fix(jetsocat): always send JSON-RPC message on MCP proxy backend disconnect#1737
Merged
Benoît Cortier (CBenoit) merged 4 commits intomasterfrom Apr 2, 2026
Merged
fix(jetsocat): always send JSON-RPC message on MCP proxy backend disconnect#1737Benoît Cortier (CBenoit) merged 4 commits intomasterfrom
Benoît Cortier (CBenoit) merged 4 commits intomasterfrom
Conversation
The --log-term flag was routing log output through stdout, polluting the MCP stdio JSON-RPC channel and any other stdout-based protocol. Fixed to use stderr, which is the correct descriptor for diagnostic output.
…onnect When the backend MCP process disconnects, the proxy now always sends a protocol-level message to the client before exiting, instead of silently closing the stdio channel: - SendError::Fatal (write to broken backend): JSON-RPC error response (-32099) with the pending request id. - ReadError::Fatal with a pending request: JSON-RPC error response (-32099) so the client can correlate it with the outstanding request. - ReadError::Fatal with no pending request: a \\$/proxy/serverDisconnected\ notification so the client is informed even without an active request.
Contributor
There was a problem hiding this comment.
Pull request overview
This PR fixes jetsocat’s MCP proxy behavior so clients always receive a protocol-level JSON-RPC frame (error response or notification) when the backend disconnects, and ensures --log-term output no longer corrupts the stdio JSON-RPC stream by moving logs to stderr.
Changes:
- Route
--log-termlogging to stderr (and update related CLI docs/tests). - Track a pending JSON-RPC request id in
mcp-proxyto synthesize either an error response (-32099) or a$/proxy/serverDisconnectednotification on fatal backend disconnect. - Replace the testsuite MCP server shutdown helper with
tokio_util::sync::CancellationTokenand adjust MCP proxy tests accordingly.
Reviewed changes
Copilot reviewed 6 out of 7 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
crates/mcp-proxy/src/lib.rs |
Adds pending-request tracking and always generates a JSON-RPC message on fatal backend disconnect. |
jetsocat/src/mcp.rs |
Ensures fatal read/write errors flush the synthesized JSON-RPC message to the client before exiting. |
jetsocat/src/main.rs |
Moves --log-term output to stderr and updates flag description. |
testsuite/tests/cli/jetsocat.rs |
Updates assertions to expect logs on stderr and strengthens MCP proxy disconnect test expectations. |
testsuite/src/mcp_server.rs |
Replaces custom shutdown signal with CancellationToken and updates server loop cancellation. |
testsuite/Cargo.toml |
Adds tokio-util dependency for CancellationToken. |
Cargo.lock |
Locks new dependency. |
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
Contributor
There was a problem hiding this comment.
Pull request overview
Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
Vladyslav Nikonov (vnikonov-devolutions)
approved these changes
Apr 2, 2026
Contributor
Vladyslav Nikonov (vnikonov-devolutions)
left a comment
Vladyslav Nikonov (vnikonov-devolutions)
left a comment
There was a problem hiding this comment.
LGTM!
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Fixes two bugs in the jetsocat MCP proxy that could cause silent failures when the backend MCP process disconnected.
--log-termwriting to stdout: log output was mixed into the MCP stdio JSON-RPC channel, corrupting the protocol stream for any client using--log-term.Silent close on backend disconnect: when the backend broke, the proxy would close the stdio channel without sending any protocol-level message, leaving the client with a raw broken-pipe or EOF error it couldn't meaningfully handle. The proxy now always sends a message:
-32099) correlated to the pending request id.$/proxy/serverDisconnectednotification so the client is still informed.Also replaces the
McpShutdownSignaltest helper (which usednotify_one()and could silently leave a waiter alive) withCancellationTokenfromtokio-util, fixing the root cause of the flakymcp_proxy_terminated_on_broken_pipeCI test.