modelcontextprotocol
diff --git a/src/mcp/server/auth/handlers/register.py b/src/mcp/server/auth/handlers/register.py
@@ -68,7 +68,7 @@ async def handle(self, request: Request) -> Response:
                     ),
                     status_code=400,
                 )
-        if set(client_metadata.grant_types) != {"authorization_code", "refresh_token"}:
+        if not {"authorization_code", "refresh_token"}.issubset(set(client_metadata.grant_types)):
             return PydanticJSONResponse(
                 content=RegistrationErrorResponse(
                     error="invalid_client_metadata",

diff --git a/src/mcp/shared/auth.py b/src/mcp/shared/auth.py
@@ -47,7 +47,7 @@ class OAuthClientMetadata(BaseModel):
     # ie: we do not support client_secret_basic
     token_endpoint_auth_method: Literal["none", "client_secret_post"] = "client_secret_post"
     # grant_types: this implementation only supports authorization_code & refresh_token
-    grant_types: list[Literal["authorization_code", "refresh_token"]] = [
+    grant_types: list[Literal["authorization_code", "refresh_token"] | str] = [
         "authorization_code",
         "refresh_token",
     ]

diff --git a/tests/server/fastmcp/auth/test_auth_integration.py b/tests/server/fastmcp/auth/test_auth_integration.py
@@ -937,6 +937,23 @@ async def test_client_registration_invalid_grant_type(self, test_client: httpx.A
         assert error_data["error"] == "invalid_client_metadata"
         assert error_data["error_description"] == "grant_types must be authorization_code and refresh_token"
 
+    @pytest.mark.anyio
+    async def test_client_registration_with_additional_grant_type(self, test_client: httpx.AsyncClient):
+        client_metadata = {
+            "redirect_uris": ["https://client.example.com/callback"],
+            "client_name": "Test Client",
+            "grant_types": ["authorization_code", "refresh_token", "urn:ietf:params:oauth:grant-type:device_code"],
+        }
+
+        response = await test_client.post("/register", json=client_metadata)
+        assert response.status_code == 201
+        client_info = response.json()
+
+        # Verify client was registered successfully
+        assert "client_id" in client_info
+        assert "client_secret" in client_info
+        assert client_info["client_name"] == "Test Client"
+
     @pytest.mark.anyio
     async def test_client_registration_with_additional_response_types(
         self, test_client: httpx.AsyncClient, mock_oauth_provider: MockOAuthProvider