Keep the Social Security wage base on current law when a reform changes only the payroll cap under a Trustees assumption

changelog.d/trustees-wage-base-cap.fixed.md

@@ -0,0 +1 @@
+Keep the Social Security wage base on each Trustees assumption's path when a reform changes only the payroll cap.

policyengine_us/parameters/uprating_extensions.py

@@ -820,14 +820,24 @@ def extend_social_security_payroll_cap(
 def extend_social_security_wage_indexed_parameters(
     parameters: ParameterNode,
     end_year: int,
+    *,
+    cap: Optional[Parameter] = None,
 ) -> None:
-    """Extend SSA wage-indexed benefit parameters with statutory lag and rounding."""
+    """Extend SSA wage-indexed benefit parameters with statutory lag and rounding.
+
+    The wage base for each year from 2027 is set to the contribution and
+    benefit base. `cap` defaults to `gov.irs.payroll.social_security.cap`.
+    Pass another parameter that holds the base, such as a copy of it, to read
+    the wage base from that parameter instead, so a reform that changes only
+    the payroll cap doesn't move the wage base when this runs after it.
+    """
     ssa = parameters.gov.ssa
     social_security = ssa.social_security
     nawi = ssa.nawi
 
     wage_base = social_security.wage_base
-    payroll_cap = parameters.gov.irs.payroll.social_security.cap
+    if cap is None:
+        cap = parameters.gov.irs.payroll.social_security.cap
     qc_threshold = social_security.quarters_of_coverage_threshold
     earnings_test = social_security.earnings_test
     pia = social_security.pia.formula_factors
@@ -839,7 +849,7 @@ def extend_social_security_wage_indexed_parameters(
 
         wage_base.update(
             period=period,
-            value=payroll_cap(f"{year}-01-01"),
+            value=cap(f"{year}-01-01"),
         )
 
         qc_threshold.update(

policyengine_us/reforms/ssa/trustees_2025.py

@@ -201,6 +201,34 @@ def apply_trustees_2025_economic_assumptions(
         last_projected_year=2035,
         end_year=end_year,
     )
-    # The wage base and the other wage-indexed amounts follow the cap and the
-    # NAWI, so re-derive them from the Trustees path as well (#10452).
-    extend_social_security_wage_indexed_parameters(parameters, end_year=end_year)
+    # Extend the owners' draw copy on the same rule from its own values, not
+    # from the live cap, so a policy reform's cap can't become the copy's
+    # no-decrease floor when the reform set runs again.
+    current_law_cap = parameters.gov.contrib.ssa.owners_draw.current_law_cap
+    extend_social_security_payroll_cap(
+        parameters,
+        last_projected_year=2035,
+        end_year=end_year,
+        cap=current_law_cap,
+    )
+    # Extend the benefit wage base on the same rule from its own values
+    # through 2035. The rule reads the wage base's 1994 base, its value for
+    # the year before and the NAWI, but neither the live cap nor the copy, so
+    # a reform that changes only one of those doesn't move the wage base when
+    # the reform set runs again. A reform that means to move it sets the wage
+    # base itself.
+    wage_base = parameters.gov.ssa.social_security.wage_base
+    extend_social_security_payroll_cap(
+        parameters,
+        last_projected_year=2035,
+        end_year=end_year,
+        cap=wage_base,
+    )
+    # The other wage-indexed amounts follow the NAWI, so re-derive them from
+    # the Trustees path as well (#10452). This reads the wage base from
+    # itself, so it rewrites the wage base with its own values.
+    extend_social_security_wage_indexed_parameters(
+        parameters,
+        end_year=end_year,
+        cap=wage_base,
+    )

policyengine_us/reforms/ssa/trustees_2026.py

@@ -39,8 +39,10 @@
 - the base's owners' draw copy, ``gov.contrib.ssa.owners_draw.current_law_cap``,
   on the same rule from the copy's own values through 2027, so a policy reform
   that changes the live base doesn't reach it;
-- the wage base, quarter-of-coverage threshold, earnings-test exempt amounts,
-  PIA bend points and SGA amounts, from 2027;
+- the wage base on the same rule from its own values through 2027, so a
+  policy reform that changes the live base or the copy doesn't reach it;
+- the quarter-of-coverage threshold, earnings-test exempt amounts, PIA bend
+  points and SGA amounts, from 2027;
 - every parameter whose ``uprating`` metadata names either path. Core's
   ``uprate_parameters`` writes their values when the system builds, so a later
   change to a path doesn't reach them. The module derives each again as core
@@ -480,7 +482,26 @@ def apply_trustees_2026_economic_assumptions(
         end_year=end_year,
         cap=parameters.gov.contrib.ssa.owners_draw.current_law_cap,
     )
-    extend_social_security_wage_indexed_parameters(parameters, end_year=end_year)
+    # Extend the benefit wage base on the same rule from its own values
+    # through 2027. The rule reads the wage base's 1994 base, its value for
+    # the year before and the NAWI, but neither the live cap nor the copy, so
+    # a reform that changes only one of those doesn't move the wage base when
+    # the reform set runs again. A reform that means to move it sets the wage
+    # base itself.
+    wage_base = parameters.gov.ssa.social_security.wage_base
+    extend_social_security_payroll_cap(
+        parameters,
+        last_projected_year=TRUSTEES_2026_LAST_DETERMINED_CAP_YEAR,
+        end_year=end_year,
+        cap=wage_base,
+    )
+    # Reads the wage base from itself, so this rewrites the wage base with its
+    # own values and extends the other wage-indexed amounts on the NAWI.
+    extend_social_security_wage_indexed_parameters(
+        parameters,
+        end_year=end_year,
+        cap=wage_base,
+    )
     apply_trustees_2026_path_readers(parameters, end_year=end_year)