---
title: debug
sidebarTitle: debug
---
# `fastmcp.server.auth.providers.debug`
Debug token verifier for testing and special cases.
This module provides a flexible token verifier that delegates validation
to a custom callable. Useful for testing, development, or scenarios where
standard verification isn't possible (like opaque tokens without introspection).
Example:
```python
from fastmcp import FastMCP
from fastmcp.server.auth.providers.debug import DebugTokenVerifier
# Accept all tokens (default - useful for testing)
auth = DebugTokenVerifier()
# Custom sync validation logic
auth = DebugTokenVerifier(validate=lambda token: token.startswith("valid-"))
# Custom async validation logic
async def check_cache(token: str) -> bool:
return await redis.exists(f"token:{token}")
auth = DebugTokenVerifier(validate=check_cache)
mcp = FastMCP("My Server", auth=auth)
```
## Classes
### `DebugTokenVerifier`
Token verifier with custom validation logic.
This verifier delegates token validation to a user-provided callable.
By default, it accepts all non-empty tokens (useful for testing).
Use cases:
- Testing: Accept any token without real verification
- Development: Custom validation logic for prototyping
- Opaque tokens: When you have tokens with no introspection endpoint
WARNING: This bypasses standard security checks. Only use in controlled
environments or when you understand the security implications.
**Methods:**
#### `verify_token`
```python
verify_token(self, token: str) -> AccessToken | None
```
Verify token using custom validation logic.
**Args:**
- `token`: The token string to validate
**Returns:**
- AccessToken if validation succeeds, None otherwise