
+          nonce: input.nonce,
+          code_challenge: challenge,
+          code_challenge_method: "S256",
+        });
+      } catch (error) {
+        if (error instanceof OidcProtocolError) throw error;
+        throw new OidcProtocolError();
+      }
+    },
+    async callback(input) {
+      if (input.currentUrl.origin !== callbackUrl.origin || input.currentUrl.pathname !== callbackUrl.pathname) {
+        throw new OidcProtocolError();
+      }
+      try {
+        const config = await configuration();
+        const tokens = await authorizationCodeGrant(config, input.currentUrl, {
+          expectedState: input.state,
+          expectedNonce: input.nonce,
+          pkceCodeVerifier: input.codeVerifier,
+          idTokenExpected: true,
+        });
+        await verifyIdTokenSignature(tokens.id_token, config, options);
+        const claims = tokens.claims();
+        if (!claims || Array.isArray(claims)) throw new OidcProtocolError();
+        return identityFromClaims(claims as Record<string, unknown>, options);
+      } catch (error) {
+        if (error instanceof OidcProtocolError) throw error;
+        throw new OidcProtocolError();
+      }
+    },
+    async diagnose(signal) {
+      signal.throwIfAborted();
+      await configuration();
+      signal.throwIfAborted();
+    },
+  };
+}
diff --git a/backend/src/auth/routes.ts b/backend/src/auth/routes.ts
index 6278a828..9fd80297 100644
--- a/backend/src/auth/routes.ts
+++ b/backend/src/auth/routes.ts
@@ -1,6 +1,6 @@
 import type { FastifyInstance, FastifyReply, FastifyRequest } from "fastify";
 import { randomBytes } from "node:crypto";
-import type { AuthenticationConfigProvider, LoadedAuthConfig } from "./types.js";
+import type { AuthenticationConfigProvider, LoadedAuthConfig, OidcAuthenticationConfig, Role } from "./types.js";
 import type { LocalUserRecord, LocalUserRegistry } from "./local-registry.js";
 import type { AuthSessionStore } from "./session-store.js";
 import { rolesToPermissions } from "./config.js";
@@ -8,12 +8,15 @@ import { captureAuthConfigSnapshot, getPrincipal, requireExactOrigin, sessionCoo
 import { requirePermission, isPrincipalContext } from "./authorization.js";
 import { deriveCsrfToken } from "./csrf.js";
 import { verifyWithDummy } from "./password.js";
+import type { OidcProtocol } from "./oidc-client.js";
 
 const TEN_MINUTES_MS = 10 * 60 * 1000;
 const REMEMBER_COOKIE_SECONDS = 2_592_000;
 const MAX_USERNAME_LENGTH = 64;
 const MAX_PASSWORD_LENGTH = 1024;
 const MAX_LIMIT_ENTRIES = 10_000;
+const MAX_OIDC_CALLBACK_QUERY_LENGTH = 4096;
+const OIDC_CALLBACK_PATH = "/api/auth/oidc/callback";
 
 export interface AuthRouteDependencies {
   authMode: "local" | "oidc" | "upstream" | "none" | "mock";
@@ -22,6 +25,8 @@ export interface AuthRouteDependencies {
   /** Test-only compatibility seam; production resolves from each loaded config snapshot. */
   localUserRegistry?: LocalUserRegistry;
   resolveLocalUserRegistry?: (loaded: LoadedAuthConfig) => LocalUserRegistry | undefined;
+  oidcProtocol?: OidcProtocol;
+  resolveOidcProtocol?: (loaded: LoadedAuthConfig) => OidcProtocol | undefined;
 }
 
 interface LoginPayload {
@@ -118,7 +123,7 @@ export function registerAuthRoutes(app: FastifyInstance, deps: AuthRouteDependen
     const snapshot = captureAuthConfigSnapshot(request, deps.authentication);
     if (!snapshot) return unavailable(reply);
     const mode = snapshot.value.mode;
-    return reply.send({ mode, localLogin: mode === "local", oidcLogin: false });
+    return reply.send({ mode, localLogin: mode === "local", oidcLogin: mode === "oidc" });
   });
 
   app.post("/auth/local/login", async (request, reply) => {
@@ -180,8 +185,83 @@ export function registerAuthRoutes(app: FastifyInstance, deps: AuthRouteDependen
     }
   });
 
-  app.get("/auth/oidc/login", async (_request, reply) => notImplemented(reply));
-  app.get("/auth/oidc/callback", async (_request, reply) => notImplemented(reply));
+  app.get("/auth/oidc/login", async (request, reply) => {
+    const loaded = captureAuthConfigSnapshot(request, deps.authentication);
+    const configured = currentOidcConfig(loaded, deps);
+    if (!configured || !deps.sessionStore) return unavailable(reply);
+    const nonce = randomOidcValue();
+    const codeVerifier = randomOidcValue();
+    try {
+      const created = await deps.sessionStore.createOidcState({
+        nonce,
+        codeVerifier,
+        returnTo: "/",
+        authConfigRevision: configured.loaded.revision,
+        issuer: configured.config.oidc.issuer,
+      });
+      try {
+        const location = await configured.protocol.authorizationUrl({ state: created.state, nonce, codeVerifier });
+        return reply.redirect(location.href);
+      } catch {
+        await deps.sessionStore.consumeOidcState(created.state).catch(() => undefined);
+        return unavailable(reply);
+      }
+    } catch {
+      return unavailable(reply);
+    }
+  });
+
+  app.get("/auth/oidc/callback", async (request, reply) => {
+    const loaded = captureAuthConfigSnapshot(request, deps.authentication);
+    if (!loaded || !deps.sessionStore) return oidcCallbackFailed(reply);
+    const callback = oidcCallbackUrl(request, loaded.value.publicUrl);
+    if (!callback) return oidcCallbackFailed(reply);
+    let state;
+    try {
+      state = await deps.sessionStore.consumeOidcState(callback.state);
+    } catch {
+      return oidcCallbackFailed(reply);
+    }
+    const configured = currentOidcConfig(loaded, deps);
+    if (!configured || !state || state.returnTo !== "/" || state.authConfigRevision !== configured.loaded.revision
+      || state.issuer !== configured.config.oidc.issuer) {
+      return oidcCallbackFailed(reply);
+    }
+    try {
+      const identity = await configured.protocol.callback({
+        currentUrl: callback.currentUrl,
+        state: callback.state,
+        nonce: state.nonce,
+        codeVerifier: state.codeVerifier,
+      });
+      if (identity.issuer !== configured.config.oidc.issuer) return oidcCallbackFailed(reply);
+      const roles = oidcRoles(identity.groups, configured.config);
+      const absoluteTtlMs = Math.min(
+        configured.config.session.oidcTtlSeconds * 1000,
+        identity.tokenExpiresAt.getTime() - Date.now(),
+      );
+      if (!Number.isSafeInteger(absoluteTtlMs) || absoluteTtlMs <= 0) return oidcCallbackFailed(reply);
+      const created = await deps.sessionStore.create({
+        principal: {
+          issuer: identity.issuer,
+          subject: identity.subject,
+          ...(identity.displayName === undefined ? {} : { displayName: identity.displayName }),
+          roles,
+          permissions: rolesToPermissions(roles),
+          isAdmin: roles.includes("admin"),
+        },
+        method: "oidc",
+        remembered: false,
+        authConfigRevision: configured.loaded.revision,
+        idleTtlMs: configured.config.session.regularIdleSeconds * 1000,
+        absoluteTtlMs,
+      });
+      reply.setCookie(sessionCookieName(), created.token, cookieOptions(configured.loaded, false));
+      return reply.redirect(state.returnTo);
+    } catch {
+      return oidcCallbackFailed(reply);
+    }
+  });
 
   app.post("/auth/logout", async (request, reply) => {
     const token = request.authSessionToken;
@@ -278,6 +358,54 @@ function cookieOptions(snapshot: LoadedAuthConfig | undefined, remembered: boole
   };
 }
 
