---
title: "Post-Call Rules"
url: "/docs/proxy/rules"
canonical_url: "https://docs.litellm.ai/docs/proxy/rules"
type: "docs"
last_updated: "2026-10-14"
summary: "Use this to fail a request based on the output of an llm api call."
related:
  - "/docs/proxy/agentic_loop_hook"
  - "/docs/proxy/worker_startup_hooks"
---
# Post-Call Rules

> Index of all LiteLLM docs: https://docs.litellm.ai/llms.txt


Use this to fail a request based on the output of an llm api call.

## Quick Start

### Step 1: Create a file (e.g. post_call_rules.py)

```python
def my_custom_rule(input): # receives the model response 
    if len(input) < 5: 
      return {
            "decision": False,
            "message": "This violates LiteLLM Proxy Rules. Response too short"
      }
    return {"decision": True}   # message not required since, request will pass
```

### Step 2. Point it to your proxy

```yaml
litellm_settings:
  post_call_rules: post_call_rules.my_custom_rule
```

### Step 3. Start + test your proxy

```bash
$ litellm /path/to/config.yaml
```

```bash
curl --location 'http://0.0.0.0:4000/v1/chat/completions' \
--header 'Content-Type: application/json' \
--header "Authorization: Bearer $LITELLM_API_KEY" \
--data '{
  "model": "gpt-5.6-luna",
  "messages": [{"role":"user","content":"What llm are you?"}],
  "temperature": 0.7,
  "max_tokens": 10,
}'
```
---

This will now check if a response is > len 5, and if it fails, it'll retry a call 3 times before failing.

### Response that fail the rule

This is the response from LiteLLM Proxy on failing a rule

```json
{
  "error":
    {
      "message":"This violates LiteLLM Proxy Rules. Response too short",
      "type":null,
      "param":null,
      "code":500
    }
}   
```

## Related pages

- [Agentic Loop Hook](https://docs.litellm.ai/docs/proxy/agentic_loop_hook.md)
- [Worker Startup Hooks](https://docs.litellm.ai/docs/proxy/worker_startup_hooks.md)