diff --git a/changelog.d/ctc-2021-refundable-split.fixed.md b/changelog.d/ctc-2021-refundable-split.fixed.md
new file mode 100644
index 00000000000..1d7fe5f8c27
--- /dev/null
+++ b/changelog.d/ctc-2021-refundable-split.fixed.md
@@ -0,0 +1,3 @@
+Correct the 2021 child tax credit split after both phaseouts so the credit for other dependents remains nonrefundable, including its effect on solar-credit ordering and Oklahoma conformity.
+
+Correct the ordinary CTC phaseout threshold for surviving spouses to $200,000 from 2018 onward, while retaining their separate $150,000 ARPA threshold.
diff --git a/policyengine_us/parameters/gov/irs/credits/ctc/phase_out/threshold.yaml b/policyengine_us/parameters/gov/irs/credits/ctc/phase_out/threshold.yaml
index 44326a91b4d..57434861b63 100644
--- a/policyengine_us/parameters/gov/irs/credits/ctc/phase_out/threshold.yaml
+++ b/policyengine_us/parameters/gov/irs/credits/ctc/phase_out/threshold.yaml
@@ -13,7 +13,7 @@ HEAD_OF_HOUSEHOLD:
   2018-01-01: 200_000
 SURVIVING_SPOUSE:
   2013-01-01: 75_000
-  2018-01-01: 400_000
+  2018-01-01: 200_000
 metadata:
   propagate_metadata_to_children: true
   unit: currency-USD
@@ -27,6 +27,8 @@ metadata:
     # TCJA:
     - title: U.S.C 26 U.S. Code § 24(h)(3)
       href: https://www.law.cornell.edu/uscode/text/26/24#h_3
+    - title: 2021 Schedule 8812, line 9
+      href: https://www.irs.gov/pub/irs-prior/f1040s8--2021.pdf#page=1
     - title: H.R.1 - One Big Beautiful Bill Act
       href: https://www.congress.gov/bill/119th-congress/house-bill/1/text
     # OBBB extends TCJA CTC phaseout thresholds.
diff --git a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/ctc.yaml b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/ctc.yaml
index cf952a3fb1f..d7e3e28ea60 100644
--- a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/ctc.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/ctc.yaml
@@ -6,3 +6,31 @@
     ctc_phase_out: 1
   output:
     ctc: 2
+
+- name: A surviving spouse phases out the 2021 child credit above 200000.
+  period: 2021
+  input:
+    people:
+      parent:
+        age: 40
+        is_tax_unit_head: true
+        employment_income: 200_001
+      child:
+        age: 5
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [parent, child]
+        filing_status: SURVIVING_SPOUSE
+    households:
+      household:
+        members: [parent, child]
+        state_code: TX
+  output:
+    ctc_arpa_phase_out_threshold: 150_000
+    ctc_arpa_phase_out: 1_600
+    ctc_phase_out_threshold: 200_000
+    ctc_phase_out: 50
+    ctc: 1_950
+    refundable_ctc: 1_950
+    non_refundable_ctc: 0
diff --git a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_phase_out_cap.yaml b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_phase_out_cap.yaml
index 56f4f243246..fab8a2ed4b2 100644
--- a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_phase_out_cap.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_phase_out_cap.yaml
@@ -6,3 +6,21 @@
     ctc_arpa_max_addition: 15_000 # e.g. 15 kids * $1,000 extra.
   output:
     ctc_arpa_phase_out_cap: 12_500 # 400k - 150k = 250k * 0.05 = 12.5k
+
+- name: The first ARPA reduction cannot exceed the child credit enhancement.
+  period: 2021
+  input:
+    filing_status: JOINT
+    ctc_arpa_max_addition: 1_600
+  output:
+    ctc_arpa_phase_out_cap: 1_600
+
+- name: Surviving spouses use the 2500 threshold-difference cap in the Line 5 Worksheet.
+  period: 2021
+  input:
+    filing_status: SURVIVING_SPOUSE
+    ctc_arpa_max_addition: 5_000
+  output:
+    ctc_arpa_phase_out_threshold: 150_000
+    ctc_phase_out_threshold: 200_000
+    ctc_arpa_phase_out_cap: 2_500
diff --git a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/phase_out/ctc_phase_out_threshold.yaml b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/phase_out/ctc_phase_out_threshold.yaml
index 27ee30edd45..e2fb8bf5d20 100644
--- a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/phase_out/ctc_phase_out_threshold.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/phase_out/ctc_phase_out_threshold.yaml
@@ -5,9 +5,40 @@
   output:
     ctc_phase_out_threshold: 200_000
 
-- name: CTC phases out at $400,000 for singles while TCJA is active.
