Handle an initialize request sent without an ID

[koic]

Motivation and Context

StreamableHTTPTransport#handle_post routes to handle_initialization via initialize_request?, which matches on the method name alone. An initialize message sent without an id (framed as a notification) therefore reaches handle_initialization. Server#init runs and marks the session initialized, but JSON-RPC emits no response for an id-less request, so handle_json returns nil.

Two things then went wrong:

• handle_initialization returned [200, headers, [nil]]. A nil body element is not a valid Rack response, so the web server raises while serializing it (NoMethodError on nil.bytesize) and the client gets a malformed response or a 500.
• The if session_id && !server_session.initialized? cleanup guard was skipped, because the session is marked initialized. The session was retained with a Mcp-Session-Id header, so a stream of id-less initialize messages accumulates sessions toward the max_sessions cap until the reaper reclaims them.

handle_initialization now treats a nil response as an id-less initialize: it discards the orphaned session and acks with 202, mirroring how handle_regular_request already handles a nil response. A well-formed initialize request (with an id) is unaffected.

How Has This Been Tested?

A new test in test/mcp/server/transports/streamable_http_transport_test.rb posts an initialize with no id and asserts a 202 with an empty body, no Mcp-Session-Id header, and an empty session map. It fails against the previous code (a 200 with a nil body element and a retained session) and passes now.

Breaking Changes

None. A conforming client sends initialize as a request with an id and is unaffected; only the malformed id-less form changes, from a broken 200 to a 202 with no retained session.

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