mock createSession hangs/times out while claude succeeds on same server (0.2.0)

[handfuloflight]

Summary

mock sessions fail to create (timeout/hang), while claude sessions create successfully on the exact same sandbox-agent server instance.

Environment

• sandbox-agent server: @sandbox-agent/cli@0.2.0
• sandbox-agent SDK: sandbox-agent@0.2.0
• Runtime host: Sprite (alpha)
• Sprite OS: Linux alpha 6.12.47-fly ... x86_64
• Node: v22.20.0
• npm: 11.10.0

Minimal repro (inside the same Sprite)

Start server:

sandbox-agent server --no-token --host 127.0.0.1 --port 8080

Run probe:

import { SandboxAgent } from "sandbox-agent";

const baseUrl = "http://127.0.0.1:8080";
const withTimeout = (p, ms, label) => Promise.race([
  p,
  new Promise((_, rej) => setTimeout(() => rej(new Error(`${label} timeout ${ms}ms`)), ms)),
]);

async function run(agent) {
  const sdk = await SandboxAgent.connect({ baseUrl });
  try {
    const session = await withTimeout(
      sdk.createSession({ id: `repro_${agent}_${Date.now()}`, agent, sessionInit: { cwd: "/", mcpServers: [] } }),
      30000,
      `createSession(${agent})`
    );
    console.log(`[${agent}] createSession ok: ${session.id}`);
  } catch (err) {
    console.log(`[${agent}] createSession error: ${err?.message || err}`);
  } finally {
    await sdk.dispose().catch(() => {});
  }
}

await run("claude");
await run("mock");

Observed output:

[claude] createSession ok: repro_claude_...
[mock] createSession error: createSession(mock) timeout 30000ms

Additional symptom seen from remote URL access

When connecting to the same server through Sprite URL (not loopback), mock also intermittently fails with:

• 502 stream error: failed writing to agent stdin: Broken pipe (os error 32)
• followed by createSession timeout

Expected

mock should create promptly and be usable as a control/test agent independent of provider credentials.

Actual

mock createSession hangs/times out, while other agents on same server can create sessions.

[linear]

RVT-5974