# AsyncFinalIteratorCallbackHandler

> **Class** in `langchain_classic`

📖 [View in docs](https://reference.langchain.com/python/langchain-classic/callbacks/streaming_aiter_final_only/AsyncFinalIteratorCallbackHandler)

Callback handler that returns an async iterator.

Only the final output of the agent will be iterated.

## Signature

```python
AsyncFinalIteratorCallbackHandler(
    self,
    *,
    answer_prefix_tokens: list[str] | None = None,
    strip_tokens: bool = True,
    stream_prefix: bool = False,
)
```

## Parameters

| Name | Type | Required | Description |
|------|------|----------|-------------|
| `answer_prefix_tokens` | `list[str] \| None` | No | Token sequence that prefixes the answer. Default is ["Final", "Answer", ":"] (default: `None`) |
| `strip_tokens` | `bool` | No | Ignore white spaces and new lines when comparing answer_prefix_tokens to last tokens? (to determine if answer has been reached) (default: `True`) |
| `stream_prefix` | `bool` | No | Should answer prefix itself also be streamed? (default: `False`) |

## Extends

- `AsyncIteratorCallbackHandler`

## Constructors

```python
__init__(
    self,
    *,
    answer_prefix_tokens: list[str] | None = None,
    strip_tokens: bool = True,
    stream_prefix: bool = False,
) -> None
```

| Name | Type |
|------|------|
| `answer_prefix_tokens` | `list[str] \| None` |
| `strip_tokens` | `bool` |
| `stream_prefix` | `bool` |


## Properties

- `answer_prefix_tokens`
- `answer_prefix_tokens_stripped`
- `last_tokens`
- `last_tokens_stripped`
- `strip_tokens`
- `stream_prefix`
- `answer_reached`

## Methods

- [`append_to_last_tokens()`](https://reference.langchain.com/python/langchain-classic/callbacks/streaming_aiter_final_only/AsyncFinalIteratorCallbackHandler/append_to_last_tokens)
- [`check_if_answer_reached()`](https://reference.langchain.com/python/langchain-classic/callbacks/streaming_aiter_final_only/AsyncFinalIteratorCallbackHandler/check_if_answer_reached)
- [`on_llm_start()`](https://reference.langchain.com/python/langchain-classic/callbacks/streaming_aiter_final_only/AsyncFinalIteratorCallbackHandler/on_llm_start)
- [`on_llm_end()`](https://reference.langchain.com/python/langchain-classic/callbacks/streaming_aiter_final_only/AsyncFinalIteratorCallbackHandler/on_llm_end)
- [`on_llm_new_token()`](https://reference.langchain.com/python/langchain-classic/callbacks/streaming_aiter_final_only/AsyncFinalIteratorCallbackHandler/on_llm_new_token)

---

[View source on GitHub](https://github.com/langchain-ai/langchain/blob/8fec4e7ceee2c368b068c49f9fed453276e210e7/libs/langchain/langchain_classic/callbacks/streaming_aiter_final_only.py#L13)