fix(auth): allow configuring OAuth prompt parameter

AuthHandler always sent prompt=consent when generating OAuth authorization URLs, which blocked silent-auth flows even when the tool configuration needed a different prompt mode.

Add an optional prompt field to OAuth2Auth, preserve consent as the default, and honor custom values such as prompt=none when building the authorization URL. Extend auth handler tests to cover both the default and a custom override.

Author: Raghu

src/google/adk/auth/auth_credential.py

@@ -87,6 +87,7 @@ class OAuth2Auth(BaseModelWithConfig):
   expires_at: int | None = None
   expires_in: int | None = None
   audience: str | None = None
+  prompt: str | None = None
   code_verifier: str | None = None
   code_challenge_method: str | None = None
   token_endpoint_auth_method: (

src/google/adk/auth/auth_handler.py

@@ -197,7 +197,7 @@ def generate_auth_uri(
     )
     params = {
         "access_type": "offline",
-        "prompt": "consent",
+        "prompt": auth_credential.oauth2.prompt or "consent",
     }
     if auth_credential.oauth2.audience:
       params["audience"] = auth_credential.oauth2.audience

tests/unittests/auth/test_auth_handler.py

@@ -66,6 +66,8 @@ def create_authorization_url(self, url, **kwargs):
     params = f"client_id={self.client_id}&scope={self.scope}"
     if kwargs.get("audience"):
       params += f"&audience={kwargs.get('audience')}"
+    if kwargs.get("prompt"):
+      params += f"&prompt={kwargs.get('prompt')}"
     return f"{url}?{params}", "mock_state"
 
   def fetch_token(
@@ -250,6 +252,25 @@ def test_generate_auth_uri_with_audience_and_prompt(
     result = handler.generate_auth_uri()
 
     assert "audience=test_audience" in result.oauth2.auth_uri
+    assert "prompt=consent" in result.oauth2.auth_uri
+
+  @patch("google.adk.auth.auth_handler.OAuth2Session", MockOAuth2Session)
+  def test_generate_auth_uri_with_custom_prompt(
+      self, openid_auth_scheme, oauth2_credentials
+  ):
+    """Test generating an auth URI with a custom prompt override."""
+    oauth2_credentials.oauth2.prompt = "none"
+    exchanged = oauth2_credentials.model_copy(deep=True)
+
+    config = AuthConfig(
+        auth_scheme=openid_auth_scheme,
+        raw_auth_credential=oauth2_credentials,
+        exchanged_auth_credential=exchanged,
+    )
+    handler = AuthHandler(config)
+    result = handler.generate_auth_uri()
+
+    assert "prompt=none" in result.oauth2.auth_uri
 
   @patch("google.adk.auth.auth_handler.OAuth2Session", MockOAuth2Session)
   def test_generate_auth_uri_openid(