+function currentOidcConfig(loaded: LoadedAuthConfig | undefined, deps: AuthRouteDependencies):
+  | { loaded: LoadedAuthConfig; config: OidcAuthenticationConfig; protocol: OidcProtocol }
+  | undefined {
+  if (!loaded || loaded.value.mode !== "oidc") return undefined;
+  const protocol = deps.resolveOidcProtocol?.(loaded) ?? deps.oidcProtocol;
+  return protocol ? { loaded, config: loaded.value, protocol } : undefined;
+}
+
+function randomOidcValue(): string {
+  return randomBytes(32).toString("base64url");
+}
+
+function oidcCallbackUrl(request: FastifyRequest, publicUrl: string): { currentUrl: URL; state: string } | undefined {
+  if (request.url.length > MAX_OIDC_CALLBACK_QUERY_LENGTH) return undefined;
+  let supplied: URL;
+  let target: URL;
+  try {
+    supplied = new URL(request.url, "http://callback.invalid");
+    target = new URL(OIDC_CALLBACK_PATH, publicUrl);
+  } catch {
+    return undefined;
+  }
+  if (supplied.pathname !== "/auth/oidc/callback") return undefined;
+  const allowed = new Set(["code", "state", "error", "error_description", "error_uri", "iss"]);
+  const copied = new URLSearchParams();
+  let state: string | undefined;
+  for (const [key, value] of supplied.searchParams) {
+    if (!allowed.has(key) || value.length > 2048 || copied.has(key)) return undefined;
+    copied.set(key, value);
+    if (key === "state") state = value;
+  }
+  if (!state || !/^[A-Za-z0-9_-]{43}$/.test(state)) return undefined;
+  target.search = copied.toString();
+  return { currentUrl: target, state };
+}
+
+function oidcCallbackFailed(reply: FastifyReply) {
+  return reply.code(401).send({ code: "oidc_callback_failed", error: "OIDC sign-in could not be completed" });
+}
+
+function oidcRoles(groups: readonly string[], config: OidcAuthenticationConfig): Role[] {
+  const roles = new Set<Role>();
+  for (const group of groups) {
+    for (const role of config.authorization.groupRoles[group] ?? []) roles.add(role);
+  }
+  return [...roles];
+}
+
 function loginPayload(request: FastifyRequest): LoginPayload {
   const body = request.body;
   if (!body || typeof body !== "object" || Array.isArray(body)) return { username: "", password: "", remember: false };
@@ -316,7 +444,3 @@ function loginLimited(reply: FastifyReply): FastifyReply {
 function unavailable(reply: FastifyReply): FastifyReply {
   return reply.code(503).send({ code: "auth_unavailable", error: "Authentication is unavailable" });
 }
-
-function notImplemented(reply: FastifyReply): FastifyReply {
-  return reply.code(501).send({ code: "auth_not_implemented", error: "OIDC login is not implemented" });
-}
diff --git a/backend/src/auth/session-store.ts b/backend/src/auth/session-store.ts
index 01d659d6..4cd6d529 100644
--- a/backend/src/auth/session-store.ts
+++ b/backend/src/auth/session-store.ts
@@ -68,6 +68,8 @@ export interface OidcStateCreateInput {
   nonce: string;
   codeVerifier: string;
   returnTo: "/";
+  authConfigRevision: string;
+  issuer: string;
 }
 
 export interface CreatedOidcState {
@@ -188,6 +190,8 @@ const oidcStateRecordSchema = z.strictObject({
   nonce: z.string().min(16).max(512).regex(/^[A-Za-z0-9_-]+$/),
   codeVerifier: z.string().min(43).max(128).regex(/^[A-Za-z0-9._~-]+$/),
   returnTo: z.literal("/"),
+  authConfigRevision: z.string().regex(/^[a-f0-9]{64}$/),
+  issuer: z.string().min(1).max(2048).refine((value) => !/\p{Cc}/u.test(value)),
   createdAt: timestamp,
   expiresAt: timestamp,
 }).superRefine((record, context) => {
@@ -226,6 +230,8 @@ const oidcStateInputSchema = z.strictObject({
   nonce: z.string().min(16).max(512).regex(/^[A-Za-z0-9_-]+$/),
   codeVerifier: z.string().min(43).max(128).regex(/^[A-Za-z0-9._~-]+$/),
   returnTo: z.literal("/"),
+  authConfigRevision: z.string().regex(/^[a-f0-9]{64}$/),
+  issuer: z.string().min(1).max(2048).refine((value) => !/\p{Cc}/u.test(value)),
 });
 
 function sameFileIdentity(left: FileIdentity, right: FileIdentity): boolean {
@@ -927,6 +933,8 @@ export function createFileAuthSessionStore(
       nonce: validated.nonce,
       codeVerifier: validated.codeVerifier,
       returnTo: validated.returnTo,
+      authConfigRevision: validated.authConfigRevision,
+      issuer: validated.issuer,
       createdAt: isoAt(nowMs),
       expiresAt: isoAt(expiresMs),
     };
diff --git a/backend/src/auth/types.ts b/backend/src/auth/types.ts
index 86b37787..ad4f4f5d 100644
--- a/backend/src/auth/types.ts
+++ b/backend/src/auth/types.ts
@@ -79,6 +79,8 @@ export interface OidcStateRecord {
   nonce: string;
   codeVerifier: string;
   returnTo: "/";
+  authConfigRevision: string;
+  issuer: string;
   createdAt: string;
   expiresAt: string;
 }
diff --git a/backend/test/app-auth-mode.test.ts b/backend/test/app-auth-mode.test.ts
index 8378e3b2..efaf958d 100644
--- a/backend/test/app-auth-mode.test.ts
+++ b/backend/test/app-auth-mode.test.ts
@@ -6,7 +6,7 @@ import { stringify } from "yaml";
 import { buildApp } from "../src/app.js";
 import { loadConfig } from "../src/config.js";
 
-test("configured OIDC starts with provider-neutral protocol placeholders that fail closed", async () => {
+test("configured OIDC advertises login but fails closed without its runtime client secret", async () => {
   const directory = mkdtempSync(join(tmpdir(), "thothii-app-oidc-mode-"));
   const file = join(directory, "auth.yaml");
   writeFileSync(file, stringify({
@@ -22,10 +22,10 @@ test("configured OIDC starts with provider-neutral protocol placeholders that fa
     const app = buildApp(loadConfig({ THT_AUTH_CONFIG_FILE: file, THT_AUTH_STATE_ROOT: join(directory, "auth-state") }));
     try {
       expect((await app.inject({ method: "GET", url: "/auth/config" })).json())
-        .toEqual({ mode: "oidc", localLogin: false, oidcLogin: false });
+        .toEqual({ mode: "oidc", localLogin: false, oidcLogin: true });
       const placeholder = await app.inject({ method: "GET", url: "/auth/oidc/login" });
-      expect(placeholder.statusCode).toBe(501);
-      expect(placeholder.json()).toEqual({ code: "auth_not_implemented", error: "OIDC login is not implemented" });
+      expect(placeholder.statusCode).toBe(503);
+      expect(placeholder.json()).toEqual({ code: "auth_unavailable", error: "Authentication is unavailable" });
     } finally {
       await app.close();
     }
diff --git a/backend/test/auth-routes-local.test.ts b/backend/test/auth-routes-local.test.ts
index 7f30eaf0..5e2bfa84 100644
--- a/backend/test/auth-routes-local.test.ts
+++ b/backend/test/auth-routes-local.test.ts
@@ -500,7 +500,7 @@ test("operational config failures return 503 and never consume login-failure cap
   expect((await login(app, { password: `${password}!` })).statusCode).toBe(429);
 });
 
