diff --git a/apps/orchestrator/src/spawner/agent-lifecycle.service.spec.ts b/apps/orchestrator/src/spawner/agent-lifecycle.service.spec.ts
index 6b359db..f4b815b 100644
--- a/apps/orchestrator/src/spawner/agent-lifecycle.service.spec.ts
+++ b/apps/orchestrator/src/spawner/agent-lifecycle.service.spec.ts
@@ -706,4 +706,233 @@ describe("AgentLifecycleService", () => {
       expect(mockSpawnerService.scheduleSessionCleanup).not.toHaveBeenCalled();
     });
   });
+
+  describe("TOCTOU race prevention (CQ-ORCH-5)", () => {
+    it("should serialize concurrent transitions to the same agent", async () => {
+      const executionOrder: string[] = [];
+
+      // Simulate state that changes after first transition completes
+      let currentStatus: "spawning" | "running" | "completed" = "spawning";
+
+      mockValkeyService.getAgentState.mockImplementation(async () => {
+        return {
+          agentId: mockAgentId,
+          status: currentStatus,
+          taskId: mockTaskId,
+        } as AgentState;
+      });
+
+      mockValkeyService.updateAgentStatus.mockImplementation(
+        async (_agentId: string, status: string) => {
+          // Simulate delay to allow interleaving if lock is broken
+          await new Promise<void>((resolve) => {
+            setTimeout(resolve, 10);
+          });
+          currentStatus = status as "spawning" | "running" | "completed";
+          executionOrder.push(`updated-to-${status}`);
+          return {
+            agentId: mockAgentId,
+            status,
+            taskId: mockTaskId,
+            startedAt: "2026-02-02T10:00:00Z",
+            ...(status === "completed" && { completedAt: "2026-02-02T11:00:00Z" }),
+          } as AgentState;
+        }
+      );
+
+      // Launch both transitions concurrently
+      const [result1, result2] = await Promise.allSettled([
+        service.transitionToRunning(mockAgentId),
+        service.transitionToCompleted(mockAgentId),
+      ]);
+
+      // First should succeed (spawning -> running)
+      expect(result1.status).toBe("fulfilled");
+
+      // Second should also succeed (running -> completed) because the lock
+      // serializes them: first one completes, updates state to running,
+      // then second reads the updated state and transitions to completed
+      expect(result2.status).toBe("fulfilled");
+
+      // Verify they executed in order, not interleaved
+      expect(executionOrder).toEqual(["updated-to-running", "updated-to-completed"]);
+    });
+
+    it("should reject second concurrent transition if first makes it invalid", async () => {
+      let currentStatus: "running" | "completed" | "killed" = "running";
+
+      mockValkeyService.getAgentState.mockImplementation(async () => {
+        return {
+          agentId: mockAgentId,
+          status: currentStatus,
+          taskId: mockTaskId,
+          startedAt: "2026-02-02T10:00:00Z",
+        } as AgentState;
+      });
+
+      mockValkeyService.updateAgentStatus.mockImplementation(
+        async (_agentId: string, status: string) => {
+          await new Promise<void>((resolve) => {
+            setTimeout(resolve, 10);
+          });
+          currentStatus = status as "running" | "completed" | "killed";
+          return {
+            agentId: mockAgentId,
+            status,
+            taskId: mockTaskId,
+            startedAt: "2026-02-02T10:00:00Z",
+            completedAt: "2026-02-02T11:00:00Z",
+          } as AgentState;
+        }
+      );
+
+      // Both try to transition from running to a terminal state concurrently
+      const [result1, result2] = await Promise.allSettled([
+        service.transitionToCompleted(mockAgentId),
+        service.transitionToKilled(mockAgentId),
+      ]);
+
+      // First should succeed (running -> completed)
+      expect(result1.status).toBe("fulfilled");
+
+      // Second should fail because after first completes,
+      // agent is in "completed" state which cannot transition to "killed"
+      expect(result2.status).toBe("rejected");
+      if (result2.status === "rejected") {
+        expect(result2.reason).toBeInstanceOf(Error);
+        expect((result2.reason as Error).message).toContain("Invalid state transition");
+      }
+    });
+
+    it("should allow concurrent transitions to different agents", async () => {
+      const agent1Id = "agent-1";
+      const agent2Id = "agent-2";
+      const executionOrder: string[] = [];
+
+      mockValkeyService.getAgentState.mockImplementation(async (agentId: string) => {
+        return {
+          agentId,
+          status: "spawning",
+          taskId: `task-for-${agentId}`,
+        } as AgentState;
+      });
+
+      mockValkeyService.updateAgentStatus.mockImplementation(
+        async (agentId: string, status: string) => {
+          executionOrder.push(`${agentId}-start`);
+          await new Promise<void>((resolve) => {
