Add end_session_confirm_message handshake

[jackcbrown89]

Summary

Previously the Python client sent end_session_message and immediately closed the WS. Any artifact_upload_request_message the runtime produced after that (from chokidar events that fired post-terminal — e.g. files created by post-agent commands like `zip`) was sent on a half-closed socket and the upload response could never make it back. Symptom:

```
Executing post-agent command: zip ...
Received new WS message
Received end_session_message. Closing session.
Waiting 3000ms for artifact watcher to drain...
WebSocket connection closed ← client closed while we were draining
Requesting upload for artifact: deterministic-test.zip ← dropped
Waiting for 1 artifact round-trips... ← hangs until timeout
```

New protocol:

• Runtime receives `end_session_message`, drains the artifact watcher, sends `end_session_confirm_message`, then closes.
• Python client sends `end_session_message`, then pumps the receive loop — dispatching late `artifact_upload_request_message`s to the session's most-recently-used callback — until the confirm arrives (or 60s timeout).

Bumps both packages to 0.12.0. No backwards compatibility.

Test plan

• `npx vitest run` — all 140 TS tests pass
• `uv run pytest test/test_client.py` — all 44 Python tests pass

---

Note

Medium Risk
Introduces a new WebSocket protocol handshake and changes session teardown semantics across both the TS runtime and Python client, which can break compatibility with older peers and affect connection lifecycle behavior.

Overview
Adds an end_session_confirm_message handshake so sessions close gracefully: the runtime drains the artifact watcher after receiving end_session_message, then confirms and closes, allowing late artifact_upload_request_messages to complete.

Updates the Python persistent-session flow to send end_session_message on context exit and keep pumping messages (responding to late artifact upload requests using the most-recent callback) until confirmation or timeout; transports now expose a new end_session() API to support this drain/confirm loop.

Bumps package versions to 0.13.0 (Python and Node) and updates test fakes to implement the new end_session method.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Unprotected cleanup in finally may suppress user exceptions

Medium Severity

_end_session() is called in a bare finally block without a try/except wrapper. While end_session() in the transport catches TimeoutError and ConnectionClosed, it does not catch all exceptions — for example, if the server sends valid JSON that isn't an object (e.g., an array), msg.get("message_type") raises AttributeError, which propagates uncaught. If the user's code also raised an exception, the cleanup exception from _end_session() would suppress the original error, making debugging very difficult.

Additional Locations (1)

• packages/runtimeuse-client-python/src/runtimeuse_client/transports/websocket_transport.py#L65-L91

 

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