diff --git a/apps/server/src/mcp/OrchestratorMcpToolkit.integration.test.ts b/apps/server/src/mcp/OrchestratorMcpToolkit.integration.test.ts
index 4c3456aec830..259232488e45 100644
--- a/apps/server/src/mcp/OrchestratorMcpToolkit.integration.test.ts
+++ b/apps/server/src/mcp/OrchestratorMcpToolkit.integration.test.ts
@@ -788,6 +788,86 @@ describe("orchestrator MCP toolkit", () => {
             yield* invoke("t3_thread_organize", { action: "unpin" });
             expect((yield* orchestrator.getThreadShell(parentThreadId))?.pinnedAt).toBeNull();
 
+            // Archiving detaches the provider, so the caller's own running turn
+            // must refuse it instead of failing mid-turn.
+            const selfArchiveCall = yield* invoke("t3_thread_organize", { action: "archive" });
+            expect(selfArchiveCall.isError).toBe(true);
+            expect(declaredFailure(selfArchiveCall)).toEqual({
+              _tag: "OrchestratorMcpFailure",
+              code: "invalid_request",
+              message:
+                "A thread cannot be archived while a turn is running. Archive it after the turn ends, from another thread or in the app.",
+            });
+            const afterSelfArchive = yield* orchestrator.getThreadProjection(parentThreadId);
+            expect(afterSelfArchive.thread.archivedAt).toBeNull();
+            expect(afterSelfArchive.runs[0]?.status).toBe("running");
+            // A detach would have dropped the session from the projection.
+            expect(parent.providerSessions).not.toHaveLength(0);
+            expect(afterSelfArchive.providerSessions.map((session) => session.id)).toEqual(
+              expect.arrayContaining(parent.providerSessions.map((session) => session.id)),
+            );
+
+            const idleThreadId = ThreadId.make("thread:mcp-idle-archive");
+            yield* orchestrator.dispatch({
+              type: "thread.create",
+              createdBy: "user",
+              creationSource: "web",
+              commandId: CommandId.make("command:mcp-idle-archive:create"),
+              threadId: idleThreadId,
+              projectId,
+              title: "Idle thread to archive",
+              modelSelection: codexSelection,
+              runtimeMode: "full-access",
+              interactionMode: "default",
+              branch: null,
+              worktreePath: cwd,
+            });
+            const archiveTargetGate = yield* Deferred.make<void>();
+            parentTerminalGates.set(idleThreadId, archiveTargetGate);
+            yield* orchestrator.dispatch({
+              type: "message.dispatch",
+              createdBy: "user",
+              creationSource: "web",
+              commandId: CommandId.make("command:mcp-idle-archive:start"),
+              threadId: idleThreadId,
+              messageId: MessageId.make("message:mcp-idle-archive:start"),
+              text: "Finish after the archive rejection.",
+              attachments: [],
+              modelSelection: codexSelection,
+              dispatchMode: { type: "start_immediately" },
+            });
+            const archiveEvents = yield* EventSink.EventSinkV2;
+            const awaitArchiveTargetStatus = (status: "running" | "completed") =>
+              archiveEvents.stream({ threadId: idleThreadId }).pipe(
+                Stream.filter(
+                  ({ event }) => event.type === "run.updated" && event.payload.status === status,
+                ),
+                Stream.take(1),
+                Stream.runDrain,
+              );
+            yield* awaitArchiveTargetStatus("running");
+            const otherArchiveCall = yield* invoke("t3_thread_organize", {
+              threadId: idleThreadId,
+              action: "archive",
+            });
+            expect(declaredFailure(otherArchiveCall)).toEqual(declaredFailure(selfArchiveCall));
+            expect(
+              (yield* orchestrator.getThreadProjection(idleThreadId)).thread.archivedAt,
+            ).toBeNull();
+            yield* Deferred.succeed(archiveTargetGate, undefined);
+            yield* awaitArchiveTargetStatus("completed");
+            const completedArchiveTarget = yield* orchestrator.getThreadProjection(idleThreadId);
+            expect(completedArchiveTarget.runs[0]?.checkpointId).not.toBeNull();
+            expect(completedArchiveTarget.runs[0]?.status).toBe("completed");
+            const idleArchiveCall = yield* invoke("t3_thread_organize", {
