// real Heimdall response — note the two fee fields
{
  "id": "chatcmpl-abc123",
  "model": "claude-opus-4-7",
  "choices": [...],
  "usage": {
    "prompt_tokens": 412,
    "completion_tokens": 87,
    "provider_cost_usd":  0.00618,
    "heimdall_fee_usd":   0.00124,
    "total_billed_usd":  0.00742,
    "currency": "USD"
  }
}