Uprate the Scottish Carer Supplement with the benefit uprating index

changelog.d/1957.fixed.md

@@ -0,0 +1 @@
+- Uprate the Scottish Carer Supplement from 2027-28 with `gov.benefit_uprating_cpi`, the benefit uprating index the Carer Support Payment component already uses as its proxy for the previous September's CPI, instead of holding it at £11.70 a week.

policyengine_uk/parameters/gov/social_security_scotland/carer_support_payment/rate.yaml

@@ -13,3 +13,7 @@ metadata:
       href: https://www.mygov.scot/carer-support-payment
     - title: The Social Security (Up-rating) (Miscellaneous Amendments) (Scotland) Regulations 2026 reg. 12(3)(a)
       href: https://www.legislation.gov.uk/ssi/2026/170/regulation/12/made
+    - title: Social Security (Scotland) Act 2018 s. 86B (duty to uprate assistance)
+      href: https://www.legislation.gov.uk/asp/2018/9/section/86B
+    - title: Social Security Assistance in Scotland - up-rating for inflation in 2026-27, section 3 (12 months to September CPI)
+      href: https://www.gov.scot/publications/social-security-assistance-scotland-up-rating-inflation-2026-27/pages/4/

policyengine_uk/parameters/gov/social_security_scotland/carer_support_payment/supplement.yaml

@@ -7,10 +7,17 @@ metadata:
   period: week
   unit: currency-GBP
   label: Scottish Carer Supplement rate
+  uprating: gov.benefit_uprating_cpi
   reference:
-    - title: New rates for carer and disability payments in 2026
-      href: https://www.socialsecurity.gov.scot/news-events/news/new-payment-rates-from-april-2026
     - title: The Carer's Assistance (Miscellaneous and Consequential Amendments, Revocation, Transitional and Saving Provisions) (Scotland) Regulations 2025 reg. 3
       href: https://www.legislation.gov.uk/ssi/2025/340/made
     - title: The Social Security (Up-rating) (Miscellaneous Amendments) (Scotland) Regulations 2026 reg. 12(3)(b)
       href: https://www.legislation.gov.uk/ssi/2026/170/regulation/12/made
+    - title: Social Security (Scotland) Act 2018 s. 86A (inflation-adjusted level)
+      href: https://www.legislation.gov.uk/asp/2018/9/section/86A
+    - title: Social Security (Scotland) Act 2018 s. 86B (duty to uprate assistance)
+      href: https://www.legislation.gov.uk/asp/2018/9/section/86B
+    - title: Social Security Assistance in Scotland - up-rating for inflation in 2026-27, section 3 (12 months to September CPI)
+      href: https://www.gov.scot/publications/social-security-assistance-scotland-up-rating-inflation-2026-27/pages/4/
+    - title: Social Security Assistance in Scotland - up-rating for inflation in 2026-27, table 5 (2026-27 rates)
+      href: https://www.gov.scot/publications/social-security-assistance-scotland-up-rating-inflation-2026-27/pages/7/

policyengine_uk/tests/policy/baseline/gov/social_security_scotland/scottish_carer_supplement.yaml

@@ -0,0 +1,56 @@
+# Social Security (Scotland) Act 2018 s.86B requires Scottish Ministers to
+# bring forward legislation raising the Scottish Carer Supplement and the
+# Carer Support Payment component to at least their s.86A inflation-adjusted
+# levels; their measure is the 12 months to September CPI (Social Security
+# Assistance in Scotland: up-rating for inflation in 2026-27, section 3).
+# From 2027 the model uprates both with gov.benefit_uprating_cpi, its proxy
+# for that measure, which is 184.3803432 in 2026, 188.6206824 in 2027 and
+# 192.3927984 in 2028 under the OBR March 2026 EFO. The model does not round
+# to the nearest 5p as Ministers do.
+
+- name: Scottish Carer Supplement is 11.70 a week in 2026
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    age: 35
+    care_hours: 40
+    country: SCOTLAND
+  output:
+    # SSI 2026/170 reg. 12(3)(b): 11.70 x 52.
+    scottish_carer_supplement: 608.40
+    carer_support_payment: 86.45 * 52
+
+- name: Scottish Carer Supplement rises with gov.benefit_uprating_cpi in 2027
+  period: 2027
+  absolute_error_margin: 0.01
+  input:
+    age: 35
+    care_hours: 40
+    country: SCOTLAND
+  output:
+    # 11.70 x 188.6206824 / 184.3803432 x 52 = 622.39, up from 608.40.
+    scottish_carer_supplement: 11.70 * 188.6206824 / 184.3803432 * 52
+    # The Carer Support Payment component rises by the same factor.
+    carer_support_payment: 86.45 * 188.6206824 / 184.3803432 * 52
+
+- name: Scottish Carer Supplement keeps rising in 2028
+  period: 2028
+  absolute_error_margin: 0.01
+  input:
+    age: 35
+    care_hours: 40
+    country: SCOTLAND
+  output:
+    # 11.70 x 192.3927984 / 184.3803432 x 52 = 634.84.
+    scottish_carer_supplement: 11.70 * 192.3927984 / 184.3803432 * 52
+
+- name: Carer in England gets no Scottish Carer Supplement in 2027
+  period: 2027
+  absolute_error_margin: 0
+  input:
+    age: 35
+    care_hours: 40
+    country: ENGLAND
+  output:
+    scottish_carer_supplement: 0
+    carer_support_payment: 0

