OAuth client sends client_id in token body under client_secret_basic (strict servers reject as multiple auth methods)

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Description

Under token_endpoint_auth_method = client_secret_basic, the OAuth client sends the client credentials in the HTTP Authorization: Basic header and also leaves client_id in the token-exchange POST body. Strict token endpoints reject this as two authentication methods in a single request.

OAuthClientProvider.prepare_token_auth (src/mcp/client/auth/oauth2.py) strips only client_secret from the body:

headers["Authorization"] = f"Basic {encoded_credentials}"
# Don't include client_secret in body for basic auth
data = {k: v for k, v in data.items() if k != "client_secret"}   # client_id remains in the body

Expected: with Basic auth, the request presents exactly one authentication method — the Authorization header — with no client credentials or identifier duplicated in the body.

Actual: client_id stays in the body.

Against Notion's MCP server (https://mcp.notion.com/mcp), where dynamic client registration returns token_endpoint_auth_method = client_secret_basic, the token exchange fails with:

Token exchange failed (400): "Client must not use multiple authentication methods"

There is also a secondary, misleading cascade (not the root cause): the failed first attempt starts a second OAuth flow that reuses the loopback callback port, and stale browser tabs redirect old approvals into it, surfacing OAuthFlowError: State parameter mismatch.

I want to frame this as an interop issue rather than a compliance accusation. RFC 6749 §2.3.1 presents the HTTP Basic header and body credentials as alternatives; sending only the header is unambiguously valid, removes the ambiguity, and loses no information (the client_id is already present, base64-encoded, in the Basic header). Stripping client_id from the body makes the SDK work against strict servers like Notion at no cost to lenient ones.

The one-line fix in the client_secret_basic branch:

data = {k: v for k, v in data.items() if k not in ("client_secret", "client_id")}

Note: the existing test test_basic_auth_token_exchange currently asserts client_id ... in content # client_id still in body — this behaviour was codified in the same commit that introduced Basic auth support (#1334), so the fix inverts that assertion.

Update: this is the CLIENT half — it depends on the server side (#1847)

The one-line client change cannot land alone. Body client_id is load-bearing on this SDK's own server side in three places, so a client that omits it fails against any server built on this SDK:

1. ClientAuthenticator.authenticate_request (src/mcp/server/auth/middleware/client_auth.py) requires client_id in the body.
2. TokenHandler's request models (AuthorizationCodeRequest / RefreshTokenRequest, src/mcp/server/auth/handlers/token.py) declare client_id: str as required — a missing value returns a Pydantic 400 invalid_request: client_id: Field required.
3. The grant-ownership checks compare auth_code.client_id != token_request.client_id (and the refresh equivalent).

The matching server-side change is exactly what #1847 already implements: it makes body client_id optional, re-sources the ownership checks from the authenticated client_info.client_id, and reads client_id from the Basic header. So the two are complementary halves of one fix:

• Client-only (this issue's change) → the SDK's own server rejects with Missing client_id. Verified.
• Client change + #1847's server change → the previously-failing Basic-auth token exchange passes end-to-end. Verified locally by grafting #1847's server diff onto main and running the client + interaction OAuth suites (green). The only residual failures were #1847's own error-message test updates, which appear stranded by a tests/server/fastmcp → tests/server/mcpserver directory rename on main (i.e. #1847 looks like it needs a rebase) — unrelated to the client change.

Questions for maintainers:

1. How would you like the client change sequenced against #1847 — folded into #1847, or as a linked follow-up PR that merges after it?
2. Which branch(es) — main (v2), and/or a v1.x backport? The live breakage is on v1 (1.27.x) via Notion.

I'm happy to open the client-side PR (one-line change + updated tests) once there's buy-in and a preferred sequencing.

Example Code

# Any client using this SDK against Notion MCP, where dynamic registration
# returns token_endpoint_auth_method = client_secret_basic. For example via mcp2cli:
#   mcp2cli --mcp https://mcp.notion.com/mcp --oauth --list
#
# 1. Complete the browser authorization.
# 2. The token exchange returns:
#    Token exchange failed (400): "Client must not use multiple authentication methods"

Python & MCP Python SDK

Python 3.10
mcp: reproduced on 1.27.2 and 1.28.1; bug also present on main @ 3a6f299
Server: Notion MCP (https://mcp.notion.com/mcp), token_endpoint_auth_method = client_secret_basic

This report was prepared with AI assistance. I've reviewed the analysis and reproduced the failure myself, and can speak to the change directly.

Start with OAuthClientProvider.prepare_token_auth in src/mcp/client/auth/oauth2.py and test_basic_auth_token_exchange. Read ClientAuthenticator.authenticate_request in src/mcp/server/auth/middleware/client_auth.py, the request models in src/mcp/server/auth/handlers/token.py, and issue #1847; done requires the client and matching server changes to pass the OAuth interaction tests.