Claude Code System Prompts

SEP-2243 standard Mcp-Method / Mcp-Name headers — presence, sentinel decoding, and Mcp-Name ↔ body cross-check — are validated by the HTTP entry on a modern-classified request after the supported-revision gate and before dispatch. The classifier’s own header-mismatch cells (protocol-version, Mcp-Method mismatch) stay on the edge era-classification rung; this rung carries the entry-layer presence/Mcp-Name half. Evaluated before the capability gate, the factory call, and the Mcp-Param-* rung so a request that fails several rungs is answered by the standard-header rung first. The documented order (after method-registry 5 and request-params 6) is NOT the observed precedence: serveModern evaluates this rung immediately after the supported-revision gate, so a request that also fails a dispatch rung is answered here before the dispatch rungs (5–6) are consulted.