While developing my Swift SDK for MCP, I encountered the following MCP spec compliance issues in the TypeScript SDK:

Tool Error Handling

When an unknown or disabled tool is called, it throws an McpError, which is caught and returned as a CallToolResult with isError: true.

The spec distinguishes between protocol errors and tool execution errors:

Protocol Errors:

1. Protocol Errors: Standard JSON-RPC errors for issues like:
   • Unknown tools
   • Invalid arguments
   • Server errors

Tool Execution Errors:

2. Tool Execution Errors: Reported in tool results with isError: true:
   • API failures
   • Invalid input data
   • Business logic errors

Protocol error example for unknown tool:

{
  "jsonrpc": "2.0",
  "id": 3,
  "error": {
    "code": -32602,
    "message": "Unknown tool: invalid_tool_name"
  }
}

I moved the unknown/disabled tool checking to before the try/catch block so that these protocol errors propagate as JSON-RPC errors rather than being caught and returned as a CallToolResult.

Resource Error Handling

The spec also requires that unknown resources return error code -32002 (ResourceNotFound) rather than -32602 (InvalidParams). This PR adds ErrorCode.ResourceNotFound to the ErrorCode enum and updates the resource handler to use it.

How Has This Been Tested?

• Updated the unknown tool test to verify the correct error code and behavior
• Added a new test for disabled tools
• Updated the resource not found test to verify the correct error code (-32002)
• Updated the prompt not found test to verify the correct error code (-32602)
• One unrelated test was failing on my machine, so I fixed it by updating the expected error message. Then I noticed it failed in CI. Apparently the error message differs depending on the environment, so it now accepts either variant. Now all tests pass.

Breaking Changes

• Users who expected CallToolResult with isError: true for unknown/disabled tools will now receive rejected promises with McpError
• Users who checked for error code -32602 on unknown resources should update to expect -32002

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