500 error in text generator with Gemini models

[progamesigner]

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Dify version

1.3.1

Cloud or Self Hosted

Self Hosted (Source), Self Hosted (Docker)

Steps to reproduce

1. Create a text generator from blank
2. Use Gemini models (I am using Gemini 2.0 Flash / Gemini 2.5 Flash Preview)
3. Type something to ask model

(Use other models work, such DeepSeek or models from OpenAI)

✔️ Expected Behavior

Should be able to response texts

❌ Actual Behavior

Do nothing with API error:

[2025-05-15 06:32:18 +0000] [31] [ERROR] Error handling request /console/api/apps/0c70a1c6-db89-4cb6-9c1d-049fb2240f8d/completion-messages
Traceback (most recent call last):
  File "/app/api/.venv/lib/python3.12/site-packages/gunicorn/workers/base_async.py", line 54, in handle
    self.handle_request(listener_name, req, client, addr)
  File "/app/api/.venv/lib/python3.12/site-packages/gunicorn/workers/ggevent.py", line 127, in handle_request
    super().handle_request(listener_name, req, sock, addr)
  File "/app/api/.venv/lib/python3.12/site-packages/gunicorn/workers/base_async.py", line 114, in handle_request
    for item in respiter:
                ^^^^^^^^
  File "/app/api/.venv/lib/python3.12/site-packages/werkzeug/wsgi.py", line 256, in __next__
    return self._next()
           ^^^^^^^^^^^^
  File "/app/api/.venv/lib/python3.12/site-packages/werkzeug/wrappers/response.py", line 32, in _iter_encoded
    for item in iterable:
                ^^^^^^^^
  File "/app/api/.venv/lib/python3.12/site-packages/flask/helpers.py", line 125, in generator
    yield from gen
  File "/app/api/libs/helper.py", line 203, in generate
    yield from response
  File "/app/api/core/app/features/rate_limiting/rate_limit.py", line 116, in __next__
    return next(self.generator)
           ^^^^^^^^^^^^^^^^^^^^
  File "/app/api/core/app/apps/base_app_generator.py", line 155, in gen
    for message in generator:
                   ^^^^^^^^^
  File "/app/api/core/app/apps/base_app_generate_response_converter.py", line 25, in _generate_full_response
    yield from cls.convert_stream_full_response(response)
  File "/app/api/core/app/apps/completion/generate_response_converter.py", line 61, in convert_stream_full_response
    for chunk in stream_response:
                 ^^^^^^^^^^^^^^^
  File "/app/api/core/app/task_pipeline/easy_ui_based_generate_task_pipeline.py", line 182, in _to_stream_response
    for stream_response in generator:
                           ^^^^^^^^^
  File "/app/api/core/app/task_pipeline/easy_ui_based_generate_task_pipeline.py", line 221, in _wrapper_process_stream_response
    for response in self._process_stream_response(publisher=publisher, trace_manager=trace_manager):
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/api/core/app/task_pipeline/easy_ui_based_generate_task_pipeline.py", line 317, in _process_stream_response
    current_content += cast(str, delta_text)
TypeError: can only concatenate str (not "list") to str