Catching a regression before it ships sounds like it should just be "write more tests," and if that were sufficient, nobody would ever reintroduce a bug they already fixed once. And yet they do, constantly, because the knowledge that a particular pattern is dangerous lives in one engineer's memory, or in a Slack thread, or in a commit message nobody rereads, rather than anywhere the codebase itself enforces. Bugweg's answer to this is the Recurrence Firewall, and it's worth walking through what it actually does, because the mechanism matters more than the label.
Here's the sequence. A bug gets fixed — a PR lands that changes some function to stop doing the wrong thing. At that point bugweg can generate a rule plus a regression test aimed specifically at that bug pattern, not just at the line that changed. Before that test is ever proposed to a human, it has to pass a fail-then-pass check: it must fail against the pre-fix commit and pass against the post-fix commit. That ordering is the whole point. A regression test that passes against both commits isn't testing anything — it's decoration. Requiring it to fail on the old code is what proves the test actually exercises the bug rather than just exercising around it.
That generated rule and test are always proposed for human review, never auto-merged. This is a deliberate tradeoff. An automatically merged regression test is faster but it's also an unreviewed claim about what the bug actually was, and bug fixes are exactly the kind of change where the stated cause and the real cause diverge more often than anyone would like. Keeping a human in the loop costs a little speed and buys back the chance to catch a test that locked in the wrong lesson.
From one fix to a standing rule
The reason this is called a firewall rather than just "regression testing" is that it's meant to generalize past the single PR. Bug Genealogy treats a bug as one continuous record — introduced, fixed, reintroduced, locked — rather than a string of unrelated review comments scattered across different pull requests over time. Once a fix is locked in by the Recurrence Firewall, a future PR that reintroduces the same pattern gets caught by the rule bugweg already generated, not by a reviewer happening to remember a six-month-old incident. Production Lock extends the same idea past code review entirely: when a team marks an error resolved in Sentry or Rollbar, that triggers the same lock-in mechanism, so a bug caught in production gets the same standing protection as one caught in a PR.
A common failure mode looks roughly like this, as a hypothetical rather than a specific incident: a team fixes some bug, ships it, and moves on, and the fix lives only in that one PR's history. Some time later, a different engineer, unaware of the earlier incident, touches the same area of code for an unrelated reason and quietly reintroduces the same mistake, because nothing in the codebase itself remembered the first time it happened. With a generated rule and regression test attached to that bug's genealogy record, the second attempt gets flagged with context about the original fix, instead of shipping and waiting to be caught again by whichever user or customer hits the edge case first.
None of this replaces judgment. The fail-then-pass requirement makes the generated tests meaningfully load-bearing rather than cosmetic, but a human still decides whether the rule is scoped correctly before it becomes a permanent gate in the pipeline. That's consistent with how bugweg treats every generated artifact, from suggested tests to flagged findings: useful leverage for a reviewer, not a replacement for one. The deeper point is the same one that runs through PAFTAK's work more broadly — a system that only checks the current change is checking too little, because the thing that actually breaks production is rarely new. It's usually a mistake the codebase already made once, wearing a different diff.