diff --git a/backend/src/app.ts b/backend/src/app.ts
index 2cd24ef3..9881fbe8 100644
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -14,6 +14,7 @@ import { loadSettings, type Settings } from "./settings/settings-store.js";
 
 export interface BuildAppDeps {
   thtRunner?: ThtRunner;
+  mgr?: PiProcessManager;
   spawnFn?: () => any;
   listModels?: ListModelsFn;
   getSettings?: () => Settings;
@@ -34,7 +35,7 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
     harnessDir: config.harnessDir,
     configPath: "config/tht.yaml",
   });
-  const mgr = new PiProcessManager(config, deps?.spawnFn ? { spawnFn: deps.spawnFn } : undefined);
+  const mgr = deps?.mgr ?? new PiProcessManager(config, deps?.spawnFn ? { spawnFn: deps.spawnFn } : undefined);
   const hub = new SseHub();
 
   const listModels = deps?.listModels ?? createPiModelLister(config);
diff --git a/backend/test/routes-sessions.test.ts b/backend/test/routes-sessions.test.ts
index 7b79d0b0..ee01fb00 100644
--- a/backend/test/routes-sessions.test.ts
+++ b/backend/test/routes-sessions.test.ts
@@ -87,6 +87,19 @@ test("DELETE /sessions/:id calls deleteSession", async () => {
   expect(deleted).toBe("s1");
 });
 
+test("DELETE /sessions/:id tears down the runtime before deleting on disk", async () => {
+  const order: string[] = [];
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: { deleteSession: async (id: string) => { order.push(`del:${id}`); } } as any,
+    mgr: { teardown: (id: string) => { order.push(`teardown:${id}`); } } as any,
+    getSettings: () => ({ workspace: "w" }) as any,
+    spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
+  });
+  const res = await app.inject({ method: "DELETE", url: "/sessions/s1" });
+  expect(res.statusCode).toBe(204);
+  expect(order).toEqual(["teardown:s1", "del:s1"]);
+});
+
 test("GET /sessions/:id/documents returns the runner output", async () => {
   const app = mutApp({ documents: async () => [{ phase: "—", key: "question", title: "t", format: "text", content: "q" }] });
   const res = await app.inject({ method: "GET", url: "/sessions/s1/documents" });