fix: guard CE sessions against concurrent-turn displacement and harden JSON-protocol parsing

Port the planning turn-admission invariant (FNXC:PlanningTurnAdmission,
2026-07-22) into the Compound Engineering orchestrator: at most one turn
(opening/answer/resume-rehydration) is admitted per CE session, reserved
synchronously and held until the turn settles — a re-entered mobile view
re-submitting a turn now gets CeTurnInProgressError (HTTP 409) instead of
displacing the in-flight turn's live agent, which surfaced as "Failed to
parse agent response: AI returned no valid JSON". cancel()/discard()
force-clear the reservation; releases are token-scoped so a stale release
can't drop a newer turn's slot.

In the engine interactive-ai-session seam: bump the reformat retry from
one to two attempts (non-Anthropic default models comply less reliably
with the JSON-only protocol), and log every failed parse with a bounded
raw-response snippet plus resolved provider/model — including a distinct
empty-assistant-message marker — so support can diagnose these reports
without a repro.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.changeset/ce-turn-admission-json-parse.md

@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": patch
+---
+
+summary: Fix Compound Engineering sessions dying with "AI returned no valid JSON" when turns race; add retry and diagnostics.
+category: fix
+dev: CE orchestrator now enforces synchronous single-turn admission per session (concurrent answer/resume gets `CeTurnInProgressError`, HTTP 409) so a re-entered mobile view cannot displace the in-flight turn's live agent. The interactive AI session seam gains a second reformat retry and logs bounded raw-response snippets with provider/model via `interactiveSessionLog` on every parse failure.

packages/engine/src/__tests__/interactive-ai-session.test.ts

@@ -8,6 +8,7 @@ import {
   type InteractiveAgentResult,
   type InteractiveAgentSession,
 } from "../interactive-ai-session.js";
+import { interactiveSessionLog } from "../logger.js";
 import type { AgentRuntime, AgentRuntimeOptions, AgentSessionResult } from "../agent-runtime.js";
 import type { AgentSession } from "@earendil-works/pi-coding-agent";
 
@@ -278,12 +279,16 @@ describe("interactive-ai-session seam", () => {
     expect(JSON.parse(lastPrompt)).toMatchObject({ type: "answer", questionId: question.id, response: answer });
   });
 
-  it("retries once on unparseable output then surfaces an error event (no hang)", async () => {
-    // First turn: garbage. Reformat retry: still garbage. → error.
-    const scripted = makeScriptedAgent(["not json at all", "still not json"]);
+  it("retries twice on unparseable output then surfaces an error event (no hang), logging each failed parse", async () => {
+    const warnSpy = vi.spyOn(interactiveSessionLog, "warn");
+    const errorSpy = vi.spyOn(interactiveSessionLog, "error");
+    // First turn: garbage. Both reformat retries: still garbage. → error.
+    const scripted = makeScriptedAgent(["not json at all", "still not json", "nope"]);
     const { session } = await createInteractiveAiSessionWith(factoryFor(scripted.session), {
       cwd: "/tmp",
       systemPrompt: "protocol",
+      defaultProvider: "openai",
+      defaultModelId: "gpt-5",
     });
 
     await session.prompt("start");
@@ -291,14 +296,38 @@ describe("interactive-ai-session seam", () => {
     expect(ev.type).toBe("error");
     expect(ev.type === "error" && ev.data.message).toMatch(/parse/i);
 
-    // The reformat-retry prompt was actually sent (2 prompts: initial + retry).
-    expect(scripted.promptCalls().length).toBe(2);
+    // Both reformat-retry prompts were actually sent (3 prompts: initial + 2 retries).
+    expect(scripted.promptCalls().length).toBe(3);
+
+    // Every failed parse attempt logged a bounded raw-response snippet with the
+    // resolved provider/model, and the terminal give-up logged at error level.
+    expect(warnSpy).toHaveBeenCalledTimes(3);
+    expect(warnSpy.mock.calls[0][0]).toContain("provider=openai, model=gpt-5");
+    expect(warnSpy.mock.calls[0][0]).toContain('"not json at all"');
+    expect(errorSpy).toHaveBeenCalledTimes(1);
+    expect(errorSpy.mock.calls[0][0]).toContain("giving up after 3 parse attempts");
 
     // Terminal: nextEvent keeps returning the error, never hangs.
     expect((await session.nextEvent()).type).toBe("error");
+    warnSpy.mockRestore();
+    errorSpy.mockRestore();
   });
 
