diff --git a/backend/src/pi/list-models.ts b/backend/src/pi/list-models.ts
index 4c62a195..cb8b31d4 100644
--- a/backend/src/pi/list-models.ts
+++ b/backend/src/pi/list-models.ts
@@ -35,6 +35,7 @@ export function createPiModelLister(cfg: AppConfig, opts: Opts = {}): () => Prom
     if (cache && now() - cache.at < ttlMs) return cache.models;
 
     const env: NodeJS.ProcessEnv = { ...process.env };
+    delete env.THT_DATA_ROOT;
     if (cfg.dataRoot !== undefined) env.THT_DATA_ROOT = cfg.dataRoot;
     const child = spawnFn(cfg.piBin, ["--mode", "rpc"], { cwd: cfg.harnessDir, env });
     child.stderr.resume();
diff --git a/backend/src/pi/pi-process-manager.ts b/backend/src/pi/pi-process-manager.ts
index a5c8106e..f2a43952 100644
--- a/backend/src/pi/pi-process-manager.ts
+++ b/backend/src/pi/pi-process-manager.ts
@@ -35,6 +35,7 @@ export class PiProcessManager {
       THT_SESSION: sessionId,
       THT_AUTHOR: author,
     };
+    delete env.THT_DATA_ROOT;
     if (this.cfg.dataRoot !== undefined) env.THT_DATA_ROOT = this.cfg.dataRoot;
     // pi 0.73 removed `--approve`: rpc mode is headless and its argv is intentionally minimal.
     const child = spawnFn(this.cfg.piBin, ["--mode", "rpc"], {
diff --git a/backend/src/tht/tht-runner.ts b/backend/src/tht/tht-runner.ts
index 0a8437b1..1406a21a 100644
--- a/backend/src/tht/tht-runner.ts
+++ b/backend/src/tht/tht-runner.ts
@@ -63,6 +63,7 @@ export class ThtRunner {
   run(args: string[], workspace?: string): Promise<{ code: number; stdout: string; stderr: string }> {
     return new Promise((resolve) => {
       const env: NodeJS.ProcessEnv = { ...process.env };
+      delete env.THT_DATA_ROOT;
       if (this.cfg.dataRoot !== undefined) env.THT_DATA_ROOT = this.cfg.dataRoot;
       const ch = spawn(this.cfg.thtBin, this.buildArgv(args, workspace), {
         cwd: this.cfg.harnessDir,
diff --git a/backend/test/list-models.test.ts b/backend/test/list-models.test.ts
index 51237ab8..7a6788e1 100644
--- a/backend/test/list-models.test.ts
+++ b/backend/test/list-models.test.ts
@@ -81,3 +81,29 @@ test("production model-list spawn preserves PATH and passes the portable data ro
     rmSync(path.dirname(script), { recursive: true, force: true });
   }
 });
+
+test("model-list spawn omits ambient THT_DATA_ROOT when config does not provide one", async () => {
+  const script = scriptWith([]);
+  const calls: any[][] = [];
+  const previousDataRoot = process.env.THT_DATA_ROOT;
+  const previousCredential = process.env.PI_PROVIDER_API_KEY;
+  process.env.THT_DATA_ROOT = "/ambient-must-not-leak";
+  process.env.PI_PROVIDER_API_KEY = "still-inherited";
+  try {
+    const lister = createPiModelLister(loadConfig({ PI_BIN: "/usr/local/bin/pi" }), {
+      spawnFn: (...args: any[]) => {
+        calls.push(args);
+        return spawn("node", [FAKE, script]) as any;
+      },
+    });
+    await lister();
+    expect(calls[0][2].env).not.toHaveProperty("THT_DATA_ROOT");
+    expect(calls[0][2].env.PI_PROVIDER_API_KEY).toBe("still-inherited");
+  } finally {
+    if (previousDataRoot === undefined) delete process.env.THT_DATA_ROOT;
+    else process.env.THT_DATA_ROOT = previousDataRoot;
+    if (previousCredential === undefined) delete process.env.PI_PROVIDER_API_KEY;
+    else process.env.PI_PROVIDER_API_KEY = previousCredential;
+    rmSync(path.dirname(script), { recursive: true, force: true });
+  }
+});
diff --git a/backend/test/pi-process-manager.test.ts b/backend/test/pi-process-manager.test.ts
index 0ecb1f4a..f5311f72 100644
--- a/backend/test/pi-process-manager.test.ts
+++ b/backend/test/pi-process-manager.test.ts
