# Fix: an Mcp-Method header disagreeing with the body method draws -32020

> Web-audit fix skill for the `mcp-modern-header-mismatch` check (MCP, SHOULD).

## Goal

Validate the SEP-2243 header mirror between Mcp-Method and the body method.

## Fix

Reject a request whose `Mcp-Method` header names a different method than the JSON-RPC body
with `error.code: -32020` (header mismatch). The SEP-2243 headers exist so infrastructure
can route without parsing bodies; a disagreeing pair means the routing layer and the
handler would act on different methods, so it must be refused rather than silently
resolved in either direction.

## Resources

- [MCP lifecycle (2026-07-28)](https://modelcontextprotocol.io/specification/2026-07-28/basic/lifecycle)

## Copy-paste prompt

Paste this into your coding agent. [Your audit](https://anc.dev/audit) adds what it observed for this check:

```text
Goal: Validate the SEP-2243 header mirror between Mcp-Method and the body method
Fix: Reject a request whose `Mcp-Method` header names a different method than the JSON-RPC body with `error.code: -32020` (header mismatch). The SEP-2243 headers exist so infrastructure can route without parsing bodies; a disagreeing pair means the routing layer and the handler would act on different methods, so it must be refused rather than silently resolved in either direction.
Skill: https://anc.dev/fix/mcp-modern-header-mismatch
Docs: https://modelcontextprotocol.io/specification/2026-07-28/basic/lifecycle
```

## Verify

Re-run the audit at [https://anc.dev/audit](https://anc.dev/audit) or call the `audit_website` MCP tool; the `mcp-modern-header-mismatch` check should report `pass`.