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

When an unknown tool is called, it throws a ToolError, which is caught by the server's exception handler and converted to a response 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"
 }
}

The same issue applies to prompts and resources. Per the spec:

• Unknown prompts should return -32602 (INVALID_PARAMS): spec reference
• Unknown resources should return -32002 (RESOURCE_NOT_FOUND): spec reference

Changes

• ToolManager: Raises McpError with INVALID_PARAMS (-32602) for unknown tools
• PromptManager: Raises McpError with INVALID_PARAMS (-32602) for unknown prompts
• ResourceManager: Raises McpError with RESOURCE_NOT_FOUND (-32002) for unknown resources
• Added RESOURCE_NOT_FOUND constant to mcp.types

How Has This Been Tested?

I updated the tests to check for the correct error handling.

Breaking Changes

Users who expected the incorrect error handling may need to change their code.

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