# Explicit cache keys (llmcpp)

**Explicit cache keys** pin a previously generated implementation independently of newly computed inputs. The modifier accepts a full hash or an unambiguous prefix:

```cpp
__llm__(key("abcdef0"), offline)
int twice(int value)
{
    Return twice value.
}
```

Here `abcdef0` is illustrative: replace it with a real cache key. `offline` makes a missing entry fail without generation. Without it, a miss can generate and store a body under the supplied key.

Keys require seven to sixty-four hexadecimal characters; ambiguous prefixes are errors. They enable caching and cannot be combined with `no_cache`. Lifetime still applies.

A pinned body must still compile in the current program, and changing the prompt does not adapt its behavior. Review the implementation when its intended contract changes.