+              threadId: idleThreadId,
+              action: "archive",
+            });
+            expect(idleArchiveCall.structuredContent).toHaveProperty("sequence");
+            expect(
+              (yield* orchestrator.getThreadProjection(idleThreadId)).thread.archivedAt,
+            ).not.toBeNull();
+
             if (parentRun === undefined || parentRun.rootNodeId === null) {
               return yield* Effect.die(new Error("Parent run missing."));
             }
diff --git a/apps/server/src/mcp/toolkits/thread/handlers.ts b/apps/server/src/mcp/toolkits/thread/handlers.ts
index 2a06c58c1b43..ac1dbfcdc568 100644
--- a/apps/server/src/mcp/toolkits/thread/handlers.ts
+++ b/apps/server/src/mcp/toolkits/thread/handlers.ts
@@ -315,7 +315,17 @@ export const layer = McpToolAccess.toLayer(ThreadToolkit, {
         default:
           command = { ...common, type: `thread.${input.action}` };
       }
-      const result = yield* threads.dispatch(command).pipe(Effect.mapError(dispatchFailure));
+      const result = yield* threads.dispatch(command).pipe(
+        Effect.mapError((error) =>
+          error._tag === "OrchestratorThreadTurnRunningError"
+            ? new OrchestratorMcpFailure({
+                code: "invalid_request",
+                message:
+                  "A thread cannot be archived while a turn is running. Archive it after the turn ends, from another thread or in the app.",
+              })
+            : dispatchFailure(error),
+        ),
+      );
       return { sequence: result.sequence };
     }),
   ),
diff --git a/apps/server/src/orchestration-v2/Orchestrator.control-reads.test.ts b/apps/server/src/orchestration-v2/Orchestrator.control-reads.test.ts
index c68fc0ace5e9..3e7edd5fce1e 100644
--- a/apps/server/src/orchestration-v2/Orchestrator.control-reads.test.ts
+++ b/apps/server/src/orchestration-v2/Orchestrator.control-reads.test.ts
@@ -266,6 +266,17 @@ it.effect(
         turnItemTypes: ["user_message"],
       });
       assert.isAbove(fresh.turnItems.at(-1)!.ordinal, 900);
+      // Archive refuses a preparing run, so end the deferred one first.
+      const deferred = (yield* projections.getThreadRecords(threadId, ["runs"])).runs.at(-1)!;
+      assert.equal(deferred.status, "preparing");
+      yield* projections.apply({
+        id: EventId.make("cancel-deferred-run"),
+        type: "run.updated",
+        threadId,
+        runId: deferred.id,
+        occurredAt: now,
+        payload: { ...deferred, status: "cancelled", completedAt: now },
+      });
       yield* orchestrator.dispatch({
         type: "thread.archive",
         commandId: CommandId.make("archive-with-old-history"),
diff --git a/apps/server/src/orchestration-v2/Orchestrator.ts b/apps/server/src/orchestration-v2/Orchestrator.ts
index 5d4b1cfda15f..77576fac0a0d 100644
--- a/apps/server/src/orchestration-v2/Orchestrator.ts
+++ b/apps/server/src/orchestration-v2/Orchestrator.ts
@@ -190,6 +190,16 @@ export class OrchestratorSubagentThreadReadOnlyError extends Schema.TaggedError<
   }
 }
 
+/** Archive detaches the thread's provider sessions, which would fail a turn in flight. */
+export class OrchestratorThreadTurnRunningError extends Schema.TaggedError<OrchestratorThreadTurnRunningError>()(
+  "OrchestratorThreadTurnRunningError",
+  { commandId: CommandId, threadId: ThreadId },
+) {
+  override get message(): string {
+    return "This thread cannot be archived while a turn is running. Archive it after the turn ends.";
+  }
+}
+
 /** The command's thread runs above the modes its sender may touch (see `DispatchModeLimit`). */
 export class OrchestratorThreadAboveModeLimitError extends Schema.TaggedError<OrchestratorThreadAboveModeLimitError>()(
   "OrchestratorThreadAboveModeLimitError",
@@ -255,6 +265,7 @@ export const OrchestratorV2Error = Schema.Union([
   OrchestratorCommandPreviouslyRejectedError,
   OrchestratorCommandIdConflictError,
   OrchestratorSubagentThreadReadOnlyError,
+  OrchestratorThreadTurnRunningError,
   OrchestratorThreadAboveModeLimitError,
 ]);
 export type OrchestratorV2Error = typeof OrchestratorV2Error.Type;
@@ -2478,6 +2489,22 @@ const makeOrchestrator = Effect.fn("orchestrationV2.Orchestrator.layer")(functio
         cause: `Thread ${command.threadId} is already archived.`,
       });
     }
+    if (command.type === "thread.archive") {
+      // Archive detaches every provider session below, which would fail a turn
+      // in flight. Queued runs are cancelled by the archive itself.