-  it("recovers when the reformat retry produces valid JSON", async () => {
+  it("flags an empty assistant message distinctly in the parse-failure log", async () => {
+    const warnSpy = vi.spyOn(interactiveSessionLog, "warn");
+    const scripted = makeScriptedAgent(["", "", ""]);
+    const { session } = await createInteractiveAiSessionWith(factoryFor(scripted.session), {
+      cwd: "/tmp",
+      systemPrompt: "protocol",
+    });
+
+    await session.prompt("start");
+    expect((await session.nextEvent()).type).toBe("error");
+    expect(warnSpy.mock.calls[0][0]).toContain("empty assistant message");
+    warnSpy.mockRestore();
+  });
+
+  it("recovers when the first reformat retry produces valid JSON", async () => {
     const question: PlanningQuestion = { id: "q1", type: "text", question: "?" };
     const scripted = makeScriptedAgent(["garbage", q(question)]);
     const { session } = await createInteractiveAiSessionWith(factoryFor(scripted.session), {
@@ -311,6 +340,20 @@ describe("interactive-ai-session seam", () => {
     expect(ev.type).toBe("question");
   });
 
+  it("recovers when only the second reformat retry produces valid JSON", async () => {
+    const question: PlanningQuestion = { id: "q1", type: "text", question: "?" };
+    const scripted = makeScriptedAgent(["garbage", "still garbage", q(question)]);
+    const { session } = await createInteractiveAiSessionWith(factoryFor(scripted.session), {
+      cwd: "/tmp",
+      systemPrompt: "protocol",
+    });
+
+    await session.prompt("start");
+    const ev = await session.nextEvent();
+    expect(ev.type).toBe("question");
+    expect(scripted.promptCalls().length).toBe(3);
+  });
+
   it("surfaces agent prompt errors as an error event without throwing", async () => {
     const throwing: InteractiveAgentSession = {
       prompt: vi.fn(async () => {

packages/engine/src/interactive-ai-session.ts

@@ -23,6 +23,7 @@ import type {
 } from "@fusion/core";
 import type { AgentRuntime } from "./agent-runtime.js";
 import { askAcpOnce } from "./cli-agent-ask.js";
+import { interactiveSessionLog } from "./logger.js";
 
 /** Minimal shape of an agent session we depend on (subset of pi's AgentSession). */
 export interface InteractiveAgentSession {
@@ -55,8 +56,12 @@ export type PlanningExecutorSelection =
   | { kind: "model" }
   | { kind: "cli-agent"; runtime: AgentRuntime };
 
-/** One bounded reformat retry, matching planning.ts's MAX_PARSE_RETRIES. */
-const MAX_PARSE_RETRIES = 1;
+/*
+FNXC:InteractiveSessionParse 2026-07-23-10:40:
+A CE Strategy session in the field died with "Failed to parse agent response: AI returned no valid JSON" on the opening turn (non-Anthropic default models comply less reliably with the JSON-only protocol).
+Two bounded reformat retries instead of one give a non-compliant model a second corrective nudge before the session goes terminal, and every failed parse attempt logs a bounded snippet of the raw assistant text plus the resolved provider/model so support can distinguish model non-compliance from an empty message (the disposed-agent race signature) without a repro.
+*/
+const MAX_PARSE_RETRIES = 2;
 
 const REFORMAT_PROMPT =
   "Your previous response could not be parsed as JSON. " +
@@ -337,6 +342,13 @@ function extractLastAssistantText(session: InteractiveAgentSession): string {
 
 type LoopState = "idle" | "awaiting_input" | "complete" | "error";
 
+/** Bounded, quoted preview of an unparseable assistant response for diagnostics. */
+function describeUnparseableResponse(text: string): string {
+  if (!text.trim()) return "(empty assistant message — possible disposed/displaced agent or a tool-call-only final turn)";
+  const preview = text.length > 400 ? `${text.slice(0, 400)}…` : text;
+  return JSON.stringify(preview);
+}
+
 /**
  * Build the interactive session over an injected agent factory.
  * Exported for direct (deterministic, fake-agent) testing.
@@ -384,6 +396,9 @@ export async function createInteractiveAiSessionWith(
         break;
       } catch (err) {
         lastError = err instanceof Error ? err : new Error(String(err));
+        interactiveSessionLog.warn(
+          `parse attempt ${attempt + 1}/${MAX_PARSE_RETRIES + 1} failed (provider=${options.defaultProvider ?? "default"}, model=${options.defaultModelId ?? "default"}): ${lastError.message} — response: ${describeUnparseableResponse(responseText)}`,
+        );
         if (attempt < MAX_PARSE_RETRIES) {
           try {
             await agent.prompt(REFORMAT_PROMPT);
@@ -398,6 +413,9 @@ export async function createInteractiveAiSessionWith(
 
