---
title: "MockChatGenerator"
id: mockchatgenerator
slug: "/mockchatgenerator"
description: "A Chat Generator that returns predefined responses without calling any API, for tests and quick prototypes."
---

# MockChatGenerator

A Chat Generator that returns predefined responses without calling any API, for tests and quick prototypes.

<div className="key-value-table">

|  |  |
| --- | --- |
| **Most common position in a pipeline** | In place of a real Chat Generator, in tests and prototypes |
| **Mandatory init variables** | None |
| **Mandatory run variables** | `messages`: A list of [`ChatMessage`](../../concepts/data-classes/chatmessage.mdx) objects |
| **Output variables** | `replies`: A list of generated `ChatMessage` objects |
| **API reference** | [Generators](/reference/generators-api) |
| **GitHub link** | https://github.com/deepset-ai/haystack/blob/main/haystack/components/generators/chat/mock.py |
| **Package name** | `haystack-ai` |

</div>

## Overview

`MockChatGenerator` is a deterministic, zero-cost drop-in replacement for real Chat Generators such as `OpenAIChatGenerator`. It implements `run`, `run_async`, streaming callbacks, and serialization but never contacts an external service, which makes it ideal for unit tests, smoke tests, and quick prototypes.

The response is selected based on how the component is configured:

- **Fixed response**: Pass a single string or `ChatMessage` via `responses`. The same reply is returned on every call. A `ChatMessage` passed as a response must have the `assistant` role.
- **Cycling responses**: Pass a list of strings and/or `ChatMessage` objects via `responses`. Each call returns the next item, wrapping around to the start once the list is exhausted. This is useful to drive multi-step flows such as Agents, where the first call returns a tool call and a later call returns the final answer.
- **Dynamic response**: Pass a `response_fn` callable that receives the input messages and returns the reply as a string or an assistant `ChatMessage`. Use this when the reply should depend on the input. To support serialization, pass a named function.
- **Echo (default)**: With no configuration, the component echoes back the text of the last message that has text content, so it is usable out of the box.

`responses` and `response_fn` are mutually exclusive.

Further optional parameters:

- `model`: The model name reported in the response metadata. Defaults to `"mock-model"`.
- `meta`: Additional metadata merged into the `meta` of every returned `ChatMessage`. A per-response `ChatMessage`'s own metadata takes precedence.
- `streaming_callback`: An optional callback invoked with `StreamingChunk` objects reconstructed from the predefined response. It lets the mock exercise streaming code paths without a real model.

## Usage

### On its own

```python
from haystack.components.generators.chat import MockChatGenerator
from haystack.dataclasses import ChatMessage

# Fixed response
generator = MockChatGenerator(responses="Hello, this is a mock response.")
result = generator.run([ChatMessage.from_user("Hi!")])
print(result["replies"][0].text)  # "Hello, this is a mock response."

# Echo mode (default): returns the last message with text content
generator = MockChatGenerator()
result = generator.run([ChatMessage.from_user("Repeat after me")])
print(result["replies"][0].text)  # "Repeat after me"
```

### Driving an Agent

Pass `ChatMessage` objects (rather than plain strings) to return tool calls or reasoning content. With cycling responses, you can script a full agent loop without a real model:

```python
from haystack.components.agents import Agent
from haystack.components.generators.chat import MockChatGenerator
from haystack.dataclasses import ChatMessage, ToolCall
from haystack.tools import tool


@tool
def search(query: str) -> str:
    """Search for information."""
    return f"Results for: {query}"


generator = MockChatGenerator(
    responses=[
        ChatMessage.from_assistant(
            tool_calls=[ToolCall(tool_name="search", arguments={"query": "Haystack"})],
        ),
        "Here is the final answer.",
    ],
)

agent = Agent(chat_generator=generator, tools=[search])
result = agent.run(messages=[ChatMessage.from_user("Tell me about Haystack")])
print(result["last_message"].text)  # "Here is the final answer."
```

### Input-dependent responses

```python
from haystack.components.generators.chat import MockChatGenerator
from haystack.dataclasses import ChatMessage


def shout_back(messages: list[ChatMessage]) -> str:
    return messages[-1].text.upper()


generator = MockChatGenerator(response_fn=shout_back)
result = generator.run([ChatMessage.from_user("hello")])
print(result["replies"][0].text)  # "HELLO"
```