fix(agent): send tau rpc prompt as string

src/process/tau-rpc.test.ts

@@ -0,0 +1,81 @@
+import { EventEmitter } from "node:events";
+import { PassThrough } from "node:stream";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { resetPiRpc, runPiRpc } from "./tau-rpc.js";
+
+vi.mock("node:child_process", () => {
+  const spawn = vi.fn();
+  return { spawn };
+});
+
+type MockChild = EventEmitter & {
+  stdin: EventEmitter & {
+    write: (chunk: string, cb?: (err?: Error | null) => void) => boolean;
+    once: (event: "drain", listener: () => void) => unknown;
+  };
+  stdout: PassThrough;
+  stderr: PassThrough;
+  killed: boolean;
+  kill: (signal?: NodeJS.Signals) => boolean;
+};
+
+function makeChild(): MockChild {
+  const child = new EventEmitter() as MockChild;
+  const stdin = new EventEmitter() as MockChild["stdin"];
+  stdin.write = (_chunk: string, cb?: (err?: Error | null) => void) => {
+    cb?.(null);
+    return true;
+  };
+  child.stdin = stdin;
+  child.stdout = new PassThrough();
+  child.stderr = new PassThrough();
+  child.killed = false;
+  child.kill = () => {
+    child.killed = true;
+    return true;
+  };
+  return child;
+}
+
+describe("tau-rpc", () => {
+  afterEach(() => {
+    resetPiRpc();
+    vi.resetAllMocks();
+  });
+
+  it("sends prompt with string message", async () => {
+    const { spawn } = await import("node:child_process");
+    const child = makeChild();
+    vi.mocked(spawn).mockReturnValue(child as never);
+
+    const writes: string[] = [];
+    child.stdin.write = (chunk: string, cb?: (err?: Error | null) => void) => {
+      writes.push(String(chunk));
+      cb?.(null);
+      return true;
+    };
+
+    const run = runPiRpc({
+      argv: ["tau", "--mode", "rpc"],
+      cwd: "/tmp",
+      timeoutMs: 500,
+      prompt: "hello",
+    });
+
+    // Allow the async `prompt()` to install the pending resolver before exiting.
+    await Promise.resolve();
+
+    expect(writes.length).toBeGreaterThan(0);
+    child.emit("exit", 0, null);
+    const res = await run;
+
+    expect(res.code).toBe(0);
+    expect(writes.length).toBeGreaterThan(0);
+    const first = writes[0]?.trim();
+    expect(first?.endsWith("\n")).toBe(false);
+    const obj = JSON.parse(first ?? "{}") as { type?: string; message?: unknown };
+    expect(obj.type).toBe("prompt");
+    expect(obj.message).toBe("hello");
+  });
+});

src/process/tau-rpc.ts

@@ -221,11 +221,10 @@ class TauRpcClient {
       const ok = child.stdin.write(
         `${JSON.stringify({
           type: "prompt",
-          // Send structured content to match tau RPC expectations and avoid
-          // empty-text bugs on older builds.
-          message: {
-            content: [{ type: "text", text: prompt }],
-          },
+          // Pi/Tau RPC expects a plain string prompt.
+          // (The structured { content: [{type:"text", text}] } shape is used by some
+          // model APIs, but is not the RPC wire format here.)
+          message: prompt,
         })}\n`,
         (err) => (err ? reject(err) : resolve()),
       );