mock ACP initialize hangs/timeouts on sandbox-agent 0.2.0 (invalid stdout + broken pipe)

[handfuloflight]

Summary

agent=mock fails ACP initialization and causes createSession to hang/time out in sandbox-agent v0.2.0.

In our Sprite environment, agent=codex works in the same server process, but agent=mock consistently fails.

Environment

• sandbox-agent: 0.2.0
• Deployment: server mode on Linux Sprite VM
• Endpoint: http://127.0.0.1:2468 (same behavior through external gateway)
• Agent path for mock (from /v1/agents?config=true): /home/sprite/.local/share/sandbox-agent/bin/mock (reported), but actual launches use .../agent_processes/mock-acp

Repro

1. Start server:
   • sandbox-agent server --host 0.0.0.0 --port 2468 --token <TOKEN>
2. Post ACP initialize for mock:
   • sandbox-agent api acp post --endpoint http://127.0.0.1:2468 --token <TOKEN> --server-id issue-mock-<ts> --agent mock --json '{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":1,"clientInfo":{"name":"issue-probe","version":"v1"}}}'
3. Observe timeout (operation timed out).
4. Using SDK (SandboxAgent.connect(...).createSession({ agent: "mock" })) also times out.

Actual Behavior

• ACP initialize does not return.
• Session creation times out.
• Follow-up requests can fail with broken pipe.

Expected Behavior

• Built-in mock should respond with valid ACP JSON and allow createSession + prompt for smoke testing.

Relevant Logs

... message="create_instance: launch spec resolved, spawning" ... program="\"/home/sprite/.local/share/sandbox-agent/bin/agent_processes/mock-acp\""
... message="agent stdout: invalid JSON" ... raw="mock agent process is in-process in sandbox-agent"
... message="post: TIMEOUT waiting for agent response" method=initialize id=0 ...
... message="post: failed to write to agent stdin" ... "Broken pipe (os error 32)"

Also, sandbox-agent install-agent mock reports:

{ "alreadyInstalled": true, "artifacts": [] }

while the runtime still fails as above.

Notes

• This appears specific to mock; in the same deployment agent=codex works end-to-end.
• We can provide additional logs if helpful.

[linear]

RVT-5972

[NathanFlurry]

will get this fixed this weekend thx