Somewhere in the space of Indian salary returns there was a point where earning ₹52,880 more computed ₹10 less tax. No CA ever hit it. No filer ever noticed it. My fuzzer found it: a one-in-350,000 floating-point rounding edge sitting in a tax engine that had already passed every hand-written test I had.
That bug is fixed and pinned as a regression test (the comment in it records the mechanism: a true half-point carried as 4.999...97 that rounded down), and it is the cleanest argument I have for the design rule behind itr-wala, an open-source skill that prepares Indian income tax returns from the terminal: the model is banned from doing math.
TL;DR The LLM reads your Form 16, AIS and 26AS, interviews you the way a good CA would, and fills in one JSON. Every rupee after that comes from plain Python: 51 golden tests hand-derived from the statute, 104 validator tests, a fuzzer past 350,000 random returns. Before anything is submitted, the portal's own computed figure has to match the engine's to the rupee, and you press every button yourself. Non-audit ITR-3/4 returns are due August 31.
The wave this rode in on
In June and July, my feed filled with people filing their ITR with bare Claude. The best-documented run I found is a 250-upvote r/IndiaTax post: a software engineer with RSUs and US stocks, documents redacted, six hours spent reviewing every section before submitting. It mostly went well. The one place the model slipped, by the author's own account, was asking for Schedule AL under a threshold that had since been raised; the filer happened to know the current rule and caught it.
