Note the tax add-back in the carer supplement invariant; record the carer_support_payment meaning change

changelog.d/pension-credit-carers-income-followups.changed.md

@@ -0,0 +1 @@
+Since 2.106.0, `carer_support_payment` means the Carer Support Payment component only, as "carer support payment" does in the reserved-benefit regulations since SI 2026/246. The Scottish Carer Supplement is the separate `scottish_carer_supplement`; code that summed `carer_support_payment` for the total paid to a Scottish carer must add both.

policyengine_uk/tests/policy/baseline/gov/dwp/pension_credit/pension_credit_carer_income.yaml

@@ -158,15 +158,14 @@
     # Credit is the maximum, 60% of (standard minimum guarantee - threshold).
     savings_credit: 0.6 * (238 - 208.07) * 52
 
-- name: Housing Benefit no longer falls where Guarantee Credit used to end for a couple whose partner receives Carer's Allowance
-  # Before this change, Pension Credit left out the partner's Carer's
-  # Allowance, so Guarantee Credit ran on to a private pension of 9,700 and the
-  # Housing Benefit passport ended at 9,800, when HB counted the allowance:
-  # net income fell by 1,779.24 with the default HB income disregard (2,254.02
-  # with the disregard set to 0, as here). Now Guarantee Credit has ended well
-  # before, and each 100 of private pension adds 100 - 20 tax - 52 HB (65%)
-  # - 16 CTR. The HB income disregard is set to 0 so that the numbers do not
-  # depend on it; the couple has no earnings.
+- name: Housing Benefit and Council Tax Reduction no longer fall where Guarantee Credit used to end for a couple whose partner receives Carer's Allowance
+  # Before #1952, Pension Credit left out the partner's Carer's Allowance, so
+  # Guarantee Credit ran on to a private pension of 9,700 and the Housing
+  # Benefit and Council Tax Reduction passports ended at 9,800, when both
+  # counted the allowance, and net income fell steeply there. Now Guarantee
+  # Credit has ended well before, and each 100 of private pension adds
+  # 100 - 20 tax - 52 HB (65%) - 16 CTR. The HB income disregard is set to 0
+  # so that the numbers do not depend on it; the couple has no earnings.
   period: 2026
   absolute_error_margin: 0.01
   input:

policyengine_uk/tests/test_pension_credit_carer_income_properties.py

@@ -3,12 +3,13 @@
 Pension Credit income left out Carer's Allowance and Carer Support Payment,
 although the State Pension Credit Regulations 2002 reg 15(1) prescribe "all
 social security benefits" as income except those they list, and neither is
-listed. Pension-age Housing Benefit counted both. While Guarantee Credit was
-paid, Housing Benefit was passported to its maximum and hid the difference;
-when Guarantee Credit ended, Housing Benefit was assessed on the carer's
-benefit that Pension Credit had ignored. A pensioner couple whose partner
-received Carer's Allowance lost £1,779.24 of net income when private pension
-rose from £9,700 to £9,800 in 2026.
+listed. Pension-age Housing Benefit counted both, and Council Tax Reduction
+counted Carer's Allowance (it does not yet count Carer Support Payment: #1955).
+While Guarantee Credit was paid, both were passported to their maximum and hid
+the difference; when Guarantee Credit ended, both were assessed on the
+Carer's Allowance that Pension Credit had ignored, so a pensioner couple whose
+partner received it lost well over the £100 rise in their private pension at
+that step.
 
 The Scottish Carer Supplement, paid with Carer Support Payment from 15 March
 2026, is the opposite case: SPC Regs reg 15(1)(ri) excepts it from Pension
@@ -31,7 +32,12 @@
    cliff, so it is excluded.
 3. Metamorphic: setting the Scottish Carer Supplement to zero changes
    Pension Credit income and Housing Benefit applicable income only through
-   income tax: each plus income tax is unchanged.
+   income tax: each plus income tax is unchanged. This encodes the model's
+   current behaviour, not the law's. The law disregards tax only on income
+   taken into account (SPC Regs reg 17(10); HB (SPC) Regs 2006 reg 33(12)),
+   so the supplement should leave both measures unchanged outright, while
+   the model deducts all income tax, including the tax on the supplement
+   (#1954). When that is fixed, assert the measures themselves are unchanged.
 
 These compare PolicyEngine's own income measures, so they hold whatever
 carer's benefit the model pays. The model does not yet apply the