+            setTimeout(resolve, 10);
+          });
+          executionOrder.push(`${agentId}-end`);
+          return {
+            agentId,
+            status,
+            taskId: `task-for-${agentId}`,
+            startedAt: "2026-02-02T10:00:00Z",
+          } as AgentState;
+        }
+      );
+
+      // Both should run concurrently since they target different agents
+      const [result1, result2] = await Promise.allSettled([
+        service.transitionToRunning(agent1Id),
+        service.transitionToRunning(agent2Id),
+      ]);
+
+      expect(result1.status).toBe("fulfilled");
+      expect(result2.status).toBe("fulfilled");
+
+      // Both should start before either finishes (concurrent, not serialized)
+      // The execution order should show interleaving
+      expect(executionOrder).toContain("agent-1-start");
+      expect(executionOrder).toContain("agent-2-start");
+    });
+
+    it("should release lock even when transition throws an error", async () => {
+      let callCount = 0;
+
+      mockValkeyService.getAgentState.mockImplementation(async () => {
+        callCount++;
+        if (callCount === 1) {
+          // First call: throw error
+          return null;
+        }
+        // Second call: return valid state
+        return {
+          agentId: mockAgentId,
+          status: "spawning",
+          taskId: mockTaskId,
+        } as AgentState;
+      });
+
+      mockValkeyService.updateAgentStatus.mockResolvedValue({
+        agentId: mockAgentId,
+        status: "running",
+        taskId: mockTaskId,
+        startedAt: "2026-02-02T10:00:00Z",
+      });
+
+      // First transition should fail (agent not found)
+      await expect(service.transitionToRunning(mockAgentId)).rejects.toThrow(
+        `Agent ${mockAgentId} not found`
+      );
+
+      // Second transition should succeed (lock was released despite error)
+      const result = await service.transitionToRunning(mockAgentId);
+      expect(result.status).toBe("running");
+    });
+
+    it("should handle three concurrent transitions sequentially for same agent", async () => {
+      const executionOrder: string[] = [];
+      let currentStatus: "spawning" | "running" | "completed" | "failed" = "spawning";
+
+      mockValkeyService.getAgentState.mockImplementation(async () => {
+        return {
+          agentId: mockAgentId,
+          status: currentStatus,
+          taskId: mockTaskId,
+          ...(currentStatus !== "spawning" && { startedAt: "2026-02-02T10:00:00Z" }),
+        } as AgentState;
+      });
+
+      mockValkeyService.updateAgentStatus.mockImplementation(
+        async (_agentId: string, status: string) => {
+          executionOrder.push(`update-${status}`);
+          await new Promise<void>((resolve) => {
+            setTimeout(resolve, 5);
+          });
+          currentStatus = status as "spawning" | "running" | "completed" | "failed";
+          return {
+            agentId: mockAgentId,
+            status,
+            taskId: mockTaskId,
+            startedAt: "2026-02-02T10:00:00Z",
+            ...(["completed", "failed"].includes(status) && {
+              completedAt: "2026-02-02T11:00:00Z",
+            }),
+          } as AgentState;
+        }
+      );
+
+      // Launch three transitions at once: spawning->running->completed, plus a failed attempt
+      const [r1, r2, r3] = await Promise.allSettled([
+        service.transitionToRunning(mockAgentId),
+        service.transitionToCompleted(mockAgentId),
+        service.transitionToFailed(mockAgentId, "late error"),
+      ]);
+
+      // First: spawning -> running (succeeds)
+      expect(r1.status).toBe("fulfilled");
+      // Second: running -> completed (succeeds, serialized after first)
+      expect(r2.status).toBe("fulfilled");
+      // Third: completed -> failed (fails, completed is terminal)
+      expect(r3.status).toBe("rejected");
+
+      // Verify sequential execution
+      expect(executionOrder[0]).toBe("update-running");
+      expect(executionOrder[1]).toBe("update-completed");
+      // Third never gets to update because validation fails
+      expect(executionOrder).toHaveLength(2);
+    });
+  });
 });
diff --git a/apps/orchestrator/src/spawner/agent-lifecycle.service.ts b/apps/orchestrator/src/spawner/agent-lifecycle.service.ts
index b2fccdc..942cb08 100644
--- a/apps/orchestrator/src/spawner/agent-lifecycle.service.ts
+++ b/apps/orchestrator/src/spawner/agent-lifecycle.service.ts
@@ -14,11 +14,21 @@ import { isValidAgentTransition } from "../valkey/types/state.types";
  * - Persists agent state changes to Valkey
  * - Emits pub/sub events on state changes
  * - Tracks agent metadata (startedAt, completedAt, error)
+ * - Uses per-agent mutex to prevent TOCTOU race conditions (CQ-ORCH-5)
  */
 @Injectable()
 export class AgentLifecycleService {
   private readonly logger = new Logger(AgentLifecycleService.name);
 
