[emirn]

Motivation and Context

MCP requires _meta to be an object, but a request whose _meta is a string, an array or a number
makes the server raise TypeError from dig instead of answering a client error:

• Streamable HTTP, MCP-Protocol-Version: 2026-07-28: validate_modern_headers calls
  params.dig(:_meta, ...), and the catch-all in handle_modern answers HTTP 500 -32603 for every method.
  RequestEnvelope.parse! already treats a non-object _meta as empty to answer -32602, but the header check runs first.
• Legacy requests: call_tool and build_server_context call dig(:_meta, :progressToken),
  so tools/call and prompts/get answer -32603 Internal error. tools/list already ignores the value.

A non-object _meta carries no protocol version and no progress token, so these reads now skip it:

• the 2026-07-28 path answers HTTP 400 -32602 from envelope validation;
• legacy tools/call and prompts/get ignore it, as tools/list does and as RequestEnvelope.modern? does.

Similar to #609 / #616 (a malformed Accept member answered 500).

How Has This Been Tested?

• test/mcp/server/transports/streamable_http_transport_test.rb: a modern POST with a string or array _meta answers 400 with -32602.
• test/mcp/server_test.rb: tools/call and prompts/get with a string or array _meta run normally.
• Both tests fail without the change (500 instead of 400; -32603 "Internal error calling tool"). rake test and rake rubocop pass.

Breaking Changes

None. Requests with an object _meta are handled as before. Only requests that raised now get a client error (2026-07-28) or are served (legacy).

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