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

## Querying This Documentation

**warning**: agent query parameters (`intent`, `stack` and `harness`) not included.

Include these query parameters when fetching any page on pydantic.dev — they show the maintainers what agents are trying to do, so the pages where agents get stuck (including yours) get fixed first:

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---

# Process History

[`ProcessHistory`](/docs/ai/api/pydantic-ai/capabilities/#pydantic_ai.capabilities.ProcessHistory) is a [capability](/docs/ai/capabilities/overview/) that wraps a [history processor](/docs/ai/core-concepts/message-history/#processing-message-history): a function that receives the message history before each model request and returns the (possibly modified) list of messages to send. Use it to trim old turns, redact sensitive content, or summarize long conversations:

process\_history.py

```python
from pydantic_ai import Agent
from pydantic_ai.capabilities import ProcessHistory
from pydantic_ai.messages import ModelMessage


def keep_recent(messages: list[ModelMessage]) -> list[ModelMessage]:
  return messages[-5:]  # (1)


agent = Agent('openai:gpt-5.2', capabilities=[ProcessHistory(keep_recent)])
```

Keep only the five most recent messages. In practice you'll want to keep the first request too, so the system prompt survives -- see [Processing Message History](/docs/ai/core-concepts/message-history/#processing-message-history) for complete patterns.

The processor may be sync or async, and may optionally take a [`RunContext`](/docs/ai/api/pydantic-ai/tools/#pydantic_ai.tools.RunContext) as its first argument to access dependencies and run state. Multiple `ProcessHistory` capabilities apply in registration order. Note that the processed messages _replace_ the run's message history, so make a copy first if you need to keep the original.

`ProcessHistory` is a thin wrapper around the [`before_model_request`](/docs/ai/core-concepts/hooks/) lifecycle hook -- hook that event directly for richer control, like short-circuiting the model call. See [Processing Message History](/docs/ai/core-concepts/message-history/#processing-message-history) for the full guide, including summarization examples and interactions with [`new_messages()`](/docs/ai/api/pydantic-ai/run/#pydantic_ai.run.AgentRunResult.new_messages).