+  /**
+   * Per-agent mutex map to serialize state transitions.
+   * Uses promise chaining so concurrent transitions to the same agent
+   * are queued and executed sequentially, preventing TOCTOU races
+   * where two concurrent requests could both read the same state,
+   * both validate it as valid, and both write, causing lost updates.
+   */
+  private readonly agentLocks = new Map<string, Promise<void>>();
+
   constructor(
     private readonly valkeyService: ValkeyService,
     @Inject(forwardRef(() => AgentSpawnerService))
@@ -27,6 +37,37 @@ export class AgentLifecycleService {
     this.logger.log("AgentLifecycleService initialized");
   }
 
+  /**
+   * Acquire a per-agent mutex to serialize state transitions.
+   * Uses promise chaining: each caller chains onto the previous lock,
+   * ensuring transitions for the same agent are strictly sequential.
+   * Different agents can transition concurrently without contention.
+   *
+   * @param agentId Agent to acquire lock for
+   * @param fn Critical section to execute while holding the lock
+   * @returns Result of the critical section
+   */
+  private async withAgentLock<T>(agentId: string, fn: () => Promise<T>): Promise<T> {
+    const previousLock = this.agentLocks.get(agentId) ?? Promise.resolve();
+
+    let releaseLock!: () => void;
+    const currentLock = new Promise<void>((resolve) => {
+      releaseLock = resolve;
+    });
+    this.agentLocks.set(agentId, currentLock);
+
+    try {
+      await previousLock;
+      return await fn();
+    } finally {
+      releaseLock();
+      // Clean up the map entry if we are the last in the chain
+      if (this.agentLocks.get(agentId) === currentLock) {
+        this.agentLocks.delete(agentId);
+      }
+    }
+  }
+
   /**
    * Transition agent from spawning to running state
    * @param agentId Unique agent identifier
@@ -34,28 +75,34 @@ export class AgentLifecycleService {
    * @throws Error if agent not found or invalid transition
    */
   async transitionToRunning(agentId: string): Promise<AgentState> {
-    this.logger.log(`Transitioning agent ${agentId} to running`);
+    return this.withAgentLock(agentId, async () => {
+      this.logger.log(`Transitioning agent ${agentId} to running`);
 
