Answer a method of an undeclared capability with Method not found

[koic]

Motivation and Context

A request for a method whose capability the server does not declare, such as prompts/list on a server declaring only tools, is answered with -32603 Internal error (issue #602). To a client that reads as a crash, while the server merely has no such method: the TypeScript and Python servers answer -32601 Method not found for a handler they never registered, and the Streamable HTTP transport of 2026-07-28 has the server answer a method it does not implement with 404 Not Found and -32601, which this transport already does for an unknown method but not for this one.

Server#handle_request now re-raises Methods::MissingRequiredCapabilityError as -32601 Method not found, keeping the capability it names in the error data, so the modern transport answers 404 as for any other method it lacks.

Fixes #602.

How Has This Been Tested?

test/mcp/server_test.rb (the three capability tests now check the code and message) and new tests in test/mcp/server/transports/streamable_http_transport_test.rb for both lifecycles. Against the previous library they fail with -32603, the modern one also with a 200.

Breaking Changes

A client that matched the -32603 of an undeclared capability's method sees -32601 now, and a 404 instead of a 200 on the modern lifecycle. The error data text is unchanged.

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