+      const { runs } = yield* loadProjectionForCommand(command, ["runs"]);
+      if (
+        runs.some(
+          (run) =>
+            run.status === "preparing" || run.status === "starting" || run.status === "running",
+        )
+      ) {
+        return yield* new OrchestratorThreadTurnRunningError({
+          commandId: command.commandId,
+          threadId: command.threadId,
+        });
+      }
+    }
     if (command.type === "thread.unarchive" && thread.archivedAt === null) {
       return yield* new OrchestratorDispatchError({
         commandId: command.commandId,
@@ -3289,10 +3316,11 @@ const makeOrchestrator = Effect.fn("orchestrationV2.Orchestrator.layer")(functio
     // Settle joins archive here: both mean "done with this
     // thread", so a live provider session must not keep running background
     // work (PR monitors, dev servers, subagent fleets) after any of them
-    // lands. The settle guard above already rejects active or blocked runs,
-    // so for settle this only ever stops an idle session; commands are
-    // decided serially against the projection, so a turn start that
-    // re-engages the thread cannot race this detach.
+    // lands. The guards above reject a preparing, starting or running run
+    // (and settle also rejects any other active or blocked run), so this
+    // never detaches a provider mid-turn; commands are decided serially
+    // against the projection, so a turn start that re-engages the thread
+    // cannot race this detach.
     const detachSessionIds = new Set(
       command.type === "thread.archive" || command.type === "thread.settle"
         ? (providerContext?.providerSessions ?? []).map((session) => session.id)
diff --git a/apps/server/src/orchestration-v2/runtimeLayer.test.ts b/apps/server/src/orchestration-v2/runtimeLayer.test.ts
index 131f05034051..aae801f65cd3 100644
--- a/apps/server/src/orchestration-v2/runtimeLayer.test.ts
+++ b/apps/server/src/orchestration-v2/runtimeLayer.test.ts
@@ -263,6 +263,41 @@ const moveProject = (projectId: ProjectId, workspaceRoot: string, updatedAt: str
     }),
   );
 
+/** Stage run rows for command-policy tests. This does not simulate provider or checkpoint effects. */
+const rewriteRuns = (
+  threadId: ThreadId,
+  commandId: string,
+  rewrite: (run: OrchestrationV2Run, now: DateTime.Utc) => OrchestrationV2Run | undefined,
+) =>
+  Effect.gen(function* () {
+    const orchestrator = yield* Orchestrator.OrchestratorV2;
+    const eventSink = yield* EventSink.EventSinkV2;
+    const projection = yield* orchestrator.getThreadProjection(threadId);
+    const now = yield* DateTime.now;
+    yield* eventSink.commitCommand({
+      commandId: CommandId.make(commandId),
+      threadId,
+      commandType: "provider-runtime.reconcile",
+      acceptedAt: now,
+      events: projection.runs.flatMap((run) => {
+        const payload = rewrite(run, now);
+        return payload === undefined
+          ? []
+          : [
+              {
+                id: EventId.make(`${commandId}:${run.id}`),
+                type: "run.updated" as const,
+                threadId,
+                runId: run.id,
+                occurredAt: now,
+                payload,
+              },
+            ];
+      }),
+      effects: [],
+    });
+  });
+
 const layerTest = Layer.mergeAll(
   RuntimeLayer.layer,
   RuntimeLayer.layerEventSink,
@@ -3548,6 +3583,13 @@ it.layer(layerTest)("OrchestrationV2LayerLive lifecycle", (it) => {
       assert.isDefined(activeRun);
       assert.isDefined(queuedRun);
 
+      // Archive refuses a starting turn, so simulate a restart: the active run
+      // ends and recovery holds the queue.
