ITC per head.claimable is computed on total tax. In a TAX_HEAD_MISMATCH the total can agree while the heads (IGST vs CGST+SGST) do not; the credit must follow 2B's heads, so correct the books entry.
180-day interest is an estimate. Interest runs on the reversed ITC at 18% p.a. from invoice date to payment date, an upper-bound proxy: the statute counts from the date ITC was availed to the date of reversal, which depends on your GSTR-3B filing dates. Re-availment after payment is not modelled. Without a payment date nothing is reversed (only a warning is raised).
Time limit uses the 30 November rule only; the alternative "date of filing the annual return, if earlier" and any special extensions are not modelled. The as-of date defaults to the end of the period; use --as-of for the actual filing date.
Blocked credits are a configurable HSN-prefix and keyword heuristic, not a legal determination. Partial blocking (for example employee insurance) and the exceptions inside sec 17(5) must be handled by editing the rules.
2B sections not read: amendments (B2BA, CDNRA), ISD, IMPGSEZ, and the ITC reversal/ineligible summary tables. Only B2B, CDNR and IMPG are parsed.
Multiple line items: the 2B rate of an invoice is the rate of its largest taxable item; books rows are one line per invoice (aggregate multi-rate invoices before import).
One-to-many search is bounded (12 candidates, subsets of up to 4, 45-day window). Larger splits stay unmatched and are listed under ONLY_IN_*.
Fuzzy pass requires taxable within 5%. An invoice with both a mistyped number and a larger amount difference is reported as one ONLY_IN_2B plus one ONLY_IN_BOOKS; gstrecon explain shows the closest candidate.
State: one stored run per period (re-running replaces it). Re-running an earlier period after later ones removes carry-forward entries that the later runs had resolved; re-run the later periods afterwards.
IMPG documents have no supplier GSTIN, so they match on bill-of-entry number within the empty-GSTIN group.
The claimable figure is a reconciliation aid for preparing GSTR-3B, not a filing.