Empty response when the finish reason is MAX_TOKENS

[UMuktesh]

Description of the bug:

When using max_output_tokens in generate_content to limit the output of the model, the following error is thrown when trying to access response.text, whereas its expected to get the output till MAX_TOKENS.
 
ValueError: The response.text quick accessor only works when the response contains a valid Part, but none was returned. Check the candidate.safety_ratings to see if the response was blocked.

The actual response:

response:
GenerateContentResponse(
    done=True,
    iterator=None,
    result=glm.GenerateContentResponse({'candidates': [{'finish_reason': 2, 'index': 0, 'safety_ratings': [{'category': 9, 'probability': 1, 'blocked': False}, {'category': 8, 'probability': 1, 'blocked': False}, {'category': 7, 'probability': 1, 'blocked': False}, {'category': 10, 'probability': 1, 'blocked': False}], 'token_count': 0, 'grounding_attributions': []}]}),
)

This is the code snippet, taken from https://ai.google.dev/tutorials/python_quickstart#generation_configuration, which shows expected behaviour is to get upto 20 tokens then cut off due to MAX_TOKENS.

import google.generativeai as genai

genai.configure(api_key='***')
model = genai.GenerativeModel('gemini-pro')
response = model.generate_content(
    'Tell me a story about a magic backpack.',
    generation_config=genai.types.GenerationConfig(
        # Only one candidate for now.
        candidate_count=1,
        # stop_sequences=['x'],     # Commenting this part as sometimes it works due to 'x' being present in output and model cuts off before MAX_TOKENS is reached.
        max_output_tokens=20,
        temperature=1.0)
)

text = response.text

if response.candidates[0].finish_reason.name == "MAX_TOKENS":
    text += '...'

print(text)

Actual vs expected behavior:

https://ai.google.dev/tutorials/python_quickstart#generation_configuration
Similar output as of here, not error when trying to access response.text, but to get the output of approx 20 tokens.

Works in AI Studio

Any other information you'd like to share?

Package information
Name: google-generativeai
Version: 0.5.0

Streaming partially works, but the last chunk with MAX_TOKENS is still empty (Could be intended behaviour)

[singhniraj08]

@UMuktesh,

Thank you for reporting this issue. I am able to replicate this error. While setting max_output_tokens to None, we are getting response from the model, but changing max_output_tokens to integer value results in no output text from model. Ref: gist
@MarkDaoust, This looks like a bug in SDK. Please have a look. Thank you!

[ClayGriifith]

Facing a similar issue and unclear on whether or not this is by-design, but I'm getting an empty 'text' response when finishReason is MAX_TOKENS.

I'd expect at least a partial response, if not (ideally) a response that is actually within the token threshold assigned. Getting nothing at all is confusing.

Can somebody clarify if this is intended behavior?

[NicolaDonelli]

Same as above.

[lgnashold]

Same as above.

[homer6]

I ran into this too and it was very hard to track down. The exception was very misleading.

[scissorstail]

"The response.text quick accessor only works when the response contains a valid Part, but none was returned. Check the candidate.safety_ratings to see if the response was blocked."

[MarkDaoust]

This is fixed.

[Akshay0799]

Did anyone manage to find a reason for why this was happening?
I am using Gemini 2.5-flash-preview and I still face this problem sometimes and I can't seem to understand why this happens at times.
There's no response from the LLM, the number of output_tokens in the usage_metadata is 0 and stop_reason is 'MAX_TOKENS'

[approxit1]

Also experiencing this problem with Gemini 2.5-flash-preview