Pass a handler-returned _meta through the subscribe result

[koic]

Motivation and Context

resources/subscribe and resources/unsubscribe answered with a hardcoded {}, discarding whatever the registered handler returned. A server had no way to attach anything to the response - not even _meta, which the specification allows on every result. The handler docstring justified the discard as "always an empty result {} per the MCP specification", but the spec does not require an empty object here: the subscribe result is an EmptyResult, and every result may carry _meta.

The one field that is interoperable is _meta. The TypeScript SDK validates the subscribe result against EmptyResultSchema.strict(), which rejects any member other than _meta, so a top-level field like a bespoke subscriptionId would be refused by a TypeScript client. The Python SDK passes the handler's EmptyResult return through, _meta included. Passing _meta matches both while staying within what the spec defines.

The subscribe and unsubscribe handlers now pass a returned _meta hash through to the result and drop anything else. subscription_result reads _meta under a symbol or string key and includes it only when it is itself a hash; every other return shape - a non-hash, or a hash without a usable _meta - keeps the empty {} result it produced before. Advisory data such as a subscription identifier goes under _meta, namespaced, rather than at the top level. This affects only 2025-11-25 and earlier connections, since resources/subscribe and resources/unsubscribe are removed from the modern lifecycle.

Fixes #508.

How Has This Been Tested?

New tests in test/mcp/server_test.rb cover a _meta hash passing through on both resources/subscribe and resources/unsubscribe, a non-_meta field being dropped, a string _meta key working, a non-hash _meta being ignored, and a non-hash return keeping the empty result. The existing subscribe and unsubscribe tests, which return {} and assert an empty result, are unchanged.

Breaking Changes

None. A handler that does not return a hash carrying a _meta hash - which is every handler written against the previous "return value is ignored" contract - still produces the empty {} result.

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