A cached prompt costs the cached rate

cached_per_1k in a price charges the prompt tokens the answer counts as
cached_tokens at their own rate; zero keeps charging them as prompt tokens.
The count reaches the cost and not a column: CostMicros holds what it
changed. Cached beyond the prompt is clamped, an engine's miscount rather
than a refund. BACKLOG L5.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/reference.md

@@ -424,6 +424,7 @@ rather than repeated.
 | `secret_file` | `string` | [Webhook](#webhook) |
 | `prompt_per_1k` | `int64` | [accounting.Price](#accountingprice) |
 | `completion_per_1k` | `int64` | [accounting.Price](#accountingprice) |
+| `cached_per_1k` | `int64` | [accounting.Price](#accountingprice) |
 | `per_image` | `int64` | [accounting.Price](#accountingprice) |
 | `per_mesh` | `int64` | [accounting.Price](#accountingprice) |
 | `name` | `string` | [admission.Band](#admissionband) |
@@ -462,6 +463,7 @@ rather than repeated.
 | `queue_ttl` | `time.Duration` | [config.Band](#configband) |
 | `prompt_per_1k` | `int64` | [config.Price](#configprice) |
 | `completion_per_1k` | `int64` | [config.Price](#configprice) |
+| `cached_per_1k` | `int64` | [config.Price](#configprice) |
 | `per_image` | `int64` | [config.Price](#configprice) |
 | `per_mesh` | `int64` | [config.Price](#configprice) |
 | `disabled` | `[]string` | [engines.Config](#enginesconfig) |
@@ -3600,6 +3602,15 @@ is sha256=<hex HMAC of the body>.
 
 `int64`
 
+### `cached_per_1k`
+
+`int64`
+
+CachedPer1k is what a prompt token costs that the engine or provider
+read from its cache instead of computing: the part of prompt_tokens
+the answer counts as cached_tokens. Zero charges those at
+PromptPer1k, as before there was a rate for them.
+
 ### `per_image`
 
 `int64`
@@ -3874,6 +3885,14 @@ had rather wait behind people than be turned away.
 
 `int64`
 
+### `cached_per_1k`
+
+`int64`
+
+CachedPer1k is the cost of a prompt token the engine or provider read
+from its cache (the answer's cached_tokens, counted within the
+prompt's). Zero charges them at prompt_per_1k.
+
 ### `per_image`
 
 `int64`