Configure a Salesforce Flow HTTP callout action to invoke an external REST API without code

In-depth guide

Steps

1. Register the external API as an External Service in Setup by providing an OpenAPI 2.0 or 3.0 schema, which generates invocable Apex actions automatically
2. Alternatively, use a Flow's HTTP Callout element (where available) by specifying the named credential, HTTP method, relative URL, and request body template using Flow merge fields
3. In the Flow, map input variables to the HTTP Callout element's request body fields and map response body fields to output variables using the response schema
4. Add a Fault connector from the HTTP Callout element to handle non-2xx responses — the fault message contains the HTTP status code and response body
5. Test the callout in Flow Builder's debug mode, inspecting the raw request and response in the debug log before activating
6. For flows that run in transaction context (record-triggered flows), ensure the callout respects the synchronous callout restriction — use an asynchronous path (After-Save with async) to avoid mixed-DML callout errors