+- name: CTC phases out at $400,000 for joint filers while TCJA is active.
   period: 2022
   input:
     filing_status: JOINT
   output:
     ctc_phase_out_threshold: 400_000
+
+# Section 24(h)(3) reserves $400,000 for joint returns. A surviving spouse
+# shares joint tax rates under section 1(a), but does not file a joint return.
+- name: Before TCJA surviving spouses retain the nonjoint threshold.
+  period: 2017
+  input:
+    filing_status: [SURVIVING_SPOUSE, JOINT]
+  output:
+    ctc_phase_out_threshold: [75_000, 110_000]
+
+- name: TCJA gives surviving spouses the ordinary nonjoint threshold.
+  period: 2018
+  input:
+    filing_status: [SURVIVING_SPOUSE, JOINT]
+  output:
+    ctc_phase_out_threshold: [200_000, 400_000]
+
+- name: ARPA does not make surviving spouses joint filers for ordinary phaseout.
+  period: 2021
+  input:
+    filing_status: [SURVIVING_SPOUSE, JOINT]
+  output:
+    ctc_phase_out_threshold: [200_000, 400_000]
+    ctc_arpa_phase_out_threshold: [150_000, 150_000]
+
+- name: OBBB retains the nonjoint ordinary threshold for surviving spouses.
+  period: 2026
+  input:
+    filing_status: [SURVIVING_SPOUSE, JOINT]
+  output:
+    ctc_phase_out_threshold: [200_000, 400_000]
diff --git a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/refundable_ctc.yaml b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/refundable_ctc.yaml
index 4b6ca135f18..05bb3efe4e2 100644
--- a/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/refundable_ctc.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/refundable_ctc.yaml
@@ -206,3 +206,118 @@
     section_911_excluded_income: 0
     # Without the section 911 bar, the same phase-in gives a $3,400 refund.
     refundable_ctc: 0
+
+# Proposed additions to:
+# policyengine_us/tests/policy/baseline/gov/irs/credits/ctc/refundable/refundable_ctc.yaml
+# Sources: 2021 IRC 24(i)(1)(B), 24(i)(4)(C)-(D), and Schedule 8812
+# https://www.govinfo.gov/content/pkg/USCODE-2021-title26/pdf/USCODE-2021-title26-subtitleA.pdf#page=57
+# https://www.govinfo.gov/content/pkg/USCODE-2021-title26/pdf/USCODE-2021-title26-subtitleA.pdf#page=58
+# https://www.irs.gov/pub/irs-prior/f1040s8--2021.pdf#page=1
+- name: ARPA phaseout cannot make other-dependent credit refundable in 2021
+  period: 2021
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        is_tax_unit_head: true
+        employment_income: 170_000
+      child:
+        age: 5
+        is_tax_unit_dependent: true
+      other_dependent:
+        age: 19
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [parent, child, other_dependent]
+        filing_status: HEAD_OF_HOUSEHOLD
+    households:
+      household:
+        members: [parent, child, other_dependent]
+        state_code: TX
+  output:
+    adjusted_gross_income: 170_000
+    # min(50 * ceil((170,000 - 112,500) / 1,000), 1,600, 4,375).
+    ctc_arpa_phase_out: 1_600
+    ctc_phase_out: 0
+    # Schedule 8812 lines 5 + 7 = (3,600 - 1,600) + 500.
+    ctc: 2_500
+    # Lines 14a = 500; 14b = 2,500 - 500 = 2,000.
+    refundable_ctc: 2_000
+    # Before the section 26(a) tax limit (not binding in this household).
+    non_refundable_ctc: 500
+    ctc_value: 2_500
+
+- name: Other-dependent credit remains nonrefundable after the child portion phases out in 2021
+  period: 2021
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        is_tax_unit_head: true
+        employment_income: 240_001
+      child:
+        age: 5
+        is_tax_unit_dependent: true
+      other_dependent:
+        age: 19
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [parent, child, other_dependent]
+        filing_status: HEAD_OF_HOUSEHOLD
+    households:
+      household:
+        members: [parent, child, other_dependent]
+        state_code: TX
+  output:
+    ctc_arpa_phase_out: 1_600
+    # 50 * ceil((240,001 - 200,000) / 1,000) = 2,050.
+    ctc_phase_out: 2_050
+    ctc: 450
+    # max(3,600 - 1,600 - 2,050, 0) = 0.
+    refundable_ctc: 0
+    non_refundable_ctc: 450
+    ctc_value: 450
+
+# This interaction proves the split affects total federal tax, not only labels.
+# 2021 IRC 25D(c), Form 5695 lines 6b/14/15 and its Credit Limit Worksheet:
+# https://www.govinfo.gov/content/pkg/USCODE-2021-title26/pdf/USCODE-2021-title26-subtitleA.pdf#page=81
+# https://www.irs.gov/pub/irs-prior/f5695--2021.pdf#page=1
+# https://www.irs.gov/pub/irs-prior/i5695--2021.pdf#page=4
+- name: Nonrefundable ODC reduces solar credit capacity before the 2021 child refund
+  period: 2021
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        is_tax_unit_head: true
+        employment_income: 170_000
+      child:
+        age: 5
+        is_tax_unit_dependent: true
+      other_dependent:
+        age: 19
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [parent, child, other_dependent]
+        filing_status: HEAD_OF_HOUSEHOLD
+        solar_electric_property_expenditures: 150_000
+    households:
+      household:
+        members: [parent, child, other_dependent]
+        state_code: TX
+  output:
+    income_tax_before_credits: 28_857
+    residential_clean_energy_credit_potential: 39_000
+    # 28,857 - 500 ODC; refundable child credit is in a different subpart.
+    residential_clean_energy_credit_credit_limit: 28_357
+    residential_clean_energy_credit: 28_357
+    refundable_ctc: 2_000
+    non_refundable_ctc: 500
+    ctc_value: 2_500
+    income_tax: -2_000
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_integration.yaml b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_integration.yaml
index ef0d960dd06..341ffdb7f0f 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_integration.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/ia/tax/income/ia_integration.yaml
@@ -49,11 +49,11 @@
   output:  # expected ia_income_tax from patched TAXSIM35 2023-05-05 version
     # federal income tax results used by Iowa income tax:
     filing_status: JOINT
-    # federal CTC was fully refundable in 2021, but composition is flawed
-    non_refundable_ctc: 0  # a negative credit is nonsensical
-    refundable_ctc: 8_000  # the correct amount in this case is $8000
+    # The child-only CTC remaining after both 2021 reductions is refundable.
+    non_refundable_ctc: 0
+    refundable_ctc: 8_000
     income_tax_before_refundable_credits: 42_502.80  #
-    # formula for ia_fedtax_deduction works around the above federal CTC flaw
+    # Iowa retains its addback of nonrefundable CTC in fully refundable years.
     additional_medicare_tax: 18.18  # neither this nor SECA tax part of fedtax
     # Iowa income tax results:
     ia_net_income: [104_010, 148_010, 0, 0, 0, 0]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ok/tax/income/credits/ok_federal_ctc.yaml b/policyengine_us/tests/policy/baseline/gov/states/ok/tax/income/credits/ok_federal_ctc.yaml
index 4f8d7ea0d38..680910cacb6 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/ok/tax/income/credits/ok_federal_ctc.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/ok/tax/income/credits/ok_federal_ctc.yaml
@@ -22,6 +22,42 @@
     income_tax_capped_non_refundable_credits: 1_000
     ok_federal_ctc: 500
 
+- name: In 2021 Oklahoma counts the child refund and only usable ODC after ARPA phaseout.
+  period: 2021
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        is_tax_unit_head: true
+        employment_income: 90_000
+      child:
+        age: 5
+        is_tax_unit_dependent: true
+      other_dependent:
+        age: 19
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [parent, child, other_dependent]
+        filing_status: SINGLE
+        # Isolate a binding federal tax limit while deriving the CTC split.
+        income_tax_before_credits: 300
+    households:
+      household:
+        members: [parent, child, other_dependent]
+        state_code: OK
+  output:
+    ctc_arpa_phase_out: 750
+    ctc_phase_out: 0
+    ctc: 3_350
+    refundable_ctc: 2_850
+    non_refundable_ctc: 500
+    income_tax_capped_non_refundable_credits: 300
+    ctc_value: 3_150
+    # Refundable child credit plus 300 of the 500 nonrefundable ODC.