-    const currentState = await this.getAgentState(agentId);
-    this.validateTransition(currentState.status, "running");
+      const currentState = await this.getAgentState(agentId);
+      this.validateTransition(currentState.status, "running");
 
-    // Set startedAt timestamp if not already set
-    const startedAt = currentState.startedAt ?? new Date().toISOString();
+      // Set startedAt timestamp if not already set
+      const startedAt = currentState.startedAt ?? new Date().toISOString();
 
-    // Update state in Valkey
-    const updatedState = await this.valkeyService.updateAgentStatus(agentId, "running", undefined);
+      // Update state in Valkey
+      const updatedState = await this.valkeyService.updateAgentStatus(
+        agentId,
+        "running",
+        undefined
+      );
 
-    // Ensure startedAt is set
-    if (!updatedState.startedAt) {
-      updatedState.startedAt = startedAt;
-      await this.valkeyService.setAgentState(updatedState);
-    }
+      // Ensure startedAt is set
+      if (!updatedState.startedAt) {
+        updatedState.startedAt = startedAt;
+        await this.valkeyService.setAgentState(updatedState);
+      }
 
-    // Emit event
-    await this.publishStateChangeEvent("agent.running", updatedState);
+      // Emit event
+      await this.publishStateChangeEvent("agent.running", updatedState);
 
-    this.logger.log(`Agent ${agentId} transitioned to running`);
-    return updatedState;
+      this.logger.log(`Agent ${agentId} transitioned to running`);
+      return updatedState;
+    });
   }
 
   /**
@@ -65,35 +112,37 @@ export class AgentLifecycleService {
    * @throws Error if agent not found or invalid transition
    */
   async transitionToCompleted(agentId: string): Promise<AgentState> {
-    this.logger.log(`Transitioning agent ${agentId} to completed`);
+    return this.withAgentLock(agentId, async () => {
+      this.logger.log(`Transitioning agent ${agentId} to completed`);
 
-    const currentState = await this.getAgentState(agentId);
-    this.validateTransition(currentState.status, "completed");
+      const currentState = await this.getAgentState(agentId);
+      this.validateTransition(currentState.status, "completed");
 
-    // Set completedAt timestamp
-    const completedAt = new Date().toISOString();
+      // Set completedAt timestamp
+      const completedAt = new Date().toISOString();
 
-    // Update state in Valkey
-    const updatedState = await this.valkeyService.updateAgentStatus(
-      agentId,
-      "completed",
-      undefined
-    );
+      // Update state in Valkey
+      const updatedState = await this.valkeyService.updateAgentStatus(
+        agentId,
+        "completed",
+        undefined
+      );
 
-    // Ensure completedAt is set
-    if (!updatedState.completedAt) {
-      updatedState.completedAt = completedAt;
-      await this.valkeyService.setAgentState(updatedState);
-    }
+      // Ensure completedAt is set
+      if (!updatedState.completedAt) {
+        updatedState.completedAt = completedAt;
+        await this.valkeyService.setAgentState(updatedState);
+      }
 
-    // Emit event
-    await this.publishStateChangeEvent("agent.completed", updatedState);
+      // Emit event
+      await this.publishStateChangeEvent("agent.completed", updatedState);
 
-    // Schedule session cleanup
-    this.spawnerService.scheduleSessionCleanup(agentId);
+      // Schedule session cleanup
+      this.spawnerService.scheduleSessionCleanup(agentId);
 
-    this.logger.log(`Agent ${agentId} transitioned to completed`);
-    return updatedState;
+      this.logger.log(`Agent ${agentId} transitioned to completed`);
+      return updatedState;
+    });
   }
 
   /**
@@ -104,31 +153,33 @@ export class AgentLifecycleService {
    * @throws Error if agent not found or invalid transition
    */
   async transitionToFailed(agentId: string, error: string): Promise<AgentState> {
-    this.logger.log(`Transitioning agent ${agentId} to failed: ${error}`);
+    return this.withAgentLock(agentId, async () => {
+      this.logger.log(`Transitioning agent ${agentId} to failed: ${error}`);
 