     if (!parsed) {
       state = "error";
+      interactiveSessionLog.error(
+        `giving up after ${MAX_PARSE_RETRIES + 1} parse attempts (provider=${options.defaultProvider ?? "default"}, model=${options.defaultModelId ?? "default"}): ${lastError?.message ?? "Unknown error"} — last response: ${describeUnparseableResponse(responseText)}`,
+      );
       const ev: InteractiveAiSessionEvent = {
         type: "error",
         data: { message: `Failed to parse agent response: ${lastError?.message ?? "Unknown error"}`, cause: lastError },

packages/engine/src/logger.ts

@@ -132,6 +132,9 @@ export const autopilotLog = createLogger("autopilot");
 /** Logger for the heartbeat execution subsystem. */
 export const heartbeatLog = createLogger("heartbeat");
 
+/** Logger for the interactive AI session seam (planning / CE stage sessions). */
+export const interactiveSessionLog = createLogger("interactive-session");
+
 /** Logger for remote node runtime/client subsystems. */
 export const remoteNodeLog = createLogger("remote-node");
 

plugins/fusion-plugin-compound-engineering/src/__tests__/orchestrator-turn-admission.test.ts

@@ -0,0 +1,192 @@
+import { afterEach, beforeEach, expect, it, vi } from "vitest";
+import type { InteractiveAiSession, InteractiveAiSessionEvent, PlanningQuestion } from "@fusion/core";
+import { CeOrchestrator, CeTurnInProgressError } from "../session/orchestrator.js";
+import { makeHarness, pgDescribe, type TestHarness } from "./_harness.js";
+
+/*
+FNXC:CompoundEngineeringConcurrency 2026-07-23-11:10:
+Port of the planning turn-admission invariant (FNXC:PlanningTurnAdmission, packages/dashboard/src/planning.ts 2026-07-22).
+Field incident: a mobile CE Strategy session died with "Failed to parse agent response: AI returned no valid JSON" — the planning analogue was a re-entered view re-submitting a turn, displacing the in-flight turn's live agent, which then read an empty assistant message.
+Invariant under test: at most one turn is admitted per CE session at a time; a concurrent answer/resume is rejected with CeTurnInProgressError and the in-flight turn settles unharmed; the reservation is released when the turn settles (including detached turns) and by explicit cancel().
+*/
+
+const QUESTION: PlanningQuestion = {
+  id: "q1",
+  type: "text",
+  question: "Direction?",
+};
+
+let h: TestHarness;
+
+beforeEach(async () => {
+  h = await makeHarness();
+});
+
+afterEach(() => {
+  h.close();
+});
+
+/**
+ * Scripted session whose ANSWER turn blocks until the test releases it: turn 1
+ * (prompt) yields the question; the answer turn parks on a gate, then yields
+ * complete. Lets the test hold a turn in flight deterministically.
+ */
+function gatedAnswerSession(): { session: InteractiveAiSession; releaseAnswer: () => void; answerCalls: () => number } {
+  let cursor = -1;
+  let release: (() => void) | undefined;
+  const gate = new Promise<void>((resolve) => {
+    release = resolve;
+  });
+  let answers = 0;
+  const session: InteractiveAiSession = {
+    prompt: vi.fn(async () => {
+      cursor++;
+    }),
+    answer: vi.fn(async () => {
+      answers++;
+      await gate;
+      cursor++;
+    }),
+    nextEvent: vi.fn(async (): Promise<InteractiveAiSessionEvent> => {
+      return cursor === 0
+        ? { type: "question", data: QUESTION }
+        : { type: "complete", data: { artifact: "# Done\n" } };
+    }),
+    dispose: vi.fn(),
+  };
+  return { session, releaseAnswer: () => release?.(), answerCalls: () => answers };
+}
+
+pgDescribe("turn admission (single in-flight turn per session)", () => {
+  it("rejects a concurrent answer with CeTurnInProgressError and the in-flight turn settles unharmed", async () => {
+    const gated = gatedAnswerSession();
+    const orch = new CeOrchestrator({
+      ctx: h.ctx,
+      createInteractiveAiSession: vi.fn(async () => ({ session: gated.session })),
+      projectRoot: h.projectRoot,
+    });
+
+    const started = await orch.start("brainstorm", { openingMessage: "kick off" });
+    expect(started.session.status).toBe("awaiting_input");
+    const id = started.session.id;
+
+    // First answer holds the turn slot (its agent turn is gated open).
+    const first = orch.answer(id, "q1", "north");
+    // Give the first entry its synchronous reservation before racing it.
+    await Promise.resolve();
+
+    // A re-submitted answer (remounted view) is rejected, NOT admitted.
+    await expect(orch.answer(id, "q1", "north")).rejects.toThrow(CeTurnInProgressError);
+    // A racing resume is rejected the same way.
+    await expect(orch.resume(id)).rejects.toThrow(CeTurnInProgressError);
+
+    // The surviving turn was never displaced: it settles cleanly.
+    gated.releaseAnswer();
+    const settled = await first;
+    expect(settled.event?.type).toBe("complete");
+    expect(settled.session.status).toBe("completed");
+    // Exactly ONE answer reached the live agent.
+    expect(gated.answerCalls()).toBe(1);
+  });
+
+  it("releases the reservation when the turn settles (terminal session accepts resume again)", async () => {
+    const gated = gatedAnswerSession();
+    const orch = new CeOrchestrator({
+      ctx: h.ctx,
+      createInteractiveAiSession: vi.fn(async () => ({ session: gated.session })),
+      projectRoot: h.projectRoot,
+    });
+
+    const started = await orch.start("brainstorm", { openingMessage: "kick off" });
+    const id = started.session.id;
+    const first = orch.answer(id, "q1", "north");
+    await Promise.resolve();
+    gated.releaseAnswer();
+    await first;
+
+    // Reservation released → resume is admitted (completed → no-op, no conflict throw).
+    const resumed = await orch.resume(id);
+    expect(resumed.session.status).toBe("completed");
+  });
+
+  it("holds the reservation across a DETACHED answer turn and releases on settle", async () => {
+    const gated = gatedAnswerSession();
+    const orch = new CeOrchestrator({
+      ctx: h.ctx,
+      createInteractiveAiSession: vi.fn(async () => ({ session: gated.session })),
+      projectRoot: h.projectRoot,
+    });
+
+    const started = await orch.start("brainstorm", { openingMessage: "kick off" });
+    const id = started.session.id;
+
+    // Detached answer returns immediately with the turn running in background…
+    const accepted = await orch.answer(id, "q1", "north", { detach: true });
+    expect(accepted.session.status).toBe("active");
+
+    // …but the turn slot stays reserved for the whole background turn.
+    await expect(orch.answer(id, "q1", "north", { detach: true })).rejects.toThrow(CeTurnInProgressError);
+    await expect(orch.resume(id, { detach: true })).rejects.toThrow(CeTurnInProgressError);
+
+    gated.releaseAnswer();
+    await vi.waitFor(async () => {
+      const state = await orch.getState(id);
+      expect(state?.status).toBe("completed");
+    });
+    // Background settle released the slot.
+    const resumed = await orch.resume(id);
+    expect(resumed.session.status).toBe("completed");
+    expect(gated.answerCalls()).toBe(1);
+  });
+
+  it("a rejected concurrent answer does not clear the awaiting question (recovery anchor intact)", async () => {
+    const gated = gatedAnswerSession();
+    const orch = new CeOrchestrator({
+      ctx: h.ctx,
+      createInteractiveAiSession: vi.fn(async () => ({ session: gated.session })),
+      projectRoot: h.projectRoot,
+    });
+
+    const started = await orch.start("brainstorm", { openingMessage: "kick off" });
+    const id = started.session.id;
+    const first = orch.answer(id, "q1", "north", { detach: true });
+    await first;
+
+    // The rejected duplicate must not have mutated persisted state mid-turn.
+    await expect(orch.answer(id, "q1", "dupe", { detach: true })).rejects.toThrow(CeTurnInProgressError);
+    const state = await orch.getState(id);
+    expect(state?.status).toBe("active"); // still the first turn's accepted state
+
+    gated.releaseAnswer();
+    await vi.waitFor(async () => {
+      expect((await orch.getState(id))?.status).toBe("completed");
+    });
+  });
+
+  it("cancel() clears the reservation so a fresh turn is admitted after explicit teardown", async () => {
+    const gated = gatedAnswerSession();
+    const orch = new CeOrchestrator({
+      ctx: h.ctx,
+      createInteractiveAiSession: vi.fn(async () => ({ session: gated.session })),
+      projectRoot: h.projectRoot,
+    });
+
+    const started = await orch.start("brainstorm", { openingMessage: "kick off" });
+    const id = started.session.id;
+    const inFlight = orch.answer(id, "q1", "north", { detach: true });
+    await inFlight;
+    await expect(orch.resume(id)).rejects.toThrow(CeTurnInProgressError);
+
+    // Explicit teardown: cancel interrupts the session AND frees the slot.
+    const cancelled = await orch.cancel(id);
+    expect(cancelled?.status).toBe("interrupted");
+    // Resume is admitted again (no CeTurnInProgressError): the interrupted
+    // session (answer already cleared currentQuestion) resumes to the "active"
+    // retry posture.
+    const resumed = await orch.resume(id);
+    expect(resumed.session.status).toBe("active");
+
+    // Unblock the zombie turn so the test run doesn't leak a pending promise.
+    gated.releaseAnswer();
+  });
+});

plugins/fusion-plugin-compound-engineering/src/routes/session-routes.ts

@@ -1,5 +1,5 @@
 import type { PluginContext, PluginRouteDefinition, PluginRouteResponse } from "@fusion/core";
-import { CeOrchestrator } from "../session/orchestrator.js";
+import { CeOrchestrator, CeTurnInProgressError } from "../session/orchestrator.js";
 import { recoverStaleSessionsForContext } from "../session/session-recovery.js";
 import { asCeSessionStatus } from "../session/session-store.js";
 import { getCePipelineStore } from "../sync/pipeline-store.js";
@@ -102,6 +102,13 @@ export function createSessionRoutes(): PluginRouteDefinition[] {
           const result = await orch.resume(id, { detach: true });
           return { status: 200, body: { session: result.session } };
         } catch (err) {
+          /*
+           * FNXC:CompoundEngineeringConcurrency 2026-07-23-11:05:
+           * A resume racing an in-flight turn is a retryable conflict (409), not a missing session (404) — the mobile client re-enters the view and re-fires resume while the previous turn is still settling.
+           */
+          if (err instanceof CeTurnInProgressError) {
+            return { status: 409, body: { error: err.message } };
+          }
           return { status: 404, body: { error: err instanceof Error ? err.message : String(err) } };
         }
       },

plugins/fusion-plugin-compound-engineering/src/session/orchestrator.ts

@@ -109,6 +109,21 @@ export class CeTurnTimeoutError extends Error {
   }
 }
 
+/*
+FNXC:CompoundEngineeringConcurrency 2026-07-23-10:55:
+Port of the planning turn-admission invariant (see FNXC:PlanningTurnAdmission in packages/dashboard/src/planning.ts, 2026-07-22).
+Mobile view unmount/remount can re-submit an answer or resume while the previous turn is still running; without a guard the second entry rehydrates or re-prompts the SAME live handle, displacing the first turn — the displaced/disposed agent then yields an empty assistant message and the surviving turn dies with "Failed to parse agent response: AI returned no valid JSON".
+Invariant: at most one turn (opening / answer / resume-rehydration) may be admitted per CE session at any time, reserved SYNCHRONOUSLY before any await and held until the turn fully settles (including detached background turns). A concurrent entry is rejected with CeTurnInProgressError (routes map it to HTTP 409) instead of displacing a healthy in-flight turn. cancel()/discard() intentionally bypass the guard — they are explicit teardown, and the displaced turn settles as interrupted through the existing no-silent-loss path.
+*/
+export class CeTurnInProgressError extends Error {
+  constructor(sessionId: string) {
+    super(
+      `A turn is already in progress for CE session ${sessionId}. Wait for it to settle (watch the live output) instead of re-submitting.`,
+    );
+    this.name = "CeTurnInProgressError";
+  }
+}
+
 /**
  * Which session backend a CE stage runs on (U9 seam).
  *
@@ -292,6 +307,14 @@ export class CeOrchestrator {
   private readonly progressPersistence = new Map<string, Promise<void>>();
   /** Process-local terminal state used only when detached failure persistence itself fails. */
   private readonly detachedFailureFallbacks = new Map<string, { session: CeSession; durableFingerprint: string }>();
+  /**
+   * Sessions with an admitted in-flight turn (see CeTurnInProgressError),
+   * keyed to a per-reservation token so a stale release (from a turn that
+   * cancel()/discard() force-cleared) can never drop a NEWER turn's
+   * reservation. Membership is toggled SYNCHRONOUSLY — no await between check
+   * and add — so two overlapping entries cannot both pass the guard.
+   */
+  private readonly turnReservations = new Map<string, symbol>();
 
   constructor(deps: OrchestratorDeps) {
     this.ctx = deps.ctx;
@@ -491,6 +514,25 @@ export class CeOrchestrator {
     };
   }
 
+  /**
+   * Synchronously reserve the session's single turn slot; returns the
+   * idempotent release fn. Throws CeTurnInProgressError when a turn is already
+   * admitted. Callers MUST release when the turn fully settles (finally on the
+   * sync path, promise-finally on the detached path).
+   */
+  private reserveTurn(sessionId: string): () => void {
+    if (this.turnReservations.has(sessionId)) {
+      throw new CeTurnInProgressError(sessionId);
+    }
+    const token = Symbol(sessionId);
+    this.turnReservations.set(sessionId, token);
+    return () => {
+      if (this.turnReservations.get(sessionId) === token) {
+        this.turnReservations.delete(sessionId);
+      }
+    };
+  }
+
   /**
    * Start a fresh session for a registered stage and run the opening turn.
    *
@@ -533,18 +575,34 @@ export class CeOrchestrator {
       ? await this.store.createWithPlanHandoffClaimAsync(sessionInput, handoffArtifactPath)
       : await this.store.createAsync(sessionInput);
     await this.store.appendHistoryAsync(session.id, { role: "user", text: opts.openingMessage, at: new Date().toISOString() });
+    // The row is brand-new, so nothing can hold this reservation yet — but the
+    // opening turn must HOLD it so an answer/resume racing in mid-turn is
+    // rejected instead of displacing the live handle.
+    const releaseTurn = this.reserveTurn(session.id);
     if (opts.detach) {
-      let accepted = await this.requireSession(session.id);
-      const turn = Promise.resolve().then(() => this.runOpeningTurn(
-        session.id,
-        stage,
-        opts.openingMessage,
-        (active) => { accepted = active; },
-      ));
+      let accepted: CeSession;
+      try {
+        accepted = await this.requireSession(session.id);
+      } catch (err) {
+        releaseTurn();
+        throw err;
+      }
+      const turn = Promise.resolve()
+        .then(() => this.runOpeningTurn(
+          session.id,
+          stage,
+          opts.openingMessage,
+          (active) => { accepted = active; },
+        ))
+        .finally(releaseTurn);
       this.detachTurn(() => accepted, "opening turn", turn);
       return { session: accepted };
     }
-    return this.runOpeningTurn(session.id, stage, opts.openingMessage);
+    try {
+      return await this.runOpeningTurn(session.id, stage, opts.openingMessage);
+    } finally {
+      releaseTurn();
+    }
   }
 
   /** Resolve the newest durable same-project Brainstorm handoff accepted for in-place Plan enrichment. */
@@ -613,35 +671,49 @@ export class CeOrchestrator {
     response: unknown,
     opts: { detach?: boolean } = {},
   ): Promise<CeStepResult> {
-    const session = await this.requireSession(sessionId);
-    if (session.status !== "awaiting_input") {
-      throw new Error(`Session ${sessionId} is not awaiting input (status=${session.status}).`);
-    }
-    // Validate the questionId BEFORE mutating any persisted state. A stale/wrong
-    // questionId must NOT clear `currentQuestion` or flip status to active —
-    // doing so would destroy the recovery anchor while the seam rejects the
-    // mismatch, leaving the DB diverged from the live session. Reject cleanly and
-    // leave `currentQuestion`/status intact so the session stays answerable.
-    if (questionId !== session.currentQuestion?.id) {
-      throw new Error(
-        `Session ${sessionId} is awaiting question "${session.currentQuestion?.id ?? "(none)"}", not "${questionId}".`,
-      );
-    }
-    const live = this.live.get(sessionId);
-    if (!live && !this.factory) {
-      throw new Error(INTERACTIVE_AI_UNAVAILABLE_MESSAGE);
-    }
+    // Turn admission FIRST, synchronously — a re-submitted answer from a
+    // remounted mobile view must be rejected before it can touch persisted
+    // state or the live handle (see CeTurnInProgressError).
+    const releaseTurn = this.reserveTurn(sessionId);
+    // Once the detached background turn is armed, its promise-finally owns the
+    // release; until then (validation throws) the finally below owns it.
+    let releaseHandedOff = false;
+    try {
+      const session = await this.requireSession(sessionId);
+      if (session.status !== "awaiting_input") {
+        throw new Error(`Session ${sessionId} is not awaiting input (status=${session.status}).`);
+      }
+      // Validate the questionId BEFORE mutating any persisted state. A stale/wrong
+      // questionId must NOT clear `currentQuestion` or flip status to active —
+      // doing so would destroy the recovery anchor while the seam rejects the
+      // mismatch, leaving the DB diverged from the live session. Reject cleanly and
+      // leave `currentQuestion`/status intact so the session stays answerable.
+      if (questionId !== session.currentQuestion?.id) {
+        throw new Error(
+          `Session ${sessionId} is awaiting question "${session.currentQuestion?.id ?? "(none)"}", not "${questionId}".`,
+        );
+      }
+      const live = this.live.get(sessionId);
+      if (!live && !this.factory) {
+        throw new Error(INTERACTIVE_AI_UNAVAILABLE_MESSAGE);
+      }
 
