diff --git a/changelog.d/fix-mt-ehrc-filer-gains-ss-roles.fixed.md b/changelog.d/fix-mt-ehrc-filer-gains-ss-roles.fixed.md
new file mode 100644
index 00000000000..6ea58342f76
--- /dev/null
+++ b/changelog.d/fix-mt-ehrc-filer-gains-ss-roles.fixed.md
@@ -0,0 +1 @@
+Montana's elderly homeowner/renter credit now counts each person's capital gains without losses netted on the same return or carried over from a prior year, and counts Social Security once when a person is input as both a tax unit dependent and the head or spouse.
diff --git a/policyengine_us/parameters/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/gross_income_sources.yaml b/policyengine_us/parameters/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/gross_income_sources.yaml
index 7c3c2184cdf..cf106919fa7 100644
--- a/policyengine_us/parameters/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/gross_income_sources.yaml
+++ b/policyengine_us/parameters/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/gross_income_sources.yaml
@@ -80,8 +80,11 @@ values:
     - mt_elderly_homeowner_or_renter_credit_received
     - mt_elderly_homeowner_or_renter_credit_federal_refundable_credits
     - mt_elderly_homeowner_or_renter_credit_property_tax_rebate_received
-# Social Security under § 15-30-2337(9)(a)(viii) is added separately in
-# mt_elderly_homeowner_or_renter_credit_gross_household_income.py because the
-# untaxed portion is computed as social_security - taxable_social_security.
+# mt_elderly_homeowner_or_renter_credit_gross_household_income.py adds three
+# items separately: a tax unit dependent's own gross income (their income is
+# on their own return, not in adjusted_gross_income_person); the untaxed
+# portion of Social Security under § 15-30-2337(9)(a)(viii), computed as
+# social_security less the taxable part already counted; and the losses
+# deducted in federal AGI.
 # This year's elderly homeowner/renter credit is excluded to avoid a circular
 # reference; the credit received during the year counts instead.
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.yaml
index 77815f66850..26c8d9b63a2 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.yaml
@@ -842,3 +842,159 @@
     mt_elderly_homeowner_or_renter_credit_pre_one_claimant: [50, 410]
     mt_elderly_homeowner_or_renter_credit_selected_claimant: [false, true]
     mt_elderly_homeowner_or_renter_credit: [0, 410]
+
+- name: 2024 Schedule 2EC, a household member's netted capital loss does not raise the credit
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 2_000
+      person2:
+        age: 30
+        long_term_capital_gains: 10_000
+        short_term_capital_gains: -4_000
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Line 5 = 20_000; line 17 = 10_000 (the gain without the loss); line
+    # 18 = 30_000; line 20 = 17_400; line 22 = 17_400 x 0.05 = 870; line 27 =
+    # 2_000 - 870 = 1_130; line 29 = 100%; line 30 = 1_130. Netting the loss
+    # would give line 18 = 26_000, line 22 = 670 and the 1_150 cap.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [870, 0]
+    mt_elderly_homeowner_or_renter_credit: [1_130, 0]
+
+- name: 2024 Schedule 2EC, a capital loss netted between spouses on a joint return is added back once
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: 10_000
+        real_estate_taxes: 2_000
+      person2:
+        age: 68
+        short_term_capital_gains: -4_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Federal AGI 20_000 + 10_000 - 4_000 = 26_000; the 4_000 loss is added
+    # back once. Line 18 = 30_000; line 22 = 870; line 30 = 1_130.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [870, 0]
+    mt_elderly_homeowner_or_renter_credit: [1_130, 0]
+
+- name: 2024 Schedule 2EC, a minor filing alone in the household counts once
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 2_000
+        is_tax_unit_dependent: false
+      person2:
+        age: 16
+        employment_income: 10_000
+        is_tax_unit_dependent: false
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Line 18 = 20_000 + 10_000 = 30_000; line 30 = 1_130. Counting the
+    # minor's wages twice would give 40_000, line 22 = 1_370 and a credit of
+    # (2_000 - 1_370) x 30% = 189.
+    mt_elderly_homeowner_or_renter_credit: [1_130, 0]
+
+- name: 2024 Schedule 2EC, a capital loss carryover does not raise the credit
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: 6_000
+        long_term_capital_loss_carryover: 4_000
+        real_estate_taxes: 2_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # Line 5 = 20_000; line 7 = the year's 10_000 gain; line 18 = 30_000;
+    # line 22 = 870; line 30 = 1_130. Counting the 6_000 net would give line
+    # 18 = 26_000, line 22 = 670 and the 1_150 cap.
