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
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
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
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
Test the callout in Flow Builder's debug mode, inspecting the raw request and response in the debug log before activating
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
Known gotchas
HTTP Callout elements in Flow do not support streaming responses or multipart bodies — the entire response must be a parseable JSON or XML document within the governor timeout
External Service actions generated from an OpenAPI schema regenerate when you reimport the schema, which can silently break existing Flow references if field names change in the new schema version
Flows with HTTP callouts count against the same per-transaction callout limit (100 callouts) as Apex; a loop in a flow that calls an HTTP element per iteration will exhaust limits quickly
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp