Replay FIX Messages

Replay helps you take a real-world issue—an unexpected reject, missing execution, or state mismatch—and turn it into a repeatable scenario you can run during development and QA.

If you are troubleshooting an integration failure during onboarding, see why FIX onboarding projects stall and how replay testing prevents repeated certification failures.

Common Reasons to Replay Messages

Reproduce intermittent bugs without relying on a live environment
Validate order lifecycle transitions end-to-end
Confirm that a fix does not introduce regressions
Test edge cases: duplicates, resends, delayed responses

Replay vs. “Message Recovery”

Message recovery (resends, gap fills) is part of the FIX session layer. Replay is an intentional testing technique: you rerun a known sequence to validate your application logic, timing assumptions, and state handling.

Turn Replays into Regression Tests

Keep the smallest message sequence that reproduces the behavior
Define expected outcomes (states, rejects, quantities)
Run the scenario after every change to your FIX layer or business logic

Replay scenarios should become part of your FIX regression testing suite so future releases cannot reintroduce the same behavior.

How FIXSIM Helps

FIXSIM can replay message sequences in a controlled environment so you can debug quickly and keep your fixes stable over time.

Run These Test Cases in a FIX Simulator

To run these scenarios in a controlled environment, use a FIX protocol simulator that can emulate counterparties, replay message flows, and validate session behavior before certification.