Handle BrokenResourceError in stdio client stdout reader - #2567
Closed
zuhabul wants to merge 2 commits into
Closed
Conversation
Copy link
Copy Markdown
Sorry, something went wrong.
…protocol#348)\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
…contextprotocol#1564)\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copy link
Copy Markdown
There was a problem hiding this comment.
Pull request overview
This PR primarily addresses issue #1564 by preventing stdio_client from crashing when the subprocess stdout stream closes abruptly, treating BrokenResourceError as a normal shutdown condition.
Changes:
- Catch anyio.BrokenResourceError in the stdio client stdout reader to avoid propagating an exception from the transport task group.
- Extend MCPServer tool result handling to interpret a 3-tuple return shape and plumb the third element into CallToolResult.is_error.
- Document the new 3-tuple tool return shape in the migration guide.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.
| File | Description |
|---|---|
| src/mcp/client/stdio.py | Swallows BrokenResourceError during stdout reads to avoid transport crashes on abrupt subprocess pipe closure. |
| src/mcp/server/mcpserver/server.py | Adds 3-tuple handling for tool call results to set CallToolResult.is_error. |
| docs/migration.md | Documents the new (unstructured, structured, is_error) 3-tuple tool return option. |
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
Sorry, something went wrong.
Comment thread
src/mcp/server/mcpserver/server.py
Comment on lines
+320
to
+334
| # Support either (unstructured_content, structured_content) or | ||
| # (unstructured_content, structured_content, is_error). The third element, | ||
| # if present, controls the CallToolResult.is_error flag. | ||
| if len(result) == 2: | ||
| unstructured_content, structured_content = result | ||
| is_error = False | ||
| elif len(result) == 3: | ||
| unstructured_content, structured_content, is_error = result | ||
| else: | ||
| # Fallback: treat as a sequence of content blocks | ||
| return CallToolResult(content=list(result)) | ||
| return CallToolResult( | ||
| content=list(unstructured_content), # type: ignore[arg-type] | ||
| structured_content=structured_content, # type: ignore[arg-type] | ||
| is_error=bool(is_error), |
Comment thread
src/mcp/server/mcpserver/server.py
Comment on lines
+319
to
+334
| if isinstance(result, tuple): | ||
| # Support either (unstructured_content, structured_content) or | ||
| # (unstructured_content, structured_content, is_error). The third element, | ||
| # if present, controls the CallToolResult.is_error flag. | ||
| if len(result) == 2: | ||
| unstructured_content, structured_content = result | ||
| is_error = False | ||
| elif len(result) == 3: | ||
| unstructured_content, structured_content, is_error = result | ||
| else: | ||
| # Fallback: treat as a sequence of content blocks | ||
| return CallToolResult(content=list(result)) | ||
| return CallToolResult( | ||
| content=list(unstructured_content), # type: ignore[arg-type] | ||
| structured_content=structured_content, # type: ignore[arg-type] | ||
| is_error=bool(is_error), |
Comment thread
docs/migration.md
Comment on lines
+1141
to
+1147
| Tools may now return a 3-tuple: (unstructured_content, structured_content, is_error). | ||
| If the third element is True, the resulting CallToolResult sent to clients will have | ||
| is_error=True. This allows returning non-text content (images, audio) while still | ||
| indicating the tool execution failed or produced an error state. | ||
|
|
||
| Alternatively, tools may return a full `CallToolResult` instance directly to control | ||
| is_error and other fields explicitly. |
Comment thread
src/mcp/client/stdio.py
Comment on lines
137
to
163
| async def stdout_reader(): | ||
| assert process.stdout, "Opened process is missing stdout" | ||
|
|
||
| try: | ||
| async with read_stream_writer: | ||
| buffer = "" | ||
| async for chunk in TextReceiveStream( | ||
| process.stdout, | ||
| encoding=server.encoding, | ||
| errors=server.encoding_error_handler, | ||
| ): | ||
| lines = (buffer + chunk).split("\n") | ||
| buffer = lines.pop() | ||
|
|
||
| for line in lines: | ||
| try: | ||
| message = types.jsonrpc_message_adapter.validate_json(line, by_name=False) | ||
| except Exception as exc: # pragma: no cover | ||
| logger.exception("Failed to parse JSONRPC message from server") | ||
| await read_stream_writer.send(exc) | ||
| continue | ||
|
|
||
| session_message = SessionMessage(message) | ||
| await read_stream_writer.send(session_message) | ||
| except anyio.ClosedResourceError: # pragma: lax no cover | ||
| except (anyio.ClosedResourceError, anyio.BrokenResourceError): # pragma: lax no cover | ||
| await anyio.lowlevel.checkpoint() | ||
|
|
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 #1564 — treat BrokenResourceError from process stdout as normal shutdown and avoid crashing the client. See issue: #1564