fix(streamable_http): include MCP method in client JSON-RPC error messages

[syf2211]

Summary

Include the originating MCP method name when wrapping JSON-RPC errors returned by the client over streamable HTTP transport.

Motivation

handleSamplingResponse is shared by sampling/createMessage, elicitation/create, and roots/list, but previously hardcoded all client-side JSON-RPC errors as sampling error %d: %s. This made it impossible to distinguish elicitation failures from sampling failures without fragile string matching.

Fixes #817

Changes

• Introduce pendingClientRequest to track both the response channel and MCP method for pending client requests
• Store the correct method when registering sampling, elicitation, and roots/list requests
• Format client JSON-RPC errors as <method> error <code>: <message> (e.g. elicitation/create error -32601: Method not found)
• Add regression test TestStreamableHTTPServer_ClientRequestErrorIncludesMethod

Tests

go test ./server/... -run 'TestStreamableHTTPServer_(ClientRequestErrorIncludesMethod|Sampling)' -count=1 -v

All tests passed.

Notes

• Sampling errors now read sampling/createMessage error ... instead of sampling error ... for accuracy. Error strings are not a stable API; callers should inspect error values rather than parse message text.
• stdio transport error messages are unchanged (out of scope for this issue).

Summary by CodeRabbit

• Bug Fixes
  • Improved error reporting for sampling-related requests so client-side errors now include the originating request type.
  • Fixed handling of pending responses in Streamable HTTP sessions to better detect missing or invalid requests.
• Tests
  • Added coverage for error messages returned from sampling requests to ensure they include the correct method context.

[mark-iii-labs-huly]

Connected to Huly®: MCP_G-476

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: c990252c-892a-4047-9c80-f7eb88bc3464

📥 Commits

Reviewing files that changed from the base of the PR and between b6e6224 and b68951d.

📒 Files selected for processing (2)

• server/streamable_http.go
• server/streamable_http_sampling_test.go

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Walkthrough

Adds a pendingClientRequest struct that pairs a response channel with the originating MCP method. RequestSampling, ListRoots, and RequestElicitation now store this struct in samplingRequests. handleSamplingResponse and deliverSamplingResponse use the stored method to produce method-specific error messages. A test verifies the method name appears in errors.

Method-aware sampling error plumbing

Layer / File(s)	Summary
pendingClientRequest struct and storage updates server/streamable_http.go	Introduces pendingClientRequest holding a response channel and MCP method string; updates RequestSampling, ListRoots, and RequestElicitation to store it in samplingRequests instead of a bare channel.
Error and delivery logic using stored method server/streamable_http.go	handleSamplingResponse loads the stored pendingClientRequest to extract the method name and formats errors as "{method} error {code}: {msg}". deliverSamplingResponse type-asserts the loaded value and adds explicit HTTP error responses for missing or invalid entries.
Test: error string includes originating method server/streamable_http_sampling_test.go	Adds TestStreamableHTTPServer_ClientRequestErrorIncludesMethod which injects a seeded pendingClientRequest for an elicitation/create request, sends a JSON-RPC error payload, and asserts the resulting error string is exactly elicitation/create error -32601: Method not found.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• mark3labs/mcp-go#764: Modifies the same handleSamplingResponse/deliverSamplingResponse code paths in server/streamable_http.go.

Suggested labels

type: bug

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[syf2211]

Closing as duplicate of #822, which already addresses the same issue with a similar approach. Thanks!