OTel client span stays Unset when the tool result has isError: true

Open Beginner friendly
  • Is something wrong?
#3,810 0 comments 0 reactions 0 assignees View on GitHub

Maintainers usually reply within 1 day

Nobody has claimed this yet.

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
79/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
backend-api-design, observability

Research direction

Start in shared/jsonrpc_dispatcher.py and read the existing handling for JSON-RPC error responses, then compare it with the tool-error handling in server/_otel.py and the guidance in docs/gen-ai/mcp.md. Exercise a successful response containing a CallToolResult with isError set; done means the client span reports error.type=tool_error and ERROR status, with a test covering the behavior.

Written by the indexing model from the issue text.

Description

bug v2

With mcp==2.3.0, the client span created in shared/jsonrpc_dispatcher.py sets ERROR status and error.type for a JSON-RPC error response (the #3174 fix), but not for a successful response whose CallToolResult.isError is true. The server span for the same call does set error.type=tool_error (server/_otel.py), so the two ends of one call disagree.

The OTel GenAI semantic conventions for MCP (docs/gen-ai/mcp.md, note on error.type) say instrumentation SHOULD set error.type to tool_error when the tool result reports an error.

Reproduction (Python client and server, Streamable HTTP, default OTel setup):

git clone https://github.com/mkwitko/mcp-reliability && cd mcp-reliability
examples/python-sdk-streamable-http/run.sh      # needs docker, uv, go
go run ./cmd/mcp-reliability doctor testdata/python-sdk-default.jsonl

Output includes:

[WARN] MCP-ERR-003: client and server spans for "tools/call fail" disagree on failure: the client span lacks error.type

Captured trace and write-up: https://github.com/mkwitko/mcp-reliability/blob/v0.1.0/docs/research/python-sdk-v2-validation.md

Suggested change: after the response is parsed in the dispatcher, if the result is a CallToolResult with isError, set error.type="tool_error" and status ERROR on the client span, matching server/_otel.py. Happy to send a PR if you agree with the direction.

Dominant language
Python
Stars
24.7k
Forks
4.1k
Avg merge
18h 27m
Merged PRs (30d)
15

Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from modelcontextprotocol/python-sdk

All issues in modelcontextprotocol/python-sdk

Similar issues

More Python issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.