Deliver a steer message that arrives while the previous turn is still starting

A steer send that reached an agent before its previous turn had a turn id
found no turn to steer and replaced the starting turn, so the earlier
message never reached the provider. Steering now waits for the starting
turn to be accepted (or fail) and steers the turn it became.

Fixes #6186

packages/server/src/server/agent/agent-manager.test.ts

@@ -956,6 +956,73 @@ test("unavailable steer interrupts once and starts one replacement turn", async
   }
 });
 
+test("steers a send that arrives while the previous turn is still starting", async () => {
+  const startHeld = deferred<void>();
+  class SlowStartSteeringSession extends SteeringTestSession {
+    override async startTurn(prompt: AgentPromptInput): Promise<{ turnId: string }> {
+      await startHeld.promise;
+      return await super.startTurn(prompt);
+    }
+  }
+  const session = new SlowStartSteeringSession({
+    provider: "codex",
+    cwd: process.cwd(),
+  });
+  const workdir = mkdtempSync(join(tmpdir(), "agent-manager-steer-starting-"));
+  const manager = new AgentManager({
+    clients: {
+      codex: new (class extends TestAgentClient {
+        override async createSession(): Promise<AgentSession> {
+          return session;
+        }
+      })(),
+    },
+    logger,
+  });
+  let agentId: string | null = null;
+  try {
+    const agent = await manager.createAgent({ provider: "codex", cwd: workdir }, undefined, {
+      workspaceId: undefined,
+    });
+    agentId = agent.id;
+    const steerOptions = (clientMessageId: string) => ({
+      replaceRunning: true,
+      activeTurnBehavior: "steer" as const,
+      runOptions: { clientMessageId },
+    });
+    await startAgentRun(manager, agent.id, "FIRST", logger, steerOptions("first-client"));
+    const second = startAgentRun(
+      manager,
+      agent.id,
+      "SECOND",
+      logger,
+      steerOptions("second-client"),
+    );
+
+    startHeld.resolve();
+    await expect(second).resolves.toEqual({ disposition: "steered" });
+
+    expect(session.startPrompts).toEqual(["FIRST"]);
+    expect(session.interruptCount).toBe(0);
+    expect(
+      manager
+        .fetchTimeline(agent.id, { limit: 0 })
+        .rows.filter((row) => row.item.type === "user_message")
+        .map((row) => ({
+          text: row.item.type === "user_message" ? row.item.text : null,
+          turnId: row.turnId,
+        })),
+    ).toEqual([
+      { text: "FIRST", turnId: "active-turn-1" },
+      { text: "SECOND", turnId: "active-turn-1" },
+    ]);
+  } finally {
+    startHeld.resolve();
+    if (agentId) await manager.closeAgent(agentId).catch(() => undefined);
+    rmSync(workdir, { recursive: true, force: true });
+  }
+});
+
 test("orders an accepted steer before output emitted while acknowledgement is pending", async () => {
   const entered = deferred<void>();
   const release = deferred<void>();

packages/server/src/server/agent/agent-manager.ts

@@ -2734,7 +2734,7 @@ export class AgentManager {
         throw error;
       }
       if (isStaleProviderSessionError(error)) {
-        pendingRun.start = { status: "failed", error: error.message };
+        this.runs.settleForegroundStart(pendingRun, { status: "failed", error: error.message });
         agent.pendingReplacement = false;
         if (!agent.activeForegroundTurnId) agent.lifecycle = "idle";
         this.runs.settleForegroundRun(agentId, pendingRun.token);
@@ -2742,7 +2742,7 @@ export class AgentManager {
       }
       agent.pendingReplacement = false;
       const errorMsg = error instanceof Error ? error.message : "Failed to start turn";
-      pendingRun.start = { status: "failed", error: errorMsg };
+      this.runs.settleForegroundStart(pendingRun, { status: "failed", error: errorMsg });
       // A terminal rejection belongs after the submitted prompt even though no provider turn exists.
       if (options?.clientMessageId) {
         this.recordSubmittedPrompt(agent, prompt, options.clientMessageId, "rejected", {
@@ -2832,7 +2832,7 @@ export class AgentManager {
         agent.pendingReplacement = false;
       }
       const turnStartedAt = new Date();
-      pendingRun.start = { status: "started", turnId };
+      this.runs.settleForegroundStart(pendingRun, { status: "started", turnId });
       agent.activeForegroundTurnId = turnId;
       this.openActiveTurn(agent, turnId, turnStartedAt);
       agent.lifecycle = "running";
@@ -3041,6 +3041,9 @@ export class AgentManager {
     prompt: AgentPromptInput,
     options?: AgentSteerOptions,
   ): Promise<ActiveTurnSteerDispatchResult> {
+    // A turn that is still starting has no id to steer yet. Replacing it would drop its
+    // prompt, so wait until it is accepted (or fails) and steer the turn it became.
+    await this.runs.waitForForegroundStart(agentId);
     const agent = this.requireSessionAgent(agentId);
     const expectedTurnId = agent.activeForegroundTurnId ?? agent.activeTurnId;
     if (!expectedTurnId) {

packages/server/src/server/agent/agent-run-state.ts

@@ -10,14 +10,18 @@ export interface ForegroundTurnWaiter {
   resolveSettled: () => void;
 }
 
+type ForegroundRunStart =
+  | { status: "pending" }
+  | { status: "started"; turnId: string }
+  | { status: "failed"; error: string };
+
 export interface PendingForegroundRun {
   token: string;
   kind: "foreground";
   stagedEvents: AgentStreamEvent[];
-  start:
-    | { status: "pending" }
-    | { status: "started"; turnId: string }
-    | { status: "failed"; error: string };
+  start: ForegroundRunStart;
+  startSettledPromise: Promise<void>;
+  resolveStartSettled: () => void;
   settled: boolean;
   settledPromise: Promise<void>;
   resolveSettled: () => void;
@@ -54,6 +58,23 @@ export class AgentRunState {
     return run?.kind === "foreground" ? run : null;
   }
 
+  settleForegroundStart(
+    run: PendingForegroundRun,
+    start: Exclude<ForegroundRunStart, { status: "pending" }>,
+  ): void {
+    run.start = start;
+    run.resolveStartSettled();
+  }
+
+  /** Resolves once the agent's foreground run has a turn id, failed to start, or ended. */
+  async waitForForegroundStart(agentId: string): Promise<void> {
+    const run = this.getPendingRun(agentId);
+    if (run?.start.status !== "pending") {
+      return;
+    }
+    await Promise.race([run.startSettledPromise, run.settledPromise]);
+  }
+
   hasPendingRun(agentId: string): boolean {
     return this.getPendingRun(agentId) !== null;
   }
@@ -277,10 +298,16 @@ export class ForegroundTurnStream {
 }
 
 function createPendingForegroundRun(): PendingForegroundRun {
+  let resolveStartSettled!: () => void;
+  const startSettledPromise = new Promise<void>((resolvePromise) => {
+    resolveStartSettled = resolvePromise;
+  });
   return {
     ...createTrackedRunState(),
     kind: "foreground",
     start: { status: "pending" },
+    startSettledPromise,
+    resolveStartSettled,
     stagedEvents: [],
   };
 }