diff --git a/src/google/adk/flows/llm_flows/functions.py b/src/google/adk/flows/llm_flows/functions.py
index 259d40b6b6..7bf597d29a 100644
--- a/src/google/adk/flows/llm_flows/functions.py
+++ b/src/google/adk/flows/llm_flows/functions.py
@@ -302,7 +302,7 @@ def build_auth_request_event(
         args=AuthToolArguments(
             function_call_id=function_call_id,
             auth_config=auth_config,
-        ).model_dump(exclude_none=True, by_alias=True),
+        ).model_dump(mode='json', exclude_none=True, by_alias=True),
     )
     long_running_tool_ids.add(request_euc_function_call.id)
     parts.append(types.Part(function_call=request_euc_function_call))
diff --git a/tests/unittests/flows/llm_flows/test_functions_request_euc.py b/tests/unittests/flows/llm_flows/test_functions_request_euc.py
index f1e1d1f610..2ab97af07c 100644
--- a/tests/unittests/flows/llm_flows/test_functions_request_euc.py
+++ b/tests/unittests/flows/llm_flows/test_functions_request_euc.py
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import json
 from typing import Any
 from typing import Optional
 
@@ -153,6 +154,68 @@ def call_external_api2(tool_context: ToolContext) -> Optional[int]:
     )
 
 
+def test_function_request_euc_args_are_json_serializable():
+  responses = [
+      [
+          types.Part.from_function_call(name='call_external_api', args={}),
+      ],
+      [
+          types.Part.from_text(text='response1'),
+      ],
+  ]
+
+  auth_config = AuthConfig(
+      auth_scheme=OAuth2(
+          flows=OAuthFlows(
+              authorizationCode=OAuthFlowAuthorizationCode(
+                  authorizationUrl='https://accounts.google.com/o/oauth2/auth',
+                  tokenUrl='https://oauth2.googleapis.com/token',
+                  scopes={
+                      'https://www.googleapis.com/auth/calendar': (
+                          'See, edit, share, and permanently delete all the'
+                          ' calendars you can access using Google Calendar'
+                      )
+                  },
+              )
+          )
+      ),
+      raw_auth_credential=AuthCredential(
+          auth_type=AuthCredentialTypes.OAUTH2,
+          oauth2=OAuth2Auth(
+              client_id='oauth_client_id',
+              client_secret='oauth_client_secret',
+          ),
+      ),
+  )
+
+  mock_model = testing_utils.MockModel.create(responses=responses)
+
+  def call_external_api(tool_context: ToolContext) -> Optional[int]:
+    tool_context.request_credential(auth_config)
+
+  agent = Agent(
+      name='root_agent',
+      model=mock_model,
+      tools=[call_external_api],
+  )
+  runner = testing_utils.InMemoryRunner(agent)
+  events = runner.run('test')
+
+  request_euc_function_call = events[1].content.parts[0].function_call
+  assert (
+      request_euc_function_call.name == functions.REQUEST_EUC_FUNCTION_CALL_NAME
+  )
+
+  # The in-memory event args must be JSON-serializable. A python-mode dump
+  # leaves auth_scheme.type as a live SecuritySchemeType enum, which breaks any
+  # consumer that json.dumps the event before it round-trips the session DB.
+  json.dumps(request_euc_function_call.args)
+  assert (
+      request_euc_function_call.args['authConfig']['authScheme']['type']
+      == 'oauth2'
+  )
+
+
 def test_function_get_auth_response():
   id_1 = 'id_1'
   id_2 = 'id_2'