# Response headers

> The headers ACOG adds to responses: the model that answered, cache hits, budget warnings, fallbacks and features removed for compatibility.

Product: AI Cost Optimizer Gateway (ACOG). Web version: https://tessarac.com/docs/acog/reference/response-headers. Last updated: 2026-10-16.

Your provider's own response headers are passed through. ACOG adds:

| Header | Value | When |
|---|---|---|
| `x-request-id` | The request's ID | Always |
| `x-acog-resolved-model` | The model that was sent the request | Compatible endpoints, including cache hits. Differs from your request for `auto` or a version alias. |
| `x-acog-cache` | `hit` | The response came from the response cache, the semantic cache or a shared in-flight request |
| `x-acog-cache-model` | A model id | A semantic-cache hit on a compatible endpoint: the model that generated the stored answer |
| `x-acog-budget-notice` | `scope/window=pct`, comma-separated, for example `org/month=80` | A budget passed 50, 80 or 90% of its limit with this response |
| `x-acog-policy-fallback` | `applied`, `skipped` or `unavailable` | The provider's safety classifier refused the request. See [Routing and failover](https://tessarac.com/docs/acog/providers/routing-and-failover.md#policy-block-fallback). |
| `x-acog-thinking-stripped` | `true` | The serving model rejected extended thinking, and ACOG retried without it |
| `x-acog-beta-stripped` | The rejected `anthropic-beta` values, or `all` | The serving platform rejected beta features, and ACOG retried without them |
| `x-acog-fields-stripped` | Comma-separated field names | The serving model rejected these request fields, and ACOG retried without them |
| `Retry-After` | Seconds | With `429 rate_limited` and `503 upstream_unavailable` |

Streaming responses also carry a keep-alive comment line (`: keep-alive`) every 15 seconds without data. It is part of the event stream, not a header.

ACOG adds no header with costs or savings. Read them from the console or the [management API](https://tessarac.com/docs/acog/api/management-api.md#usage-and-savings).