Direct answer: When an integration breaks, save one real request and response, validate the JSON shape, strip optional fields until you have the smallest failing payload, then fix the contract or client against that fixture. Guessing in application code without a minimal reproduction usually wastes the first hour.
Most “app bugs” at API boundaries are contract drift: a field changed type, a new required parameter appeared, or an HTTP 200 wrapped a nested error. Those problems are invisible if you only stare at the final UI exception. Capture method, URL, behavior-affecting headers, redacted body, status code, and the full JSON response. Format and validate the payload so type and shape changes jump out.
Next, delete optional fields one at a time until the failure still reproduces. That minimal request tells you whether the issue is auth, serialization, validation, or the downstream service. Share the fixture with a teammate or vendor without sharing your whole codebase. Turn the finding into a guardrail: schema validation at the boundary, log the upstream request ID, and keep a sanitized regression case.
GetCodingTools helps this loop—format/validate JSON, compare payloads, inspect URL encoding, and craft a clean minimal request—without installing another one-off utility. Keep the checklist next to your webhook and API tester bookmarks so the next incident starts with evidence, not speculation.