## Motivation and Context
The modern Streamable HTTP path validated the SEP-2243 mirror headers only when present:
an absent `Mcp-Method` or `Mcp-Name` was silently tolerated. The 2026-07-28 specification
requires `Mcp-Method` on every request and `Mcp-Name` on `tools/call`, `resources/read`,
and `prompts/get`, and lists a missing required standard header among the validation failures
a server MUST reject; the TypeScript SDK enforces presence with `-32020` the same way.
Tolerating absence also defeats the headers' purpose silently: they exist so intermediaries
can route and inspect requests without parsing bodies.
A modern POST without `Mcp-Method`, or a name-bearing modern POST whose body carries
a target name without `Mcp-Name`, now answers HTTP 400 with `-32020` naming the missing header,
exactly like a mismatch. `Mcp-Name` stays unrequired when the body carries no target name,
matching the TypeScript SDK. The bundled `MCP::Client::HTTP` already sends both headers on
every modern request, so SDK-to-SDK traffic is unaffected.
## How Has This Been Tested?
New tests in `test/mcp/server/transports/streamable_http_transport_test.rb` cover
the missing `Mcp-Method` and missing `Mcp-Name` rejections, asserting the `-32020` code
and the header name in the message. The `modern_rack_request` helper now mirrors
a conforming client by deriving `Mcp-Method` from the body (with `method_header: nil` to omit it),
so the existing modern-path tests exercise the requirement on every request.
## Breaking Changes
None for conforming clients, which must already send these headers.
Hand-rolled modern clients that omitted them are now rejected with `-32020` instead of being served;
add the headers, or use the legacy lifecycle, to proceed.
5f3a131e1a