AGENTS.md: what makes a fork worth reporting, and three sharper rules

The ambiguity convention added earlier covered where to emit and how to
test presence. Six review rounds and Derek's scope call exposed four
gaps, three of them things I got wrong AFTER writing the rule.

- A kind is worth adding only if a reader would hesitate too. The
  existing rule said where to emit, never whether a fork deserves a
  kind at all. "Smith, John V" reads as a middle initial to anyone, so
  reporting it is noise that teaches callers to ignore the field --
  reachability of the second branch is necessary, not sufficient. This
  is what stopped the feature growing indefinitely; every reviewer kept
  finding more reachable forks, and reachability was the wrong measure.

- A branch that runs but changes nothing is not a decision. merge(k, j)
  executes when j == k + 1, folding a piece into itself; keying on "the
  code got here" reported a fork for all 39 ambiguous particles on
  "Dr. Van Jr." and double-reported with _assign. Third instance of the
  same mistake, the last one after the rule was written down.

- Pin the decision, not the vocabulary. The only titled-particle test
  used an UNAMBIGUOUS particle, so it walked the right code path and
  proved nothing about the branch under test -- two criticals passed
  1539 tests.

- Document the positive direction of a partial property. "A non-empty
  ambiguities is a signal" is checkable; "an empty one means no fork
  occurred" is a universal negative needing exhaustive verification. It
  was written twice and falsified twice, at sites I had not audited.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: derek73

