chore(mcp): check MCP error code over brittle string match

[jackwotherspoon]

Summary

This PR enhances the stability of MCP server discovery by using specific MCP error codes instead of brittle string matching.

Previously

MCP server failure on prompt discovery for "Method not supported" showed a brittle match on "Method not found" so "Method not supported" was not being picked up.

Now

MCP prompts use proper error code to check discovery and return empty on error so tools can properly work.

Details

• Replaced string-based error checking for "Method not found" with explicit McpError and ErrorCode.MethodNotFound checks.

Related Issues

Fixes #25107

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the stability and resilience of MCP server discovery. By modifying prompt and resource discovery functions to gracefully handle failures without halting the entire process, it ensures that tool discovery can proceed even if other discovery steps encounter issues. The changes also refine error handling by moving from string-based checks to explicit error codes, leading to more reliable and maintainable code.

Highlights

• Improved MCP Server Discovery Resilience: Failures during prompt or resource discovery no longer prevent the successful discovery of tools, enhancing overall stability.
• Enhanced Error Handling: Error checking for 'Method not found' has been refactored to use explicit McpError and ErrorCode.MethodNotFound types, providing more robust and specific error management.
• Comprehensive Unit Testing: New unit tests have been added to validate the improved resilience patterns and error handling mechanisms.

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[github-actions]

Size Change: +6 B (0%)

Total Size: 34.1 MB

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[gemini-code-assist]

Code Review

This pull request improves the robustness of MCP (Model Context Protocol) discovery by refactoring error handling. It replaces string-based error checking with explicit McpError and ErrorCode.MethodNotFound checks in discoverTools, discoverPrompts, and listResources. Additionally, discoverPrompts and listResources now return an empty array instead of throwing an error upon failure, ensuring that discovery failures do not interrupt the overall process. Corresponding unit tests have been updated to reflect these changes and verify diagnostic behavior. I have no feedback to provide.

[abhipatel12]

Approved! One nit

[github-actions]

✅ 59 tests passed successfully on gemini-3-flash-preview.

🧠 Model Steering Guidance

This PR modifies files that affect the model's behavior (prompts, tools, or instructions).

• ⚠️ Consider adding Evals: No behavioral evaluations (evals/*.eval.ts) were added or updated in this PR. Consider adding a test case to verify the new behavior and prevent regressions.
• 🚀 Maintainer Reminder: Please ensure that these changes do not regress results on benchmark evals before merging.

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This is an automated guidance message triggered by steering logic signatures.