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## Description
OpenRouter returns `cost` and `cost_details` in its API response `usage`
object, providing the actual cost of each API call. Currently,
`_create_usage_metadata()` only extracts token counts and drops these
cost fields.
This PR surfaces both `cost` and `cost_details` in `response_metadata`
for both non-streaming and streaming paths, allowing users to access
actual API costs directly from the response without manual estimation
from token counts.
**Example response from OpenRouter:**
```json
{
"usage": {
"prompt_tokens": 100,
"completion_tokens": 50,
"cost": 0.000075,
"cost_details": {
"upstream_inference_cost": 0.00007745,
"upstream_inference_prompt_cost": 0.00000895,
"upstream_inference_completions_cost": 0.0000685
}
}
}
```
**After this change:**
```python
result = chat.invoke("hello")
result.response_metadata["cost"] # 0.000075
result.response_metadata["cost_details"] # {...}
```
## Changes
- **`_create_chat_result`**: Surface `cost` and `cost_details` from
`token_usage` into `response_metadata` (non-streaming)
- **`_convert_chunk_to_message_chunk`**: Same for streaming
`AIMessageChunk`
- Added `PLR0912` to `noqa` comments (new branches pushed count over
threshold)
- Added two unit tests: one verifying cost fields are present when
returned, one verifying they're absent when not in usage
## Issue
N/A — discovered while integrating OpenRouter in a production pipeline.
The cost data is already returned by the API but was being silently
dropped.
## Dependencies
None.
## Twitter handle
@hamza_kyamanywa
---------
Co-authored-by: Mason Daugherty <mason@langchain.dev>
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