+    ok_federal_ctc: 3_150
+
 - name: A credit on Credit Limit Worksheet A, line 2, is applied before the CTC when credits exceed tax.
   period: 2025
   input:
diff --git a/policyengine_us/tests/test_ctc_2021_refundable_split.py b/policyengine_us/tests/test_ctc_2021_refundable_split.py
new file mode 100644
index 00000000000..14a419e0dc4
--- /dev/null
+++ b/policyengine_us/tests/test_ctc_2021_refundable_split.py
@@ -0,0 +1,155 @@
+"""Vectorized invariants for the 2021 Schedule 8812 refundable/ODC split.
+
+The YAML cases cover complete households. This grid checks the partition across
+combinations of child credit, other dependent credit, both reductions and the
+refundable cap, using one Core simulation for every year. Inputs stop above the
+split so these tests exercise its dependency wiring and the pre-liability
+``non_refundable_ctc`` definition without building tax-return branches.
+
+2021 Schedule 8812 lines 12, 14a and 14b, and IRC 24(h)(4), (i)(1)(B),
+(i)(4)(D) independently determine the child-only refund and remaining ODC.
+Other years retain the existing section 24(d) tax-limit-increase calculation.
+"""
+
+from itertools import product
+
+import numpy as np
+import pytest
+from policyengine_core.simulations import SimulationBuilder
+
+from policyengine_us.system import system
+
+
+OTHER_YEARS = (2018, 2020, 2022, 2025, 2026)
+# First-reduction fractions include both ends of the ARPA-increase cap. The
+# ordinary reduction extends beyond all child credit and then all ODC.
+GRID = np.asarray(
+    list(
+        product(
+            (0, 1),  # children under 6
+            (0, 1),  # children 6 through 17
+            (0, 1, 2),  # other dependents
+            (0, 0.5, 1),  # share of the ARPA increase removed
+            (0, 50, 500, 2_000, 4_000, 8_000, 20_000),
+        )
+    )
+)
+
+
+@pytest.fixture(scope="module")
+def split_grid():
+    # A second copy gives each case a smaller refundable maximum. This guards
+    # the existing cap contract without making a second, mutable reform tree.
+    cases = np.concatenate((GRID, GRID))
+    young, older, other_dependents, first_share, ordinary = cases.T
+    children = young + older
+    count = len(cases)
+    baseline = np.arange(count) < len(GRID)
+    simulation = SimulationBuilder().build_default_simulation(system, count)
+    phase_in = np.resize([0, 300, 1_500, 10_000], count)
+    liability = np.resize([0, 250, 2_000, 20_000], count)
+    barred = np.resize([False, False, False, True, False], count)
+    references = {}
+
+    for year in (2021, *OTHER_YEARS):
+        p = system.parameters(f"{year}-01-01").gov.irs.credits.ctc
+        base_per_child = p.amount.base.calc(np.array([5]))[0]
+        base_children = children * base_per_child
+        other_credit = other_dependents * p.amount.adult_dependent
+        if year == 2021:
+            child_credit = young * 3_600 + older * 3_000
+            addition = child_credit - base_children
+            first = addition * first_share
+            maximum = np.where(baseline, child_credit, child_credit / 2)
+        else:
+            child_credit = base_children
+            addition = np.zeros(count)
+            first = np.zeros(count)
+            maximum = children * min(base_per_child, p.refundable.individual_max)
+
+        inputs = {
+            "filer_meets_ctc_identification_requirements": np.ones(count, dtype=bool),
+            "ctc_maximum": base_children + other_credit,
+            "ctc_arpa_max_addition": addition,
+            "ctc_arpa_phase_out": first,
+            "ctc_phase_out": ordinary,
+            "ctc_refundable_maximum": maximum,
+            "ctc_phase_in": phase_in,
+            "ctc_limiting_tax_liability": liability,
+            "refundable_ctc_barred_by_section_911_exclusion": barred,
+        }
+        for name, values in inputs.items():
+            simulation.set_input(name, year, values)
+        references[year] = {
+            "child": child_credit,
+            "other": other_credit,
+            "first": first,
+            "ordinary": ordinary,
+            "maximum": maximum,
+            "total": np.maximum(child_credit + other_credit - first - ordinary, 0),
+        }
+
+    return simulation, references, baseline, phase_in, liability, barred
+
+
+def test_2021_schedule_8812_partition_before_tax_limit(split_grid):
+    simulation, references, baseline, _, liability, _ = split_grid
+    r = references[2021]
+    total = simulation.calculate("ctc", 2021)
+    refundable = simulation.calculate("refundable_ctc", 2021)
+    non_refundable = simulation.calculate("non_refundable_ctc", 2021)
+    child_after_reductions = np.maximum(r["child"] - r["first"] - r["ordinary"], 0)
+
+    # Schedule 8812 line 14b excludes ODC and applies both reductions. Identity
+    # alone would miss the old bug: its refundable and non-refundable amounts
+    # added up correctly while some ODC was incorrectly refundable.
