litellm-proxy-authentication-hooks

name	LiteLLM Proxy Authentication Hooks
description	How custom_auth, CustomLogger hooks, and FastAPI middleware interact with standard and passthrough endpoints in LiteLLM proxy
topics	litellm, proxy, authentication, passthrough, anthropic, custom_auth, hooks
created	"2026-04-19T00:00:00.000Z"
updated	"2026-04-19T00:00:00.000Z"
scratchpad	.specs/scratchpad/516cd93e.md

LiteLLM Proxy Authentication Hooks

Overview

LiteLLM proxy has two distinct auth hook systems. custom_auth (via general_settings) fires for ALL routes including true HTTP passthrough endpoints. CustomLogger.async_pre_call_hook only fires when LiteLLM routes the request through its SDK — it is silently skipped for true passthrough routes like /anthropic/v1/messages.

---

Key Concepts

• custom_auth: A FastAPI Depends(user_api_key_auth) dependency injected into every route handler — fires before any routing decision
• async_pre_call_hook: A CustomLogger callback that fires inside the LiteLLM SDK call — not triggered for HTTP passthrough
• True passthrough: Routes like /anthropic/{endpoint:path} that forward raw HTTP to the upstream provider — LiteLLM SDK is not involved
• Unified /v1/messages: A beta endpoint that routes through the LiteLLM SDK (pre_call_hook fires here), distinct from /anthropic/v1/messages

---

Auth Hook Coverage Matrix

Hook	/v1/chat/completions	/v1/messages (beta)	/anthropic/v1/messages (passthrough)
custom_auth (general_settings)	YES	YES	YES
async_pre_call_hook (CustomLogger)	YES	YES	NO
FastAPI BaseHTTPMiddleware	YES	YES	YES

Source: verified in litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py and litellm/proxy/anthropic_endpoints/endpoints.py on BerriAI/litellm main branch.

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Documentation & References

Resource	Description	Link
Custom Auth docs	Function signature, config options, modes	https://docs.litellm.ai/docs/proxy/custom_auth
Anthropic Passthrough docs	/anthropic/{endpoint} passthrough usage	https://docs.litellm.ai/docs/pass_through/anthropic_completion
Pass-through endpoints docs	Custom passthrough config, auth field	https://docs.litellm.ai/docs/proxy/pass_through
All settings	Full config.yaml reference	https://docs.litellm.ai/docs/proxy/config_settings
Source: passthrough routes	llm_passthrough_endpoints.py	https://github.com/BerriAI/litellm/blob/main/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
Source: user_api_key_auth	auth/user_api_key_auth.py	https://github.com/BerriAI/litellm/blob/main/litellm/proxy/auth/user_api_key_auth.py

---

custom_auth: Function Signature and Config

config.yaml

general_settings:
  custom_auth: mymodule.my_auth_function
  custom_auth_settings:
    mode: "on"   # "on" = only custom auth; "auto" = custom + LiteLLM both; "off" = disabled

Function signature

from fastapi import Request
from litellm.proxy._types import UserAPIKeyAuth

async def my_auth_function(request: Request, api_key: str) -> UserAPIKeyAuth:
    # request.headers contains all HTTP headers (Authorization, x-api-key, etc.)
    # api_key is extracted by LiteLLM from the Authorization / x-api-key header
    token = request.headers.get("authorization", "").removeprefix("Bearer ").strip()
    if token != "my-expected-secret":
        from litellm.proxy._types import ProxyException
        raise ProxyException(message="Unauthorized", type="auth_error", param=None, code=401)
    return UserAPIKeyAuth(api_key=api_key, user_id="my-user")

• Must be async
• request: FastAPI Request — access headers via request.headers
• api_key: already extracted by LiteLLM from Authorization: Bearer ... or x-api-key header
• Return UserAPIKeyAuth or a str (a valid LiteLLM key to substitute)
• Raise ProxyException (not a plain Exception) for structured error responses to the client

ProxyException for clean error responses

from litellm.proxy._types import ProxyException
raise ProxyException(
    message="Unauthorized: invalid token",
    type="auth_error",
    param=None,
    code=401,
)

---

How user_api_key_auth Calls custom_auth (Source-Verified)

The flow in litellm/proxy/auth/user_api_key_auth.py:

user_api_key_auth(request, api_key, ...)
  └── if enterprise_custom_auth: call it
  └── elif user_custom_auth:            # ← your custom_auth function
        response = await user_custom_auth(request=request, api_key=api_key)
  └── else: standard LiteLLM key check

user_custom_auth is populated at startup from general_settings.custom_auth via get_instance_fn().

