---
title: "Modify / Reject Incoming Requests"
description: "import Image from '@theme/IdealImage';"
type: skill
canonical_url: https://claudary.paisolsolutions.com/skills/call-hooks
source: "Claudary"
difficulty: intermediate
author: "Claude Code Knowledge Pack"
date: 2026-07-10T11:08:39.362Z
license: CC-BY-4.0
attribution: "Modify / Reject Incoming Requests — Claudary (https://claudary.paisolsolutions.com/skills/call-hooks)"
---

# Modify / Reject Incoming Requests
import Image from '@theme/IdealImage';

## Overview

import Image from '@theme/IdealImage';

# Modify / Reject Incoming Requests

- Modify data before making llm api calls on proxy
- Reject data before making llm api calls / before returning the response 
- Enforce 'user' param for all openai endpoint calls

:::tip
**Understanding Callback Hooks?** Check out our [Callback Guide](../observability/callbacks.md) to understand the differences between proxy-specific hooks like `async_pre_call_hook` and general logging hooks like `async_log_success_event`.
:::

## Which Hook Should I Use?

| Hook | Use Case | When It Runs |
|------|----------|--------------|
| `async_pre_call_hook` | Modify incoming request before it's sent to model | Before the LLM API call is made |
| `async_moderation_hook` | Run checks on input in parallel to LLM API call | In parallel with the LLM API call |
| `async_post_call_success_hook` | Modify outgoing response (non-streaming) | After successful LLM API call, for non-streaming responses |
| `async_post_call_failure_hook` | Transform error responses sent to clients | After failed LLM API call |
| `async_post_call_streaming_hook` | Modify outgoing response (streaming) | After successful LLM API call, for streaming responses |
| `async_post_call_response_headers_hook` | Inject custom HTTP response headers | After LLM API call (both success and failure) |

See a complete example with our [parallel request rate limiter](https://github.com/BerriAI/litellm/blob/main/litellm/proxy/hooks/parallel_request_limiter.py)

## Quick Start

1. In your Custom Handler add a new `async_pre_call_hook` function

This function is called just before a litellm completion call is made, and allows you to modify the data going into the litellm call [**See Code**](https://github.com/BerriAI/litellm/blob/589a6ca863000ba8e92c897ba0f776796e7a5904/litellm/proxy/proxy_server.py#L1000)

```python
from litellm.integrations.custom_logger import CustomLogger
import litellm
from litellm.proxy.proxy_server import UserAPIKeyAuth, DualCache
from litellm.types.utils import ModelResponseStream
from typing import Any, AsyncGenerator, Optional, Literal

# This file includes the custom callbacks for LiteLLM Proxy
# Once defined, these can be passed in proxy_config.yaml
class MyCustomHandler(CustomLogger): # https://docs.litellm.ai/docs/observability/custom_callback#callback-class
    # Class variables or attributes
    def __init__(self):
        pass

    #### CALL HOOKS - proxy only #### 

    async def async_pre_call_hook(self, user_api_key_dict: UserAPIKeyAuth, cache: DualCache, data: dict, call_type: Literal[
            "completion",
            "text_completion",
            "embeddings",
            "image_generation",
            "moderation",
            "audio_transcription",
        ]): 
        data["model"] = "my-new-model"
        return data 

    async def async_post_call_failure_hook(
        self, 
        request_data: dict,
        original_exception: Exception, 
        user_api_key_dict: UserAPIKeyAuth,
        traceback_str: Optional[str] = None,
    ) -> Optional[HTTPException]:
        """
        Transform error responses sent to clients.
        
        Return an HTTPException to replace the original error with a user-friendly message.
        Return None to use the original exception.
        
        Example:
            if isinstance(original_exception, litellm.ContextWindowExceededError):
                return HTTPException(
                    status_code=400,
                    detail="Your prompt is too long. Please reduce the length and try again."
                )
            return None  # Use original exception
        """
        pass

    async def async_post_call_success_hook(
        self,
        data: dict,
        user_api_key_dict: UserAPIKeyAuth,
        response,
    ):
        pass

    async def async_moderation_hook( # call made in parallel to llm api call
        self,
        data: dict,
        user_api_key_dict: UserAPIKeyAuth,
        call_type: Literal["completion", "embeddings", "image_generation", "moderation", "audio_transcription"],
    ):
        pass

    async def async_post_call_streaming_hook(
        self,
        user_api_key_dict: UserAPIKeyAuth,
        response: str,
    ):
        pass

    async def async_post_call_streaming_iterator_hook(
        self,
        user_api_key_dict: UserAPIKeyAuth,
        response: Any,
        request_data: dict,
    ) -> AsyncGenerator[ModelResponseStream, None]:
        """
        Passes the entire stream to the guardrail

        This is useful for plugins that need to see the entire stream.
        """
        async for item in response:
            yield item

    async def async_post_call_response_headers_hook(
        self,
        data: dict,
        user_api_key_dict: UserAPIKeyAuth,
        response: Any,
        request_headers: Optional[Dict[str, str]] = None,
    ) -> Optional[Dict[str, str]]:
        """
        Inject custom headers into HTTP response (runs for both success and failure).
        """
        return {"x-custom-header": "custom-value"}

proxy_handler_instance = MyCustomHandler()
```