+    mt_elderly_homeowner_or_renter_credit: 1_130
+
+- name: 2024 Schedule 2EC, a carryover against a net loss supplied directly adds no gain
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 30_000
+        capital_gains: -5_000
+        long_term_capital_loss_carryover: 5_000
+        real_estate_taxes: 2_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # Line 18 = 30_000; line 22 = 17_400 x 0.05 = 870; line 30 = 1_130.
+    # Counting the carryover as a gain would give 35_000 and 880 x 40% = 352.
+    mt_elderly_homeowner_or_renter_credit: 1_130
+
+- name: 2024 Schedule 2EC, a dependent's gain counts when another person's capital_gains is supplied
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        capital_gains: 0
+        real_estate_taxes: 2_000
+      person2:
+        age: 16
+        long_term_capital_gains: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Line 18 = 20_000 + 10_000 = 30_000; line 22 = 870; line 30 = 1_130.
+    mt_elderly_homeowner_or_renter_credit: [1_130, 0]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.yaml
index ec00f8c2fad..88c8b004aa0 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.yaml
@@ -336,3 +336,449 @@
     # extent that the amount recovered reduced the claimant's Montana income
     # tax in a prior year".
     mt_elderly_homeowner_or_renter_credit_gross_household_income: 10_000
+
+- name: A filer's short-term loss netted against their own long-term gain does not reduce gross household income
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+      person2:
+        age: 30
+        long_term_capital_gains: 10_000
+        short_term_capital_gains: -4_000
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # § 15-30-2337(9)(a): federal AGI "without regard to loss"; 2024
+    # Schedule 2EC line 7: "do not include any capital losses". The member's
+    # own return nets the 4_000 loss into a 6_000 gain, so no loss reaches
+    # loss_ald; line 17 still carries the 10_000 gain.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+
+- name: A claimant's short-term loss netted against their own long-term gain does not reduce gross household income
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: 10_000
+        short_term_capital_gains: -4_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # Line 5 pensions 20_000 plus line 7 capital gain 10_000, without the
+    # 4_000 loss: 30_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 30_000
+
+- name: A minor alone in a tax unit and not input as a dependent counts once
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        is_tax_unit_dependent: false
+      person2:
+        age: 16
+        employment_income: 10_000
+        is_tax_unit_dependent: false
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # The minor is neither head (no adult in the unit), spouse nor
+    # dependent. irs_gross_income counts people who are not dependents and
+    # dependent_gross_income people who are not the head or spouse, so only
+    # the dependent gate keeps the 10_000 wages from counting twice.
+    is_tax_unit_head: [true, false]
+    is_tax_unit_dependent: [false, false]
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+
+- name: Social Security counts in full for a person input as a dependent who is also the head
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        is_tax_unit_dependent: false
+      person2:
+        age: 40
+        social_security_retirement: 60_000
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    marital_units:
+      marital_unit1:
+        members: [person1]
+      marital_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Every adult in tax_unit2 is input as a dependent, so person2 is also
+    # its head. Neither irs_gross_income (dependents) nor
+    # dependent_gross_income (heads) counts their taxable benefits, so none
+    # may be subtracted: § 15-30-2337(9)(a)(viii) counts all 60_000.
+    is_tax_unit_head: [true, true]
+    is_tax_unit_dependent: [false, true]
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 60_000]
+
+- name: A business loss on another household member's return does not reduce gross household income
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+      person2:
+        age: 40
+        taxable_interest_income: 10_000
+        self_employment_income: -5_000
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # The member's federal AGI is 10_000 - 5_000 = 5_000; the 5_000 loss is
+    # added back on their own return's head. 2024 Schedule 2EC line 10:
+    # "Do not include any losses generated from the business."
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+
+- name: A dependent in another household member's tax unit counts
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 15_000
+      person2:
+        age: 35
+        employment_income: 25_000
+      person3:
+        age: 5
+      person4:
+        age: 8
+        employment_income: 3_000
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2, person3, person4]
+    households:
+      household:
+        members: [person1, person2, person3, person4]
+        state_name: MT
+  output:
+    # Three generations. The daughter's line 8 credits: federal EITC with
+    # two children 6_960 - 21.06% x (25_000 - 22_720) = 6_479.83; refundable
+    # child tax credit min(2 x 1_700, 15% x (25_000 - 2_500)) = 3_375;
+    # Montana EITC 10% = 647.98. Her total: 25_000 + 6_479.83 + 3_375 +
+    # 647.98 = 35_502.81. The grandchild is her dependent; their 3_000 of
+    # wages count through the dependent path.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [15_000, 35_502.81, 0, 3_000]
+
+- name: A prior-year capital loss carryover does not reduce the year's gain
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        # A 10_000 gain this year less a 4_000 carryover (Schedule D line 15).