+    np.testing.assert_allclose(total[baseline], r["total"][baseline])
+    np.testing.assert_allclose(refundable[baseline], child_after_reductions[baseline])
+    np.testing.assert_allclose(
+        non_refundable[baseline], np.minimum(r["other"], r["total"])[baseline]
+    )
+    np.testing.assert_allclose(refundable + non_refundable, total)
+    assert (refundable >= 0).all()
+    assert (refundable <= child_after_reductions).all()
+    assert (refundable[r["child"] == 0] == 0).all()
+    # The stored non-refundable amount is before section 26(a), even where
+    # the tax unit cannot use all its other-dependent credit.
+    assert (baseline & (non_refundable > liability)).any()
+    assert (baseline & (r["first"] > 0) & (r["other"] > 0) & (refundable > 0)).any()
+    assert (
+        baseline & (r["ordinary"] > 0) & (refundable == 0) & (non_refundable > 0)
+    ).any()
+
+
+def test_2021_preserves_lower_refundable_maximum(split_grid):
+    simulation, references, baseline, _, _, _ = split_grid
+    r = references[2021]
+    expected = np.minimum(
+        np.maximum(r["maximum"] - r["first"] - r["ordinary"], 0), r["total"]
+    )
+    actual = simulation.calculate("refundable_ctc", 2021)
+    np.testing.assert_allclose(actual[~baseline], expected[~baseline])
+    assert (actual <= r["maximum"]).all()
+    assert ((~baseline) & (actual > 0) & (actual < r["child"])).any()
+
+
+@pytest.mark.parametrize("year", OTHER_YEARS)
+def test_other_years_preserve_partial_refundability(split_grid, year):
+    simulation, references, _, phase_in, liability, barred = split_grid
+    r = references[year]
+    # The legacy section 24(d) calculation refunds the gain in allowed CTC
+    # when its tax limit increases by the phase-in, subject to its child cap
+    # and the separate section 911 bar.
+    increase = np.minimum(r["total"], liability + phase_in) - np.minimum(
+        r["total"], liability
+    )
+    expected = np.where(barred, 0, np.minimum(r["maximum"], increase))
+    total = simulation.calculate("ctc", year)
+    refundable = simulation.calculate("refundable_ctc", year)
+    non_refundable = simulation.calculate("non_refundable_ctc", year)
+    np.testing.assert_allclose(total, r["total"])
+    np.testing.assert_allclose(refundable, expected)
+    np.testing.assert_allclose(refundable + non_refundable, total)
+    assert (refundable <= r["child"]).all()
+    assert ((refundable > 0) & (refundable < total)).any()
+    assert ((total > 0) & (refundable == 0)).any()
diff --git a/policyengine_us/variables/gov/irs/credits/ctc/non_refundable_ctc.py b/policyengine_us/variables/gov/irs/credits/ctc/non_refundable_ctc.py
index d384b2c4dad..683d9fc8ab5 100644
--- a/policyengine_us/variables/gov/irs/credits/ctc/non_refundable_ctc.py
+++ b/policyengine_us/variables/gov/irs/credits/ctc/non_refundable_ctc.py
@@ -6,7 +6,15 @@ class non_refundable_ctc(Variable):
     entity = TaxUnit
     label = "non-refundable CTC"
     unit = USD
-    documentation = "The portion of the Child Tax Credit that is not refundable."
+    documentation = (
+        "The nonrefundable Child Tax Credit entitlement before the section "
+        "26(a) tax-liability limit. In 2021 this is the credit for other "
+        "dependents on Schedule 8812 line 14a, before the limit on line 14d."