+      yield* rewriteRuns(threadId, "runtime-layer-archive-queued-restart", (run, now) =>
+        run.status === "queued"
+          ? { ...run, queueHeld: true }
+          : { ...run, status: "cancelled", completedAt: now },
+      );
       yield* orchestrator.dispatch({
         type: "thread.archive",
         commandId: CommandId.make("runtime-layer-archive-queued-archive"),
@@ -3580,6 +3622,116 @@ it.layer(layerTest)("OrchestrationV2LayerLive lifecycle", (it) => {
     }),
   );
 
+  const startArchiveTestThread = (name: string) =>
+    Effect.gen(function* () {
+      const orchestrator = yield* Orchestrator.OrchestratorV2;
+      const threadId = ThreadId.make(`runtime-layer-archive-${name}-thread`);
+      yield* orchestrator.dispatch({
+        type: "thread.create",
+        createdBy: "user",
+        creationSource: "web",
+        commandId: CommandId.make(`runtime-layer-archive-${name}-create`),
+        threadId,
+        projectId: ProjectId.make(`runtime-layer-archive-${name}-project`),
+        title: `Archive ${name}`,
+        modelSelection,
+        runtimeMode: "full-access",
+        interactionMode: "default",
+        branch: null,
+        worktreePath: `/tmp/runtime-layer-archive-${name}`,
+      });
+      yield* orchestrator.dispatch({
+        type: "message.dispatch",
+        createdBy: "user",
+        creationSource: "web",
+        commandId: CommandId.make(`runtime-layer-archive-${name}-message`),
+        threadId,
+        messageId: MessageId.make(`runtime-layer-archive-${name}-message`),
+        text: "Keep the provider occupied.",
+        attachments: [],
+        modelSelection,
+        dispatchMode: { type: "start_immediately" },
+      });
+      const started = yield* orchestrator.getThreadProjection(threadId);
+      assert.deepEqual(
+        started.runs.map((run) => run.status),
+        ["starting"],
+      );
+      return threadId;
+    });
+
+  it.effect.each(["preparing", "starting", "running"] as const)(
+    "refuses to archive a thread while its turn is %s, then archives it once the turn ends",
+    (status) =>
+      Effect.gen(function* () {
+        const orchestrator = yield* Orchestrator.OrchestratorV2;
+        const eventSink = yield* EventSink.EventSinkV2;
+        const outbox = yield* EffectOutbox.EffectOutboxV2;
+        const threadId = yield* startArchiveTestThread(status);
+        if (status !== "starting") {
+          yield* rewriteRuns(threadId, `runtime-layer-archive-${status}-stage`, (run) => ({
+            ...run,
+            status,
+          }));
+        }
+        const previousSequence = yield* orchestrator.getThreadEventSequence(threadId);
+
+        const commandId = CommandId.make(`runtime-layer-archive-${status}-archive`);
+        const error = yield* orchestrator
+          .dispatch({ type: "thread.archive", commandId, threadId })
+          .pipe(Effect.flip);
+        assert.instanceOf(error, Orchestrator.OrchestratorThreadTurnRunningError);
+        assert.equal(
+          error.message,
+          "This thread cannot be archived while a turn is running. Archive it after the turn ends.",
+        );
+        assert.equal(yield* orchestrator.getThreadEventSequence(threadId), previousSequence);
+        assert.deepEqual(
+          yield* eventSink.readByCommandId({ commandId }).pipe(Stream.runCollect),
+          [],
+        );
+        assert.deepEqual(yield* outbox.listByCommandId(commandId), []);
+        const refused = yield* orchestrator.getThreadProjection(threadId);
+        assert.isNull(refused.thread.archivedAt);
+        assert.deepEqual(
+          refused.runs.map((run) => run.status),
+          [status],
+        );
+
+        yield* rewriteRuns(threadId, `runtime-layer-archive-${status}-complete`, (run, now) => ({
+          ...run,
+          status: "completed",
+          completedAt: now,
+        }));
+        yield* orchestrator.dispatch({
+          type: "thread.archive",
+          commandId: CommandId.make(`runtime-layer-archive-${status}-archive-after-turn`),
+          threadId,
+        });
+        assert.isNotNull((yield* orchestrator.getThreadProjection(threadId)).thread.archivedAt);
+      }),
+  );
+
+  it.effect("archives a thread whose run is waiting", () =>
+    Effect.gen(function* () {
+      const orchestrator = yield* Orchestrator.OrchestratorV2;
+      const threadId = yield* startArchiveTestThread("waiting");
+      yield* rewriteRuns(threadId, "runtime-layer-archive-waiting-stage", (run) => ({
+        ...run,
+        status: "waiting",
+      }));
+
+      yield* orchestrator.dispatch({
+        type: "thread.archive",
+        commandId: CommandId.make("runtime-layer-archive-waiting-archive"),
+        threadId,
+      });
+      const archived = yield* orchestrator.getThreadProjection(threadId);
+      assert.isNotNull(archived.thread.archivedAt);
+      assert.equal(archived.runs[0]?.status, "waiting");
+    }),
+  );
+
   it.effect.each([false, true])(
     "promotes only one queued run after each terminal run (notification: %s)",
     (automatic) =>
diff --git a/apps/web/src/components/threadActionMenu.logic.ts b/apps/web/src/components/threadActionMenu.logic.ts
index 1b9ada254f14..a34a76a89ab9 100644
--- a/apps/web/src/components/threadActionMenu.logic.ts
+++ b/apps/web/src/components/threadActionMenu.logic.ts
@@ -87,7 +87,7 @@ export interface ThreadActionMenuState {
   readonly isSnoozed: boolean;
   readonly canSnoozeNow: boolean;
   readonly isRegeneratingTitle: boolean;
-  /** Archive rejects a thread with an attached provider, so disable it here rather than let the action fail. */
+  /** The server rejects archive while a run is preparing, starting or running (idle attached providers still archive), so disable it here. */
   readonly isRunning: boolean;
   readonly supports: {
     readonly settlement: boolean;