Module headers 

The standard request headers HTTP transports carry from 2026-07-28 onward (SEP-2243), and the validation a server owes them.

Mcp-Method mirrors the body’s method on every request; Mcp-Name mirrors params.name or params.uri on the three methods that address a specific tool, prompt, or resource. Both are required for compliance with 2026-07-28 and later — earlier protocol revisions do not define them, so validate only requires them of a request whose MCP-Protocol-Version header declares a revision that does.

Their whole purpose is redundancy: an intermediary — a load balancer, a router, a rate limiter — can act on the header without parsing the body, so the two must agree or different components end up working from different sources of truth. That is why the client’s “what should I send” and the server’s “does this match” are the same derivation, and why they live in one module rather than one on each side of the crate.

This is not part of handle_request

crate::handle_request takes a decoded request and never sees a header — deliberately, since it is transport-agnostic and does no I/O. Headers belong to the transport, so the transport is what calls validate, which is a free function for exactly that reason.

Both halves of a wire, one derivation

// Client: send what the body implies.
for (name, value) in headers::standard_headers(message) {
    // request.set_header(name, value)
    let _ = (name, value);
}
// Server: check the headers that arrived against the same derivation.
if let Err(error) = headers::validate(message, |name| {
    request_headers.get(name).map(String::as_str)
}) {
    // 400 Bad Request + JsonRpcResponse::error(id, error)
    let _ = error;
}

Constants

MCP_METHOD

The header naming the body’s method. Required on every request.

MCP_NAME

The header naming the addressed tool, prompt, or resource.

MCP_PROTOCOL_VERSION

The header carrying the client’s declared protocol version, sent on every HTTP request since 2025-06-18.

Functions

decode_value

The inverse of encode_value: unwrap a sentinel-encoded value, or hand back what was there.

encode_value

A header value per the specification’s Value Encoding rules: as-is when it can ride raw, and =?base64?<base64 of the UTF-8 bytes>?= when it cannot.

name_for

The Mcp-Name source value for a method, or None if that method does not address a specific thing.

standard_headers

The headers a client must send alongside message, already encoded.

validate

Check that the headers a request arrived with agree with its body.