[MaxGhenis]

Settles the NZ dependent-child rule for benefit units (method card MC7) and makes unit construction apply it. This is package G8-u of #1183.

build/nz/benefit_unit_rule.json had age_limit_years: null in an "UNVERIFIED proposal", and transport/compose.py refuses a null limit. So no NZ benefit unit could be built on real data.

The rule, from the Social Security Act 2018 Schedule 2 (as at 1 April 2026)

The hub read the statute and settled the open cases; that is item B8 of the G8 method addendum. The spec now declares:

• "Child": single and under 18 (exclusive age limit 18), except a 16- or 17-year-old who is financially independent.
• Financially independent by work: a 16- or 17-year-old whose donor record shows at least 30 hours a week ("full employment") forms their own unit. Hours come from the donor's usual weekly hours, carried as the usual_weekly_hours concept (benefit_unit_rule.json:47).
• Teen parents:
  • A 16- or 17-year-old with a co-resident child of their own forms their own unit with that child.
  • A parent under 16 stays a dependent child in their own parent's unit, and their child joins that same unit.
  • These branches decide the teen parent's own dependency. When the child's two co-resident parents end up in separate units, the child follows the declared parent-1 placement.
• Two co-resident parents who are not partners: the child joins parent 1's unit (unchanged, :119).
• A child with no co-resident parent: joins the household reference person's unit. Construction refuses when that person is themselves a dependent child (:64).

Disclosed, not applied (:59, :124):

• Donor hours in worked weeks only approximate the statutory average; contracts and the averaging period aren't observed.
• These limbs aren't observable and are not applied: student allowance and analogous schemes, other main-benefit receipt, contractual full-time exceptions, and orphan's or unsupported child's benefit receipt.
• Single status (Sch 2 DLM6784713/DLM6784537, s 8) is approximated by the co-resident partner pointer. A young person whose spouse or partner lives elsewhere is treated as single and can be a dependent child. Legal marital status isn't recovered by the donor decoder, and MSD's section 8 determinations aren't observable (412d4a7).
• Teen parenthood is inferred from co-resident child pointers.

Every number lives in the spec. The shared code reads it and stays country-neutral (transport/cli.py:266, column_kernels.py:134, population_kernels.py:225 share one spec parser).

Code

microcosm/frame/unit_construction.py:

• parses the cited country declarations and keeps serialized-rule compatibility (:380);
• reads the teen-parent age (:775) and the employment threshold (:785);
• follows dependent-parent chains and refuses dependent reference caregivers (:820);
• reports diagnostics (:609): 16–17-year-olds made independent by hours, teen parents in each branch, non-partnered co-parent households, and children placed with the reference person.

The CREATE receipt carries those counts (population_kernels.py:289).

Invariants (tested as Hypothesis properties over random households, test_transport_dependent_children.py:352, :437)

• Every person is in exactly one unit.
• Every unit has an adult head.
• Partners share a unit.
• Units nest within households.
• Inherited family weights sum correctly.
• The result doesn't depend on row order.

Tests

Hub re-run at this head, one file per run:

File	Result
test_transport_dependent_children.py (new)	24 passed
test_unit_construction.py	43 passed
test_units.py	26 passed
test_transport_graph.py	15 passed
test_nz_spec_package.py	72 passed
test_country_spec.py	135 passed

The new file has a fixture for each case:

• a 17-year-old working 35 hours, and one working 20;
• a 16-year-old parent and a 15-year-old parent, each with a baby;
• two non-partnered co-resident parents;
• a child living only with a grandparent;
• a dependent-child reference person, which is refused.

It also has boundary cases, spec-variation witnesses and round-trips. The lane also ran the CREATE and column integration tests (2 + 2), the concept tests (26 + 26) and the unit-construction compatibility test (24).

Mutants, each failing a named test with a passing control:

• age limit 19 → test_eighteen_year_old_is_an_adult_even_without_work;
• hours limb ignored → test_seventeen_year_old_hours_determine_independence[35-hours];
• teen parents left dependent → test_teen_parent_and_baby_join_the_declared_unit[age16];
• parentless child left unassigned → test_child_without_parent_joins_grandparent_reference_person;
• hours threshold hard-coded → test_hours_threshold_is_read_from_spec[threshold10].

Also run: ruff check ., tools/ci_test_plan.py verify and git diff --check, all passing. At the current head 4cd53ce (main merged, including #1219), the CI plan verifies 682 files.