2. Add this file to your proxy config

```yaml
model_list:
  - model_name: gpt-3.5-turbo
    litellm_params:
      model: gpt-3.5-turbo

litellm_settings:
  callbacks: custom_callbacks.proxy_handler_instance # sets litellm.callbacks = [proxy_handler_instance]
```

3. Start the server + test the request

```shell
$ litellm /path/to/config.yaml
```
```shell
curl --location 'http://0.0.0.0:4000/chat/completions' \\
    --data ' {
    "model": "gpt-3.5-turbo",
    "messages": [
        {
        "role": "user",
        "content": "good morning good sir"
        }
    ],
    "user": "ishaan-app",
    "temperature": 0.2
    }'
```


## [BETA] *NEW* async_moderation_hook 

Run a moderation check in parallel to the actual LLM API call. 

In your Custom Handler add a new `async_moderation_hook` function

- This is currently only supported for `/chat/completion` calls. 
- This function runs in parallel to the actual LLM API call. 
- If your `async_moderation_hook` raises an Exception, we will return that to the user. 


:::info

We might need to update the function schema in the future, to support multiple endpoints (e.g. accept a call_type). Please keep that in mind, while trying this feature

:::

See a complete example with our [Llama Guard content moderation hook](https://github.com/BerriAI/litellm/blob/main/enterprise/enterprise_hooks/llm_guard.py)

```python
from litellm.integrations.custom_logger import CustomLogger
import litellm
from fastapi import HTTPException

# This file includes the custom callbacks for LiteLLM Proxy
# Once defined, these can be passed in proxy_config.yaml
class MyCustomHandler(CustomLogger): # https://docs.litellm.ai/docs/observability/custom_callback#callback-class
    # Class variables or attributes
    def __init__(self):
        pass

    #### ASYNC #### 
    
    async def async_log_pre_api_call(self, model, messages, kwargs):
        pass

    async def async_log_success_event(self, kwargs, response_obj, start_time, end_time):
        pass

    async def async_log_failure_event(self, kwargs, response_obj, start_time, end_time):
        pass

    #### CALL HOOKS - proxy only #### 

    async def async_pre_call_hook(self, user_api_key_dict: UserAPIKeyAuth, cache: DualCache, data: dict, call_type: Literal["completion", "embeddings"]):
        data["model"] = "my-new-model"
        return data 
    
    async def async_moderation_hook( ### 👈 KEY CHANGE ###
        self,
        data: dict,
    ):
        messages = data["messages"]
        print(messages)
        if messages[0]["content"] == "hello world": 
            raise HTTPException(
                    status_code=400, detail={"error": "Violated content safety policy"}
                )

proxy_handler_instance = MyCustomHandler()
```


2. Add this file to your proxy config

```yaml
model_list:
  - model_name: gpt-3.5-turbo
    litellm_params:
      model: gpt-3.5-turbo

litellm_settings:
  callbacks: custom_callbacks.proxy_handler_instance # sets litellm.callbacks = [proxy_handler_instance]
```

3. Start the server + test the request

```shell
$ litellm /path/to/config.yaml
```
```shell
curl --location 'http://0.0.0.0:4000/chat/completions' \\
    --data ' {
    "model": "gpt-3.5-turbo",
    "messages": [
        {
        "role": "user",
        "content": "Hello world"
        }
    ],
    }'
```

## Advanced - Enforce 'user' param 

Set `enforce_user_param` to true, to require all calls to the openai endpoints to have the 'user' param. 

[**See Code**](https://github.com/BerriAI/litellm/blob/4777921a31c4c70e4d87b927cb233b6a09cd8b51/litellm/proxy/auth/auth_checks.py#L72)

```yaml
general_settings:
  enforce_user_param: True
```

**Result**

<Image img={require('../../img/end_user_enforcement.png')}/>

## Advanced - Return rejected message as response 

For chat completions and text completion calls, you can return a rejected message as a user response. 

