A ₹50 checkout bug taught me what a coding agent should report back

A developer built ReproRoom, a hackathon workflow that pairs a Mattermost chat room with a local Codex coding agent to reproduce, fix, and verify a deliberately planted checkout bug in a Node.js app. Given a report specifying an expected ₹900 total versus an observed ₹950, the agent wrote a failing regression test before editing code, then corrected the free-shipping threshold to compare against the original subtotal; the repair commit 658f2a9 passed all 10 tests. The developer noted the workflow lacked status signals like Received or Working, forcing a direct chat to confirm the agent had picked up the request.

Five tests passed. Then I entered SAVE10 on a ₹1,000 cart and watched the shipping line change from Free to ₹50 . The coupon had done its job. It took ₹100 off the item. But the checkout used the discounted amount to decide whether I still qualified for free shipping. My total came to ₹950. It should have been ₹900. I deliberately put that bug in a small Node.js app for the Flip the Switch Hackathon. I wanted to see whether someone who spotted a wrong total could report it in team chat, ask a coding agent for evidence, and decide for themselves whether the repair worked. I called the workflow ReproRoom . Watch the demo https://youtu.be/XqM5f-dg35M · Explore the code https://github.com/aditya052003/ReproRoom I set up a private Mattermost room called reproroom . In Switch Console, I connected a local Codex agent, repro-agent , to the room and gave it access to the checkout repository. I played the reporter and the reviewer. My report specified one item, one coupon, and two numbers: I expected ₹900. I got ₹950. I asked the agent to create a repair branch and run a regression test before touching the shipping calculation. That order mattered. The five baseline tests already passed. If the agent edited the code first and handed me a green test afterward, I’d still have to ask whether the test could catch the original bug. The first run gave me an answer I could check. Checkout amounts were stored in integer paise: | | Shipping | Total | |---|---|---| | Expected | 0 paise | 90,000 paise | | Observed before the fix | 5,000 paise | 95,000 paise | The focused test failed. The ₹5,000-paise difference in the total came entirely from shipping. The business rule was simple: a cart qualifies for free shipping when its original item subtotal reaches ₹1,000. The baseline compared the threshold against ₹900 after the coupon. The repair compared it against the original ₹1,000 instead. Fixing the reported cart was only half the job. A change from “greater than” to “greater than or equal to” can decide who pays for shipping. Here’s what I wanted the tests to establish: | Original subtotal | Correct shipping, with or without SAVE10 | |---|---| | ₹999.99 | ₹50 | | ₹1,000.00 | Free | | ₹1,000.01 | Free | At repair commit 658f2a9 , the previously failing test passed. The full suite reported 10 passed, 0 failed . The repair report https://github.com/aditya052003/ReproRoom/blob/fix/checkout-free-shipping-eligibility/REPAIR REPORT.md includes the command, the original failure, and the result after the fix. You can also inspect the regression test https://github.com/aditya052003/ReproRoom/blob/fix/checkout-free-shipping-eligibility/test/checkout.test.js directly. One awkward moment changed how I’d design this workflow. I posted the repair request in the room, but I couldn’t tell whether the agent had picked it up. I opened a direct chat to check. I wanted to avoid giving the same agent a second repair job while the first might still be running. A small improvement would have helped: put Received , Working , or Needs attention under the original request, with a link to the run. When you’re waiting on a teammate, “Did you get this?” is a different question from “Have you fixed it?” The part I’d keep was the review. Back in reproroom , I asked the agent to verify the exact repair commit, rerun the suite, and report which thresholds it covered. I could then do my part without reading a Git diff: open the browser, enter SAVE10 , and check the shipping line. This time it said Free , and the total was ₹900 . I recorded my acceptance in the room. The repair stayed on its branch, with merge and deployment subject to separate approval. The README https://github.com/aditya052003/ReproRoom readme explains how to set up the room and run both versions locally. To see the failure, use baseline abd68ed . To see the repair, use 658f2a9 . Keep the quantity at one and use the same coupon both times. If you build a similar workflow, start with one bug whose outcome the reporter can verify independently. For my demo, the best acceptance check was the simplest one: the person who saw ₹950 could repeat the same checkout and see ₹900.