Mcp-Name accepts an orphan header, while Mcp-Param-* rejects it as a routing spoof

Empieza en src/mcp/shared/inbound.py, en la rama classify_inbound_request alrededor de la línea 447, y después lee tests/shared/test_inbound.py, especialmente test_validate_mcp_param_headers_rejects_orphan_header_for_absent_or_null_argument y el test existente de Mcp-Name. Confirma con los maintainers la postura prevista; se considera terminado si hay una prueba de regresión y la validación correspondiente, o un docstring de prueba que explique la asimetría intencionada.

What

validate_mcp_param_headers rejects an orphan Mcp-Param-* header, one present when the body argument is absent or null. classify_inbound_request accepts an orphan Mcp-Name header.

The Mcp-Param-* posture is deliberate and documented, in tests/shared/test_inbound.py:

"""SDK-defined posture on a spec gap: an orphan header is the routing-spoof case; go rejects too, ts skips."""

The Mcp-Name path has no equivalent branch. src/mcp/shared/inbound.py, line 447 on main:

body_value = cast("Mapping[str, Any]", body["params"]).get(name_key)
if body_value is not None and decode_header_value(headers.get(MCP_NAME_HEADER)) != body_value:

When body_value is None the whole check is skipped, including the case where a header claims a name.

Reproducer

from mcp.shared.inbound import (
    MCP_METHOD_HEADER, MCP_NAME_HEADER, MCP_PROTOCOL_VERSION_HEADER,
    InboundModernRoute, classify_inbound_request,
)

V = "2026-07-28"
body = {"jsonrpc": "2.0", "id": 1, "method": "tools/call", "params": {
    "arguments": {},
    "_meta": {"io.modelcontextprotocol/protocolVersion": V,
              "io.modelcontextprotocol/clientCapabilities": {}}}}

result = classify_inbound_request(body, headers={
    MCP_PROTOCOL_VERSION_HEADER: V,
    MCP_METHOD_HEADER: "tools/call",
    MCP_NAME_HEADER: "ping",          # claims a tool the body never names
})
assert isinstance(result, InboundModernRoute)   # passes today

Same shape for resources/read with uri.

Why this looks like an inconsistency rather than a decision

The existing Mcp-Name test covers a different case:

def test_header_rung_does_not_require_name_header_when_body_omits_the_named_param() -> None:
    """SDK-defined: ... the param's absence is INVALID_PARAMS later, not HEADER_MISMATCH here."""

matching_headers omits Mcp-Name when the body lacks the param, so this pins "body omits the param and no header is sent". That rationale reads correctly for that case: nobody asserted a name, so the defect is the missing param and INVALID_PARAMS is the right answer.

It does not obviously extend to a client that did assert one. There the header and body disagree, which is the condition the routing-spoof posture exists for, and Mcp-Name is the header an intermediary is most likely to route on, since it names the tool or resource rather than a secondary parameter.

To be clear about impact: the malformed request fails INVALID_PARAMS downstream either way, so nothing executes. The concrete cost is an intermediary routing or rate-limiting on a name the body never contained.

Ask

Is the Mcp-Param-* posture intended to apply to Mcp-Name? If so the fix mirrors the sibling branch, rejecting when body_value is None and the header is present, with a test alongside test_validate_mcp_param_headers_rejects_orphan_header_for_absent_or_null_argument.

If the asymmetry is intentional, a note in that test's docstring saying so would prevent the next reader drawing the same conclusion I did.

Happy to open the PR either way once you say which you would prefer.

AI disclosure per AI_POLICY: I directed this investigation and reviewed the result. The research and drafting were done by Claude Code, which found this while probing SEP-2243 handling with adversarial tests, having implemented the same validation in a TypeScript resource server. 87 probes against encode_header_value and decode_header_value found no defects there, including non-canonical base64, invalid UTF-8 and sentinel-literal collisions, so this is the only finding. Verified against main rather than the released 2.0.0 wheel.