Claude Code System Prompts

SEP-2243 Mcp-Param-* headers are validated against the named tool’s x-mcp-header declarations and the body arguments after the tool registry is known and before dispatch reaches the handler; a missing/disagreeing/malformed header is rejected 400 / -32020 with the same shape as the standard-header cross-checks. The documented order (after method resolution and params validation) is preserved observably only when the body arguments would otherwise validate: the check runs pre-dispatch, so a tools/call that fails BOTH this rung and a dispatch-time rung (e.g. order-6 request-params, -32602) is answered by this gate first with 400 / -32020, not by the earlier-ordered rung.