# RuntimeError: Dependency cache key error

- **ID:** `python/fastapi-runtimeerror-dependency-cache-key-error`
- **Domain:** python
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

FastAPI's dependency injection cache has a conflict, often due to using mutable default arguments in dependency functions.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **Use immutable defaults or None** (90% success)
   ```
   def dependency(items: list = None):
    if items is None:
        items = []
   ```
2. **Clear dependency cache manually** (50% success)
   ```
   from fastapi import Depends
# Not recommended; better to fix dependency function
   ```

## Dead Ends

- **Ignoring mutable defaults in dependencies** — Mutable defaults are shared across requests, causing cache issues. (70% fail)
- **Using global variables in dependencies** — Global state can cause cache key collisions. (60% fail)