diff --git a/changelog.d/fix-ctc-ss-tax-filer-spouse.fixed.md b/changelog.d/fix-ctc-ss-tax-filer-spouse.fixed.md
new file mode 100644
index 00000000000..b7b37243622
--- /dev/null
+++ b/changelog.d/fix-ctc-ss-tax-filer-spouse.fixed.md
@@ -0,0 +1 @@
+The refundable child tax credit's social security taxes (26 U.S.C. 24(d)(2)) and the Additional Medicare Tax (Form 8959) now count only the head's and spouse's wages and self-employment income, not a tax unit dependent's. The CRFB AGI surtax's expanded base now adds only the head's and spouse's contributions, exclusions and tax-exempt Social Security.
diff --git a/policyengine_us/parameters/gov/contrib/crfb/surtax/increased_base/in_effect.yaml b/policyengine_us/parameters/gov/contrib/crfb/surtax/increased_base/in_effect.yaml
index 2edbf44becf..842c1ab91d5 100644
--- a/policyengine_us/parameters/gov/contrib/crfb/surtax/increased_base/in_effect.yaml
+++ b/policyengine_us/parameters/gov/contrib/crfb/surtax/increased_base/in_effect.yaml
@@ -3,6 +3,9 @@ metadata:
   unit: bool
   period: year
   label: CRFB surtax expanded base in effect
+  reference:
+    - title: "Committee for a Responsible Federal Budget, Break Glass: A Plan for the Next Economic Shock, footnote 2"
+      href: https://www.crfb.org/papers/break-glass-plan-next-economic-shock
 
 values:
   0000-01-01: false
diff --git a/policyengine_us/parameters/gov/contrib/crfb/surtax/increased_base/sources.yaml b/policyengine_us/parameters/gov/contrib/crfb/surtax/increased_base/sources.yaml
index 9bcb0de8221..a978ed74f69 100644
--- a/policyengine_us/parameters/gov/contrib/crfb/surtax/increased_base/sources.yaml
+++ b/policyengine_us/parameters/gov/contrib/crfb/surtax/increased_base/sources.yaml
@@ -1,6 +1,18 @@
 description: Additional income sources for CRFB surtax expanded base beyond IRS gross income.
 values:
   2010-01-01:
+    # CRFB's Break Glass plan (footnote 2) applies the surtax to "a broad
+    # definition of income that includes Adjusted Gross Income (AGI) plus"
+    # retirement contributions, HSA/MSA contributions, deductible student
+    # loan interest, Social Security not counted toward AGI, tax-free bond
+    # interest and employer-provided health insurance. It does not address
+    # dependents. The base starts from the return's AGI, so person-level
+    # sources are summed over the head and spouse only, the people whose AGI
+    # it is; that attribution is inferred. Tax-unit-level sources describe
+    # the head's and spouse's own return. Which items belong, and which
+    # variable stands for each, is a separate question
+    # (PolicyEngine/policyengine-us#9763); the footnote does not list the
+    # foreign earned income exclusion.
     # Retirement contributions (IRA, 401k, 403b)
     - traditional_ira_contributions
     - traditional_401k_contributions
@@ -10,7 +22,7 @@ values:
     # Deductible student loan interest
     - student_loan_interest_ald
     # Social Security not counted toward AGI
-    - tax_exempt_social_security
+    - filer_tax_exempt_social_security
     # Foreign earnings exclusion (tax unit level, may need special handling)
     - foreign_earned_income_exclusion
     # Tax-free bond interest
@@ -22,3 +34,6 @@ metadata:
   unit: list
   period: year
   label: CRFB surtax expanded base additional sources
+  reference:
+    - title: "Committee for a Responsible Federal Budget, Break Glass: A Plan for the Next Economic Shock, footnote 2"
+      href: https://www.crfb.org/papers/break-glass-plan-next-economic-shock
diff --git a/policyengine_us/parameters/gov/irs/credits/ctc/refundable/social_security/add.yaml b/policyengine_us/parameters/gov/irs/credits/ctc/refundable/social_security/add.yaml
index 6a191940a77..53123c3cadf 100644
--- a/policyengine_us/parameters/gov/irs/credits/ctc/refundable/social_security/add.yaml
+++ b/policyengine_us/parameters/gov/irs/credits/ctc/refundable/social_security/add.yaml
@@ -1,17 +1,19 @@
 description: The IRS defines social security taxes under the Child Tax Credit to include the following variables.
 values:
   2018-01-01:
-    # Person:
+    # Person (summed over the head and spouse only):
     - employee_social_security_tax
     - employee_medicare_tax
+    - self_employment_tax_ald_person
+    # Tax Unit (describes the head's and spouse's own return):
     - unreported_payroll_tax
-    # Tax Unit:
-    - self_employment_tax_ald
     - additional_medicare_tax
 metadata:
   unit: list
   period: year
-  label: Child Tax Credit added social security taxes 
+  label: Child Tax Credit added social security taxes
   reference:
     - title: 26 U.S. Code § 24(d)(2)(A)
       href: https://www.law.cornell.edu/uscode/text/26/24#d_2_A
+    - title: 2024 Schedule 8812 (Form 1040), Part II-B, lines 21 to 23
+      href: https://www.irs.gov/pub/irs-prior/f1040s8--2024.pdf#page=2
diff --git a/policyengine_us/reforms/biden/budget_2025/medicare_and_investment_tax_increase.py b/policyengine_us/reforms/biden/budget_2025/medicare_and_investment_tax_increase.py
index 50bf3c0a703..853fa1f257e 100644
--- a/policyengine_us/reforms/biden/budget_2025/medicare_and_investment_tax_increase.py
+++ b/policyengine_us/reforms/biden/budget_2025/medicare_and_investment_tax_increase.py
@@ -19,7 +19,8 @@ def formula(tax_unit, period, parameters):
                 "payroll_tax_gross_wages",
                 "taxable_self_employment_income",
             ]
-            wages_plus_se = add(tax_unit, period, ELEMENTS)
+            # As in the baseline, only the head's and spouse's earnings.
+            wages_plus_se = tax_unit_non_dep_add(tax_unit, period, ELEMENTS)
             exclusion = amc.exclusion[tax_unit("filing_status", period)]
             base = max_(0, wages_plus_se - exclusion)
             base_tax = amc.rate * base
diff --git a/policyengine_us/reforms/congress/wyden_smith/ctc_expansion.py b/policyengine_us/reforms/congress/wyden_smith/ctc_expansion.py
index e4b770f5f5f..8d39284ee7e 100644
--- a/policyengine_us/reforms/congress/wyden_smith/ctc_expansion.py
+++ b/policyengine_us/reforms/congress/wyden_smith/ctc_expansion.py
@@ -57,22 +57,10 @@ def formula(tax_unit, period, parameters):
 
             relevant_earnings = earnings_over_threshold * phase_in_rate
 
-            # Compute "Social Security taxes" as defined in the US Code for the ACTC.
-            # This includes OASDI and Medicare payroll taxes, as well as half
-            # of self-employment taxes.
-            SS_ADD_VARIABLES = [
-                # Person:
-                "employee_social_security_tax",
-                "employee_medicare_tax",
-                "unreported_payroll_tax",
-                # Tax unit:
-                "self_employment_tax_ald",
-                "additional_medicare_tax",
-            ]
-            SS_SUBTRACT_VARIABLES = ["excess_payroll_tax_withheld"]
-            social_security_tax = add(tax_unit, period, SS_ADD_VARIABLES) - add(
-                tax_unit, period, SS_SUBTRACT_VARIABLES
-            )
+            # "Social Security taxes" as defined in 26 U.S.C. 24(d)(2) for the
+            # ACTC: the head's and spouse's OASDI and Medicare payroll taxes and
+            # half of their self-employment taxes, as in the baseline.
+            social_security_tax = tax_unit("ctc_social_security_tax", period)
             eitc = tax_unit("eitc", period)
             social_security_excess = max_(0, social_security_tax - eitc)
             qualifying_children = tax_unit("ctc_qualifying_children", period)
diff --git a/policyengine_us/reforms/crfb/agi_surtax.py b/policyengine_us/reforms/crfb/agi_surtax.py
index dc7d492de85..f10e1f63f72 100644
--- a/policyengine_us/reforms/crfb/agi_surtax.py
+++ b/policyengine_us/reforms/crfb/agi_surtax.py
@@ -15,7 +15,15 @@ def formula(tax_unit, period, parameters):
             agi = tax_unit("adjusted_gross_income", period)
             p = parameters(period).gov.contrib.crfb.surtax
             if p.increased_base.in_effect:
-                additional_sources = add(tax_unit, period, p.increased_base.sources)
+                # CRFB's Break Glass plan (footnote 2) defines the base as
+                # AGI plus other income. AGI is this return's, so the base
+                # adds the head's and spouse's own amounts; a tax unit
+                # dependent's income and deductions are on the dependent's
+                # own return. The plan does not address dependents, so this
+                # attribution is inferred.
+                additional_sources = tax_unit_non_dep_add(
+                    tax_unit, period, p.increased_base.sources
+                )
                 agi = agi + additional_sources
             filing_status = tax_unit("filing_status", period)
             joint = filing_status == filing_status.possible_values.JOINT
diff --git a/policyengine_us/tests/core/test_dependent_payroll_taxes_ctc_and_surtax_invariants.py b/policyengine_us/tests/core/test_dependent_payroll_taxes_ctc_and_surtax_invariants.py
new file mode 100644
index 00000000000..3a71438bb9e
--- /dev/null
+++ b/policyengine_us/tests/core/test_dependent_payroll_taxes_ctc_and_surtax_invariants.py
@@ -0,0 +1,454 @@
+"""Invariants for a tax unit dependent's payroll taxes and contributions.
+
+26 U.S.C. 24(d)(1)(B)(ii) lets a taxpayer with three or more qualifying
+children figure the refundable child tax credit from "the taxpayer's social
+security taxes" less the EITC, and 24(d)(2)(A) defines those as the taxes on
+amounts the taxpayer received and on the taxpayer's self-employment income.
+Schedule 8812 line 21 adds the spouse's on a joint return. The Additional
+Medicare Tax among them comes from the return's own Form 8959, which counts
+the wages of the taxpayer and, on a joint return, the spouse. A dependent's
+wages are on the dependent's own Form W-2 and return. The CRFB AGI surtax's
+expanded base adds amounts to this return's AGI, so it counts the head's and
+spouse's own contributions, exclusions and deductions too.
+
+Each case runs every tax unit twice in one vectorized simulation: once with
+the dependents' amounts as drawn and once with them set to zero. For every
+tax unit:
+
+1. A dependent's wages, self-employment income, retirement contributions,
+   tax-exempt interest, Social Security benefits and health insurance
+   premiums never change `ctc_social_security_tax`, the Puerto Rico
+   equivalent, `additional_medicare_tax` or the CRFB surtax's additions to
+   AGI.
+2. A dependent's wages, traditional IRA contributions, 401(k) and 403(b)
+   deferrals and Social Security benefits, none of which enters the filer's
+   AGI or deductions, never change the filer's AGI, refundable or
+   non-refundable CTC, federal income tax or CRFB surtax. A dependent's IRA
+   deduction is on the dependent's own return (#9801), so the surtax neither
+   subtracts it through AGI nor adds it back.
+3. Differential: `ctc_social_security_tax`, the Puerto Rico equivalent,
+   `additional_medicare_tax` and the surtax equal an independent numpy
+   calculation over the head and spouse.
+
+Invariant 2 leaves out three kinds of dependent amount, which invariant 1
+checks with AGI and its taxable Social Security given as inputs:
+
+- A dependent's self-employment income still adds to the filer's qualified
+  business income deduction, which sums every member's `qbid_amount`.
+- A dependent's tax-exempt interest counts toward the filer's EITC
+  investment income limit (#9635).
+- Health insurance premiums a parent pays for a dependent may be the
+  parent's deductible medical expenses.
+"""
+
+from functools import cache
+
+import numpy as np
+from hypothesis import HealthCheck, given, settings
+from hypothesis import strategies as st
+
+from policyengine_us import Simulation
+from policyengine_us.system import CountryTaxBenefitSystem
+
+TOLERANCE = 0.01  # dollars
+YEARS = [2019, 2020, 2024, 2026]
+
+# The reform's in_effect parameter also applies its structural reform.
