Known divergences from python-ags4

laterite is a clean-room re-implementation of the AGS4 rules, validated against the incumbent python-ags4 on its own test corpus (see Cross-surface parity). Where the two deliberately differ, the difference is catalogued as an O-N observation. This is the user-facing list; the full catalogue (including internal NOTE/SPEC entries) lives in OBSERVATIONS.md in the repo.

Every one of these is intentional and tested — the compliance harness reconciles them so they never read as a regression.

Encoding & edition

#	Divergence
O-32	Non-UTF-8 input is decoded lossily (like python's errors="replace"), not refused.
O-1	Rule 1's "entirely ASCII" is treated as an FYI for extended-ASCII, not a hard error.
O-30	Edition is selected from TRAN_AGS; some deliberate differences from python's mapping.
O-42	TRAN_AGS="4.0" resolves to 4.0.4 (superset-safe); python's static 4.0→4.0.3 over-reports Rule 10c.
O-10 / O-20 / O-25 / O-28	Dictionary selection is TRAN_AGS-driven and self-contained; the external --dict override is deferred.

Rule attribution & value checks

#	Divergence
O-2	Rule 6 is a no-op in python-ags4; laterite implements the embedded-CR check.
O-8	python-ags4's rule_7_2 can raise IndexError on duplicate headings; laterite handles it.
O-12 / O-33	The DT/datetime validity engine differs from pandas (and is bounded to pandas' Timestamp range for parity).
O-31	Rule 8 flags an empty DT UNIT (closing a python-parity gap).

Leniency & structure

#	Divergence
O-37	The native parser is lenient where python-ags4 raises hard (duplicate GROUP, ragged rows) — findings-first, never crash.
O-41	Rows before the first GROUP are reported as Rule 2 findings, not a parser crash.
O-34	A non-AGS4 file surfaces as a clean NotAgs4 error, reconciled against python's "missing mandatory groups".

Checks laterite adds

#	Divergence
O-43	A self-declared but non-standard PA abbreviation → a laterite FYI (Related to Rule 16). python-ags4 has no such check.
O-44	Structural validation of a file-level DICT group → a laterite WARNING (Related to Rule 18).
O-45	An unrecognised TRAN_AGS edition → a laterite WARNING (Related to Rule 14), shown by default.

Reading the tiers

Whether a divergence surfaces as an error, warning, or FYI follows laterite's severity tiers.