-test("public auth configuration is safe and OIDC protocol placeholders fail closed", async () => {
+test("public auth configuration is safe and unavailable OIDC login fails closed", async () => {
   const { app } = await createLocalApp();
   const configuration = await app.inject({ method: "GET", url: "/auth/config" });
   expect(configuration.statusCode).toBe(200);
@@ -508,6 +508,6 @@ test("public auth configuration is safe and OIDC protocol placeholders fail clos
   expect(JSON.stringify(configuration.json())).not.toContain("users.yaml");
 
   const placeholder = await app.inject({ method: "GET", url: "/auth/oidc/login" });
-  expect(placeholder.statusCode).toBe(501);
-  expect(placeholder.json()).toEqual({ code: "auth_not_implemented", error: "OIDC login is not implemented" });
+  expect(placeholder.statusCode).toBe(503);
+  expect(placeholder.json()).toEqual({ code: "auth_unavailable", error: "Authentication is unavailable" });
 });
diff --git a/backend/test/auth-routes-oidc.test.ts b/backend/test/auth-routes-oidc.test.ts
new file mode 100644
index 00000000..55af1294
--- /dev/null
+++ b/backend/test/auth-routes-oidc.test.ts
@@ -0,0 +1,215 @@
+import Fastify from "fastify";
+import cookie from "@fastify/cookie";
+import { afterEach, expect, test } from "vitest";
+import { registerAuthRoutes } from "../src/auth/routes.js";
+import type { LoadedAuthConfig, OidcStateRecord } from "../src/auth/types.js";
+import type { AuthSessionStore } from "../src/auth/session-store.js";
+import type { OidcProtocol } from "../src/auth/oidc-client.js";
+
+const revision = "a".repeat(64);
+const issuer = "https://issuer.example.test";
+const state = "s".repeat(43);
+const nonce = "n".repeat(43);
+const verifier = "v".repeat(43);
+const createdApps: Array<ReturnType<typeof Fastify>> = [];
+
+function config(overrides: Partial<LoadedAuthConfig["value"]> = {}): LoadedAuthConfig {
+  return {
+    revision,
+    sourcePath: "/private/auth.yaml",
+    value: {
+      version: 1,
+      mode: "oidc",
+      publicUrl: "https://thothii.example.test",
+      session: {
+        regularTtlSeconds: 3600, regularIdleSeconds: 300,
+        rememberTtlSeconds: 3600, rememberIdleSeconds: 300, oidcTtlSeconds: 3600,
+      },
+      oidc: {
+        issuer, clientId: "thothii", clientSecretRef: "THT_OIDC_CLIENT_SECRET",
+        scopes: ["openid", "profile"], groupsClaim: "groups",
+      },
+      groupCatalog: { driver: "authentik", baseUrl: issuer, apiTokenRef: "THT_AUTHENTIK_API_TOKEN" },
+      authorization: { groupRoles: { Users: ["user"], Admins: ["admin"] } },
+      ...overrides,
+    },
+  } as LoadedAuthConfig;
+}
+
+function stateRecord(extra: Partial<OidcStateRecord> = {}): OidcStateRecord {
+  return {
+    version: 1, nonce, codeVerifier: verifier, returnTo: "/",
+    authConfigRevision: revision, issuer,
+    createdAt: "2030-01-01T00:00:00.000Z", expiresAt: "2030-01-01T00:10:00.000Z",
+    ...extra,
+  } as OidcStateRecord;
+}
+
+function fixture(options: {
+  loaded?: LoadedAuthConfig;
+  identity?: Awaited<ReturnType<OidcProtocol["callback"]>>;
+  callbackFailure?: boolean;
+  stateReturnTo?: string;
+} = {}) {
+  let loaded = options.loaded ?? config();
+  let protocolAvailable = true;
+  let storedState: OidcStateRecord | undefined;
+  const stateInputs: Array<Record<string, unknown>> = [];
+  const creates: Array<Record<string, unknown>> = [];
+  const callbacks: URL[] = [];
+  const protocol: OidcProtocol = {
+    authorizationUrl: async ({ state: received, nonce: receivedNonce, codeVerifier }) => {
+      expect(received).toBe(state);
+      expect(receivedNonce).toHaveLength(43);
+      expect(codeVerifier).toHaveLength(43);
+      return new URL(`https://issuer.example.test/authorize?state=${received}`);
+    },
+    callback: async ({ currentUrl }) => {
+      callbacks.push(currentUrl);
+      if (options.callbackFailure) throw new Error("provider failure with access-token-must-not-leak");
+      return options.identity ?? {
+        issuer, subject: "user-123", displayName: "Ada", groups: ["Users", "Admins", "Unmapped"],
+        tokenExpiresAt: new Date(Date.now() + 120_000),
+      };
+    },
+    diagnose: async () => undefined,
+  };
+  const store = {
+    createOidcState: async (input: Record<string, unknown>) => {
+      stateInputs.push(input);
+      const record = stateRecord({
+        nonce: input.nonce as string,
+        codeVerifier: input.codeVerifier as string,
+        authConfigRevision: input.authConfigRevision as string,
+        issuer: input.issuer as string,
+      });
+      if (options.stateReturnTo) (record as { returnTo: string }).returnTo = options.stateReturnTo;
+      storedState = record;
+      return { state, record: storedState };
+    },
+    consumeOidcState: async (received: string) => {
+      if (received !== state) return undefined;
+      const consumed = storedState;
+      storedState = undefined;
+      return consumed;
+    },
+    create: async (input: Record<string, unknown>) => {
+      creates.push(input);
+      return { token: "opaque-session-token", csrfToken: "c".repeat(43), record: {} };
+    },
+  } as unknown as AuthSessionStore;
+  const app = Fastify();
+  app.decorateRequest("authConfigSnapshot", undefined);
+  app.decorateRequest("authConfigSnapshotCaptured", false);
+  app.decorateRequest("authConfigSnapshotUnavailable", false);
+  app.register(cookie);
+  registerAuthRoutes(app, {
+    authMode: "oidc",
+    authentication: { current: () => loaded },
+    sessionStore: store,
+    resolveOidcProtocol: () => protocolAvailable ? protocol : undefined,
+  });
+  createdApps.push(app);
+  return {
+    app, creates, callbacks, stateInputs,
+    setConfig(next: LoadedAuthConfig) { loaded = next; },
+    setProtocolAvailable(available: boolean) { protocolAvailable = available; },
+    stateWasConsumed: () => storedState === undefined,
+  };
+}
+
+afterEach(async () => {
+  await Promise.all(createdApps.splice(0).map((app) => app.close()));
+});
+
+test("creates digest-only bound state, maps exact groups, creates a cookie session, and redirects safely", async () => {
+  const subject = fixture();
+  const start = await subject.app.inject({ method: "GET", url: "/auth/oidc/login" });
+  expect(start.statusCode).toBe(302);
+  expect(new URL(start.headers.location ?? "").searchParams.get("state")).toBe(state);
+  expect(subject.stateInputs[0]).toMatchObject({ returnTo: "/", authConfigRevision: revision, issuer });
+
+  const callback = await subject.app.inject({
+    method: "GET",
+    url: `/auth/oidc/callback?code=good&state=${state}`,
+    headers: { host: "attacker.example.test" },
+  });
+  expect(callback.statusCode).toBe(302);
+  expect(callback.headers.location).toBe("/");
+  expect(callback.headers["set-cookie"]).toContain("HttpOnly");
+  expect(callback.headers["set-cookie"]).toContain("SameSite=Lax");
+  expect(callback.headers["set-cookie"]).toContain("Secure");
+  expect(subject.callbacks[0]?.href).toBe(`https://thothii.example.test/api/auth/oidc/callback?code=good&state=${state}`);
+  expect(subject.creates).toHaveLength(1);
+  expect(subject.creates[0]).toMatchObject({
+    method: "oidc", remembered: false, authConfigRevision: revision,
+    principal: { issuer, subject: "user-123", roles: ["user", "admin"] },
+    idleTtlMs: 300_000,
+  });
+  const absoluteTtlMs = subject.creates[0]?.absoluteTtlMs;
+  expect(typeof absoluteTtlMs).toBe("number");
+  expect(absoluteTtlMs as number).toBeGreaterThan(0);
+  expect(absoluteTtlMs as number).toBeLessThanOrEqual(120_000);
+  expect(JSON.stringify(subject.creates)).not.toContain("access-token-must-not-leak");
+  expect(JSON.stringify(subject.creates)).not.toContain("refresh-token-must-not-leak");
+});
+
+test("consumes state on callback failure and refuses replay", async () => {
+  const subject = fixture({ callbackFailure: true });
+  await subject.app.inject({ method: "GET", url: "/auth/oidc/login" });
+  const failed = await subject.app.inject({ method: "GET", url: `/auth/oidc/callback?code=good&state=${state}` });
+  expect(failed.statusCode).toBe(401);
+  const replay = await subject.app.inject({ method: "GET", url: `/auth/oidc/callback?code=good&state=${state}` });
+  expect(replay.statusCode).toBe(401);
+  expect(subject.callbacks).toHaveLength(1);
+});
+
+test("consumes state when the protocol becomes unavailable before callback", async () => {
+  const subject = fixture();
+  await subject.app.inject({ method: "GET", url: "/auth/oidc/login" });
+  subject.setProtocolAvailable(false);
+  const failed = await subject.app.inject({ method: "GET", url: `/auth/oidc/callback?code=good&state=${state}` });
+  expect(failed.statusCode).toBe(401);
+  expect(subject.stateWasConsumed()).toBe(true);
+});
+
+test("rejects a consumed state with a non-root return target", async () => {
+  const subject = fixture({ stateReturnTo: "https://attacker.example.test" });
+  await subject.app.inject({ method: "GET", url: "/auth/oidc/login" });
+  const callback = await subject.app.inject({ method: "GET", url: `/auth/oidc/callback?code=good&state=${state}` });
+  expect(callback.statusCode).toBe(401);
+  expect(subject.callbacks).toEqual([]);
+  expect(subject.creates).toEqual([]);
+});
+
+test("rejects an OIDC state when its configuration revision changes before callback", async () => {
+  const subject = fixture();
+  await subject.app.inject({ method: "GET", url: "/auth/oidc/login" });
+  subject.setConfig({ ...config(), revision: "b".repeat(64) });
+  const callback = await subject.app.inject({ method: "GET", url: `/auth/oidc/callback?code=good&state=${state}` });
+  expect(callback.statusCode).toBe(401);
+  expect(subject.callbacks).toEqual([]);
+});
+
+test("rejects an OIDC state when its issuer changes before callback", async () => {
+  const subject = fixture();
+  await subject.app.inject({ method: "GET", url: "/auth/oidc/login" });
+  const previous = config();
+  subject.setConfig({
+    ...previous,
+    value: { ...previous.value, oidc: { ...previous.value.oidc, issuer: "https://other.example.test" } },
+  } as LoadedAuthConfig);
+  const callback = await subject.app.inject({ method: "GET", url: `/auth/oidc/callback?code=good&state=${state}` });
+  expect(callback.statusCode).toBe(401);
+  expect(subject.callbacks).toEqual([]);
+});
+
+test("creates an authenticated but forbidden principal for extra unmapped groups", async () => {
+  const subject = fixture({ identity: {
+    issuer, subject: "user-123", groups: ["Unmapped"], tokenExpiresAt: new Date(Date.now() + 60_000),
+  } });
+  await subject.app.inject({ method: "GET", url: "/auth/oidc/login" });
+  const callback = await subject.app.inject({ method: "GET", url: `/auth/oidc/callback?code=good&state=${state}` });
+  expect(callback.statusCode).toBe(302);
+  expect(subject.creates[0]).toMatchObject({ principal: { roles: [], permissions: [], isAdmin: false } });
+});
diff --git a/backend/test/auth-session-store.test.ts b/backend/test/auth-session-store.test.ts
index 9be986e1..e5687cbe 100644
--- a/backend/test/auth-session-store.test.ts
+++ b/backend/test/auth-session-store.test.ts
@@ -69,6 +69,10 @@ const base = new Date("2030-01-02T03:04:05.000Z");
 const revision = "a".repeat(64);
 const validLocalUser = { enabled: true, authRevision: 7, roles: ["admin"] as const };
 
