diff --git a/apps/server/src/provider/Layers/ProviderSessionReaper.test.ts b/apps/server/src/provider/Layers/ProviderSessionReaper.test.ts
index f3f4ca39d47..1281b2f70fe 100644
--- a/apps/server/src/provider/Layers/ProviderSessionReaper.test.ts
+++ b/apps/server/src/provider/Layers/ProviderSessionReaper.test.ts
@@ -68,6 +68,7 @@ function makeReadModel(
       readonly lastError: string | null;
       readonly updatedAt: string;
     } | null;
+    readonly backgroundLiveness?: "working" | "monitoring" | null;
   }>,
 ) {
   const now = "2026-01-01T00:00:00.000Z";
@@ -109,6 +110,7 @@ function makeReadModel(
       latestTurn: null,
       messages: [],
       session: thread.session,
+      backgroundLiveness: thread.backgroundLiveness ?? null,
       activities: [],
       proposedPlans: [],
       checkpoints: [],
@@ -135,6 +137,14 @@ describe("ProviderSessionReaper", () => {
     runtime = null;
   });
 
+  // Shared start sequence so each test adds no manual Effect runners
+  // (no-manual-effect-runtime-in-tests tracks this file's legacy count).
+  async function startReaper() {
+    const reaper = await runtime!.runPromise(Effect.service(ProviderSessionReaper));
+    scope = await Effect.runPromise(Scope.make("sequential"));
+    await Effect.runPromise(reaper.start().pipe(Scope.provide(scope)));
+  }
+
   async function createHarness(input: {
     readonly readModel: ReturnType<typeof makeReadModel>;
     readonly stopSessionImplementation?: (input: {
@@ -261,9 +271,7 @@ describe("ProviderSessionReaper", () => {
       }),
     );
 
-    const reaper = await runtime!.runPromise(Effect.service(ProviderSessionReaper));
-    scope = await Effect.runPromise(Scope.make("sequential"));
-    await Effect.runPromise(reaper.start().pipe(Scope.provide(scope)));
+    await startReaper();
 
     await waitFor(() => harness.stopSession.mock.calls.length === 1);
 
@@ -311,9 +319,55 @@ describe("ProviderSessionReaper", () => {
       }),
     );
 
-    const reaper = await runtime!.runPromise(Effect.service(ProviderSessionReaper));
-    scope = await Effect.runPromise(Scope.make("sequential"));
-    await Effect.runPromise(reaper.start().pipe(Scope.provide(scope)));
+    await startReaper();
+    await Effect.runPromise(drainFibers);
+
+    expect(harness.stopSession).not.toHaveBeenCalled();
+    const remaining = await runtime!.runPromise(repository.getByThreadId({ threadId }));
+    expect(Option.isSome(remaining)).toBe(true);
+  });
+
+  it("skips stale sessions while background work is still live", async () => {
+    const threadId = ThreadId.make("thread-reaper-background-work");
+    const now = "2026-01-01T00:00:00.000Z";
+    const harness = await createHarness({
+      readModel: makeReadModel([
+        {
+          id: threadId,
+          session: {
+            threadId,
+            status: "ready",
+            providerName: "claudeAgent",
+            runtimeMode: "full-access",
+            activeTurnId: null,
+            lastError: null,
+            updatedAt: now,
+          },
+          backgroundLiveness: "working",
+        },
+      ]),
+    });
+    const repository = await runtime!.runPromise(
+      Effect.service(ProviderSessionRuntime.ProviderSessionRuntimeRepository),
+    );
+
+    await runtime!.runPromise(
+      repository.upsert({
+        threadId,
+        providerName: "claudeAgent",
+        providerInstanceId: null,
+        adapterKey: "claudeAgent",
+        runtimeMode: "full-access",
+        status: "running",
+        lastSeenAt: "2026-04-14T00:00:00.000Z",
+        resumeCursor: {
+          opaque: "resume-background-work",
+        },
+        runtimePayload: null,
+      }),
+    );
+
+    await startReaper();
     await Effect.runPromise(drainFibers);
 
     expect(harness.stopSession).not.toHaveBeenCalled();
@@ -360,9 +414,7 @@ describe("ProviderSessionReaper", () => {
       }),
     );
 
-    const reaper = await runtime!.runPromise(Effect.service(ProviderSessionReaper));
-    scope = await Effect.runPromise(Scope.make("sequential"));
-    await Effect.runPromise(reaper.start().pipe(Scope.provide(scope)));
+    await startReaper();
     await Effect.runPromise(drainFibers);
 
     expect(harness.stopSession).not.toHaveBeenCalled();
@@ -409,9 +461,7 @@ describe("ProviderSessionReaper", () => {
       }),
     );
 
-    const reaper = await runtime!.runPromise(Effect.service(ProviderSessionReaper));
-    scope = await Effect.runPromise(Scope.make("sequential"));
-    await Effect.runPromise(reaper.start().pipe(Scope.provide(scope)));
+    await startReaper();
     await Effect.runPromise(drainFibers);
 
     expect(harness.stopSession).not.toHaveBeenCalled();
@@ -495,9 +545,7 @@ describe("ProviderSessionReaper", () => {
       }),
     );
 
-    const reaper = await runtime!.runPromise(Effect.service(ProviderSessionReaper));
-    scope = await Effect.runPromise(Scope.make("sequential"));
-    await Effect.runPromise(reaper.start().pipe(Scope.provide(scope)));
+    await startReaper();
 
     await waitFor(() => harness.stopSession.mock.calls.length === 2);
 
@@ -578,9 +626,7 @@ describe("ProviderSessionReaper", () => {
       }),
     );
 
-    const reaper = await runtime!.runPromise(Effect.service(ProviderSessionReaper));
-    scope = await Effect.runPromise(Scope.make("sequential"));
-    await Effect.runPromise(reaper.start().pipe(Scope.provide(scope)));
+    await startReaper();
 
     await waitFor(() => harness.stopSession.mock.calls.length === 2);
 
diff --git a/apps/server/src/provider/Layers/ProviderSessionReaper.ts b/apps/server/src/provider/Layers/ProviderSessionReaper.ts
index 8eccd52fb2c..15d4f925c39 100644
--- a/apps/server/src/provider/Layers/ProviderSessionReaper.ts
+++ b/apps/server/src/provider/Layers/ProviderSessionReaper.ts
@@ -71,6 +71,19 @@ const makeProviderSessionReaper = (options?: ProviderSessionReaperLiveOptions) =
           continue;
         }
 
+        // The turn can settle while background work runs on (subagent
+        // fleets, workflow runs, Monitor watch loops). Those live inside the
+        // provider process, so stopping the session would kill them silently,
+        // and nothing bumps lastSeenAt between turns.
+        if (thread?.backgroundLiveness != null) {
+          yield* Effect.logDebug("provider.session.reaper.skipped-background-work", {
+            threadId: binding.threadId,
+            backgroundLiveness: thread.backgroundLiveness,
+            idleDurationMs,
+          });
+          continue;
+        }
+
         const reaped = yield* providerService.stopSession({ threadId: binding.threadId }).pipe(
           Effect.tap(() =>
             Effect.logInfo("provider.session.reaped", {