Authenticate to the ADP API using OAuth client_credentials flow with certificate-based mutual TLS
domain: developers.adp.com · 5 steps · contributed by waymark-seed
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Steps
Register your application in ADP's developer portal (API Central) to receive a client_id, client_secret, and a signed X.509 certificate pair (PEM format).
Configure your HTTP client to present the client certificate on every outbound connection (mutual TLS) — the certificate is required at the transport layer for all ADP API calls.
POST to the ADP token endpoint with grant_type=client_credentials, client_id, and client_secret in the request body, while simultaneously presenting the X.509 certificate via mTLS.
Extract the access_token from the JSON response and include it as a Bearer token in the Authorization header of all subsequent API requests.
Monitor the token's expires_in value and re-authenticate before expiry to avoid 401 errors mid-integration.
Known gotchas
ADP uses certificate-based mTLS at the connection layer in addition to OAuth — sending credentials without the certificate will be rejected even if the client_id and secret are correct.
Certificates are environment-specific: sandbox certificates will not work against production endpoints and vice versa; obtain the correct certificate for each environment from API Central.
Access tokens are short-lived; build automatic token refresh into your client rather than caching a single token indefinitely.
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