-    if (opts.detach) {
-      // If the process lost its live handle, rehydration can take time. Mirror
-      // resume(detach): mark the row active immediately while the background
-      // turn re-creates the handle and converges through persisted state.
-      const accepted = await this.store.updateAsync(sessionId, { status: "active", currentQuestion: null, error: null }) ?? session;
-      const turn = Promise.resolve().then(() => this.runAnswerTurn(accepted, questionId, response));
-      this.detachTurn(() => accepted, "answer turn", turn);
-      return { session: accepted };
+      if (opts.detach) {
+        // If the process lost its live handle, rehydration can take time. Mirror
+        // resume(detach): mark the row active immediately while the background
+        // turn re-creates the handle and converges through persisted state.
+        const accepted = await this.store.updateAsync(sessionId, { status: "active", currentQuestion: null, error: null }) ?? session;
+        const turn = Promise.resolve()
+          .then(() => this.runAnswerTurn(accepted, questionId, response))
+          .finally(releaseTurn);
+        releaseHandedOff = true;
+        this.detachTurn(() => accepted, "answer turn", turn);
+        return { session: accepted };
+      }
+      return await this.runAnswerTurn(session, questionId, response);
+    } finally {
+      if (!releaseHandedOff) releaseTurn();
     }
-    return this.runAnswerTurn(session, questionId, response);
   }
 
   private async runAnswerTurn(session: CeSession, questionId: string, response: unknown): Promise<CeStepResult> {
@@ -691,62 +763,72 @@ export class CeOrchestrator {
    * left `interrupted` with a clear error explaining it can't be continued here.
    */
   async resume(sessionId: string, opts: { detach?: boolean } = {}): Promise<CeStepResult> {
-    const session = await this.requireSession(sessionId);
+    // Turn admission FIRST, synchronously — a resume racing an in-flight
+    // opening/answer turn (mobile re-entry, double-tapped Resume) must not
+    // rehydrate a second live handle over the one the running turn is using.
+    const releaseTurn = this.reserveTurn(sessionId);
+    let releaseHandedOff = false;
+    try {
+      const session = await this.requireSession(sessionId);
 
-    // Terminal / already-answerable-with-a-live-handle cases need no rehydration.
-    if (session.status === "completed") return { session };
-    if (session.status === "awaiting_input" && this.live.has(sessionId)) {
-      return { session }; // already live + answerable.
-    }
-
-    // No pending question → nothing to re-prime to. Mark active so the caller can
-    // re-run the turn with fresh input (retry for `error`, resume for others).
-    if (!session.currentQuestion) {
-      const next = await this.store.updateAsync(sessionId, { status: "active", error: null }) ?? session;
-      return { session: next };
-    }
-
-    // A live handle already exists (e.g. interrupted but not disposed) — just
-    // restore the answerable status.
-    if (this.live.has(sessionId)) {
-      const next = await this.store.updateAsync(sessionId, { status: "awaiting_input", error: null }) ?? session;
-      return { session: next };
-    }
-
-    // Rehydration path: re-create the live session and replay history back to the
-    // current question.
-    if (!this.factory) {
-      // Honest status: we cannot back an answerable state in this process, so do
-      // not pretend the session is resumable here. Surface a clear error.
-      const next =
-        await this.store.updateAsync(sessionId, {
-          status: "interrupted",
-          error: INTERACTIVE_AI_UNAVAILABLE_MESSAGE,
-        }) ?? session;
-      return { session: next };
-    }
-
-    const rehydration = (async (): Promise<CeStepResult> => {
-      try {
-        await this.rehydrate(session);
-      } catch (err) {
-        // Rehydration failed — keep progress, surface the failure, do not
-        // advertise an answerable status we can't back.
-        return { session: await this.interruptSession(sessionId, err) };
+      // Terminal / already-answerable-with-a-live-handle cases need no rehydration.
+      if (session.status === "completed") return { session };
+      if (session.status === "awaiting_input" && this.live.has(sessionId)) {
+        return { session }; // already live + answerable.
       }
-      const next = await this.store.updateAsync(sessionId, { status: "awaiting_input", error: null }) ?? session;
-      return { session: next };
-    })();
 
