> ## Documentation Index
> Fetch the complete documentation index at: https://docs.prisminference.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Prompt caching

> Reuse stable prompt prefixes across multi-turn Prism requests.

Prism automatically caches eligible prompt prefixes. Repeated system prompts,
tool definitions, repository context, and conversation history can reuse work
when their leading tokens stay identical.

## Keep the prefix stable

Put stable content first and frequently changing content last:

1. System instructions
2. Tool definitions
3. Repository or reference context
4. Conversation history
5. The newest user message

Changing one token near the beginning of the prompt prevents reuse for the
content that follows it.

## Set a conversation key

For Chat Completions, send the same `prompt_cache_key` on every turn in one
conversation:

```typescript theme={"theme":{"light":"github-light","dark":"github-dark"}}
const response = await client.chat.completions.create({
  model: "prism-glm53",
  prompt_cache_key: "conversation_7f3a2c",
  cache_ttl: "1h",
  messages,
  tools,
});
```

Use an opaque, stable identifier. Do not put email addresses, API keys, or
other secrets in the cache key. `prompt_cache_key` accepts up to 512 bytes.

`cache_ttl` accepts `5m`, `30m`, `1h`, `6h`, or `24h`. Invalid values return
`400`.

You can also send the identifier as a header:

```http theme={"theme":{"light":"github-light","dark":"github-dark"}}
x-session-id: conversation_7f3a2c
```

When both are present, `x-session-id` takes precedence over
`prompt_cache_key`. Use one mechanism consistently for a conversation. A cache
key provides affinity and is not an idempotency key.

## Preserve byte-stable inputs

* Serialize tool schemas deterministically.
* Keep tool order stable between turns.
* Avoid timestamps and request IDs in system prompts.
* Append new conversation turns instead of rebuilding older messages.
* Reuse the same model within a conversation.
* Keep large, shared context before the newest user input.

## Multi-tenant safety

Generate a different key for each tenant and conversation. Never reuse one
customer's key for another customer, even when their prompts look similar.

The cache is an inference optimization. Your application remains responsible
for conversation storage, retention, and access control.