Mock agent process crashes on session creation (ECONNRESET)

[marcus-sa]

Environment

• SDK version: sandbox-agent@0.4.0
• Platform: macOS (Darwin 25.1.0, arm64)
• Runtime: Bun 1.3.8 and Node.js 25.2.1
• Agent: mock (installed via sandbox-agent install-agent mock)

Symptoms

Creating a session with agent: "mock" causes the mock agent process to crash immediately after the ACP connection is established. The SDK receives an ECONNRESET / "other side closed" error.

The claude agent works perfectly with the same setup.

Reproduction

import { SandboxAgent } from "sandbox-agent";
import { local } from "sandbox-agent/local";

const sdk = await SandboxAgent.start({ sandbox: local() });
console.log("Started:", sdk.sandboxId);

// This hangs then errors with ECONNRESET
const session = await sdk.createSession({ agent: "mock", cwd: "/" });

Error output:

ACP write error: error: The socket connection was closed unexpectedly.
  path: "http://127.0.0.1:<port>/v1/acp/sdk-mock-<uuid>?agent=mock",
  code: "ECONNRESET"

Same error occurs with both Bun and Node.js, confirming it's not runtime-specific.

Expected behavior

createSession({ agent: "mock" }) should return a session handle that responds to prompts with mock data, as documented in the quickstart.

What works

• SandboxAgent.start() succeeds
• sdk.listAgents() shows mock as installed with all capabilities
• sdk.getHealth() returns { status: "ok" }
• createSession({ agent: "claude" }) works perfectly
• session.prompt(...) works with claude agent

Impact

The mock agent is intended for SDK/integration testing without provider credentials. Without it, tests must use the claude agent with real API keys.

[linear]

RVT-6124