[koic]

Motivation and Context

_meta is an object on every request (MetaObject in the schema). After #650, a String, Array, or number there no longer raises from dig, and the modern lifecycle answers -32602 because the envelope keys are missing; the legacy lifecycle, though, went on to run the handler. That left two gaps. The value still reached server_context[:_meta], where the server_context.dig(:_meta, ...) the server context page shows raises TypeError on a String or an Array, so a handler written as documented still turned the request into -32603. And the TypeScript and Python servers refuse such a request as invalid params on both lifecycles, through schema validation of params and their inbound checks, so the Ruby server answered differently from both.

The request is now refused with -32602 "Invalid params: _meta must be an object" before the envelope is read and before any handler runs, on both lifecycles. A notification handed to a handler is dropped the same way, without a response, as the TypeScript server drops a notification with invalid params; notifications/cancelled is served before the check and reads no _meta. A null _meta reads as absent, as it does for the Python server's request model. The server context page states the rule, since it tells handlers to read _meta from server_context.

How Has This Been Tested?

test/mcp/server_test.rb and test/mcp/server/transports/streamable_http_transport_test.rb. Before the fix, legacy tools/call with _meta: "not-an-object" ran the tool, and a tool reading server_context.dig(:_meta, :key) answered -32603 "Internal error" with "Internal error calling tool meta_reader" as the data.

Breaking Changes

A legacy-lifecycle request whose _meta is not an object now answers -32602. Released versions served such a request for methods that never read _meta, such as tools/list, and answered -32603 for tools/call and prompts/get.

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