Update tools/call input validation errors to be tool execution errors

[fredericbarthelet]

Reporting input validation errors as Tool Execution Errors instead of Protocol Errors in order to increase model self-recovery success rate.

Motivation and Context

The tools error handling specs is quite vague on wether input validation errors shall be treated as Protocol Errors (following the Invalid arguments exemple) or as Tool Execution Errors (following the Invalid input data exemple).

Only Tool Execution Errors are injected back into the LLM context window, just like successful responses. Input validation error messages can be leveraged by the model as much as any other prompt, giving it a chance to recover from the error without human intervention.

How Has This Been Tested?

The should validate tool args test suite in src/server/mcp.test.ts has been updated to reflect the new error reponse format.

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

I am the author of the following article on MCP tool execution errors : Better MCP tools/call Error Responses: Help Your AI Recover Gracefully

[domdomegg]

Hey @fredericbarthelet, thanks for raising this PR!

I think as written, the spec would include these errors as protocol errors, as the fall under 'Invalid arguments'. So at this time I don't think we'd be willing to accept this PR.

However, it does raise a good point: it adds complexity to have two types of errors which are inconsistently handled in clients - resulting in the behaviour where models can't retry failed things in a range of cases.

Would you be open to raising a spec proposal in the main repo on this? https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/CONTRIBUTING.md