+SURTAX_ON = {
+    "gov.contrib.crfb.surtax.in_effect": {"2000-01-01.2100-12-31": True},
+    "gov.contrib.crfb.surtax.increased_base.in_effect": {"2000-01-01.2100-12-31": True},
+}
+
+# Dependent amounts that never enter the filer's AGI, deductions or credits.
+OFF_RETURN_INPUTS = [
+    "employment_income",
+    "traditional_ira_contributions_desired",
+    "traditional_401k_contributions_desired",
+    "traditional_403b_contributions_desired",
+    "social_security_dependents",
+]
+# Dependent amounts that reach the filer's return elsewhere: see the module
+# docstring.
+OTHER_INPUTS = [
+    "self_employment_income",
+    "tax_exempt_interest_income",
+    "health_insurance_premiums",
+]
+PERSON_PAYROLL_TAXES = [
+    "employee_social_security_tax",
+    "employee_medicare_tax",
+    "self_employment_tax_ald_person",
+]
+PERSON_SURTAX_SOURCES = [
+    "traditional_ira_contributions",
+    "traditional_401k_contributions",
+    "traditional_403b_contributions",
+    "student_loan_interest_ald",
+    "tax_exempt_interest_income",
+    "health_insurance_premiums",
+]
+
+money = st.integers(0, 60_000).map(float)
+
+
+def maybe(strategy):
+    """Zero or a draw from the strategy."""
+    return st.one_of(st.just(0.0), strategy)
+
+
+@cache
+def surtax_system():
+    """The CRFB surtax with its expanded base, built once for every case."""
+    return CountryTaxBenefitSystem(reform=SURTAX_ON)
+
+
+@st.composite
+def filer(draw):
+    return {
+        "age": draw(st.integers(25, 64)),
+        "employment_income": draw(maybe(st.integers(1, 300_000).map(float))),
+        "self_employment_income": draw(maybe(st.integers(-5_000, 60_000).map(float))),
+        # Over the EITC's investment income limit, the alternative refundable
+        # CTC formula decides more refunds.
+        "taxable_interest_income": draw(maybe(money)),
+        "social_security_disability": draw(maybe(money)),
+        "traditional_ira_contributions_desired": draw(maybe(st.just(5_000.0))),
+        "traditional_401k_contributions_desired": draw(maybe(money)),
+        "tax_exempt_interest_income": draw(maybe(money)),
+        "health_insurance_premiums": draw(maybe(st.integers(1, 9_000).map(float))),
+    }
+
+
+@st.composite
+def dependent(draw, *, every_amount):
+    # Under 18, so the model never treats a dependent as a spouse.
+    person = {
+        "age": draw(st.integers(0, 17)),
+        "employment_income": draw(maybe(money)),
+        "traditional_ira_contributions_desired": draw(
+            maybe(st.integers(1, 7_000).map(float))
+        ),
+        "traditional_401k_contributions_desired": draw(maybe(money)),
+        "traditional_403b_contributions_desired": draw(maybe(money)),
+        "social_security_dependents": draw(maybe(st.integers(1, 15_000).map(float))),
+    }
+    if every_amount:
+        person["self_employment_income"] = draw(
+            maybe(st.integers(-5_000, 60_000).map(float))
+        )
+        person["tax_exempt_interest_income"] = draw(maybe(money))
+        person["health_insurance_premiums"] = draw(
+            maybe(st.integers(1, 9_000).map(float))
+        )
+    return person
+
+
+@st.composite
+def tax_unit(draw, *, every_amount):
+    unit = {
+        "head": draw(filer()),
+        "spouse": draw(filer()) if draw(st.booleans()) else None,
+        "dependents": draw(
+            st.lists(dependent(every_amount=every_amount), min_size=1, max_size=5)
+        ),
+        "excess_payroll_tax_withheld": draw(maybe(st.integers(1, 2_000).map(float))),
+    }
+    if every_amount:
+        unit["adjusted_gross_income"] = draw(st.integers(0, 1_500_000).map(float))
+    return unit
+
+
+def without_dependent_amounts(unit, inputs):
+    dependents = [
+        {k: (0.0 if k in inputs else v) for k, v in d.items()}
+        for d in unit["dependents"]
+    ]
+    return {**unit, "dependents": dependents}
+
+
+def build(units, year):
+    """A situation with every tax unit, and which people are head or spouse."""
+    people, tax_units, marital_units = {}, {}, {}
+    groups = {"households": {}, "spm_units": {}, "families": {}}
+    filer_flags, unit_index = [], []
+    for i, unit in enumerate(units):
+        members = []
+        filers = [("head", unit["head"])]
+        if unit["spouse"] is not None:
+            filers.append(("spouse", unit["spouse"]))
+        roles = filers + [
+            (f"dependent{j}", d) for j, d in enumerate(unit["dependents"])
+        ]
+        for role, values in roles:
+            name = f"{role}_{i}"
+            people[name] = {k: {year: v} for k, v in values.items()}
+            members.append(name)
+            filer_flags.append(not role.startswith("dependent"))
+            unit_index.append(i)
+            if role.startswith("dependent"):
+                marital_units[f"marital_unit_{name}"] = {"members": [name]}
+        marital_units[f"marital_unit_{i}"] = {"members": members[: len(filers)]}
+        tax_units[f"tax_unit_{i}"] = {
+            "members": members,
+            "excess_payroll_tax_withheld": {year: unit["excess_payroll_tax_withheld"]},
+        }
+        if "adjusted_gross_income" in unit:
+            # AGI and the Social Security in it are given, so the filer's
+            # AGI cannot carry a dependent's deductions into the surtax.
+            taxable_social_security = 0.5 * sum(
+                values.get("social_security_disability", 0.0) for _, values in filers
+            )
+            tax_units[f"tax_unit_{i}"].update(
+                adjusted_gross_income={year: unit["adjusted_gross_income"]},
+                tax_unit_taxable_social_security={year: taxable_social_security},
+            )
+        groups["households"][f"household_{i}"] = {
+            "members": members,
+            "state_code": {year: "TX"},
+        }
+        groups["spm_units"][f"spm_unit_{i}"] = {"members": members}
+        groups["families"][f"family_{i}"] = {"members": members}
+    situation = {
+        "people": people,
+        "tax_units": tax_units,
+        "marital_units": marital_units,
+        **groups,
+    }
+    return situation, np.array(filer_flags), np.array(unit_index)
+
+
+def filer_sum(values, filers, unit_index):
+    """Sum a person-level array over each tax unit's head and spouse."""