+        long_term_capital_gains: 6_000
+        long_term_capital_loss_carryover: 4_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # Line 7 counts the year's gain, 10_000, without the carried loss.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 30_000
+
+- name: A carryover larger than the year's gain leaves the gain counted and the rest unused
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        # A 2_000 gain this year less a 10_000 carryover.
+        long_term_capital_gains: -8_000
+        long_term_capital_loss_carryover: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # 20_000 + the year's 2_000 gain; the unused 8_000 of carryover counts
+    # neither as income nor as a loss.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 22_000
+
+- name: A carryover with no gain this year adds nothing
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: -5_000
+        long_term_capital_loss_carryover: 5_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 20_000
+
+- name: A dependent's capital loss carryover does not reduce their gain
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+      person2:
+        age: 16
+        long_term_capital_gains: 6_000
+        long_term_capital_loss_carryover: 4_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Line 17: the dependent's 10_000 gain this year, as on a filer's return.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+
+- name: A long-term loss netted against a short-term gain does not reduce gross household income
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: -4_000
+        short_term_capital_gains: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 30_000
+
+- name: A gain is counted when the person's capital losses exceed it
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: -10_000
+        short_term_capital_gains: 4_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # Federal AGI 20_000 - 3_000 (the § 1211(b) limit); the 3_000 is added
+    # back and the 4_000 short-term gain counts: 24_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 24_000
+
+- name: Social Security counts in full for a spouse who is also input as a dependent
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        is_tax_unit_dependent: false
+      person2:
+        age: 50
+        social_security_retirement: 60_000
+        is_tax_unit_dependent: true
+      person3:
+        age: 45
+        social_security_retirement: 60_000
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2, person3]
+    marital_units:
+      marital_unit1:
+        members: [person1]
+      marital_unit2:
+        members: [person2, person3]
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_name: MT
+  output:
+    # Both adults of tax_unit2 are input as dependents, so they stay head and
+    # spouse as well. § 15-30-2337(9)(a)(viii) counts all of each one's
+    # benefits.
+    is_tax_unit_head: [true, true, false]
+    is_tax_unit_spouse: [false, false, true]
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 60_000, 60_000]
+
+- name: A carryover with a net capital loss supplied directly counts no gain
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 30_000
+        # A net amount supplied directly, without long-term and short-term
+        # parts; with the 5_000 carryover added back, the year's net is zero.
+        capital_gains: -5_000
+        long_term_capital_loss_carryover: 5_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # Federal AGI 30_000 - 3_000 (§ 1211(b)); the 3_000 is added back. No
+    # gain this year, so line 7 is zero: 30_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 30_000
+
+- name: A carryover with a net capital gain supplied directly restores the year's gain
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        capital_gains: 6_000
+        long_term_capital_loss_carryover: 4_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # The year's net gain is 6_000 + 4_000 = 10_000: 30_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 30_000
+
+- name: A dependent's carryover with a net capital loss supplied directly counts no gain
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+      person2:
+        age: 16
+        employment_income: 10_000
+        capital_gains: -5_000
+        long_term_capital_loss_carryover: 5_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+
+- name: A dependent's long-term gain counts when another person's capital_gains is supplied
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        capital_gains: 0
+      person2:
+        age: 16
+        long_term_capital_gains: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # capital_gains is an input for person1, so person2 holds its default of
+    # zero rather than the sum of their parts. The parts decide: line 17
+    # counts the dependent's 10_000 gain.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+
+- name: The long-term and short-term parts decide the gains when capital_gains disagrees with them
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: 10_000
+        short_term_capital_gains: -4_000
+        capital_gains: 20_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # The parts show a 10_000 gain and a 4_000 loss; line 7 counts 10_000,
+    # not the inconsistent 20_000 net: 30_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 30_000
diff --git a/policyengine_us/tests/test_mt_elderly_credit_capital_gains.py b/policyengine_us/tests/test_mt_elderly_credit_capital_gains.py
new file mode 100644
index 00000000000..2377af122f6
--- /dev/null
+++ b/policyengine_us/tests/test_mt_elderly_credit_capital_gains.py
@@ -0,0 +1,222 @@
+"""Invariant tests: capital gains in Montana elderly homeowner/renter credit
+income are counted without losses, however they are supplied.
