# Cache identity (llmcpp)

**Cache identity** connects an implementation to the inputs that produced it. The generation pass computes its key from the target, prompt, effective policy, and compiler context:

```cpp
t.m_key =
    sha256_hex(std::string(CacheVersion) + "\n" + t.m_name + "\n" + t.m_signature +
               "\n" + t.m_prompt_text + "\n" + policy + "\n" + t.m_cache_salt + "\n" +
               sha256_hex(t.m_options.m_system_prompt) + "\n" +
               sha256_hex(t.m_options.m_agent_config) + "\n" + t.m_options.m_backend +
               "\n" + contextDigest +
               (!t.m_agent_override ? "" : "\nagent:" + t.m_options.m_agent_command));
```

The inputs include the signature, prompt, cache salt, system instructions, agent configuration, backend, and compilation-context digest. Attached references are incorporated afterward.

This deliberately invalidates conservatively: changing a header can cause a miss even when the old body might still be usable. Output-path changes do not define a different implementation. Full hashes remain in metadata even when filenames display abbreviated prefixes. [[llmcpp-explicit-cache-keys|Explicit keys]] select a stored implementation independently of this computed identity.

Source: [generation_pass.cpp](https://github.com/yanevskiv/llmcpp/blob/main/src/llmcpp/generation_pass.cpp).