+    return np.bincount(
+        unit_index, weights=values * filers, minlength=unit_index.max() + 1
+    )
+
+
+def run_twice(units, inputs, year, surtax=False):
+    """Calculate each unit with its dependents' amounts and without them."""
+    zeroed = [without_dependent_amounts(unit, inputs) for unit in units]
+    situation, filers, unit_index = build(units + zeroed, year)
+    if surtax:
+        simulation = Simulation(situation=situation, tax_benefit_system=surtax_system())
+    else:
+        simulation = Simulation(situation=situation)
+    return simulation, filers, unit_index
+
+
+def calculate(simulation, variable, year):
+    return np.asarray(simulation.calculate(variable, year))
+
+
+def assert_unchanged(simulation, variables, year, n):
+    for variable in variables:
+        values = calculate(simulation, variable, year)
+        np.testing.assert_allclose(
+            values[:n], values[n:], atol=TOLERANCE, err_msg=variable
+        )
+
+
+def assert_payroll_taxes_match_reference(simulation, filers, unit_index, year):
+    person = {
+        v: calculate(simulation, v, year)
+        for v in [
+            *PERSON_PAYROLL_TAXES,
+            "payroll_tax_gross_wages",
+            "taxable_self_employment_income",
+        ]
+    }
+    filing_status = simulation.calculate("filing_status", year).decode_to_str()
+    p = simulation.tax_benefit_system.parameters(year).gov.irs.payroll.medicare
+    threshold = p.additional.exclusion[np.asarray(filing_status)]
+    earnings = filer_sum(
+        person["payroll_tax_gross_wages"] + person["taxable_self_employment_income"],
+        filers,
+        unit_index,
+    )
+    additional_medicare_tax = p.additional.rate * np.maximum(0, earnings - threshold)
+    np.testing.assert_allclose(
+        calculate(simulation, "additional_medicare_tax", year),
+        additional_medicare_tax,
+        atol=TOLERANCE,
+    )
+    payroll_taxes = (
+        sum(filer_sum(person[v], filers, unit_index) for v in PERSON_PAYROLL_TAXES)
+        + calculate(simulation, "unreported_payroll_tax", year)
+        + additional_medicare_tax
+    )
+    np.testing.assert_allclose(
+        calculate(simulation, "pr_refundable_ctc_social_security_tax", year),
+        payroll_taxes,
+        atol=TOLERANCE,
+    )
+    np.testing.assert_allclose(
+        calculate(simulation, "ctc_social_security_tax", year),
+        payroll_taxes - calculate(simulation, "excess_payroll_tax_withheld", year),
+        atol=TOLERANCE,
+    )
+
+
+def assert_surtax_matches_reference(simulation, filers, unit_index, year):
+    sources = sum(
+        filer_sum(calculate(simulation, v, year), filers, unit_index)
+        for v in PERSON_SURTAX_SOURCES
+    )
+    social_security = filer_sum(
+        calculate(simulation, "social_security", year), filers, unit_index
+    )
+    base = (
+        calculate(simulation, "adjusted_gross_income", year)
+        + sources
+        + calculate(simulation, "health_savings_account_ald", year)
+        + calculate(simulation, "foreign_earned_income_exclusion", year)
+        + social_security
+        - calculate(simulation, "tax_unit_taxable_social_security", year)
+    )
+    p = simulation.tax_benefit_system.parameters(year).gov.contrib.crfb.surtax
+    filing_status = np.asarray(
+        simulation.calculate("filing_status", year).decode_to_str()
+    )
+    expected = np.where(
+        filing_status == "JOINT", p.rate.joint.calc(base), p.rate.single.calc(base)
+    )
+    np.testing.assert_allclose(
+        calculate(simulation, "agi_surtax", year), expected, atol=TOLERANCE
+    )
+
+
+def check_off_return_amounts(units, year):
+    """Invariants 2 and 3; returns both simulations."""
+    n = len(units)
+    baseline, filers, unit_index = run_twice(units, OFF_RETURN_INPUTS, year)
+    assert_unchanged(
+        baseline,
+        [
+            "adjusted_gross_income",
+            "ctc_social_security_tax",
+            "pr_refundable_ctc_social_security_tax",
+            "additional_medicare_tax",
+            "ctc_phase_in",
+            "refundable_ctc",
+            "non_refundable_ctc",
+            "income_tax",
+        ],
+        year,
+        n,
+    )
+    assert_payroll_taxes_match_reference(baseline, filers, unit_index, year)
+    reformed, filers, unit_index = run_twice(
+        units, OFF_RETURN_INPUTS, year, surtax=True
+    )
+    assert_unchanged(reformed, ["agi_surtax"], year, n)
+    assert_surtax_matches_reference(reformed, filers, unit_index, year)
+    return baseline, reformed
+
+
+def check_every_amount_with_agi_given(units, year):
+    """Invariants 1 and 3."""
+    n = len(units)
+    simulation, filers, unit_index = run_twice(
+        units, OFF_RETURN_INPUTS + OTHER_INPUTS, year, surtax=True
+    )
+    assert_unchanged(
+        simulation,
+        [
+            "ctc_social_security_tax",
+            "pr_refundable_ctc_social_security_tax",
+            "additional_medicare_tax",
+            "agi_surtax",
+        ],
+        year,
+        n,
+    )
+    assert_payroll_taxes_match_reference(simulation, filers, unit_index, year)
+    assert_surtax_matches_reference(simulation, filers, unit_index, year)
+
+
+# Each case runs one or two vectorized simulations of up to 24 tax units.
+# derandomize keeps CI runs reproducible.