+
+MCA 15-30-2337(9)(a) defines income as "federal adjusted gross income,
+without regard to loss", and 2024 Schedule 2EC line 7 says "do not include
+any capital losses". So a person's capital gains enter gross household income
+as the year's gains without losses. Three losses can hide them:
+- a short-term loss netted against a long-term gain, or the reverse;
+- the prior-year long-term capital loss carryover, which
+  long_term_capital_gains already includes;
+- losses netted against capital gain distributions.
+
+The model counts gains through federal AGI and the loss_ald add-back for
+filers, and through dependent_gross_income for tax unit dependents, and
+restores what each path nets away.
+
+The gain is a piecewise-linear function of a few amounts. So instead of
+random draws, one vectorized simulation covers a grid that reaches every
+sign region of the long-term amount plus carryover, the short-term amount,
+and a net amount, with and without distributions. It supplies them in three
+styles:
+
+- parts: long-term and short-term amounts, with capital_gains set to their
+  sum;
+- sparse parts: long-term and short-term amounts with capital_gains omitted.
+  Because capital_gains is an input for other people in the same
+  simulation, these people hold its default of zero, not the sum of their
+  parts (review round 3 of #10139);
+- net: capital_gains alone, without parts.
+
+Each case appears three ways: (A) a household member who is the claimant's
+dependent, (B) a member who files their own return, and (C) the claimant
+themselves.
+
+1. Gross household income is the claimant's pension plus the gains without
+   losses plus distributions. The reference is written from Schedule 2EC
+   line 7, independently of the model: with parts,
+   max(0, long-term + carryover) + max(0, short-term); without them,
+   max(0, net + carryover).
+2. The claimant's credit is the same in A and B.
+
+The model computes in single precision, so comparisons allow one cent or
+eight float32 spacings at the case's total, whichever is larger.
+"""
+
+import itertools
+
+import numpy as np
+import pytest
+
+from policyengine_us import Simulation
+
+YEAR = 2024
+PENSION = 20_000
+PROPERTY_TAX = 2_000
+TOLERANCE = 0.01
+
+LONG_TERM = [-12_000, -3_000, 0, 4_000, 11_000]
+SHORT_TERM = [-7_000, 0, 2_000, 9_000]
+NET = [-9_000, -2_500, 0, 3_000, 14_000]
+CARRYOVER = [0, 2_500, 15_000]
+DISTRIBUTIONS = [0, 4_000]
+
+
+def grid():
+    cases = []
+    for style in ["parts", "sparse"]:
+        for lt, st, co, dist in itertools.product(
+            LONG_TERM, SHORT_TERM, CARRYOVER, DISTRIBUTIONS
+        ):
+            if lt == 0 and st == 0:
+                continue  # no parts: covered by the net style
+            cases.append(
+                dict(style=style, long_term=lt, short_term=st, net=lt + st)
+                | dict(carryover=co, distributions=dist)
+            )
+    for net, co, dist in itertools.product(NET, CARRYOVER, DISTRIBUTIONS):
+        cases.append(
+            dict(style="net", long_term=0, short_term=0, net=net)
+            | dict(carryover=co, distributions=dist)
+        )
+    return cases
+
+
+def capital_inputs(c):
+    inputs = {
+        "long_term_capital_loss_carryover": {YEAR: c["carryover"]},
+        "non_sch_d_capital_gains": {YEAR: c["distributions"]},
+    }
+    if c["style"] != "net":
+        inputs["long_term_capital_gains"] = {YEAR: c["long_term"]}
+        inputs["short_term_capital_gains"] = {YEAR: c["short_term"]}
+    if c["style"] != "sparse":
+        inputs["capital_gains"] = {YEAR: c["net"]}
+    return inputs
+
+
+NO_CAPITAL = dict(style="parts", long_term=0, short_term=0, net=0) | dict(
+    carryover=0, distributions=0
+)
+
+
+def build_situation(cases):
+    people, tax_units, marital_units, households = {}, {}, {}, {}
+    for i, c in enumerate(cases):
+        for arrangement in "ABC":
+            key = f"{arrangement}{i}"
+            claimant = f"claimant_{key}"
+            people[claimant] = {
+                "age": {YEAR: 70},
+                "taxable_pension_income": {YEAR: PENSION},
+                "real_estate_taxes": {YEAR: PROPERTY_TAX},
+                # Set for everyone: an input for some people gives the others
+                # its default, not its formula.
