FIX QA Regression Testing

FIX regressions are rarely “one bug.” They usually show up as subtle state changes: sequence handling, resend side effects, or order lifecycle transitions that drift after a release. Regression testing works best when scenarios are repeatable and deterministic.

What to Regression Test in FIX

Session stability: logon rules, heartbeats, disconnects, reconnect behavior
Recovery: resend requests, duplicates, gap fill rules
Order lifecycle: ack → partial fills → fills → cancels → cancel rejects
Validation: required tags, custom tags, supported MsgTypes
Timing edge cases: delayed responses, message bursts, out-of-order delivery

For certification-focused regression suites, also include the FIX certification checklist scenarios so onboarding does not regress.

Build a Scenario Library

Your best regression suite is a library of message sequences that represent real workflows: common order paths, known failure cases, and certification sequences.

Keep scenarios small and focused (one behavior per test)
Version scenarios alongside code
Include both “happy path” and “failure path” tests

A simple starting point is to take real incident logs and replay FIX messages until the behavior is deterministic, then lock it in as a regression scenario.

Make Tests Deterministic

Control sequence numbers and reset behavior
Fix response timing where possible
Use consistent IDs (ClOrdID, ExecID strategy) per scenario
Validate outputs with clear assertions (states, quantities, rejects)

How FIXSIM Helps

A simulator makes regression practical by providing a controlled counterparty. You can replay scenarios, generate deterministic responses, and verify outcomes repeatedly after code changes.

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.