cursor agent rewrote my stripe webhook and dropped the idempotency key
Friday night prompt was literally "just clean up the webhook handler, don't change behavior."
It extracted the Stripe signature check into a helper (fine), then quietly deleted the Redis idempotency key because "unused after refactor." Same checkout.session.completed hit twice. Two invoices. CI stayed green because every test mocks Stripe.
I now keep a kill-list.md in the repo root: payments, auth, migrations. Agent can read. Agent cannot write. Anyone else doing hard denylists, or am I just bitter?
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this is why payment + auth paths should be read-only to agents by default. green CI with mocked Stripe is not a security check, it is theater.
we put
stripe/,auth/, and anything touchingrefundin a path denylist. agent can propose a patch in chat; human applies it. blast radius of a "cleanup" deleting idempotency is a real incident, not a style nit.same energy as when Make's AI builder invented a filter that silently dropped half my HubSpot rows. looked "cleaner." was not.
i screenshot the before state now, every time, before anyone hits Run. also: if a test suite mocks the thing that can charge money, that suite does not get to call itself coverage.
Denylist helps, but it is incomplete on its own. We put
/billingunder CODEOWNERS and added a CI grep that fails the PR if the diff removesidempotencyorSETNX.Caught two agent "cleanup" PRs that way last month. The tell is almost always the same: tiny diff, green mocks, no mention of money in the description.
same. agent once deleted our stripe event dedupe table because "webhook id already in logs". logs are not a database. three double charges before I saw it sunday morning.
if CI mocks Stripe you are testing the mock. I run one replay against a Stripe test clock on every payment PR now. annoying, cheaper than refunds.
we are two people shipping fintech. denylist alone was not enough — Claude Code still opened a "simplify" PR that collapsed idempotency into an in-memory Map the night before launch.
rule now: anything under payments needs a short plan doc in the PR or it gets closed. no exceptions, even at 2am Lagos time. bitter is the correct posture.