Support RFC 8693 token exchange for enterprise IdP flows (SEP-990) (#2988)

Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

src/mcp/server/auth/handlers/register.py

@@ -12,7 +12,7 @@ from mcp.server.auth.errors import stringify_pydantic_error
 from mcp.server.auth.json_response import PydanticJSONResponse
 from mcp.server.auth.provider import OAuthAuthorizationServerProvider, RegistrationError, RegistrationErrorCode
 from mcp.server.auth.settings import ClientRegistrationOptions
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata
+from mcp.shared.auth import JWT_BEARER_GRANT_TYPE, OAuthClientInformationFull, OAuthClientMetadata
 
 # this alias is a no-op; it's just to separate out the types exposed to the
 # provider from what we use in the HTTP handler
@@ -79,6 +79,21 @@ class RegistrationHandler:
                 status_code=400,
             )
 
+        # SEP-990 §5.1 / draft-ietf-oauth-identity-assertion-authz-grant §8.1: the ID-JAG flow is
+        # for confidential clients provisioned out of band. Refuse to grant it through DCR so a
+        # self-registered client cannot reach the identity-assertion provider hook.
+        if JWT_BEARER_GRANT_TYPE in client_metadata.grant_types:
+            return PydanticJSONResponse(
+                content=RegistrationErrorResponse(
+                    error="invalid_client_metadata",
+                    error_description=(
+                        f"grant_types must not include '{JWT_BEARER_GRANT_TYPE}'; "
+                        "the identity-assertion grant requires a pre-registered client"
+                    ),
+                ),
+                status_code=400,
+            )
+
         # The MCP spec requires servers to use the authorization `code` flow
         # with PKCE
         if "code" not in client_metadata.response_types: