Support client_credentials flow with JWT and Basic auth

[pcarleton]

Summary

Implements SEP-1046 OAuth client_credentials flow support with simplified providers for machine-to-machine authentication.

New OAuth Providers

• ClientCredentialsOAuthProvider: For client_credentials grant with client_id + client_secret

  • Supports client_secret_basic (default) and client_secret_post auth methods
  • Sets client_info directly, bypassing dynamic client registration

• PrivateKeyJWTOAuthProvider: For client_credentials grant with private_key_jwt authentication (RFC 7523 Section 2.2)

  • Takes an assertion_provider callback that receives the audience (authorization server's issuer identifier per RFC 7523bis) and returns a JWT
  • Designed for workload identity federation (GCP, AWS IAM, Azure AD)

• SignedJWTParameters: Helper class for SDK-signed JWT assertions

  • create_assertion_provider() returns a callback for use with PrivateKeyJWTOAuthProvider

• static_assertion_provider(): Helper for pre-built JWTs that don't need the audience parameter

Deprecation

RFC7523OAuthClientProvider is now deprecated with a DeprecationWarning.

The original implementation incorrectly used RFC 7523 Section 2.1 (jwt-bearer authorization grant where the JWT itself is the authorization) instead of the intended Section 2.2 (private_key_jwt client authentication with grant_type=client_credentials).

Use ClientCredentialsOAuthProvider or PrivateKeyJWTOAuthProvider instead.

Example Usage

# Simple client credentials with client_id + secret
provider = ClientCredentialsOAuthProvider(
    server_url="https://api.example.com",
    storage=my_token_storage,
    client_id="my-client-id",
    client_secret="my-client-secret",
)

# Private key JWT with workload identity federation
async def get_workload_identity_token(audience: str) -> str:
    return await fetch_token_from_identity_provider(audience=audience)

provider = PrivateKeyJWTOAuthProvider(
    server_url="https://api.example.com",
    storage=my_token_storage,
    client_id="my-client-id",
    assertion_provider=get_workload_identity_token,
)

# Private key JWT with SDK-signed assertions
jwt_params = SignedJWTParameters(
    issuer="my-client-id",
    subject="my-client-id",
    signing_key=private_key_pem,
)
provider = PrivateKeyJWTOAuthProvider(
    server_url="https://api.example.com",
    storage=my_token_storage,
    client_id="my-client-id",
    assertion_provider=jwt_params.create_assertion_provider(),
)

Testing

• Unit tests with 100% coverage for new providers
• Conformance tests pending

[felixweinberger]

Left a comment regarding something that's pre-existing, otherwise LGTM

[felixweinberger]

Not an issue with this PR, but something pre-existing @claude pointed out here as a discrepancy between TS & Python:

prepare_token_auth doesn't include client_id for the client_secret_post auth method. According to RFC 6749 both _id and _secret should be in the request body for this method.

TS does that as it does different things for applyBasicAuth and applyPostAuth: https://github.com/modelcontextprotocol/typescript-sdk/blob/de41e4703c937753de5299b11255656efae59bc4/src/client/auth.ts#L310

While Python seems to use prepare_token_auth in both cases.

Might be worth a follow-up?

[pcarleton]

huh... yea we should fix that! i've got a draft conformance test for it here which should catch this:
modelcontextprotocol/conformance#48

the client credentials one uses client_secret_basic which would be why this didn't catch it