+    )
     definition_period = YEAR
+    reference = (
+        "https://www.law.cornell.edu/uscode/text/26/26#a",
+        "https://www.irs.gov/pub/irs-prior/f1040s8--2021.pdf#page=1",
+    )
     adds = ["ctc"]
     subtracts = ["refundable_ctc"]
diff --git a/policyengine_us/variables/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_max_addition.py b/policyengine_us/variables/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_max_addition.py
index f5f3bb3c7e9..79234777e52 100644
--- a/policyengine_us/variables/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_max_addition.py
+++ b/policyengine_us/variables/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_max_addition.py
@@ -11,7 +11,7 @@ class ctc_arpa_max_addition(Variable):
     )
     definition_period = YEAR
     # Defined on Line 5 worksheet of 2021 Instructions for Schedule 8812.
-    reference = "https://www.irs.gov/pub/irs-pdf/i1040s8.pdf#page=4"
+    reference = "https://www.irs.gov/pub/irs-prior/i1040s8--2021.pdf#page=4"
 
     def formula(tax_unit, period, parameters):
         # Logic sequence follows the form, which is clearer than the IRC.
diff --git a/policyengine_us/variables/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_phase_out_cap.py b/policyengine_us/variables/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_phase_out_cap.py
index 2ffdb47ce97..38a4aa37cee 100644
--- a/policyengine_us/variables/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_phase_out_cap.py
+++ b/policyengine_us/variables/gov/irs/credits/ctc/phase_out/arpa/ctc_arpa_phase_out_cap.py
@@ -11,7 +11,7 @@ class ctc_arpa_phase_out_cap(Variable):
     )
     definition_period = YEAR
     # Defined on Line 5 worksheet of 2021 Instructions for Schedule 8812.
-    reference = "https://www.irs.gov/pub/irs-pdf/i1040s8.pdf#page=4"
+    reference = "https://www.irs.gov/pub/irs-prior/i1040s8--2021.pdf#page=4"
 
     def formula(tax_unit, period, parameters):
         # Logic sequence follows the form, which is clearer than the IRC.
diff --git a/policyengine_us/variables/gov/irs/credits/ctc/refundable/refundable_ctc.py b/policyengine_us/variables/gov/irs/credits/ctc/refundable/refundable_ctc.py
index e2a65576a39..56602e9fa5f 100644
--- a/policyengine_us/variables/gov/irs/credits/ctc/refundable/refundable_ctc.py
+++ b/policyengine_us/variables/gov/irs/credits/ctc/refundable/refundable_ctc.py
@@ -12,6 +12,10 @@ class refundable_ctc(Variable):
         "https://www.law.cornell.edu/uscode/text/26/24#d",
         # No refundable CTC for filers electing a section 911 exclusion.
         "https://www.law.cornell.edu/uscode/text/26/24#d_3",
+        # 2021 section 24(h)(4), (i)(1)(B) and (i)(4)(C)-(D), PDF pages 57-58.
+        "https://www.govinfo.gov/content/pkg/USCODE-2021-title26/pdf/USCODE-2021-title26-subtitleA.pdf#page=57",
+        "https://www.irs.gov/pub/irs-prior/f1040s8--2021.pdf#page=1",
+        "https://www.irs.gov/pub/irs-prior/i1040s8--2021.pdf#page=4",
     )
 
     def formula(tax_unit, period, parameters):
@@ -28,7 +32,13 @@ def formula(tax_unit, period, parameters):
         total_ctc = tax_unit("ctc", period)
 
         if ctc.refundable.fully_refundable:
-            reduction = tax_unit("ctc_phase_out", period)
+            # Schedule 8812 lines 12 and 14a-14b: refund only the child
+            # credit after both the first ARPA reduction and ordinary
+            # phaseout. Section 24(i)(1)(B) excludes the section 24(h)(4)
+            # credit for other dependents from refundability.
+            reduction = tax_unit("ctc_arpa_phase_out", period) + tax_unit(
+                "ctc_phase_out", period
+            )
             reduced_max_amount = max_(0, maximum_amount - reduction)
             return min_(reduced_max_amount, total_ctc)
 
diff --git a/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_fedtax_deduction.py b/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_fedtax_deduction.py
index 647a9b01594..36d77dca2d5 100644
--- a/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_fedtax_deduction.py
+++ b/policyengine_us/variables/gov/states/ia/tax/income/including_married_filing_separately/deductions/ia_fedtax_deduction.py
@@ -27,7 +27,8 @@ def formula(person, period, parameters):
         # (line 12) is not added back. With payments equal to the liability,
         # the deduction is the federal income tax without the added-back taxes.
         ustax = person.tax_unit("income_tax_before_refundable_credits", period)
-        # correct ustax amount for flaw in CTC refundable/nonrefundable split
+        # Retain the existing addback in fully refundable years, including
+        # any nonrefundable credit for other dependents in 2021.
         ctc = parameters(period).gov.irs.credits.ctc
         ctc_fully_refundable = ctc.refundable.fully_refundable
         if ctc_fully_refundable: