Output Length field (max_output_tokens parameter) does not affect response, setting it causes empty or missing outputs #89
Description
Description of the bug:
When using the official Python Gemini SDK, setting the max_output_tokens (called “Output Length” in UI) parameter in the GenerateContentConfig object does not work as expected.
If I provide a value for max_output_tokens (e.g., 400 or 1024), the model response is empty or contains no text, and response.candidates[0].content.parts is either empty or missing.
This happens even for very short or simple prompts and regardless of the output length requested.
CODE:
from google.genai import types
from google import genai
response = client_gemini.models.generate_content(
model=gemini_model,
contents='hi, is sky blue?',
config=types.GenerateContentConfig(
max_output_tokens=MAX_OUTPUT_TOKENS,
temperature=TEMPERATURE,
),
)
print(response)
OUTPUT:
sdk_http_response=HttpResponse(
headers=<dict len=11>
) candidates=[Candidate(
content=Content(
role='model'
),
finish_reason=<FinishReason.MAX_TOKENS: 'MAX_TOKENS'>,
index=0
)] create_time=None response_id=None model_version='gemini-2.5-pro' prompt_feedback=None usage_metadata=GenerateContentResponseUsageMetadata(
prompt_token_count=7,
prompt_tokens_details=[
ModalityTokenCount(
modality=<MediaModality.TEXT: 'TEXT'>,
token_count=7
),
],
thoughts_token_count=399,
total_token_count=406
) automatic_function_calling_history=[] parsed=None
Actual vs expected behavior:
If I leave max_output_tokens unset (i.e., use the default), responses are returned normally.
CODE:
from google.genai import types
from google import genai
response = client_gemini.models.generate_content(
model=gemini_model,
contents='hi, is sky blue?',
config=types.GenerateContentConfig(
temperature=TEMPERATURE,
),
)
print(response)
OUTPUT
sdk_http_response=HttpResponse(
headers=<dict len=11>
) candidates=[Candidate(
content=Content(
parts=[
Part(
text="""Yes, on a clear day, the sky is blue.
The reason for this is a fascinating bit of physics! Here's a simple breakdown:
1. **Sunlight is White Light:** Sunlight may look yellow or white, but it's actually made up of all the colors of the rainbow (red, orange, yellow, green, blue, violet).
2. **Earth Has an Atmosphere:** Our planet is wrapped in a blanket of gases and tiny particles.
3. **Light Scatters:** When sunlight enters the atmosphere, it bumps into the tiny gas molecules. This causes the light to scatter in all directions.
4. **Blue Scatters Best:** Shorter wavelengths of light (blue and violet) are scattered much more easily and widely by these gas molecules than the longer wavelengths (like red and orange). This effect is called **Rayleigh scattering**.
Because this blue light is scattered all over the sky, that's the color we see when we look up.
### So, why isn't the sky violet?
That's a great question! Violet light has an even shorter wavelength and is scattered even more than blue. However, two things are happening:
* The sun emits less violet light than blue light.
* Our eyes are much more sensitive to blue light than they are to violet.
The combination of these two factors means our eyes and brain perceive the sky as blue.
### When is the sky NOT blue?
* **At Sunrise and Sunset:** The sky often looks red, orange, or pink. This is because the sun is low on the horizon, and its light has to travel through much more of the atmosphere to reach you. By the time it gets to your eyes, most of the blue light has been scattered away, leaving the longer, reddish wavelengths to come straight through.
* **On a Cloudy Day:** The sky looks white or gray. Clouds are made of water droplets, which are much larger than gas molecules. They scatter all colors of light about equally. When all the colors mix back together, we see white."""
),
],
role='model'
),
finish_reason=<FinishReason.STOP: 'STOP'>,
index=0
)] create_time=None response_id=None model_version='gemini-2.5-pro' prompt_feedback=None usage_metadata=GenerateContentResponseUsageMetadata(
candidates_token_count=435,
prompt_token_count=7,
prompt_tokens_details=[
ModalityTokenCount(
modality=<MediaModality.TEXT: 'TEXT'>,
token_count=7
),
],
thoughts_token_count=1392,
total_token_count=1834
) automatic_function_calling_history=[] parsed=None