Implement dual-rate billing for Gemini image-output models

[adambalogh]

Summary

Gemini image-output models (gemini-2.5-flash-image, gemini-3.1-flash-image) bill output tokens at two different rates: image-modality tokens at a premium rate ($30/MTok) and text/thinking tokens at the standard rate ($1.50–$3/MTok). This PR implements proper split-rate billing by:

1. Adding image_output_price_usd field to ModelConfig for dual-rate models
2. Extracting and surfacing reasoning_tokens (thinking) through the usage pipeline
3. Splitting output token billing in compute_session_cost() based on reasoning count
4. Updating Gemini image model pricing to reflect Google's actual dual-rate structure

Key Changes

• model_registry.py:

  • Added image_output_price_usd optional field to ModelConfig for image-modality token pricing
  • Updated GEMINI_2_5_FLASH_IMAGE and GEMINI_3_1_FLASH_IMAGE to use correct dual-rate pricing (text/thinking at output_price_usd, images at image_output_price_usd)

• llm_backend.py:

  • Modified extract_usage() to extract and return reasoning_tokens from output_token_details nested in usage metadata

• pricing.py:

  • Implemented dual-rate billing logic in compute_session_cost(): when image_output_price_usd is set, reasoning tokens are billed at output_price_usd and remaining output tokens (images + captions) at image_output_price_usd
  • Conservative approach: never undercharges image tokens and stays well below the previous behavior of billing all output at the image rate

• chat_controller.py:

  • Modified _create_non_streaming_response() to surface only the standard OpenAI usage triple (prompt/completion/total tokens) while passing reasoning tokens separately to the cost calculator
  • Updated streaming response handling in generate() to extract and accumulate reasoning tokens from output_token_details in both non-streaming and streaming paths
  • Pass reasoning_tokens to compute_session_cost() via a separate cost_usage dict to avoid polluting the OpenAI-compatible response

• test_image_billing.py:

  • Updated test documentation to reflect dual-rate billing model
  • Modified _usage_dict() to use extract_usage() which now carries reasoning tokens
  • Renamed test_generated_image_is_charged_as_output_tokens() → test_generated_image_is_charged_at_image_rate() with updated assertions
  • Added test_thinking_tokens_billed_at_text_rate() to verify thinking tokens use the cheaper text rate
  • Added test_thinking_is_cheaper_than_billing_all_at_image_rate() regression test ensuring the fix doesn't revert to the old buggy behavior

• test_price_feed.py:

  • Updated mock model config to include image_output=False and image_output_price_usd=None for single-rate test models

Implementation Details

• Reasoning token extraction: LangChain breaks out thinking tokens in output_token_details.reasoning but folds them into the main output_tokens count. The billing split uses this breakdown to charge thinking at the text rate and the remainder (image + any caption) at the image rate.
• Conservative billing: The split never undercharges image tokens and is far below the previous behavior of billing all output at the image rate, ensuring we don't lose revenue on image generation.
• OpenAI compatibility: The response surface maintains the standard OpenAI usage triple; reasoning tokens ride along to the cost calculator only, not exposed in the API response.

https://claude.ai/code/session_01GDGKRki93xtXCFUNDkcEyq