[koic]

Motivation and Context

StdioTransport reads $stdin as UTF-8 and strips each frame before parsing it, in the read loop of #open and in the loop of #send_request that reads the client's reply. String#strip raises for a UTF-8 string holding an invalid byte at either end on every supported Ruby, and anywhere in it from Ruby 3.2 on, and nothing rescued it there: one frame such as \xff[], or a request with an invalid byte in a string value, ended the read loop and the server process, before the parser could answer it. JSON text must be UTF-8 (RFC 8259, section 8.1), and the Python SDK replaces such bytes on its stdio streams instead of stopping.

Frames are now stripped as bytes. A frame that is not JSON gets the -32700 parse error any other unparsable frame gets, and a value holding such a byte reaches the handlers, which serve or refuse it as over HTTP.

How Has This Been Tested?

test/mcp/server/transports/stdio_transport_test.rb: a frame starting with an invalid byte, a frame holding one in a value, and a client reply holding one in a value. Against the previous library all three raise from String#strip (the value tests from Ruby 3.2 on).

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