[waldekmastykarz]

Closes #1757

devproxy stdio treated every raw stdin read as one message. JSON-RPC messages written back-to-back (e.g. notifications/initialized immediately followed by tools/call) landed in a single read, so MockStdioResponsePlugin couldn't parse the body as JSON and left @stdin.body.id unresolved.

Changes

• Add StdioLineFramer, which splits stdin on newlines (JSON-RPC over stdio / MCP framing). Partial lines are buffered until the newline arrives; any remainder is flushed on EOF. Messages keep their trailing newline so forwarded bytes are unchanged.
• ProxySession.ForwardStdinAsync now runs the plugin pipeline (logging, plugins, forward/mock, request log) once per framed message.
• Stdout and stderr handling is unchanged.

Testing

• Unit tests for StdioLineFramer (multiple lines per chunk, a line split across chunks, CRLF, a line over 4 KB, flush on EOF)
• Integration test confirming @stdin.body.id resolves for a newline-terminated JSON-RPC message
• The repro from the issue now returns "id":2

[Copilot]

🟡 Changes recommended

Mandatory newline framing can stall non-line-oriented executable sessions, and the added test does not exercise the stdin forwarding pipeline needed to catch the coalesced-input regression.

2 open issues

• Make newline framing optional for arbitrary executable sessions · New
• Test coalesced notifications and requests through the forwarding pipeline · New

What changed in this PR

This PR addresses #1757 by framing stdin into newline-delimited messages before running Dev Proxy’s stdio plugin pipeline, allowing back-to-back JSON-RPC messages to be processed separately.

Changes:

• Adds byte-based framing that preserves line endings, buffers partial messages, and flushes remaining data on EOF.
• Processes logging, plugins, forwarding, and mocking once per stdin message; stdout and stderr remain unchanged.
• Adds framing unit tests and a plugin test for placeholder resolution with newline-terminated JSON.

File	Description
DevProxy/​Stdio/​StdioLineFramer.cs	Adds newline framing and partial-message buffering.
DevProxy/​Stdio/​ProxySession.cs	Dispatches each framed stdin message through the plugin pipeline.
DevProxy.Tests/​StdioLineFramerTests.cs	Tests chunk boundaries, CRLF, large messages, and flushing.
DevProxy.Integration.Tests/​MockStdioResponsePluginTests.cs	Tests ID placeholder resolution in a newline-terminated request.

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

A shared write semaphore can deadlock large framed messages with streaming children by blocking stdout drainage while stdin forwarding waits for the child to read more input.

1 open issue

• Separate stdin and stdout write gates to prevent pipe deadlocks · New

2 resolved since last review

• Make newline framing optional for arbitrary executable sessions
• Test coalesced notifications and requests through the forwarding pipeline

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

No active review issues are identified for the stdin message-framing change.

No open issues 🎉

1 resolved since last review

• Separate stdin and stdout write gates to prevent pipe deadlocks

_{Review effort: Balanced}