+function oidcInput(nonce: string, codeVerifier: string) {
+  return { nonce, codeVerifier, returnTo: "/" as const, authConfigRevision: revision, issuer: "https://issuer.example.test" };
+}
+
 afterEach(() => {
   fsHooks.afterRead = undefined;
   fsHooks.afterWrite = undefined;
@@ -258,7 +262,7 @@ describe("file-backed auth session store", () => {
     const store = validStore(storageRoot);
     const revoked = await create(store);
     const expired = await create(store, { idleTtlMs: 60_000, absoluteTtlMs: 60_000 });
-    const oidc = await store.createOidcState({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" }, base);
+    const oidc = await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
 
     await store.revoke(revoked.token);
     await expect(store.resolve(revoked.token)).resolves.toBeUndefined();
@@ -272,20 +276,19 @@ describe("file-backed auth session store", () => {
   test("creates bounded OIDC state records that expire and are single-use", async () => {
     const storageRoot = root();
     const store = validStore(storageRoot);
-    const created = await store.createOidcState({
-      nonce: "n".repeat(43),
-      codeVerifier: "v".repeat(43),
-      returnTo: "/",
-    }, base);
+    const created = await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
     const path = digestPath(storageRoot, "oidc", created.state);
 
     expect(created.state).toMatch(/^[A-Za-z0-9_-]{43}$/);
     expect(readFileSync(path, "utf8")).not.toContain(created.state);
     await expect(store.consumeOidcState(created.state, new Date(base.getTime() + 9 * 60_000)))
-      .resolves.toMatchObject({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" });
+      .resolves.toMatchObject({
+        nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/",
+        authConfigRevision: revision, issuer: "https://issuer.example.test",
+      });
     await expect(store.consumeOidcState(created.state)).resolves.toBeUndefined();
 
-    const expired = await store.createOidcState({ nonce: "x".repeat(43), codeVerifier: "y".repeat(43), returnTo: "/" }, base);
+    const expired = await store.createOidcState(oidcInput("x".repeat(43), "y".repeat(43)), base);
     await expect(store.consumeOidcState(expired.state, new Date(base.getTime() + 10 * 60_000)))
       .resolves.toBeUndefined();
   });
@@ -293,7 +296,7 @@ describe("file-backed auth session store", () => {
   test("fails closed when an OIDC state already has an atomic filesystem claim", async () => {
     const storageRoot = root();
     const store = validStore(storageRoot);
-    const created = await store.createOidcState({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" }, base);
+    const created = await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
     const statePath = digestPath(storageRoot, "oidc", created.state);
     linkSync(statePath, claimPath(storageRoot, created.state));
 
@@ -304,7 +307,7 @@ describe("file-backed auth session store", () => {
   test("treats a competing OIDC claim installed between availability and state checks as unavailable", async () => {
     const storageRoot = root();
     const store = validStore(storageRoot);
-    const created = await store.createOidcState({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" }, base);
+    const created = await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
     const statePath = digestPath(storageRoot, "oidc", created.state);
     const stateClaimPath = claimPath(storageRoot, created.state);
     fsHooks.beforeLstat = (observed) => {
@@ -321,7 +324,7 @@ describe("file-backed auth session store", () => {
   test("prunes an expired OIDC state abandoned after an atomic claim", async () => {
     const storageRoot = root();
     const store = validStore(storageRoot);
-    const created = await store.createOidcState({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" }, base);
+    const created = await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
     const statePath = digestPath(storageRoot, "oidc", created.state);
     const stateClaimPath = claimPath(storageRoot, created.state);
     linkSync(statePath, stateClaimPath);
@@ -334,7 +337,7 @@ describe("file-backed auth session store", () => {
   test("retains an in-flight orphan claim but removes it after the bounded recovery window", async () => {
     const storageRoot = root();
     const store = validStore(storageRoot);
-    const created = await store.createOidcState({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" }, base);
+    const created = await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
     const statePath = digestPath(storageRoot, "oidc", created.state);
     const stateClaimPath = claimPath(storageRoot, created.state);
     linkSync(statePath, stateClaimPath);
@@ -349,7 +352,7 @@ describe("file-backed auth session store", () => {
   test("allows exactly one separate Node isolate to consume an OIDC state", async () => {
     const storageRoot = root();
     const store = validStore(storageRoot);
-    const created = await store.createOidcState({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" }, base);
+    const created = await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
     const [first, second] = await Promise.all([
       isolatedOidcConsumer(storageRoot, created.state),
       isolatedOidcConsumer(storageRoot, created.state),
@@ -587,12 +590,12 @@ describe("file-backed auth session store", () => {
       await store.revoke(session.token);
       await expect(store.resolve(session.token)).resolves.toBeUndefined();
 
