[vahid-ahmadi]

Corrects the UK pipeline alignment planning page (docs/book/assumptions/uk-pipeline-alignment-plan.md), written for #1621.

The page itself is already on main: it shipped in #2386 (merged 21 October 2026), and its original .added changelog entry was released in 2.143.0. This branch has merged main, dropped the released fragment and now carries only corrections to main's copy of the page.

What this PR corrects (from its changelog fragment):

Corrected the UK pipeline alignment planning page (docs/book/assumptions/uk-pipeline-alignment-plan.md): five would_claim_* formulas read reported receipt (Council Tax Reduction was missing); in data-built simulations claims_all_entitled_benefits is false and Housing Benefit and Council Tax Reduction ignore it, and eligibility for tax credits, Income Support and working-age Housing Benefit needs a reported award; policyengine-uk-data anchored take-up (#359), imputed gift aid (#347) and added a second-stage imputation (#362) in April 2026; the residual aggregates are #1621's April 2026 figures; the Income Support phase-out landed in #1631; CTC_child_element is not gated on is_CTC_eligible, so in tax-credit years an ineligible Child Tax Credit claimant's tax_credits enters its Housing Benefit and Council Tax Reduction means tests; and the third small item in #1621 was a ~-on-bool warning, not state_pension_type.

The claim-by-claim review behind these corrections, with sources, is in the "Ready for hub review" comment on this PR.

The page also records a model defect outside this docs PR: CTC_child_element is not gated on is_CTC_eligible, so in tax-credit years an ineligible Child Tax Credit claimant gets tax_credits that enter its Housing Benefit and Council Tax Reduction means tests. That needs its own issue and fix.

• Changelog fragment: changelog.d/1621-pipeline-alignment-page-corrections.changed.md (changed).
• Docs and changelog only; no code changes.

🤖 Generated with Claude Code

[MaxGhenis]

UK hub (Claude build session, for Max)
Ready for hub review

Head: 4b08daa739c7adede799bdeb55c5375a819c6390

Context. This page already reached main, byte for byte, through #2386 (merged 21 October 2026; batch commit d50ee85), and its 1621.added.md entry is already in CHANGELOG.md under 2.143.0. I merged origin/main into this branch (no rebase, no force-push), deleted the released .added fragment, and corrected the page. The PR's net diff against main is now just those corrections plus changelog.d/1621-pipeline-alignment-page-corrections.changed.md. Suggested title: "Correct the UK pipeline alignment planning page".

Review. I had a verifier extract and check every claim, checked each correction myself, and then ran independent adversarial reviews (with household runs on main). Rounds 1 to 3 asked for changes, and round 4 approved this head.

Corrected (17)

Gap 1 (would_claim_* and claims_all_entitled_benefits)

1. Five would_claim_* formulas read _reported, not four: would_claim_council_tax_reduction was missing (would_claim_council_tax_reduction.py:18-24).
2. The runtime pattern differs by simulation type (claims_all_entitled_benefits.py:26-52):
   • hand-entered: flag | reported, with the flag true for families reporting under £1 across seven benefits;
   • data-built: the flag is false, and HB and CTR ignore it (would_claim_housing_benefit.py:21-22).
3. Setting the flag does not reach non-reporters. The eligibility tests for CTC, WTC, IS and working-age HB also need a reported award (is_CTC_eligible.py:12-17, is_WTC_eligible.py:46-53, income_support_eligible.py:73-75, housing_benefit_eligible.py:59-65).
4. New: CTC_child_element is not gated on is_CTC_eligible. In tax-credit years an ineligible claimant gets tax_credits but no child_tax_credit, and that amount enters the HB and CTR means tests (housing_benefit_applicable_income.py:94, council_tax_reduction_applicable_income.py:207) and the 2024 winter payment tests (utils/winter_heating.py:77-80). This already happens on main for hand-entered families with children that report nothing.
5. The Fix paragraph: the conversion changes awards only for CTR and pension-age HB, and needs that CTC gate first.

Gaps 2 and 3 (take-up anchoring and the second stage)
6. The Enhanced FRS already anchored UC, PC and CB take-up on reported receipt in policyengine-uk-data#359 (merged 18 April 2026), not only Microcosm.
7. The gap 2 fix is marked done in uk-data#359.
8. uk-data#347 (17 April) imputed gift aid on SPI rows.
9. uk-data#362 (18 April) added a narrower second-stage QRF. The old description is now in the past tense.
10. In Microcosm, gift_aid is a stage-1 SPI output; stage 2 trains on FRS rows and predicts for SPI rows (source_stages.json at 75167a68).
11. Microcosm's stage 2 matched #362 at 75167a6, less two reports, and has since added eight outputs (at 54b514f).

Residual aggregates
12. The non-State-Pension residuals are #1621's April 2026 figures on Enhanced FRS 1.50.3, which predates #347, #359 and #362; they are not "May 2026 measurements".
13. The same dating fixed in the admonition.
14. "Most of" the State Pension shortfall arises in building the Enhanced FRS (state-pension.md:399-405).

Other
15. #1621's third small item is the ~-on-bool DeprecationWarning, with no fix tracked; state_pension_type was a separate fix (#1618). The heading is renamed to match.
16. The Income Support phase-out landed in #1631 (parameters/gov/dwp/income_support/active.yaml: false from 2026-04-01).
17. Replaced the released .added fragment with a .changed fragment.

Checked and correct (unchanged)

• would_claim_uc and would_claim_pc are input-only with default True.
• gov.dwp.tax_credits.active is false from 2025-04-06, and tax credits ended on 5 April 2025 (GOV.UK).
• The Microcosm stage names and order are as described (test_uk_source_stages.py).
• policyengine-us-data#589 and assign_takeup_with_reported_anchors exist.
• Fix state_pension_type classifying all pensioners as BASIC #1618, Fix misleading parameter/variable metadata (Tax Credits min_benefit currency, benunit_weekly_hours label) #1627 and Pro-rate new_state_pension by reported amount #1634 are merged; Align UK model/data pipeline with US patterns: reported-anchoring, second-stage imputation, and downstream bugs #1621, State pension undershoot: -£12bn vs OBR, ASP/Protected Payment under-captured #1632 and uk-data#493 are open.
• The min_benefit unit and the benunit_weekly_hours label are fixed.
• The State Pension row matches state-pension.md.

Follow-up for the hub (model defect, outside this docs PR)

• The ungated CTC_child_element in item 4 is a live model defect for tax-credit years. A household run (2024, Welsh lone parent on £8,000, no reports) gives tax_credits £3,455, CTC £0 and CTR £0, against £574.96 CTR when would_claim_CTC is false.
• I found no open issue for it. It needs an issue and a fix PR: gate CTC_child_element on is_CTC_eligible, as the family and disabled child elements are.