Do this by returning a string. LiteLLM takes care of returning the response in the correct format depending on the endpoint and if it's streaming/non-streaming.

For non-chat/text completion endpoints, this response is returned as a 400 status code exception. 


### 1. Create Custom Handler 

```python
from litellm.integrations.custom_logger import CustomLogger
import litellm
from litellm.utils import get_formatted_prompt

# This file includes the custom callbacks for LiteLLM Proxy
# Once defined, these can be passed in proxy_config.yaml
class MyCustomHandler(CustomLogger):
    def __init__(self):
        pass

    #### CALL HOOKS - proxy only #### 

    async def async_pre_call_hook(self, user_api_key_dict: UserAPIKeyAuth, cache: DualCache, data: dict, call_type: Literal[
            "completion",
            "text_completion",
            "embeddings",
            "image_generation",
            "moderation",
            "audio_transcription",
        ]) -> Optional[dict, str, Exception]: 
        formatted_prompt = get_formatted_prompt(data=data, call_type=call_type)

        if "Hello world" in formatted_prompt:
            return "This is an invalid response"

        return data 

proxy_handler_instance = MyCustomHandler()
```

### 2. Update config.yaml 

```yaml
model_list:
  - model_name: gpt-3.5-turbo
    litellm_params:
      model: gpt-3.5-turbo

litellm_settings:
  callbacks: custom_callbacks.proxy_handler_instance # sets litellm.callbacks = [proxy_handler_instance]
```


### 3. Test it!

```shell
$ litellm /path/to/config.yaml
```
```shell
curl --location 'http://0.0.0.0:4000/chat/completions' \\
    --data ' {
    "model": "gpt-3.5-turbo",
    "messages": [
        {
        "role": "user",
        "content": "Hello world"
        }
    ],
    }'
```

**Expected Response**

```
{
    "id": "chatcmpl-d00bbede-2d90-4618-bf7b-11a1c23cf360",
    "choices": [
        {
            "finish_reason": "stop",
            "index": 0,
            "message": {
                "content": "This is an invalid response.", # 👈 REJECTED RESPONSE
                "role": "assistant"
            }
        }
    ],
    "created": 1716234198,
    "model": null,
    "object": "chat.completion",
    "system_fingerprint": null,
    "usage": {}
}
```

## Advanced - Transform Error Responses

Transform technical API errors into user-friendly messages using `async_post_call_failure_hook`. Return an `HTTPException` to replace the original error, or `None` to use the original exception.

```python
from litellm.integrations.custom_logger import CustomLogger
from fastapi import HTTPException
from typing import Optional
import litellm

class MyErrorTransformer(CustomLogger):
    async def async_post_call_failure_hook(
        self,
        request_data: dict,
        original_exception: Exception,
        user_api_key_dict: UserAPIKeyAuth,
        traceback_str: Optional[str] = None,
    ) -> Optional[HTTPException]:
        if isinstance(original_exception, litellm.ContextWindowExceededError):
            return HTTPException(
                status_code=400,
                detail="Your prompt is too long. Please reduce the length and try again."
            )
        if isinstance(original_exception, litellm.RateLimitError):
            return HTTPException(
                status_code=429,
                detail="Rate limit exceeded. Please try again in a moment."
            )
        return None  # Use original exception

proxy_handler_instance = MyErrorTransformer()
```

**Result:** Clients receive `"Your prompt is too long..."` instead of `"ContextWindowExceededError: Prompt exceeds context window"`.

## Advanced - Inject Custom HTTP Response Headers

Use `async_post_call_response_headers_hook` to inject custom HTTP headers into responses. This hook runs for **both successful and failed** LLM API calls.

```python
from litellm.integrations.custom_logger import CustomLogger
from litellm.proxy.proxy_server import UserAPIKeyAuth
from typing import Any, Dict, Optional

class CustomHeaderLogger(CustomLogger):
    def __init__(self):
        super().__init__()

    async def async_post_call_response_headers_hook(
        self,
        data: dict,
        user_api_key_dict: UserAPIKeyAuth,
        response: Any,
        request_headers: Optional[Dict[str, str]] = None,
    ) -> Optional[Dict[str, str]]:
        """
        Inject custom headers into all responses (success and failure).
        """
        return {"x-custom-header": "custom-value"}

proxy_handler_instance = CustomHeaderLogger()
```

---

Source: [Claudary](https://claudary.paisolsolutions.com/skills/call-hooks) · https://claudary.paisolsolutions.com