AGENTS.md

@@ -119,11 +119,12 @@ The 2.0 rewrite lands as underscore-private modules alongside the v1 code. These
 - **Canonical field order** — the seven roles in `Role` enum declaration order, defined once and derived everywhere (properties, `as_dict`, reprs, `comparison_key`). Never restate the order literally.
 - **Method organization**, fixed section order in every class: fields + `__post_init__` validation → alternative constructors → dunders (construction/equality → protocol → operators) → properties → public methods by concern (access → editing → comparison → rendering delegates) → private helpers last, except a helper serving exactly one section may sit at that section's head. Sanctioned deviation, facade layer only: `HumanName` and the shim `Constants` organize by v1 concern groups (`# -- render defaults --`, `# -- config / parsing --`, `# -- fields --`, ..., dunders and pickle last) — the classes mirror v1's own surface and die in 3.0; the canonical order still binds every core type.
 - **Validation is eager and fail-loud**: every `raise` states the offending value, the expected form, and the fix. Exception taxonomy: wrong type — including wrong element type inside a collection, bare `str` where an iterable of strings is expected, or a `Mapping` where a plain iterable is expected — raises `TypeError`; well-typed but unacceptable values raise `ValueError`; failed enum lookups stay `ValueError` for any input (stdlib `EnumType` precedent).
-- **Ambiguities are emitted at the DECISION site**: an `Ambiguity` records a fork the parse had to call, not a token that sits in an ambiguous vocabulary. Emit where the branch is taken — the trailing-suffix peel in `_assign`, the delimiter escape's follow-up in `classify` — never by scanning for a `vocab:*-ambiguous` tag. The same tagged token is a genuine fork in one position and unremarkable in another (`do` mid-name in "Joao da Silva do Amaral de Souza" chooses nothing), and structure often settles the question before it arises, which is why `PARTICLE_OR_GIVEN` is deliberately not emitted on the `FAMILY_COMMA` path and `SUFFIX_OR_NAME` is not emitted for "Ma, Jack". The decision site also has the token index and the detail text in hand, which the tag scan would have to reconstruct. **If a fork's two branches are taken in DIFFERENT stages, every one of them needs the emitter** -- `PARTICLE_OR_GIVEN` is decided in `_assign` when the ambiguous particle stays a lone leading piece and in `_group` when a title shifts it off index 0 and the prefix chain claims it, so both report; for two years only the first did. The stage-ownership map in `tests/v2/pipeline/test_state.py` must list `ambiguities` for each such stage, and it passes vacuously until a case row exercises the path, so add the row too. Report BOTH directions of a two-way fork — "John Smith MA" (read as a suffix) and "Jack MA" (read as the family name) are equally guesses. Every kind needs a trigger in `tests/v2/test_contracts.py::_AMBIGUITY_TRIGGERS` (an explicit `None`, strict-xfail, while reserved), and case-table rows pin expected kinds exactly, so a new emitter shows up in both immediately.
+- **Ambiguities are emitted at the DECISION site**: an `Ambiguity` records a fork the parse had to call, not a token that sits in an ambiguous vocabulary. Emit where the branch is taken — the trailing-suffix peel in `_assign`, the delimiter escape's follow-up in `classify` — never by scanning for a `vocab:*-ambiguous` tag. The same tagged token is a genuine fork in one position and unremarkable in another (`do` mid-name in "Joao da Silva do Amaral de Souza" chooses nothing). **A branch that runs but changes nothing is not a decision either** -- the prefix chain's `merge(k, j)` executes even when `j == k + 1`, folding a piece into itself, and keying on "the code got here" reported a fork for all 39 ambiguous particles on "Dr. Van Jr.", where the particle stayed the GIVEN name and `_assign` reported the same token again. Check that the branch actually claimed something (`j > k + 1`) before recording. Structure also structure often settles the question before it arises, which is why `PARTICLE_OR_GIVEN` is deliberately not emitted on the `FAMILY_COMMA` path and `SUFFIX_OR_NAME` is not emitted for "Ma, Jack". The decision site also has the token index and the detail text in hand, which the tag scan would have to reconstruct. **If a fork's two branches are taken in DIFFERENT stages, every one of them needs the emitter** -- `PARTICLE_OR_GIVEN` is decided in `_assign` when the ambiguous particle stays a lone leading piece and in `_group` when a title shifts it off index 0 and the prefix chain claims it, so both report; for two years only the first did. The stage-ownership map in `tests/v2/pipeline/test_state.py` must list `ambiguities` for each such stage, and it passes vacuously until a case row exercises the path, so add the row too. Report BOTH directions of a two-way fork — "John Smith MA" (read as a suffix) and "Jack MA" (read as the family name) are equally guesses. Every kind needs a trigger in `tests/v2/test_contracts.py::_AMBIGUITY_TRIGGERS` (an explicit `None`, strict-xfail, while reserved), and case-table rows pin expected kinds exactly, so a new emitter shows up in both immediately. **Pin the decision, not the vocabulary**: the only titled-particle test used an UNAMBIGUOUS particle, so it walked the right code path and proved nothing about the branch under test -- two criticals passed 1539 tests. A row contrasting the two readings ("John Smith V" against "John Smith B") is what makes an emitter's absence meaningful.
+- **A kind is worth adding only if a reader would hesitate too**: the test is not "does the code take a branch" but whether a person reading that input would genuinely be unsure. "Smith, John V" reads as a middle initial to anyone -- the comma settles it -- so reporting it would be noise that teaches callers to ignore the field, which costs more than the missing report. Reachability of the second branch is necessary, not sufficient. Prefer leaving a fork silent and documenting the omission (see the comma paths in concepts.rst) over emitting on input nobody finds ambiguous.
 - **Invariants guard harm, not no-ops**: add a constructor check when violating it produces a *wrong parse*, not when it produces *nothing*. A false positive costs a working configuration; a true positive on an inert condition costs the user nothing, so that trade is never worth taking. `suffix_acronyms_ambiguous ∩ suffix_words` is guarded because the overlap loses a family name; `given_name_titles` is not, because an unreachable entry is simply never consulted (see Gotchas). Before adding one, construct the config it forbids and check what actually breaks.
 - **The shim TRANSLATES; it never raises on a config v1 accepted, and never silently changes the parse**: `Constants._snapshot()` is a translation boundary between v1's model and v2's invariants, and every transformation there carries its v1-reachability argument in a comment. Four exist today — `first_name_titles` re-folded per word (v1 joins-then-`lc`, v2 normalizes-then-joins), `suffix_acronyms_ambiguous ∩ acronyms` (a provable no-op), `suffix_words − ambiguous` (v1 already accepts the word via the acronym branch, so the addition is inert there), and `particles_ambiguous ∪ (bound ∩ particles)` (a pinned deviation, `test_bound_never_given_prefix_deviates_on_two_pieces`). When a v1 config cannot satisfy a v2 invariant, work out what v1 actually *does* with it — usually nothing — and reproduce that; weakening the invariant or letting the raise through are both wrong. **Test the case the translation decides**, not one where both branches agree: a test using an input v1 parses identically with and without the config pins nothing.
 - **Reprs are bounded**: render which fields deviate from a named baseline and by how much, never contents (`Lexicon(default + titles: +2)`).
-- **Typing/docs**: `from __future__ import annotations`; `frozen=True, slots=True` on every public dataclass; strict-profile mypy flags via per-module overrides in pyproject (`strict = true` itself is not valid per-module). Docstrings state contracts in prose with **no doctest blocks** — `--doctest-modules` makes every example a test; behavior examples go to unit tests per the lean-docs rule.
+- **Typing/docs**: `from __future__ import annotations`; `frozen=True, slots=True` on every public dataclass; strict-profile mypy flags via per-module overrides in pyproject (`strict = true` itself is not valid per-module). Docstrings state contracts in prose with **no doctest blocks** — `--doctest-modules` makes every example a test; behavior examples go to unit tests per the lean-docs rule. **Document the positive direction of a partial property**: "a non-empty `ambiguities` is a signal to act on" is checkable, while "an empty one means no fork occurred" is a universal negative needing exhaustive verification -- that claim was written twice and falsified twice, at sites the author had not audited.
 - **Pickling**: v2 types must round-trip (`Parser` will be picklable by construction, and it holds a `Lexicon`). Every frozen type assigns `_guarded_getstate`/`_guarded_setstate` (`_types.py`) in its class body (`@dataclass(slots=True)` would override inherited pickle methods) — unpickling fails at the LOAD site on field-layout skew, and values are deliberately NOT re-validated (pickle is not a security boundary; canonical state comes from a validated instance). `Lexicon` keeps its own copy of the guard (layering) plus the `mappingproxy` slot rebuild; a new unpicklable slot type needs the same treatment plus a round-trip test.
 - **One sanctioned global**: the (future) cached default `Parser`. Lazily cached FROZEN singletons (`Lexicon.default()`'s `functools.cache`, the future default parser) are constants, not state; any second piece of module-level MUTABLE state requires amending the conventions doc, on purpose, in review. Sanctioned exceptions, facade layer only: `_config_shim.CONSTANTS` (the v1 shared singleton, mutable by design) and `_facade._WARNED_SUBCLASSES` (the once-per-subclass hook-warning dedup set) — both deleted with the layer in 3.0.
 - **Tests**: all v2 tests live in the `tests/v2/` package (its `conftest.py` overrides the v1 dual-run fixture — v2 code never reads shared `CONSTANTS`), one test module per source module plus cross-cutting `test_reprs.py` and `test_layering.py`, names stating behavior. Never assert `Lexicon.default()` contents; the narrow sourcing spot-checks in `test_default_sources_v1_vocabulary` that pin the v1→v2 migration contract (e.g. the flipped `particles_ambiguous` model) are the sanctioned exception.