The NZ golden country spec is regenerated (ef0be9a). Only the benefit_unit_rule.json resource hash and the spec fingerprint change, and the new hash equals the committed file's sha256.

How it was built

• A GPT-6.1 Sol lane wrote the spec, code and tests (f58fcbd).
• The hub reports that a machine restart at 11:42Z cut the run and that it resumed from its worktree. No tracked record proves this; the test results above were re-run at this branch by the hub and by the reviewer.
• The hub re-ran the two suites the restart interrupted and regenerated the golden.
• 5488d5e dropped the settled age from _GAPS in test_transport_composed_graph.py, which CI had flagged. 951091c and 4cd53ce merged main and regenerated the golden.

Review

• r1, 20261012-184520 (GPT-6.1 Sol) at 951091c: APPROVE_WITH_NITS.
  • It found no correctness, safety or honesty defect.
  • It confirmed against the PCO XML (2026-04-01): age 18 exclusive, 16/17 financial independence, the 30-hour threshold, and every cited identifier.
  • 898 baseline tests passed. Mutants for all six invariant classes, and all five named mutants, failed as expected. 1,000 old-rule comparisons matched main.
• Its three P3 nits are applied:
  • the single-status proxy disclosure in the spec;
  • the reference-person docstring;
  • evidence refreshed here (682-file CI plan at this head, attributed history labelled).
• Its "Needs Max" flag was for publishing a settled statutory method in the public spec. The hub judged no: the NZ package is inactive and publishes no number, the statutory parts are verified, the placements are disclosed modelling conventions, and the spec is reversible data.
• r2, 20261012-212506 (GPT-6.1 Sol), delta at 4cd53ce: APPROVE_WITH_NITS.
  • 231 tests passed and 6/6 mutation guards failed as expected. The single-status disclosure matches the code.
  • Its two P3 nits are applied in ce7b8fc:
    • the teen-parent wording is qualified with the parent-1 placement;
    • the spec's two local absolute paths are replaced: the Schedule 2 XML is now cited by its pinned axiom-corpus release (nz-rulespec-2026-07-25), path and sha256 (ce02ac72…), and the B8 settlement by the new in-repo docs/nz-benefit-unit-rule.md.
  • The NZ golden is regenerated (benefit_unit_rule hash and fingerprint only). Tests: dependent_children 24, nz_spec 72, country_spec 135; the CI plan verifies.
  • Its Needs Max flag is the same as r1's and is judged no for the same reasons.

🤖 Generated with Claude Code

[MaxGhenis]

Merged by the NZ hub (session local_e407a527) at ce7b8fc with --squash. Gates: gh pr checks exit 0; MERGEABLE; not draft; no CHANGES_REQUESTED review. Reviewed head: ce7b8fc. Reviews (GPT-6.1 Sol): r1 20261012-184520 APPROVE_WITH_NITS at 951091c (statute verified against the 2026-04-01 PCO XML; 898 tests; all mutants); its nits applied in 412d4a7. r2 20261012-212506 APPROVE_WITH_NITS at 4cd53ce (231 tests; 6/6 mutation guards); its two P3 nits applied in ce7b8fc (teen-parent wording; local paths replaced by the pinned corpus release + sha256 and docs/nz-benefit-unit-rule.md; golden regenerated). ce7b8fc changes only spec text, a new doc and the golden hash; hub re-ran dependent_children 24, nz_spec 72, country_spec 135. Both reviews' 'Needs Max' flags (publishing a settled statutory method in the public spec) judged no by the hub: the NZ package is inactive and publishes no number, statute verified, placements disclosed, reversible spec data.

[MaxGhenis]

Hub note: this merged at ce7b8fc before CI had registered checks for that commit. The hub's land script treated "no checks reported" right after the push as a repo with no CI, which was a script bug and is now fixed. ce7b8fc changed only spec text (citations and teen-parent wording), added docs/nz-benefit-unit-rule.md and regenerated the golden hash. The hub ran dependent_children (24), nz_spec (72) and country_spec (135) locally, and the previous head 4cd53ce was green apart from one pending engine-us job. Main's CI on the merge commit 001814f is being watched; any failure gets an immediate fix-forward PR.

[MaxGhenis]

Follow-up to the note above: main's CI on the merge commit 001814f is fully green. All 10 checks passed: engine-free, engine-uk and engine-us on 3.13 and 3.14, integration-uk, lint, select-countries and wheels. No fix-forward needed.