+SETTINGS = settings(
+    max_examples=5,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow, HealthCheck.data_too_large],
+)
+
+
+@SETTINGS
+@given(
+    st.lists(tax_unit(every_amount=False), min_size=1, max_size=12),
+    st.sampled_from(YEARS),
+)
+def test_dependents_off_return_amounts_never_change_filer_ctc_tax_or_surtax(
+    units, year
+):
+    check_off_return_amounts(units, year)
+
+
+@SETTINGS
+@given(
+    st.lists(tax_unit(every_amount=True), min_size=1, max_size=12),
+    st.sampled_from(YEARS),
+)
+def test_dependents_amounts_never_change_payroll_taxes_or_surtax_additions(units, year):
+    check_every_amount_with_agi_given(units, year)
+
+
+def low_wage_family(married, dependent_wages):
+    """Three qualifying children, the EITC barred by interest income and
+    wages low enough that payroll taxes decide the refundable CTC."""
+    head = {"age": 40, "employment_income": 1_000.0, "taxable_interest_income": 46e3}
+    spouse = {"age": 40, "employment_income": 3_000.0} if married else None
+    if not married:
+        head["employment_income"] = 4_000.0
+    children = [
+        {"age": 16, "employment_income": dependent_wages},
+        {"age": 10},
+        {"age": 8},
+    ]
+    return {
+        "head": head,
+        "spouse": spouse,
+        "dependents": children,
+        "excess_payroll_tax_withheld": 0.0,
+    }
+
+
+# The dependent's wages would push the parents over the joint Additional
+# Medicare Tax threshold, their IRA deduction would lower the parents' AGI,
+# and their contributions and benefits would add to the surtax base.
+HIGH_INCOME_COUPLE = {
+    "head": {"age": 50, "employment_income": 150_000.0},
+    "spouse": {"age": 48, "employment_income": 90_000.0},
+    "dependents": [
+        {
+            "age": 17,
+            "employment_income": 25_000.0,
+            "traditional_ira_contributions_desired": 5_000.0,
+            "traditional_401k_contributions_desired": 5_000.0,
+            "social_security_dependents": 6_000.0,
+        }
+    ],
+    "excess_payroll_tax_withheld": 0.0,
+}
+
+
+def test_fixed_households_where_the_alternative_formula_decides():
+    units = [
+        low_wage_family(married, wages)
+        for married in (False, True)
+        for wages in (8_000.0, 30_000.0)
+    ] + [HIGH_INCOME_COUPLE]
+    for year in YEARS:
+        baseline, reformed = check_off_return_amounts(units, year)
+        # The alternative formula decides each low-wage family's refund.
+        ss_tax = calculate(baseline, "ctc_social_security_tax", year)
+        eitc = calculate(baseline, "eitc", year)
+        earned = calculate(baseline, "ctc_phase_in_relevant_earnings", year)
+        refundable = calculate(baseline, "refundable_ctc", year)
+        assert (ss_tax[:4] - eitc[:4] > earned[:4] + 1).all()
+        np.testing.assert_allclose(refundable[:4], ss_tax[:4], atol=TOLERANCE)
+        # The dependent's wages would have raised the couple's tax.
+        assert calculate(baseline, "additional_medicare_tax", year)[4] == 0
+        # The surtax applies to the couple.
+        assert calculate(reformed, "agi_surtax", year)[4] > 0
diff --git a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/ctc_social_security_tax.yaml b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/ctc_social_security_tax.yaml
index 00ca526300b..5b49a8fa388 100644
--- a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/ctc_social_security_tax.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/ctc_social_security_tax.yaml
@@ -5,7 +5,7 @@
     employee_social_security_tax: 0
     employee_medicare_tax: 0
     unreported_payroll_tax: 0
-    self_employment_tax_ald: 0
+    self_employment_tax_ald_person: 0
     additional_medicare_tax: 0
     # SS_SUBTRACT_VARIABLES
     excess_payroll_tax_withheld: 0
@@ -19,7 +19,7 @@
     employee_social_security_tax: 100
     employee_medicare_tax: 100
     unreported_payroll_tax: 0
-    self_employment_tax_ald: 50
+    self_employment_tax_ald_person: 50
     additional_medicare_tax: 25
     # SS_SUBTRACT_VARIABLES
     excess_payroll_tax_withheld: 200
@@ -33,9 +33,43 @@
     employee_social_security_tax: 100
     employee_medicare_tax: 100
     unreported_payroll_tax: 0
-    self_employment_tax_ald: 50
+    self_employment_tax_ald_person: 50
     additional_medicare_tax: 25
     # SS_SUBTRACT_VARIABLES
     excess_payroll_tax_withheld: 450
   output:
     ctc_social_security_tax: -175
+
+- name: Only the head's and spouse's payroll taxes count, not a dependent's
+  period: 2024
+  input:
+    people:
+      head:
+        age: 40
+        employee_social_security_tax: 300
+        employee_medicare_tax: 70
+        self_employment_tax_ald_person: 0
+      spouse:
+        age: 40
+        employee_social_security_tax: 100
+        employee_medicare_tax: 30
+        self_employment_tax_ald_person: 50
+      dependent:
+        age: 16
+        employee_social_security_tax: 400
+        employee_medicare_tax: 90
+        self_employment_tax_ald_person: 25
+    tax_units:
+      tax_unit:
+        members: [head, spouse, dependent]
+        unreported_payroll_tax: 10
+        additional_medicare_tax: 0
+        excess_payroll_tax_withheld: 20
+    marital_units:
+      marital_unit:
+        members: [head, spouse]
+      dependent_marital_unit:
+        members: [dependent]
+  output:
+    # 300 + 70 + 100 + 30 + 50 + 10 - 20; the dependent's $515 is left out.
+    ctc_social_security_tax: 540
diff --git a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/ctc_social_security_tax_dependents.yaml b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/ctc_social_security_tax_dependents.yaml
new file mode 100644
index 00000000000..13418144c55
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/ctc_social_security_tax_dependents.yaml
@@ -0,0 +1,140 @@
+# 26 U.S.C. 24(d)(1)(B)(ii) and (d)(2)(A): a taxpayer with three or more
+# qualifying children can have the refundable credit figured from the
+# taxpayer's own social security taxes less the EITC (Schedule 8812 Part II-B).
+# On a joint return the spouse's taxes count too ("If married filing jointly,
+# include your spouse's amounts with yours", line 21). A tax unit dependent's
+# payroll taxes are on the dependent's own Form W-2 and return, so they never
+# count.
+#
+# Each household has $46,000 of taxable interest, which is more than the EITC's
+# investment income limit, so the EITC is zero, and wages low enough that
+# social security taxes exceed 15% of earned income over $2,500: the
+# alternative formula decides the refund. One qualifying child, aged 16, has
+# $8,000 of wages, with $612 (7.65%) of payroll tax that must not count.
+
+- name: Head of household with three children before 2021, a working child's payroll taxes do not count
+  period: 2020
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        employment_income: 4_000
+        taxable_interest_income: 46_000
+      child1:
+        age: 16
+        employment_income: 8_000
+      child2:
+        age: 10
+      child3:
+        age: 8
+    tax_units:
+      tax_unit:
+        members: [parent, child1, child2, child3]
+    households:
+      household:
+        members: [parent, child1, child2, child3]
+        state_code: TX
+  output:
+    filing_status: HEAD_OF_HOUSEHOLD
+    eitc: 0
+    # AGI $50,000 less the $18,650 standard deduction is $31,350 of taxable
+    # income: 10% of $14,100 plus 12% of $17,250.
+    income_tax_before_credits: 3_480
+    # 3 x $2,000.
+    ctc: 6_000
+    # 15% x ($4,000 - $2,500).
+    ctc_phase_in_relevant_earnings: 225
+    # The parent's 7.65% x $4,000 (lines 21 to 23); not the child's $612.
+    ctc_social_security_tax: 306
+    # Line 26: the larger of $225 and $306 - $0 EITC.
+    ctc_phase_in: 306
+    # Line 27: the smaller of line 17 ($4,200 = 3 x $1,400) and line 26.
+    refundable_ctc: 306
+
+- name: Head of household with three children after 2025, a working child's payroll taxes do not count
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        employment_income: 4_000
+        taxable_interest_income: 46_000
+      child1:
+        age: 16
+        employment_income: 8_000
+      child2:
+        age: 10
+      child3:
+        age: 8
+    tax_units:
+      tax_unit:
+        members: [parent, child1, child2, child3]
+    households:
+      household:
+        members: [parent, child1, child2, child3]
+        state_code: TX
+  output:
+    filing_status: HEAD_OF_HOUSEHOLD
+    eitc: 0
+    # AGI $50,000 less the $24,150 standard deduction is $25,850 of taxable
+    # income: 10% of $17,700 plus 12% of $8,150.
+    income_tax_before_credits: 2_748
+    # 3 x $2,200.
+    ctc: 6_600
+    ctc_phase_in_relevant_earnings: 225
+    ctc_social_security_tax: 306
+    ctc_phase_in: 306
+    # The credit left after the $2,748 tax limit is $3,852, more than $306.
+    refundable_ctc: 306
+
+- name: Joint return with three children, the spouse's payroll taxes count and a working child's do not
+  period: 2019
+  absolute_error_margin: 0.01
+  input:
+    people:
+      head:
+        age: 40
+        employment_income: 1_000
+        taxable_interest_income: 46_000
+      spouse:
+        age: 40
+        employment_income: 3_000
+      child1:
+        age: 16
+        employment_income: 8_000
+      child2:
+        age: 10
+      child3:
+        age: 8
+    tax_units:
+      tax_unit:
+        members: [head, spouse, child1, child2, child3]
+    marital_units:
+      marital_unit:
+        members: [head, spouse]
+      child1_marital_unit:
+        members: [child1]
+      child2_marital_unit:
+        members: [child2]
+      child3_marital_unit:
+        members: [child3]
+    households:
+      household:
+        members: [head, spouse, child1, child2, child3]
+        state_code: TX
+  output:
+    filing_status: JOINT
+    eitc: 0
+    # AGI $50,000 less the $24,400 standard deduction is $25,600 of taxable
+    # income: 10% of $19,400 plus 12% of $6,200.
+    income_tax_before_credits: 2_684
+    ctc: 6_000
+    # 15% x ($4,000 - $2,500).
+    ctc_phase_in_relevant_earnings: 225
+    # 7.65% x ($1,000 + $3,000). The head's own $76.50 alone would leave the
+    # earned income formula in charge.
+    ctc_social_security_tax: 306
+    ctc_phase_in: 306
+    refundable_ctc: 306
diff --git a/policyengine_us/tests/policy/baseline/gov/irs/tax/federal_income/additional_medicare_tax.yaml b/policyengine_us/tests/policy/baseline/gov/irs/tax/federal_income/additional_medicare_tax.yaml
index 47fd92327c2..5f9d5634d3e 100644
--- a/policyengine_us/tests/policy/baseline/gov/irs/tax/federal_income/additional_medicare_tax.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/irs/tax/federal_income/additional_medicare_tax.yaml
@@ -29,3 +29,50 @@
         filing_status: SINGLE
   output:
     additional_medicare_tax: 450 # (250k - 200k) * 0.9%
+
+- name: Joint return counts both spouses' wages
+  period: 2024
+  input:
+    people:
+      head:
+        age: 45
+        employment_income: 200_000
+      spouse:
+        age: 45
+        employment_income: 100_000
+    tax_units:
+      tax_unit:
+        members: [head, spouse]
+    marital_units:
+      marital_unit:
+        members: [head, spouse]
+  output:
+    filing_status: JOINT
+    additional_medicare_tax: 450 # (300k - 250k) * 0.9%
+
+- name: A dependent's wages are on the dependent's own Form 8959, not the parents'
+  period: 2024
+  input:
+    people:
+      head:
+        age: 45
+        employment_income: 140_000
+      spouse:
+        age: 45
+        employment_income: 100_000
+      dependent:
+        age: 16
+        employment_income: 20_000
+    tax_units:
+      tax_unit:
+        members: [head, spouse, dependent]
+    marital_units:
+      marital_unit:
+        members: [head, spouse]
+      dependent_marital_unit:
+        members: [dependent]
+  output:
+    filing_status: JOINT
+    # The parents' $240,000 is under the $250,000 joint threshold; the
+    # dependent's $20,000 would otherwise add $90.