@@ -160,3 +160,23 @@ test("production spawn uses explicit Pi path and passes portable data root witho
     vi.unstubAllEnvs();
   }
 });
+
+test("session Pi spawn omits ambient THT_DATA_ROOT when config does not provide one", async () => {
+  vi.stubEnv("THT_DATA_ROOT", "/ambient-must-not-leak");
+  vi.stubEnv("PI_PROVIDER_API_KEY", "still-inherited");
+  const calls: any[][] = [];
+  const child = recordingChild();
+  child.stderr.resume = () => {};
+  const mgr = new PiProcessManager(loadConfig({ PI_BIN: "/usr/local/bin/pi" }), {
+    spawnFn: (...args: any[]) => { calls.push(args); return child as any; },
+  });
+
+  try {
+    await mgr.spawnFor("no-data-root", {});
+    expect(calls[0][2].env).not.toHaveProperty("THT_DATA_ROOT");
+    expect(calls[0][2].env.PI_PROVIDER_API_KEY).toBe("still-inherited");
+  } finally {
+    mgr.teardown("no-data-root");
+    vi.unstubAllEnvs();
+  }
+});
diff --git a/backend/test/tht-runner.test.ts b/backend/test/tht-runner.test.ts
index 39c3f709..753160aa 100644
--- a/backend/test/tht-runner.test.ts
+++ b/backend/test/tht-runner.test.ts
@@ -24,6 +24,54 @@ test("sessionNew parses id from JSON", async () => {
   expect(await r.sessionNew({ question: "q" })).toEqual({ id: "2026-06-27-100000-x" });
 });
 
+test("run passes configured THT_DATA_ROOT and preserves the remaining environment", async () => {
+  const previousDataRoot = process.env.THT_DATA_ROOT;
+  const previousCa = process.env.NODE_EXTRA_CA_CERTS;
+  process.env.THT_DATA_ROOT = "/ambient";
+  process.env.NODE_EXTRA_CA_CERTS = "/certs/company-ca.pem";
+  try {
+    (spawn as any).mockClear();
+    const r = new ThtRunner({
+      thtBin: "/opt/venv/bin/tht",
+      harnessDir: "/app/harness",
+      configPath: "config/tht.yaml",
+      dataRoot: "/configured",
+    });
+    await r.run(["session", "list", "--json"]);
+    const [bin, , options] = (spawn as any).mock.calls[0];
+    expect(bin).toBe("/opt/venv/bin/tht");
+    expect(options.env).toMatchObject({
+      THT_DATA_ROOT: "/configured",
+      NODE_EXTRA_CA_CERTS: "/certs/company-ca.pem",
+    });
+  } finally {
+    if (previousDataRoot === undefined) delete process.env.THT_DATA_ROOT;
+    else process.env.THT_DATA_ROOT = previousDataRoot;
+    if (previousCa === undefined) delete process.env.NODE_EXTRA_CA_CERTS;
+    else process.env.NODE_EXTRA_CA_CERTS = previousCa;
+  }
+});
+
+test("run omits ambient THT_DATA_ROOT when config does not provide one", async () => {
+  const previousDataRoot = process.env.THT_DATA_ROOT;
+  const previousCredential = process.env.PI_PROVIDER_API_KEY;
+  process.env.THT_DATA_ROOT = "/ambient-must-not-leak";
+  process.env.PI_PROVIDER_API_KEY = "still-inherited";
+  try {
+    (spawn as any).mockClear();
+    const r = new ThtRunner({ thtBin: "tht", harnessDir: "/h", configPath: "config/tht.yaml" });
+    await r.run(["session", "list", "--json"]);
+    const options = (spawn as any).mock.calls[0][2];
+    expect(options.env).not.toHaveProperty("THT_DATA_ROOT");
+    expect(options.env.PI_PROVIDER_API_KEY).toBe("still-inherited");
+  } finally {
+    if (previousDataRoot === undefined) delete process.env.THT_DATA_ROOT;
+    else process.env.THT_DATA_ROOT = previousDataRoot;
+    if (previousCredential === undefined) delete process.env.PI_PROVIDER_API_KEY;
+    else process.env.PI_PROVIDER_API_KEY = previousCredential;
+  }
+});
+
 test("run with exit != 0 propagates error with stderr", async () => {
   const r = new ThtRunner({ thtBin: "tht", harnessDir: "/h", configPath: "config/tht.yaml" });
   r.run = async () => ({ code: 1, stdout: "", stderr: "ERRORE: boom" });