---

Passthrough Route Auth (Source-Verified)

/anthropic/{endpoint:path} in llm_passthrough_endpoints.py:

@router.api_route(
    "/anthropic/{endpoint:path}",
    methods=["GET", "POST", "PUT", "DELETE", "PATCH"],
)
async def anthropic_proxy_route(
    endpoint: str,
    request: Request,
    fastapi_response: Response,
    user_api_key_dict: UserAPIKeyAuth = Depends(user_api_key_auth),  # auth fires here
):
    ...
    # Then forwards raw HTTP to api.anthropic.com — no LiteLLM SDK call

Because user_api_key_auth is a Depends, FastAPI resolves it before the handler runs. custom_auth is called within user_api_key_auth. This is why custom_auth covers passthrough and async_pre_call_hook does not.

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Common Pitfalls & Solutions

Issue	Impact	Solution
Using async_pre_call_hook for auth expecting it to cover passthrough	Auth bypass on /anthropic/*	Migrate auth to custom_auth in general_settings
Raising plain Exception in custom_auth	LiteLLM may return 500 instead of 401	Raise ProxyException with correct code
Reading headers from data["proxy_server_request"] in pre_call_hook	Only works in SDK path, not passthrough	In custom_auth, read from request.headers directly
mode: "auto" when master_key is set	LiteLLM also validates its own keys	Use mode: "on" to rely solely on custom auth
Custom pass-through endpoints (not built-in) need auth: true	Requests bypass auth	Add auth: true to each entry under general_settings.pass_through_endpoints — Enterprise feature

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Patterns & Best Practices

Migrating from async_pre_call_hook to custom_auth

The existing pattern in request_transformer.py reads headers from data["proxy_server_request"]["headers"] inside async_pre_call_hook. To cover passthrough, the auth check must move to a custom_auth function that receives request: Request directly:

# OLD (misses passthrough):
async def async_pre_call_hook(self, user_api_key_dict, cache, data, call_type):
    headers = data.get("proxy_server_request", {}).get("headers", {})
    token = headers.get("x-litellm-api-key", "")
    if token != self.static_token:
        raise Exception("Unauthorized")

# NEW (covers all routes including passthrough):
async def my_auth(request: Request, api_key: str) -> UserAPIKeyAuth:
    token = request.headers.get("x-litellm-api-key", "") or api_key
    if token != os.environ.get("PROXY_STATIC_TOKEN", ""):
        raise ProxyException(message="Unauthorized", type="auth_error", param=None, code=401)
    return UserAPIKeyAuth(api_key=token)

Coexistence: custom_auth + CustomLogger for non-auth transforms

Keep CustomLogger (callbacks) for request/response transforms and token rotation. Move only the auth/rejection logic to custom_auth. The two systems are independent and complementary.

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Sources & Verification

Source	Type	Last Verified
https://docs.litellm.ai/docs/proxy/custom_auth	Official	2026-04-19
https://github.com/BerriAI/litellm/blob/main/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py	Primary source	2026-04-19
https://github.com/BerriAI/litellm/blob/main/litellm/proxy/auth/user_api_key_auth.py	Primary source	2026-04-19
https://github.com/BerriAI/litellm/blob/main/litellm/proxy/anthropic_endpoints/endpoints.py	Primary source	2026-04-19
https://github.com/BerriAI/litellm/blob/main/litellm/proxy/proxy_server.py	Primary source	2026-04-19

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Changelog

Date	Changes
2026-04-19	Initial creation — LiteLLM custom auth coverage for passthrough endpoints