+    additional_medicare_tax: 0
diff --git a/policyengine_us/tests/policy/contrib/crfb/agi_surtax.yaml b/policyengine_us/tests/policy/contrib/crfb/agi_surtax.yaml
index d34fd248f3f..f505b72c1c1 100644
--- a/policyengine_us/tests/policy/contrib/crfb/agi_surtax.yaml
+++ b/policyengine_us/tests/policy/contrib/crfb/agi_surtax.yaml
@@ -109,3 +109,145 @@
   output:
     # Only AGI 80k used, below 100k threshold
     agi_surtax: 0
+
+# The expanded base adds amounts to this return's AGI, so it counts the head's
+# and spouse's own contributions, exclusions and deductions. A tax unit
+# dependent's are on the dependent's own return. AGI is an input here, so these
+# cases test the additions alone, whichever way the dependents' deductions
+# enter AGI.
+
+- name: Expanded base leaves out a dependent's traditional IRA contribution
+  period: 2026
+  reforms: policyengine_us.reforms.crfb.agi_surtax.agi_surtax_reform_object
+  input:
+    gov.contrib.crfb.surtax.in_effect: true
+    gov.contrib.crfb.surtax.increased_base.in_effect: true
+    people:
+      head:
+        age: 45
+      spouse:
+        age: 45
+      dependent:
+        age: 17
+        employment_income: 10_000
+        traditional_ira_contributions_desired: 5_000
+    tax_units:
+      tax_unit:
+        members: [head, spouse, dependent]
+        adjusted_gross_income: 300_000
+    marital_units:
+      marital_unit:
+        members: [head, spouse]
+      dependent_marital_unit:
+        members: [dependent]
+  output:
+    filing_status: JOINT
+    # The base stays at AGI, $300,000: (300k - 200k) * 1%. Adding the
+    # dependent's $5,000 would make it $1,050.
+    agi_surtax: 1_000
+
+- name: Expanded base leaves out a dependent's Social Security benefits
+  period: 2026
+  reforms: policyengine_us.reforms.crfb.agi_surtax.agi_surtax_reform_object
+  input:
+    gov.contrib.crfb.surtax.in_effect: true
+    gov.contrib.crfb.surtax.increased_base.in_effect: true
+    people:
+      head:
+        age: 45
+      spouse:
+        age: 45
+      dependent:
+        age: 10
+        social_security_dependents: 10_000
+    tax_units:
+      tax_unit:
+        members: [head, spouse, dependent]
+        adjusted_gross_income: 250_000
+    marital_units:
+      marital_unit:
+        members: [head, spouse]
+      dependent_marital_unit:
+        members: [dependent]
+  output:
+    filing_status: JOINT
+    # The child's benefits are the child's income, not tax-exempt income of
+    # this return: (250k - 200k) * 1% = 500, not 600.
+    filer_tax_exempt_social_security: 0
+    agi_surtax: 500
+
+- name: Expanded base counts the spouse's items and leaves out the dependent's
+  period: 2026
+  reforms: policyengine_us.reforms.crfb.agi_surtax.agi_surtax_reform_object
+  input:
+    gov.contrib.crfb.surtax.in_effect: true
+    gov.contrib.crfb.surtax.increased_base.in_effect: true
+    people:
+      head:
+        age: 45
+        social_security_retirement: 20_000
+      spouse:
+        age: 45
+        employment_income: 50_000
+        traditional_ira_contributions_desired: 5_000
+        tax_exempt_interest_income: 2_000
+      dependent:
+        age: 17
+        employment_income: 20_000
+        traditional_401k_contributions_desired: 3_000
+        tax_exempt_interest_income: 1_000
+        social_security_dependents: 4_000
+    tax_units:
+      tax_unit:
+        members: [head, spouse, dependent]
+        adjusted_gross_income: 300_000
+        tax_unit_taxable_social_security: 17_000
+    marital_units:
+      marital_unit:
+        members: [head, spouse]
+      dependent_marital_unit:
+        members: [dependent]
+  output:
+    filing_status: JOINT
+    # The head's $20,000 of benefits less the $17,000 (85%) in AGI.
+    filer_tax_exempt_social_security: 3_000
+    # 300k + 5k + 2k + 3k = 310k: (310k - 200k) * 1% = 1,100. The dependent's
+    # $3,000, $1,000 and $4,000 would make it $1,180.
+    agi_surtax: 1_100
+
+# Here AGI is calculated. The dependent's IRA deduction is on the dependent's
+# own return, so it is neither subtracted from the filer's AGI nor added back
+# to the base (PolicyEngine/policyengine-us#9801 keeps it off the filer's AGI).
+
+- name: Expanded base with AGI calculated leaves a dependent's IRA contribution out of both AGI and the additions
+  period: 2024
+  reforms: policyengine_us.reforms.crfb.agi_surtax.agi_surtax_reform_object
+  input:
+    gov.contrib.crfb.surtax.in_effect: true
+    gov.contrib.crfb.surtax.increased_base.in_effect: true
+    people:
+      head:
+        age: 45
+        employment_income: 150_000
+      dependent:
+        age: 17
+        employment_income: 10_000
+        traditional_ira_contributions_desired: 5_000
+    tax_units:
+      tax_unit:
+        members: [head, dependent]
+    marital_units:
+      marital_unit:
+        members: [head]
+      dependent_marital_unit:
+        members: [dependent]
+  output:
+    filing_status: HEAD_OF_HOUSEHOLD
+    is_tax_unit_dependent: [false, true]
+    traditional_ira_contributions: [0, 5_000]
+    # Only the head's $150,000 of wages. Subtracting the dependent's $5,000
+    # IRA deduction would make it $145,000.
+    adjusted_gross_income: 150_000
+    # (150k - 100k) * 1% = 500. With the deduction in AGI and no add-back,
+    # the base would be $145,000 and the surtax $450.
+    agi_surtax: 500
diff --git a/policyengine_us/variables/gov/irs/credits/ctc/refundable/ctc_social_security_tax.py b/policyengine_us/variables/gov/irs/credits/ctc/refundable/ctc_social_security_tax.py
index c54bb7ab002..5cb3eac362d 100644
--- a/policyengine_us/variables/gov/irs/credits/ctc/refundable/ctc_social_security_tax.py
+++ b/policyengine_us/variables/gov/irs/credits/ctc/refundable/ctc_social_security_tax.py
@@ -7,10 +7,22 @@ class ctc_social_security_tax(Variable):
     label = "Refundable Child Tax Credit Social Security Tax"
     unit = USD
     documentation = (
-        "Social Security taxes considered in the Child Tax Credit calculation"
+        "Social Security taxes considered in the Child Tax Credit calculation: "
+        "the head's and, on a joint return, the spouse's (Schedule 8812 "
+        "Part II-B, lines 21 to 23, less the excess withheld on Schedule 3). "
+        "Section 24(d)(2)(A) counts the taxpayer's own payroll taxes, so a "
+        "tax unit dependent's payroll taxes, which are figured on the "
+        "dependent's own wages and own return, are left out."
