ger, environment variables, etc.) . [(2) ]()

### Answer Retell custom functions do not natively support OAuth 2.0 Client Credentials authentication with automatic token refresh and secure credential storage. [(1) ]() Custom functions allow you to set request headers, but these can only be static values or dynamic variables—there's no built-in OAuth flow, token management, or encrypted secret storage within the Retell dashboard. Retell's recommended production pattern is to use a wrapper API that you host and control. [(1) ]() [(2) ]() Here's how this works: 1. Host a wrapper API on your own infrastructure (serverless functions, cloud platforms, etc.) [(2) ]() 2. **Handle OAuth authentication in your wrapper**: Your wrapper API securely stores the Cognito

client_id

and

client_secret

, requests access tokens from the Cognito token endpoint, manages token expiration/refresh, and calls your protected API Gateway endpoint with the

Authorization: Bearer <access_token>

header 3. **Configure Retell to call your wrapper**: Add your wrapper API URL as a custom function endpoint in the Retell dashboard [(1) ]() [(2) ]() 4. **Verify requests from Retell**: Use the

X-Retell-Signature

header to verify that incoming requests to your wrapper are actually from Retell [(1) ]() This approach keeps your OAuth credentials and token logic completely outside Retell, in an environment you control with proper secret management (AWS Secrets Mana

​ To learn more, see the sources I considered: [[1] Integrate any system with custom function](https://docs.retellai.com/build/single-multi-prompt/custom-function) [[2] Zapier, ServiceTitan and Retell](https://discord.com/channels/1186424376300679209/1305569900680708116) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.