test(agent-sdk): add session.followup queueing test

sdks/typescript-agent-sdk/tests/followup.test.ts

@@ -0,0 +1,57 @@
+/**
+ * Validation #5 — session.followup() queues correctly.
+ *
+ * Sends a long prompt via send(); while it's running, calls followup().
+ * Expect: no 409, follow-up message appears in conversation after current
+ * run terminal.
+ */
+
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { Agent } from "../src/index.js";
+import { MockProxy } from "./mock-proxy.js";
+
+describe("session.followup()", () => {
+  let proxy: MockProxy;
+  let baseUrl: string;
+
+  beforeEach(async () => {
+    proxy = new MockProxy();
+    baseUrl = await proxy.start();
+  });
+
+  afterEach(async () => {
+    await proxy.stop();
+  });
+
+  it("accepts a follow-up message while a run is in flight without a 409", async () => {
+    const agent = await Agent.create({
+      apiKey: "test-key",
+      baseUrl,
+      name: "follower",
+      model: { id: "test-model" },
+    });
+    const session = await agent.createSession();
+    const run = await session.send("the long initial prompt");
+
+    // Simulate run-in-progress by leaving status non-terminal until we say so.
+    // followup() should NOT throw a 409.
+    await expect(
+      session.followup("please also handle the empty-input case")
+    ).resolves.toBeUndefined();
+
+    // Drive the run to completion.
+    proxy.emit(run.id, "delta", { text: "ok" });
+    proxy.complete(run.id, "done");
+    await run.wait();
+
+    const turns = await session.conversation();
+    const userTurns = turns.filter((t) => t.role === "user").map((t) => t.content);
+    expect(userTurns).toContain("the long initial prompt");
+    expect(userTurns).toContain("please also handle the empty-input case");
+
+    // Follow-up should be ordered after the initial prompt.
+    const initialIdx = userTurns.indexOf("the long initial prompt");
+    const followupIdx = userTurns.indexOf("please also handle the empty-input case");
+    expect(followupIdx).toBeGreaterThan(initialIdx);
+  });
+});