[Feat][CLI] enforce-include-usage

[max-wittig]

Currently, when streaming the usage is always null. This prevents the usage of limits per user and is a bit unexpected as without using streaming, the usage is always returned.

This can be used, if vllm is between a router, such as vllm-router or litellm and serves many users and is important to detect abuse, divide costs etc.

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Always returns tokens which can be used by various systems for:

• Detecting abuse by users
• Billing or dividing cost company internally

In addition this align the behavior with the non-streaming mode where the usage is always returned

Test Plan

1. Setup this commit and run vllm
2. Send the follow request and notice how usage is returned in the last segment

curl --request POST \
  --url https://yourhost.example.com/llm/v1/completions \
  --header 'apikey: {{token}}' \
  --header 'content-type: application/json' \
  --data '{
  "model": "qwen3-30b-a3b",
  "max_tokens": 100,
  "presence_penalty": 0,
  "frequency_penalty": 0,
  "temperature": 0.1,
  "prompt": "def write_hello():",
  "stream": true
}'

---

data: {"id":"cmpl-ef83ad46-8ca3-49dc-8371-790f281f60a1#8733163","object":"text_completion","created":1750142471,"model":"qwen3-30b-a3b","choices":[],"usage":{"prompt_tokens":4,"total_tokens":104,"completion_tokens":100}}

Test Result

(Optional) Documentation Update

[gemini-code-assist]

Summary of Changes

Hello @max-wittig, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request adds a new configuration option to enforce the inclusion of usage statistics in API responses, specifically addressing the current limitation where usage is not reported during streaming. This feature is intended to support use cases where vLLM is used behind a router or load balancer and per-request usage tracking is required for billing, rate limiting, or abuse detection.

Highlights

• New Configuration Option: Introduced a new boolean option enforce_include_usage in ModelConfig to control whether usage statistics are always included in API responses, even for streaming requests.
• Streaming Usage Inclusion: Modified the streaming response generators (chat_completion_stream_generator and completion_stream_generator) to include usage statistics in the final chunk if the new enforce_include_usage option is enabled, overriding the default behavior for streaming.
• API Server Integration: The new enforce_include_usage configuration is read from the model config and passed down to the OpenAIServingChat and OpenAIServingCompletion instances during API server initialization.

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[gemini-code-assist]

Code Review

This pull request introduces a new configuration option enforce_include_usage to consistently include usage statistics in API responses, particularly for streaming scenarios. The changes are well-contained and logically propagate the new flag through the relevant serving classes.

Key areas for improvement:

• Please add a docstring for the new enforce_include_usage field in ModelConfig to clarify its purpose.
• Consider if Optional[bool] is strictly necessary for enforce_include_usage in ModelConfig or if bool would be more appropriate.
• The __init__ method parameter enforce_include_usage in OpenAIServingChat is missing its type hint.

Additionally, please ensure the PR description (purpose, test plan, test results) is thoroughly filled out. Adding unit tests to verify the behavior of this new flag, especially its interaction with stream_options.include_usage, would be beneficial for long-term maintainability.

[max-wittig]

Local testing blocked by #15985

[aarnphm]

some light comments.

[max-wittig]

@aarnphm Thanks for the review! Let me know, if I should squash my commits or if any other changes are required!

[aarnphm]

For some reason I forgot to include these comments. Thanks for the perseverance.

[aarnphm]

Actually this seems to be strictly frontend, we proably don't want it to be in model_config here

[aarnphm]

I think it is better to include it in the CLI, see the enable-prompt-tokens-details options.

maybe we can also have a enable-force-include-usage here.

[aarnphm]

fwiw I think we should also add an options to the request level, where some might want certain request to have usage, some requests without. Given that usage will affect throughput quite a lot.

[max-wittig]

From our perspective as an operator, we always want to see the usage. I'm not sure how people would use the flag with half usage data. That might be unexpected.

[aarnphm]

Ok, I'm fine with having this as the CLI.

[max-wittig]

@aarnphm Thank you! Is there a place where I could put some docs for this feature?

[aarnphm]

No need to, on https://docs.vllm.ai/en/latest/cli/index.html we mentioned for --help, which you already include the helpstring for it.