-      const oidc = await store.createOidcState({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" }, base);
+      const oidc = await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
       await expect(store.consumeOidcState(oidc.state, new Date(base.getTime() + 9 * 60_000)))
         .resolves.toMatchObject({ nonce: "n".repeat(43) });
       await expect(store.consumeOidcState(oidc.state)).resolves.toBeUndefined();
       await create(store, { idleTtlMs: 60_000, absoluteTtlMs: 60_000 });
-      await store.createOidcState({ nonce: "x".repeat(43), codeVerifier: "y".repeat(43), returnTo: "/" }, base);
+      await store.createOidcState(oidcInput("x".repeat(43), "y".repeat(43)), base);
       await expect(store.prune(new Date(base.getTime() + 11 * 60_000))).resolves.toBe(2);
       expect(calls).toEqual(expect.arrayContaining(["create", "read", "replace", "remove", "claim-consume", "list"]));
       expect(records).toHaveLength(0);
@@ -651,7 +654,7 @@ describe("file-backed auth session store", () => {
         findLocalUser: async () => validLocalUser,
       }, { windowsStorageBridge: bridge });
       await create(store, { idleTtlMs: 60_000, absoluteTtlMs: 60_000 });
-      await store.createOidcState({ nonce: "n".repeat(43), codeVerifier: "v".repeat(43), returnTo: "/" }, base);
+      await store.createOidcState(oidcInput("n".repeat(43), "v".repeat(43)), base);
 
       await expect(store.prune(new Date(base.getTime() + 11 * 60_000))).resolves.toBe(2);
       expect(records).toHaveLength(0);
diff --git a/backend/test/oidc-client.test.ts b/backend/test/oidc-client.test.ts
new file mode 100644
index 00000000..d7c87333
--- /dev/null
+++ b/backend/test/oidc-client.test.ts
@@ -0,0 +1,142 @@
+import { createSign, generateKeyPairSync } from "node:crypto";
+import { expect, test } from "vitest";
+import { createOidcProtocol, OidcProtocolError } from "../src/auth/oidc-client.js";
+
+const issuer = "https://issuer.example.test";
+const clientId = "thothii";
+const callbackUrl = "https://thothii.example.test/api/auth/oidc/callback";
+const verifier = "v".repeat(43);
+const nonce = "n".repeat(43);
+const state = "s".repeat(43);
+
+const keys = generateKeyPairSync("rsa", { modulusLength: 2048 });
+const jwk = { ...keys.publicKey.export({ format: "jwk" }), kid: "test-key", use: "sig", alg: "RS256" };
+
+function token(claims: Record<string, unknown>, invalidSignature = false): string {
+  const encode = (value: unknown) => Buffer.from(JSON.stringify(value)).toString("base64url");
+  const input = `${encode({ alg: "RS256", kid: "test-key", typ: "JWT" })}.${encode(claims)}`;
+  const signer = createSign("RSA-SHA256");
+  signer.update(input);
+  signer.end();
+  const signature = signer.sign(keys.privateKey).toString("base64url");
+  const corruptedSignature = signature.startsWith("A") ? `B${signature.slice(1)}` : `A${signature.slice(1)}`;
+  return `${input}.${invalidSignature ? corruptedSignature : signature}`;
+}
+
+function protocol(options: {
+  claims?: Record<string, unknown>;
+  discoveryIssuer?: string;
+  invalidSignature?: boolean;
+  seen?: URL[];
+} = {}) {
+  const now = Math.floor(Date.now() / 1000);
+  const claims = {
+    iss: issuer,
+    sub: "user-123",
+    aud: clientId,
+    exp: now + 300,
+    iat: now,
+    nonce,
+    name: "Ada Lovelace",
+    groups: ["TOT Users", "Unmapped group"],
+    ...options.claims,
+  };
+  const fetch = async (input: RequestInfo | URL) => {
+    const url = new URL(input instanceof Request ? input.url : typeof input === "string" ? input : input.toString());
+    options.seen?.push(url);
+    if (url.pathname.includes(".well-known/")) {
+      return Response.json({
+        issuer: options.discoveryIssuer ?? issuer,
+        authorization_endpoint: `${issuer}/authorize`,
+        token_endpoint: `${issuer}/token`,
+        jwks_uri: `${issuer}/jwks`,
+        response_types_supported: ["code"],
+        grant_types_supported: ["authorization_code"],
+        subject_types_supported: ["public"],
+        id_token_signing_alg_values_supported: ["RS256"],
+      });
+    }
+    if (url.pathname === "/jwks") return Response.json({ keys: [jwk] });
+    if (url.pathname === "/token") {
+      return Response.json({
+        token_type: "Bearer",
+        access_token: "access-token-must-not-be-persisted",
+        refresh_token: "refresh-token-must-not-be-persisted",
+        id_token: token(claims, options.invalidSignature),
+      });
+    }
+    return new Response(null, { status: 404 });
+  };
+  return createOidcProtocol({
+    issuer,
+    clientId,
+    clientSecret: "client-secret-must-not-be-persisted",
+    callbackUrl,
+    scopes: ["openid", "profile"],
+    groupsClaim: "groups",
+    fetch,
+  });
+}
+
+async function callback(subject = protocol()) {
+  return subject.callback({
+    currentUrl: new URL(`${callbackUrl}?code=good&state=${state}`), state, nonce, codeVerifier: verifier,
+  });
+}
+
+test("uses HTTPS discovery, Authorization Code, and PKCE S256 without external network", async () => {
+  const seen: URL[] = [];
+  const subject = protocol({ seen });
+  const authorization = await subject.authorizationUrl({ state, nonce, codeVerifier: verifier });
+
+  expect(authorization.origin).toBe(issuer);
+  expect(authorization.pathname).toBe("/authorize");
+  expect(Object.fromEntries(authorization.searchParams)).toMatchObject({
+    response_type: "code", client_id: clientId, redirect_uri: callbackUrl, state, nonce,
+    code_challenge_method: "S256", scope: "openid profile",
+  });
+  expect(authorization.searchParams.get("code_challenge")).not.toBe(verifier);
+  await expect(callback(subject)).resolves.toEqual({
+    issuer, subject: "user-123", displayName: "Ada Lovelace",
+    groups: ["TOT Users", "Unmapped group"], tokenExpiresAt: expect.any(Date),
+  });
+  expect(seen.map((url) => url.origin)).toEqual([issuer, issuer, issuer]);
+});
+
+test("rejects non-HTTPS issuer configuration and a discovery issuer mismatch", async () => {
+  expect(() => createOidcProtocol({
+    issuer: "http://issuer.example.test", clientId, clientSecret: "secret", callbackUrl,
+    scopes: ["openid"], groupsClaim: "groups",
+  })).toThrow(OidcProtocolError);
+  await expect(protocol({ discoveryIssuer: "https://other.example.test" }).authorizationUrl({ state, nonce, codeVerifier: verifier }))
+    .rejects.toThrow(OidcProtocolError);
+});
+
+test.each([
+  ["state", new URL(`${callbackUrl}?code=good&state=wrong`), {}],
+  ["nonce", new URL(`${callbackUrl}?code=good&state=${state}`), { nonce: "wrong" }],
+  ["audience", new URL(`${callbackUrl}?code=good&state=${state}`), { aud: "someone-else" }],
+  ["issuer", new URL(`${callbackUrl}?code=good&state=${state}`), { iss: "https://other.example.test" }],
+  ["expiry", new URL(`${callbackUrl}?code=good&state=${state}`), { exp: Math.floor(Date.now() / 1000) - 1 }],
+  ["subject", new URL(`${callbackUrl}?code=good&state=${state}`), { sub: undefined }],
+])("rejects invalid %s claims or callback bindings", async (_label, currentUrl, claims) => {
+  const subject = protocol({ claims });
+  await expect(subject.callback({ currentUrl, state, nonce, codeVerifier: verifier })).rejects.toThrow(OidcProtocolError);
+});
+
+test("rejects invalid ID-token signatures", async () => {
+  await expect(callback(protocol({ invalidSignature: true }))).rejects.toThrow(OidcProtocolError);
+});
+
+test.each([
+  ["absent", { groups: undefined }],
+  ["non-array", { groups: "TOT Users" }],
+  ["empty", { groups: [""] }],
+  ["duplicate", { groups: ["TOT Users", "TOT Users"] }],
+  ["control", { groups: ["TOT\u0000Users"] }],
+  ["oversized", { groups: ["x".repeat(257)] }],
+  ["distributed", { _claim_names: { groups: "source" }, _claim_sources: { source: { endpoint: "https://issuer.example.test/claims" } } }],
+  ["overage", { hasgroups: true }],
+])("rejects %s mandatory groups claims", async (_label, claims) => {
+  await expect(callback(protocol({ claims }))).rejects.toThrow(OidcProtocolError);
+});
diff --git a/frontend/src/api/auth.test.ts b/frontend/src/api/auth.test.ts
new file mode 100644
index 00000000..5e022557
--- /dev/null
+++ b/frontend/src/api/auth.test.ts
@@ -0,0 +1,64 @@
+import { beforeEach, expect, test, vi } from "vitest";
+import { http, HttpResponse } from "msw";
+import { beginOidcLogin, logout } from "./auth";
+import { server } from "../test/msw";
+import { clearAuthState, setAuthState } from "../auth/authState";
+
+const user = {
+  issuer: "local", subject: "user-a", roles: ["user"] as const,
+  permissions: ["session.use"], isAdmin: false, csrfToken: "a".repeat(43), session: null,
+};
+
+function deferred() {
+  let resolve!: () => void;
+  const promise = new Promise<void>((onResolve) => { resolve = onResolve; });
+  return { promise, resolve };
+}
+
+beforeEach(() => {
+  clearAuthState();
+  setAuthState(user);
+});
+
+test("OIDC navigation waits for a successful pending logout response", async () => {
+  const gate = deferred();
+  let logoutStarted!: () => void;
+  const started = new Promise<void>((resolve) => { logoutStarted = resolve; });
+  server.use(http.post("/api/auth/logout", async () => {
+    logoutStarted();
+    await gate.promise;
+    return new HttpResponse(null, { status: 204 });
+  }));
+  const logoutPromise = logout();
+  await started;
+  const navigate = vi.fn();
+  const oidc = beginOidcLogin(navigate);
+
+  await Promise.resolve();
+  expect(navigate).not.toHaveBeenCalled();
+  gate.resolve();
+  await Promise.all([logoutPromise, oidc]);
+  expect(navigate).toHaveBeenCalledOnce();
+  expect(navigate).toHaveBeenCalledWith("/api/auth/oidc/login");
+});
+
+test("OIDC navigation waits for a failed pending logout response before continuing", async () => {
+  const gate = deferred();
+  let logoutStarted!: () => void;
+  const started = new Promise<void>((resolve) => { logoutStarted = resolve; });
+  server.use(http.post("/api/auth/logout", async () => {
+    logoutStarted();
+    await gate.promise;
+    return HttpResponse.json({ code: "auth_unavailable" }, { status: 503 });
+  }));
+  const logoutPromise = logout().catch(() => undefined);
+  await started;
+  const navigate = vi.fn();
+  const oidc = beginOidcLogin(navigate);
+
+  await Promise.resolve();
+  expect(navigate).not.toHaveBeenCalled();
+  gate.resolve();
+  await Promise.all([logoutPromise, oidc]);
+  expect(navigate).toHaveBeenCalledWith("/api/auth/oidc/login");
+});
diff --git a/frontend/src/api/auth.ts b/frontend/src/api/auth.ts
index 77636168..baac8539 100644
--- a/frontend/src/api/auth.ts
+++ b/frontend/src/api/auth.ts
@@ -6,6 +6,7 @@ const authModes = new Set<AuthPublicConfig["mode"]>(["local", "oidc", "upstream"
 const roles = new Set<AuthRole>(["user", "admin"]);
 const sessionMethods = new Set<AuthSessionInfo["method"]>(["local", "oidc", "upstream"]);
 let pendingLogoutResponse: Promise<void> | null = null;
+const oidcLoginPath = "/api/auth/oidc/login";
 