+                "is_tax_unit_dependent": {YEAR: False},
+                **capital_inputs(c if arrangement == "C" else NO_CAPITAL),
+            }
+            marital_units[f"marital_unit_{key}"] = {"members": [claimant]}
+            members = [claimant]
+            if arrangement == "C":
+                tax_units[f"tax_unit_{key}"] = {"members": [claimant]}
+            else:
+                member = f"member_{key}"
+                people[member] = {
+                    "age": {YEAR: 30},
+                    "is_tax_unit_dependent": {YEAR: arrangement == "A"},
+                    **capital_inputs(c),
+                }
+                marital_units[f"marital_unit_{key}_member"] = {"members": [member]}
+                members.append(member)
+                if arrangement == "A":
+                    tax_units[f"tax_unit_{key}"] = {"members": members}
+                else:
+                    tax_units[f"tax_unit_{key}"] = {"members": [claimant]}
+                    tax_units[f"tax_unit_{key}_member"] = {"members": [member]}
+            households[f"household_{key}"] = {
+                "members": members,
+                "state_code": {YEAR: "MT"},
+            }
+    return {
+        "people": people,
+        "tax_units": tax_units,
+        "marital_units": marital_units,
+        "households": households,
+    }
+
+
+def gains_without_losses(c):
+    """Schedule 2EC line 7 (line 17 for another member) plus distributions."""
+    if c["style"] == "net":
+        gains = max(0, c["net"] + c["carryover"])
+    else:
+        gains = max(0, c["long_term"] + c["carryover"]) + max(0, c["short_term"])
+    return gains + c["distributions"]
+
+
+@pytest.fixture(scope="module")
+def results():
+    cases = grid()
+    simulation = Simulation(situation=build_situation(cases))
+    shape = (len(cases), 3)
+    income = np.asarray(
+        simulation.calculate(
+            "mt_elderly_homeowner_or_renter_credit_gross_household_income",
+            YEAR,
+            map_to="household",
+        )
+    ).reshape(shape)
+    credit = np.asarray(
+        simulation.calculate(
+            "mt_elderly_homeowner_or_renter_credit", YEAR, map_to="household"
+        )
+    ).reshape(shape)
+    return cases, income, credit
+
+
+def tolerance(c):
+    scale = PENSION + sum(
+        abs(c[k]) for k in ["long_term", "short_term", "net", "carryover"]
+    )
+    scale += c["distributions"]
+    return max(TOLERANCE, 8 * float(np.spacing(np.float32(scale))))
+
+
+def test_gains_count_without_losses_on_every_path(results):
+    cases, income, _ = results
+    for i, c in enumerate(cases):
+        expected = PENSION + gains_without_losses(c)
+        assert income[i] == pytest.approx([expected] * 3, abs=tolerance(c)), c
+
+
+def test_credit_does_not_depend_on_how_the_member_files(results):
+    cases, _, credit = results
+    for i, c in enumerate(cases):
+        assert credit[i, 0] == pytest.approx(credit[i, 1], abs=tolerance(c)), c
+
+
+def test_hand_checked_cases(results):
+    """Two grid cases are checked by hand. A long-term amount of 11,000 that
+    already nets a 15,000 carryover, with a 9,000 short-term gain and no net
+    supplied (sparse): the year's long-term gain is 26,000, so 20,000 +
+    26,000 + 9,000 = 55,000, and the credit is 0 (55,000 is at least
+    45,000). A net -2,500 supplied
+    directly with a 2,500 carryover: no gain, so 20,000; (20,000 - 12,600) x
+    0.035 = 259, and min(2,000 - 259, 1,150) = 1,150."""
+    cases, income, credit = results
+
+    def find(**kw):
+        return next(i for i, c in enumerate(cases) if kw.items() <= c.items())
+
+    i = find(
+        style="sparse",
+        long_term=11_000,
+        short_term=9_000,
+        carryover=15_000,
+        distributions=0,
+    )
+    assert income[i].tolist() == pytest.approx([55_000] * 3)
+    assert credit[i].tolist() == pytest.approx([0] * 3)
+    j = find(style="net", net=-2_500, carryover=2_500, distributions=0)
+    assert income[j].tolist() == pytest.approx([20_000] * 3)
+    assert credit[j].tolist() == pytest.approx([1_150] * 3, abs=0.01)
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.py b/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.py
index c6f1065035f..4f6ba36ed86 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.py
@@ -53,16 +53,20 @@ def formula(person, period, parameters):
             person("dependent_gross_income", period) - own_deductions
         )
         # The income above counts only the taxable portion of Social
-        # Security: a filer's in federal AGI, a dependent's in their own
-        # gross income. Add the untaxed portion so all SS is counted per
-        # § 15-30-2337(9)(a)(viii).