     )
     definition_period = YEAR
-    reference = "https://www.law.cornell.edu/uscode/text/26/24#d_2"
+    reference = (
+        "https://www.law.cornell.edu/uscode/text/26/24#d_2",
+        "https://www.irs.gov/pub/irs-prior/f1040s8--2024.pdf#page=2",
+        "https://www.irs.gov/pub/irs-prior/i1040s8--2024.pdf#page=9",
+    )
 
-    adds = "gov.irs.credits.ctc.refundable.social_security.add"
-    subtracts = "gov.irs.credits.ctc.refundable.social_security.subtract"
+    def formula(tax_unit, period, parameters):
+        p = parameters(period).gov.irs.credits.ctc.refundable.social_security
+        added = tax_unit_non_dep_add(tax_unit, period, p.add)
+        subtracted = tax_unit_non_dep_add(tax_unit, period, p.subtract)
+        return added - subtracted
diff --git a/policyengine_us/variables/gov/irs/income/taxable_income/adjusted_gross_income/irs_gross_income/social_security/filer_tax_exempt_social_security.py b/policyengine_us/variables/gov/irs/income/taxable_income/adjusted_gross_income/irs_gross_income/social_security/filer_tax_exempt_social_security.py
new file mode 100644
index 00000000000..1bba32c881d
--- /dev/null
+++ b/policyengine_us/variables/gov/irs/income/taxable_income/adjusted_gross_income/irs_gross_income/social_security/filer_tax_exempt_social_security.py
@@ -0,0 +1,20 @@
+from policyengine_us.model_api import *
+
+
+class filer_tax_exempt_social_security(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "Filer's tax-exempt Social Security"
+    unit = USD
+    documentation = (
+        "Social Security benefits of the head and spouse that section 86 "
+        "leaves out of this return's AGI. A tax unit dependent's benefits are "
+        "the dependent's own income, as in "
+        "tax_unit_social_security_for_taxability. Unlike "
+        "tax_exempt_social_security, this leaves dependents' benefits out."
+    )
+    definition_period = YEAR
+    reference = "https://www.law.cornell.edu/uscode/text/26/86"
+
+    adds = ["tax_unit_social_security_for_taxability"]
+    subtracts = ["tax_unit_taxable_social_security"]
diff --git a/policyengine_us/variables/gov/irs/tax/federal_income/additional_medicare_tax.py b/policyengine_us/variables/gov/irs/tax/federal_income/additional_medicare_tax.py
index 49fc65c7bd6..dd30d2bda54 100644
--- a/policyengine_us/variables/gov/irs/tax/federal_income/additional_medicare_tax.py
+++ b/policyengine_us/variables/gov/irs/tax/federal_income/additional_medicare_tax.py
@@ -7,13 +7,23 @@ class additional_medicare_tax(Variable):
     definition_period = YEAR
     label = "Additional Medicare Tax"
     unit = USD
-    documentation = "Additional Medicare Tax from Form 8959 (included in payrolltax)"
+    documentation = (
+        "Additional Medicare Tax from Form 8959 (included in payrolltax), on "
+        "the Medicare wages and self-employment income of the head and, on a "
+        "joint return, the spouse. A tax unit dependent's wages are reported "
+        "on the dependent's own Form W-2 and own return, so they are left out."
+    )
+    reference = (
+        "https://www.law.cornell.edu/uscode/text/26/3101#b_2",
+        "https://www.law.cornell.edu/uscode/text/26/1401#b_2",
+        "https://www.irs.gov/pub/irs-prior/i8959--2024.pdf#page=3",
+    )
 
     def formula(tax_unit, period, parameters):
         amc = parameters(period).gov.irs.payroll.medicare.additional
         # Wage and self-employment income are taxed the same.
         ELEMENTS = ["payroll_tax_gross_wages", "taxable_self_employment_income"]
-        wages_plus_se = add(tax_unit, period, ELEMENTS)
+        wages_plus_se = tax_unit_non_dep_add(tax_unit, period, ELEMENTS)
         exclusion = amc.exclusion[tax_unit("filing_status", period)]
         base = max_(0, wages_plus_se - exclusion)
         return amc.rate * base
diff --git a/policyengine_us/variables/gov/territories/pr/tax/income/credits/refundable_ctc/pr_refundable_ctc_social_security_tax.py b/policyengine_us/variables/gov/territories/pr/tax/income/credits/refundable_ctc/pr_refundable_ctc_social_security_tax.py
index 105f3bc114e..e809c2aac0a 100644
--- a/policyengine_us/variables/gov/territories/pr/tax/income/credits/refundable_ctc/pr_refundable_ctc_social_security_tax.py
+++ b/policyengine_us/variables/gov/territories/pr/tax/income/credits/refundable_ctc/pr_refundable_ctc_social_security_tax.py
@@ -7,12 +7,20 @@ class pr_refundable_ctc_social_security_tax(Variable):
     label = (
         "Puerto Rico social security and medicare taxes for refundable Child Tax Credit"
     )
+    documentation = (
+        "The head's and, on a joint return, the spouse's social security and "
+        "Medicare taxes. Section 24(d)(2)(A) counts the taxpayer's own "
+        "payroll taxes, so a tax unit dependent's are left out."
+    )
     unit = USD
     definition_period = YEAR
     reference = (
         "https://www.irs.gov/pub/irs-pdf/f1040s8.pdf",
         "https://www.law.cornell.edu/uscode/text/26/24#h_4_A",
+        "https://www.law.cornell.edu/uscode/text/26/24#d_2",
     )
 
     # line 23
-    adds = "gov.irs.credits.ctc.refundable.social_security.add"
+    def formula(tax_unit, period, parameters):
+        p = parameters(period).gov.irs.credits.ctc.refundable.social_security
+        return tax_unit_non_dep_add(tax_unit, period, p.add)