policyengine_uk/tests/test_scottish_carer_supplement_uprating.py

@@ -0,0 +1,91 @@
+"""Uprating of the Scottish Carer Supplement.
+
+Social Security (Scotland) Act 2018 s.86B requires Scottish Ministers to bring
+forward legislation raising each figure of assistance given under Chapter 2 of
+Part 2, carer's assistance included, to at least its s.86A inflation-adjusted
+level. Their measure is the 12 months to September CPI, rounded to the nearest
+5p (Social Security Assistance in Scotland: up-rating for inflation in
+2026-27, sections 2 and 3). SSI 2026/170 reg 12(3) raised the Carer Support
+Payment component from 83.30 to 86.45 and the Scottish Carer Supplement from
+11.29 to 11.70: both are the 3.8% September 2025 rate, rounded to 5p. The
+supplement had no uprating metadata, so the model froze it at 11.70 a week from
+2027 while the Carer Support Payment component kept rising.
+
+Invariants, for every year from 2027 to 2039, the last year of the uprating
+index:
+
+1. Differential: the supplement and the Carer Support Payment component grow
+   by the same factor from their April 2026 rates.
+2. That factor is gov.benefit_uprating_cpi relative to 2026, so the supplement
+   never falls while the index does not.
+3. Metamorphic: setting CPI growth in the year before to g scales both
+   components by 1 + g on the year, to within the rounding of the index.
+
+The YAML tests in tests/policy/baseline/gov/social_security_scotland check
+that a Carer Support Payment recipient in Scotland gets 52 weeks of the rate.
+"""
+
+import pytest
+from hypothesis import HealthCheck, given, settings
+from hypothesis import strategies as st
+
+from policyengine_uk import CountryTaxBenefitSystem
+
+BASE_YEAR = 2026
+YEARS = range(BASE_YEAR + 1, 2040)
+SUPPLEMENT_2026 = 11.70
+CSP_RATE_2026 = 86.45
+
+
+@pytest.fixture(scope="module")
+def parameters():
+    return CountryTaxBenefitSystem().parameters
+
+
+@pytest.mark.parametrize("year", YEARS)
+def test_supplement_uprates_with_carer_support_payment(parameters, year):
+    csp = parameters.gov.social_security_scotland.carer_support_payment
+    index = parameters.gov.benefit_uprating_cpi
+    factor = index(str(year)) / index(str(BASE_YEAR))
+
+    assert csp.supplement(str(BASE_YEAR)) == SUPPLEMENT_2026
+    assert csp.rate(str(BASE_YEAR)) == CSP_RATE_2026
+    assert csp.supplement(str(year)) / SUPPLEMENT_2026 == pytest.approx(
+        csp.rate(str(year)) / CSP_RATE_2026, rel=1e-12
+    )
+    assert csp.supplement(str(year)) == pytest.approx(
+        SUPPLEMENT_2026 * factor, rel=1e-12
+    )
+    if index(str(year)) >= index(str(year - 1)):
+        assert csp.supplement(str(year)) >= csp.supplement(str(year - 1))
+
+
+@settings(
+    max_examples=8,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow],
+)
+@given(
+    year=st.integers(min_value=BASE_YEAR + 1, max_value=2035),
+    growth=st.floats(min_value=0, max_value=0.12),
+)
+def test_supplement_follows_prior_year_cpi(year, growth):
+    system = CountryTaxBenefitSystem()
+    system.reset_parameters()
+    cpi = system.parameters.gov.economic_assumptions.yoy_growth.obr
+    shocked = [
+        value_at_instant
+        for value_at_instant in cpi.consumer_price_index.values_list
+        if value_at_instant.instant_str == f"{year - 1}-01-01"
+    ]
+    assert len(shocked) == 1
+    shocked[0].value = growth
+    system.process_parameters()
+
+    csp = system.parameters.gov.social_security_scotland.carer_support_payment
+    supplement_growth = csp.supplement(str(year)) / csp.supplement(str(year - 1))
+    rate_growth = csp.rate(str(year)) / csp.rate(str(year - 1))
+
+    assert supplement_growth == pytest.approx(1 + growth, abs=1e-4)
+    assert supplement_growth == pytest.approx(rate_growth, rel=1e-12)