+        # Security: a filer's in federal AGI (irs_gross_income counts people
+        # who are not dependents), a dependent's in their own gross income
+        # (dependent_gross_income counts people who are not the head or
+        # spouse). Subtract only the part one of them counted, then add the
+        # rest, so all SS is counted once per § 15-30-2337(9)(a)(viii) even
+        # when a person is input as both a dependent and the head.
         social_security = person("social_security", period)
-        taxable_social_security = add(
-            person,
-            period,
-            ["taxable_social_security", "dependent_taxable_social_security"],
+        head_or_spouse = person("is_tax_unit_head_or_spouse", period)
+        counted_social_security = where(
+            is_dependent,
+            ~head_or_spouse * person("dependent_taxable_social_security", period),
+            person("taxable_social_security", period),
         )
-        untaxed_social_security = max_(social_security - taxable_social_security, 0)
+        untaxed_social_security = max_(social_security - counted_social_security, 0)
         # Income is federal AGI "without regard to loss" (§ 15-30-2337(9)(a)),
         # so the business and capital losses deducted in computing federal
         # AGI are added back (2023 Elderly Homeowner/Renter Credit Schedule,
@@ -70,4 +74,59 @@ def formula(person, period, parameters):
         # losses."). The return's losses are added once, to the head.
         head = person("is_tax_unit_head", period)
         losses = head * person.tax_unit("loss_ald", period)
-        return sources + dependent_income + untaxed_social_security + losses
+        # Count each person's capital gains without losses, as 2024 Schedule
+        # 2EC line 7 requires ("do not include any capital losses"). Two
+        # losses reduce the gains the income paths count without reaching
+        # loss_ald, which holds only the capital loss deducted in AGI:
+        # - a short-term loss netted against the same person's long-term gain
+        #   (or the reverse) inside capital_gains;
+        # - the prior-year long-term capital loss carryover, which
+        #   long_term_capital_gains already includes as a loss, as Schedule D
+        #   line 15 does. Adding it back gives the year's own long-term gain
+        #   (as az_property_tax_credit_capital_gains does); only the gain it
+        #   hid is restored, never the unused loss.
+        # The long-term and short-term parts, when either is nonzero, decide
+        # the gains: max(0, long-term + carryover) + max(0, short-term).
+        # capital_gains can disagree with them: once it is an input for
+        # anyone in a simulation, everyone without one holds its default of
+        # zero, not the sum of their parts. Without parts, capital_gains is a
+        # net amount supplied directly, and the gain is max(0, net +
+        # carryover); the carryover is never counted as a gain the parts did
+        # not show.
+        carryover = max_(0, person("long_term_capital_loss_carryover", period))
+        long_term_capital_gains = person("long_term_capital_gains", period)
+        short_term_capital_gains = person("short_term_capital_gains", period)
+        short_term_gains = max_(0, short_term_capital_gains)
+        capital_gains = person("capital_gains", period)
+        parts_supplied = (long_term_capital_gains != 0) | (
+            short_term_capital_gains != 0
+        )
+        gains_without_losses = where(
+            parts_supplied,
+            max_(0, long_term_capital_gains + carryover) + short_term_gains,
+            max_(0, capital_gains + carryover),
+        )
+        # The gains each income path already counted: irs_gross_income
+        # floors the net, dependent_gross_income floors each part. The
+        # difference replaces them with the gains above; it is negative only
+        # when an inconsistent capital_gains input exceeds the parts. A person
+        # input as both a dependent and the head is counted by neither path,
+        # so nothing is restored for them.
+        counted_gains = where(
+            is_dependent,
+            ~head_or_spouse
+            * max_(
+                max_(0, long_term_capital_gains) + short_term_gains,
+                capital_gains,
+            ),
+            max_(0, capital_gains),
+        )
+        counted_by_a_path = ~(is_dependent & head_or_spouse)
+        lost_capital_gains = counted_by_a_path * (gains_without_losses - counted_gains)
+        return (
+            sources
+            + dependent_income
+            + untaxed_social_security
+            + losses
+            + lost_capital_gains
+        )