[Server][Client][Transport] SEP-2243: Emit Mcp-Method and Mcp-Name HTTP headers

[chr-hertel]

Implements SEP-2243 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #336.

Spec summary

Surfaces the JSON-RPC method and target name as HTTP headers (e.g. Mcp-Method: tools/call, Mcp-Name: <tool name>) so middleboxes (proxies, gateways, observability) can route/trace/prioritize without parsing the body. Gated on protocol version — non-breaking.

PHP SDK changes

• src/Server/Transport/StreamableHttpTransport.php — set/echo Mcp-Method / Mcp-Name headers on responses; read them on requests.
• Add a small middleware under src/Server/Transport/Http/Middleware/ (e.g. McpMethodHeaderMiddleware.php).
• src/Client/Transport/HttpTransport.php — set Mcp-Method and Mcp-Name on outgoing client POSTs.
• Gate on negotiated protocol version (works with SEP-2575's per-request version).
• Update conformance/inspector tests.

Related

• Umbrella [2026-07-28] Transport & observability headers/metadata #336
• Gated by SEP-2575 protocol version (umbrella [2026-07-28] Stateless Core overhaul #334)