-    if (opts.detach) {
-      // Rehydration replays the conversation against the live model and can be
-      // slow; the route posture marks the session active and converges via
-      // push/poll.
-      const next = await this.store.updateAsync(sessionId, { status: "active", error: null }) ?? session;
-      this.detachTurn(() => next, "rehydration", rehydration);
-      return { session: next };
+      // No pending question → nothing to re-prime to. Mark active so the caller can
+      // re-run the turn with fresh input (retry for `error`, resume for others).
+      if (!session.currentQuestion) {
+        const next = await this.store.updateAsync(sessionId, { status: "active", error: null }) ?? session;
+        return { session: next };
+      }
+
+      // A live handle already exists (e.g. interrupted but not disposed) — just
+      // restore the answerable status.
+      if (this.live.has(sessionId)) {
+        const next = await this.store.updateAsync(sessionId, { status: "awaiting_input", error: null }) ?? session;
+        return { session: next };
+      }
+
+      // Rehydration path: re-create the live session and replay history back to the
+      // current question.
+      if (!this.factory) {
+        // Honest status: we cannot back an answerable state in this process, so do
+        // not pretend the session is resumable here. Surface a clear error.
+        const next =
+          await this.store.updateAsync(sessionId, {
+            status: "interrupted",
+            error: INTERACTIVE_AI_UNAVAILABLE_MESSAGE,
+          }) ?? session;
+        return { session: next };
+      }
+
+      const rehydration = (async (): Promise<CeStepResult> => {
+        try {
+          await this.rehydrate(session);
+        } catch (err) {
+          // Rehydration failed — keep progress, surface the failure, do not
+          // advertise an answerable status we can't back.
+          return { session: await this.interruptSession(sessionId, err) };
+        }
+        const next = await this.store.updateAsync(sessionId, { status: "awaiting_input", error: null }) ?? session;
+        return { session: next };
+      })().finally(releaseTurn);
+      releaseHandedOff = true;
+
+      if (opts.detach) {
+        // Rehydration replays the conversation against the live model and can be
+        // slow; the route posture marks the session active and converges via
+        // push/poll.
+        const next = await this.store.updateAsync(sessionId, { status: "active", error: null }) ?? session;
+        this.detachTurn(() => next, "rehydration", rehydration);
+        return { session: next };
+      }
+      return await rehydration;
+    } finally {
+      if (!releaseHandedOff) releaseTurn();
     }
-    return rehydration;
   }
 
   /**
@@ -873,6 +955,11 @@ export class CeOrchestrator {
     // before disposeLive clears the transient buffers (same as runTurn failure).
     const interrupted = await this.interruptSession(sessionId, new Error("Cancelled by user"));
     this.disposeLive(sessionId);
+    // Explicit teardown clears the turn reservation: the displaced turn's agent
+    // is disposed (it settles as interrupted), and the session must accept a
+    // fresh turn even if that turn's own release was wedged (e.g. a hung
+    // rehydration, which runs without a watchdog).
+    this.turnReservations.delete(sessionId);
     return interrupted;
   }
 
@@ -885,6 +972,9 @@ export class CeOrchestrator {
   async discard(sessionId: string): Promise<boolean> {
     await this.drainProgressPersistence(sessionId);
     this.disposeLive(sessionId);
+    // Same reservation clear as cancel(): the row is going away; never leave a
+    // stale reservation behind for a reused orchestrator instance.
+    this.turnReservations.delete(sessionId);
     const deleted = await this.store.deleteAsync(sessionId);
     if (deleted) this.detachedFailureFallbacks.delete(sessionId);
     return deleted;