-    const currentState = await this.getAgentState(agentId);
-    this.validateTransition(currentState.status, "failed");
+      const currentState = await this.getAgentState(agentId);
+      this.validateTransition(currentState.status, "failed");
 
-    // Set completedAt timestamp
-    const completedAt = new Date().toISOString();
+      // Set completedAt timestamp
+      const completedAt = new Date().toISOString();
 
-    // Update state in Valkey
-    const updatedState = await this.valkeyService.updateAgentStatus(agentId, "failed", error);
+      // Update state in Valkey
+      const updatedState = await this.valkeyService.updateAgentStatus(agentId, "failed", error);
 
-    // Ensure completedAt is set
-    if (!updatedState.completedAt) {
-      updatedState.completedAt = completedAt;
-      await this.valkeyService.setAgentState(updatedState);
-    }
+      // Ensure completedAt is set
+      if (!updatedState.completedAt) {
+        updatedState.completedAt = completedAt;
+        await this.valkeyService.setAgentState(updatedState);
+      }
 
-    // Emit event
-    await this.publishStateChangeEvent("agent.failed", updatedState, error);
+      // Emit event
+      await this.publishStateChangeEvent("agent.failed", updatedState, error);
 
-    // Schedule session cleanup
-    this.spawnerService.scheduleSessionCleanup(agentId);
+      // Schedule session cleanup
+      this.spawnerService.scheduleSessionCleanup(agentId);
 
-    this.logger.error(`Agent ${agentId} transitioned to failed: ${error}`);
-    return updatedState;
+      this.logger.error(`Agent ${agentId} transitioned to failed: ${error}`);
+      return updatedState;
+    });
   }
 
   /**
@@ -138,31 +189,33 @@ export class AgentLifecycleService {
    * @throws Error if agent not found or invalid transition
    */
   async transitionToKilled(agentId: string): Promise<AgentState> {
-    this.logger.log(`Transitioning agent ${agentId} to killed`);
+    return this.withAgentLock(agentId, async () => {
+      this.logger.log(`Transitioning agent ${agentId} to killed`);
 
-    const currentState = await this.getAgentState(agentId);
-    this.validateTransition(currentState.status, "killed");
+      const currentState = await this.getAgentState(agentId);
+      this.validateTransition(currentState.status, "killed");
 
-    // Set completedAt timestamp
-    const completedAt = new Date().toISOString();
+      // Set completedAt timestamp
+      const completedAt = new Date().toISOString();
 
-    // Update state in Valkey
-    const updatedState = await this.valkeyService.updateAgentStatus(agentId, "killed", undefined);
+      // Update state in Valkey
+      const updatedState = await this.valkeyService.updateAgentStatus(agentId, "killed", undefined);
 
-    // Ensure completedAt is set
-    if (!updatedState.completedAt) {
-      updatedState.completedAt = completedAt;
-      await this.valkeyService.setAgentState(updatedState);
-    }
+      // Ensure completedAt is set
+      if (!updatedState.completedAt) {
+        updatedState.completedAt = completedAt;
+        await this.valkeyService.setAgentState(updatedState);
+      }
 
-    // Emit event
-    await this.publishStateChangeEvent("agent.killed", updatedState);
+      // Emit event
+      await this.publishStateChangeEvent("agent.killed", updatedState);
 
-    // Schedule session cleanup
-    this.spawnerService.scheduleSessionCleanup(agentId);
+      // Schedule session cleanup
+      this.spawnerService.scheduleSessionCleanup(agentId);
 
-    this.logger.warn(`Agent ${agentId} transitioned to killed`);
-    return updatedState;
+      this.logger.warn(`Agent ${agentId} transitioned to killed`);
+      return updatedState;
+    });
   }
 
   /**