 function record(value: unknown): Record<string, unknown> | undefined {
   return value && typeof value === "object" && !Array.isArray(value)
@@ -86,6 +87,23 @@ export async function loginLocal(username: string, password: string, remember: b
   return user;
 }
 
+/**
+ * OIDC creates its browser session through a top-level same-origin navigation. A previous logout
+ * response may still carry a clearing Set-Cookie, so it must settle before this navigation begins.
+ */
+export async function beginOidcLogin(navigate: (path: string) => void = (path) => window.location.assign(path)): Promise<void> {
+  const pending = pendingLogoutResponse;
+  if (pending) {
+    try {
+      await pending;
+    } catch {
+      // A failed logout must release the coordinator; the current browser cookie remains the
+      // backend's authority during the following OIDC handshake.
+    }
+  }
+  navigate(oidcLoginPath);
+}
+
 export async function logout(): Promise<boolean> {
   const logoutGeneration = getAuthGeneration();
   if (pendingLogoutResponse) {
diff --git a/frontend/src/auth/LoginPage.test.tsx b/frontend/src/auth/LoginPage.test.tsx
index 37392323..7ff73e9f 100644
--- a/frontend/src/auth/LoginPage.test.tsx
+++ b/frontend/src/auth/LoginPage.test.tsx
@@ -76,14 +76,12 @@ describe("LoginPage", () => {
     expect(screen.getByLabelText(/password/i)).toHaveValue("");
   });
 
-  test("shows OIDC only when public configuration enables it and uses same-origin navigation", () => {
+  test("shows OIDC only when public configuration enables it", () => {
     const { rerender } = render(<LoginPage config={localConfig} onAuthenticated={vi.fn()} />);
     expect(screen.queryByRole("link", { name: /single sign-on/i })).not.toBeInTheDocument();
 
     rerender(<LoginPage config={oidcConfig} onAuthenticated={vi.fn()} />);
-    expect(screen.getByRole("link", { name: /single sign-on/i })).toHaveAttribute(
-      "href", "/api/auth/oidc/login",
-    );
+    expect(screen.getByRole("button", { name: /single sign-on/i })).toBeEnabled();
   });
 
   test("does not dispatch local login until an in-flight logout response settles", async () => {
diff --git a/frontend/src/auth/LoginPage.tsx b/frontend/src/auth/LoginPage.tsx
index ed81f8bd..a1be1c6e 100644
--- a/frontend/src/auth/LoginPage.tsx
+++ b/frontend/src/auth/LoginPage.tsx
@@ -2,7 +2,7 @@ import { useEffect, useRef, useState } from "react";
 import type { FormEvent } from "react";
 import { AlertTriangle, ArrowRight, LockKeyhole } from "lucide-react";
 import { ApiError } from "../api/client";
-import { loginLocal } from "../api/auth";
+import { beginOidcLogin, loginLocal } from "../api/auth";
 import type { AuthenticatedUser, AuthPublicConfig } from "../api/types";
 import { Button } from "../components/ui/button";
 
@@ -24,6 +24,7 @@ function loginError(error: unknown): { message: string; retry: boolean } {
 
 export function LoginPage({ config, onAuthenticated, onRetry }: LoginPageProps) {
   const localLogin = config.mode === "local" && config.localLogin;
+  const oidcLogin = config.mode === "oidc" && config.oidcLogin;
   const formRef = useRef<HTMLFormElement>(null);
   const passwordRef = useRef<HTMLInputElement>(null);
   const mountedRef = useRef(true);
@@ -65,6 +66,10 @@ export function LoginPage({ config, onAuthenticated, onRetry }: LoginPageProps)
     }
   }
 
+  function startOidcLogin() {
+    void beginOidcLogin();
+  }
+
   return (
     <main className="min-h-screen bg-background px-5 py-8 text-foreground sm:px-8 sm:py-12">
       <div className="mx-auto grid min-h-[calc(100vh-4rem)] max-w-5xl items-center gap-12 lg:grid-cols-[minmax(0,1fr)_26rem]">
@@ -138,6 +143,12 @@ export function LoginPage({ config, onAuthenticated, onRetry }: LoginPageProps)
               </Button>
             </form>
           )}
+          {oidcLogin && (
+            <Button type="button" size="lg" className="w-full" onClick={startOidcLogin}>
+              Continue with single sign-on
+              <ArrowRight aria-hidden="true" />
+            </Button>
+          )}
 
           {config.oidcLogin && (
             <a
diff --git a/frontend/src/shell/AppShell.session-target.test.tsx b/frontend/src/shell/AppShell.session-target.test.tsx
new file mode 100644
index 00000000..e4cba8f3
--- /dev/null
+++ b/frontend/src/shell/AppShell.session-target.test.tsx
@@ -0,0 +1,108 @@
+import { act, render, screen, waitFor } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { beforeEach, expect, test, vi } from "vitest";
+import { http, HttpResponse } from "msw";
+import { AppShell } from "./AppShell";
+import { server } from "../test/msw";
+import { FakeEventSource } from "../test/fakeEventSource";
+import { clearAuthState, setAuthState } from "../auth/authState";
+import { useSessionStore } from "../store/sessionStore";
+
+function renderShell() {
+  const client = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return render(<QueryClientProvider client={client}><AppShell /></QueryClientProvider>);
+}
+
+function deferred() {
+  let resolve!: () => void;
+  const promise = new Promise<void>((onResolve) => { resolve = onResolve; });
+  return { promise, resolve };
+}
+
+beforeEach(() => {
+  clearAuthState();
+  setAuthState({
+    issuer: "local", subject: "user-a", roles: ["user"], permissions: ["session.use"],
+    isAdmin: false, csrfToken: null, session: null,
+  });
+  FakeEventSource.instances = [];
+  (globalThis as { EventSource: typeof EventSource }).EventSource = FakeEventSource as unknown as typeof EventSource;
+  useSessionStore.getState().resetSession();
+  server.use(
+    http.get("/api/me", () => HttpResponse.json({ issuer: "local", subject: "user-a", isAdmin: false })),
+    http.get("/api/settings", () => HttpResponse.json({ workspace: "default", provider: "test", model: "test", thinking: "low" })),
+    http.get("/api/workspaces", () => HttpResponse.json([])),
+    http.get("/api/models", () => HttpResponse.json({ models: [] })),
+    http.post("/api/runtime/prewarm", () => new HttpResponse(null, { status: 202 })),
+  );
+});
+
+test("a held stop for s1 cannot reset the newer active s2 session", async () => {
+  const closeGate = deferred();
+  const closeStarted = deferred();
+  const active = (id: string, question: string) => ({
+    id, status: "open", question, summary: null, created_at: "2026-01-02T00:00:00Z",
+    updated_at: null, author: null, name: null, group: null, archived: false, active: true,
+  });
+  server.use(
+    http.get("/api/sessions", () => HttpResponse.json([active("s1", "Active one"), active("s2", "Active two")])),
+    http.post("/api/sessions/:id/resume", ({ params }) => HttpResponse.json({ id: params.id, alreadyActive: false })),
+    http.get("/api/sessions/:id", ({ params }) => HttpResponse.json({ id: params.id, status: "open", phase: 1 })),
+    http.post("/api/sessions/s1/close", async () => {
+      closeStarted.resolve();
+      await closeGate.promise;
+      return new HttpResponse(null, { status: 204 });
+    }),
+  );
+  renderShell();
+  await userEvent.click(await screen.findByTestId("session-item-s1"));
+  await waitFor(() => expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s1/events"));
+  await userEvent.click(screen.getByRole("button", { name: /stop and save session/i }));
+  await userEvent.click(await screen.findByRole("button", { name: "Stop & save" }));
+  await closeStarted.promise;
+
+  await userEvent.click(screen.getByTestId("session-item-s2"));
+  await waitFor(() => expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s2/events"));
+  act(() => useSessionStore.setState({ currentPhase: "F2" }));
+  closeGate.resolve();
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s2/events");
+  expect(FakeEventSource.instances.at(-1)?.closed).toBe(false);
+  expect(useSessionStore.getState().currentPhase).toBe("F2");
+});
+
+test("a held new-session completion cannot replace the newer active s2 target", async () => {
+  const createGate = deferred();
+  const createStarted = deferred();
+  server.use(
+    http.get("/api/sessions", () => HttpResponse.json([{
+      id: "s2", status: "open", question: "Current session", summary: null,
+      created_at: "2026-01-02T00:00:00Z", updated_at: null, author: null, name: null,
+      group: null, archived: false, active: true,
+    }])),
+    http.post("/api/sessions", async () => {
+      createStarted.resolve();
+      await createGate.promise;
+      return HttpResponse.json({ id: "s3" });
+    }),
+    http.post("/api/sessions/:id/resume", ({ params }) => HttpResponse.json({ id: params.id, alreadyActive: false })),
+    http.get("/api/sessions/:id", ({ params }) => HttpResponse.json({ id: params.id, status: "open", phase: 1 })),
+  );
+  renderShell();
+  const composer = screen.getByRole("textbox", { name: /new question/i });
+  await userEvent.type(composer, "Held new question");
+  await userEvent.click(screen.getByRole("button", { name: /send/i }));
+  await createStarted.promise;
+
+  await userEvent.click(screen.getByTestId("session-item-s2"));
+  await waitFor(() => expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s2/events"));
+  act(() => useSessionStore.setState({ currentPhase: "F2" }));
+  createGate.resolve();
+  await new Promise((resolve) => setImmediate(resolve));
+
+  expect(FakeEventSource.instances.at(-1)?.url).toContain("/sessions/s2/events");
+  expect(FakeEventSource.instances.at(-1)?.closed).toBe(false);
+  expect(useSessionStore.getState().currentPhase).toBe("F2");
+});
diff --git a/frontend/src/shell/AppShell.tsx b/frontend/src/shell/AppShell.tsx
index 889ffabc..7041678e 100644
--- a/frontend/src/shell/AppShell.tsx
+++ b/frontend/src/shell/AppShell.tsx
@@ -80,6 +80,8 @@ export function AppShell() {
   } as CSSProperties;
   const [activeSessionId, setActiveSessionId] = useState<string | null>(null);
   const activeSessionIdRef = useRef<string | null>(null);
+  const activeSessionEpochRef = useRef(0);
+  const newSessionOperationRef = useRef<{ target: string | null; epoch: number } | null>(null);
   const resumeInvocationRef = useRef(0);
   const latestResumeIntentRef = useRef<{ token: number; id: string } | null>(null);
   const resumeInFlightRef = useRef(new Map<string, {
@@ -165,6 +167,7 @@ export function AppShell() {
 
   function selectActiveSession(id: string | null) {
     // Keep async Resume completions synchronized before React commits the state update.
+    if (activeSessionIdRef.current !== id) activeSessionEpochRef.current += 1;
     activeSessionIdRef.current = id;
     setActiveSessionId(id);
   }
@@ -505,6 +508,7 @@ export function AppShell() {
 
   function startNewSession() {
     invalidateResumeIntent();
+    newSessionOperationRef.current = null;
     resetSession();
     // Starting a new question closes any open session detail panel: the reader is
     // moving away from that session, so its left-hand box must not linger.
@@ -519,11 +523,18 @@ export function AppShell() {
   }
 
   function beginSessionCreation() {
+    newSessionOperationRef.current = {
+      target: activeSessionIdRef.current,
+      epoch: activeSessionEpochRef.current,
+    };
     setAwaitingQuestion(false);
     setCreatingSession(true);
   }
 
   function finishSessionCreation(id: string) {
+    const operation = newSessionOperationRef.current;
+    newSessionOperationRef.current = null;
+    if (!operation || operation.target !== activeSessionIdRef.current || operation.epoch !== activeSessionEpochRef.current) return;
     // React batches these updates, preserving the provisional session view
     // while useSessionStream opens the durable session's SSE channel.
     selectActiveSession(id);
@@ -533,21 +544,25 @@ export function AppShell() {
   }
 
   function failSessionCreation(message?: string) {
+    const operation = newSessionOperationRef.current;
+    newSessionOperationRef.current = null;
+    if (!operation || operation.target !== activeSessionIdRef.current || operation.epoch !== activeSessionEpochRef.current) return;
     setCreatingSession(false);
     resetSession();
     toast.error(message ?? "Failed to create session. Your question is ready to retry.");
   }
 
   async function stopSession() {
-    if (!activeSessionId) return;
-    const id = activeSessionId;
+    const id = activeSessionIdRef.current;
+    if (!id) return;
     const guard = captureAuthOperation({ sessionId: id, disposalEpoch: operationEpochRef.current });
     if (!guard) return;
     invalidateResumeIntent();
     try {
       await closeSession(id);
     } finally {
-      if (!isAuthOperationCurrent(guard, { sessionId: id, disposalEpoch: operationEpochRef.current })) return;
+      if (!isAuthOperationCurrent(guard, { sessionId: id, disposalEpoch: operationEpochRef.current })
+        || activeSessionIdRef.current !== id) return;
       resetSession();
       selectActiveSession(null);
       setAwaitingQuestion(false);
@@ -643,7 +658,7 @@ export function AppShell() {
                   </header>
                 )}
                 <CentralStatus working={working} />
-                {activeSessionId && <WidgetHost sessionId={activeSessionId} />}
+                {activeSessionId && <WidgetHost key={`widget:${activeSessionId}:${activeSessionEpochRef.current}`} sessionId={activeSessionId} />}
                 {finalized && !agentActive && (
                   <div className="rounded-2xl border border-border/80 bg-card p-5 text-center shadow-md">
                     <p className="text-sm text-muted-foreground">
@@ -668,6 +683,7 @@ export function AppShell() {
             <div className="rounded-2xl border border-border/80 bg-card shadow-md transition-colors focus-within:border-primary/50 focus-within:ring-3 focus-within:ring-ring/15">
               <div className="px-2.5 py-2">
                 <SteerInput
+                  key={`steer:${activeSessionId ?? "new"}:${activeSessionEpochRef.current}`}
                   sessionId={activeSessionId}
                   onSessionCreating={beginSessionCreation}
                   onSessionCreated={finishSessionCreation}
diff --git a/frontend/src/shell/SteerInput.test.tsx b/frontend/src/shell/SteerInput.test.tsx
index 2aa4b120..ea000f71 100644
--- a/frontend/src/shell/SteerInput.test.tsx
+++ b/frontend/src/shell/SteerInput.test.tsx
@@ -113,6 +113,38 @@ test("a delayed steer from user A cannot mutate user B's store or composer", asy
   view.unmount();
 });
 
+test("a held steer for s1 cannot complete into the active s2 operation scope", async () => {
+  let release!: () => void;
+  let started!: () => void;
+  let settled!: () => void;
+  const held = new Promise<void>((resolve) => { release = resolve; });
+  const requestStarted = new Promise<void>((resolve) => { started = resolve; });
+  const requestSettled = new Promise<void>((resolve) => { settled = resolve; });
+  server.use(http.post("/api/sessions/s1/steer", async () => {
+    started();
+    try {
+      await held;
+      return new HttpResponse(null, { status: 204 });
+    } finally {
+      settled();
+    }
+  }));
+
+  const view = render(<SteerInput sessionId="s1" />);
+  const input = screen.getByRole("textbox");
+  await userEvent.type(input, "Keep s1 isolated");
+  await userEvent.click(screen.getByRole("button", { name: /invia|send|steer/i }));
+  await requestStarted;
+
+  view.rerender(<SteerInput sessionId="s2" />);
+  act(() => useSessionStore.getState().setLastUserEntry({ kind: "input", text: "s2 owned" }));
+  release();
+  await act(async () => { await requestSettled; });
+
+  expect(useSessionStore.getState().lastUserEntry).toEqual({ kind: "input", text: "s2 owned" });
+  expect(input).toHaveValue("Keep s1 isolated");
+});
+
 test("a settings preflight from user A prevents session POST after user B logs in", async () => {
   let releaseSettings!: () => void;
   let settingsStarted!: () => void;
diff --git a/frontend/src/shell/SteerInput.tsx b/frontend/src/shell/SteerInput.tsx
index 7873911f..2f51ecb6 100644
--- a/frontend/src/shell/SteerInput.tsx
+++ b/frontend/src/shell/SteerInput.tsx
@@ -51,6 +51,12 @@ export function SteerInput({
   const taRef = useRef<HTMLTextAreaElement | null>(null);
   const operationEpochRef = useRef(0);
   useEffect(() => () => { operationEpochRef.current += 1; }, []);
+  useEffect(() => {
+    // A new active session owns a new composer operation scope. Invalidate any held request
+    // before it can write the old session's completion into the new target.
+    operationEpochRef.current += 1;
+    setBusy(false);
+  }, [sessionId]);
 
   // Merge our own ref (for autosizing) with the forwarded one (parent focus).
   function attachRef(el: HTMLTextAreaElement | null) {
diff --git a/frontend/src/shell/WidgetHost.response.test.tsx b/frontend/src/shell/WidgetHost.response.test.tsx
index 6e14919d..8cd39907 100644
--- a/frontend/src/shell/WidgetHost.response.test.tsx
+++ b/frontend/src/shell/WidgetHost.response.test.tsx
@@ -92,3 +92,37 @@ test("a delayed gate response from user A cannot clear user B's pending gate", a
   expect(useSessionStore.getState().pendingWidget?.id).toBe("gate-b");
   expect(useSessionStore.getState().lastUserEntry).toBeNull();
 });
+
+test("a held s1 gate response cannot clear the pending s2 gate", async () => {
+  let release!: () => void;
+  let started!: () => void;
+  let settled!: () => void;
+  const held = new Promise<void>((resolve) => { release = resolve; });
+  const requestStarted = new Promise<void>((resolve) => { started = resolve; });
+  const requestSettled = new Promise<void>((resolve) => { settled = resolve; });
+  server.use(http.post("/api/sessions/s1/response", async () => {
+    started();
+    try {
+      await held;
+      return new HttpResponse(null, { status: 204 });
+    } finally {
+      settled();
+    }
+  }));
+  useSessionStore.setState({
+    pendingWidget: { id: "gate-s1", widget: "select", options: [{ id: "s1", label: "Answer s1" }] },
+  });
+  const view = render(<WidgetHost sessionId="s1" />);
+  await userEvent.click(screen.getByRole("button", { name: "Answer s1" }));
+  await requestStarted;
+
+  view.rerender(<WidgetHost sessionId="s2" />);
+  act(() => useSessionStore.setState({
+    pendingWidget: { id: "gate-s2", widget: "select", options: [{ id: "s2", label: "Answer s2" }] },
+  }));
+  release();
+  await act(async () => { await requestSettled; });
+
+  expect(useSessionStore.getState().pendingWidget?.id).toBe("gate-s2");
+  expect(useSessionStore.getState().lastUserEntry).toBeNull();
+});
diff --git a/frontend/src/shell/WidgetHost.tsx b/frontend/src/shell/WidgetHost.tsx
index 1c7e1cfc..7b5a60be 100644
--- a/frontend/src/shell/WidgetHost.tsx
+++ b/frontend/src/shell/WidgetHost.tsx
@@ -15,6 +15,12 @@ export function WidgetHost({ sessionId }: { sessionId: string | null }) {
   const responseInFlight = useRef(false);
   const operationEpochRef = useRef(0);
   useEffect(() => () => { operationEpochRef.current += 1; }, []);
+  useEffect(() => {
+    // A gate belongs to its active session target, not merely the authenticated user.
+    operationEpochRef.current += 1;
+    responseInFlight.current = false;
+    setResponding(false);
+  }, [sessionId]);
   if (!pending) return null;
   const Renderer = resolve(pending.widget);
   const onRespond = async (r: UiResponse) => {