diff --git a/changelog.d/mt-elderly-credit-government-assistance.fixed.md b/changelog.d/mt-elderly-credit-government-assistance.fixed.md
new file mode 100644
index 00000000000..5c1b0d95f6f
--- /dev/null
+++ b/changelog.d/mt-elderly-credit-government-assistance.fixed.md
@@ -0,0 +1 @@
+Montana's elderly homeowner/renter credit now counts SSI, TANF, workers' compensation and guaranteed income payments received by household members in gross household income (MCA 15-30-2337(9)(a); Schedule 2EC line 16).
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..fb84aef0128 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
@@ -4,27 +4,31 @@ metadata:
   period: year
   label: Montana elderly homeowner or renter credit gross income sources
   reference:
-  - title: HB191 (2021), section 1(9), all nontaxable income
-    href: https://archive.legmt.gov/bills/2021/HB0199/HB0191_X.pdf#page=2
-  - title: Montana 2022 Form2 Individual Income Tax Instructions, Page47, Gross Household Income
-    href: https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2022_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=47
+  - title: MCA 15-30-2337(9)(a), all nontaxable income, including (vi) cash public assistance and relief
+    href: https://mca.legmt.gov/bills/mca/title_0150/chapter_0300/part_0230/section_0370/0150-0300-0230-0370.html
+  - title: MCA 15-30-2337(9)(a) (2009), the same definition
+    href: https://archive.legmt.gov/bills/2009/mca/15/30/15-30-2337.htm
   - title: 2022 Montana Code Annotated, Title 15, Chapter 30, Part 23, 15-30-2337 (9)
     href: https://law.justia.com/codes/montana/2022/title-15/chapter-30/part-23/section-15-30-2337/
-  - title: 2024 Montana Individual Income Tax Return
-    href: https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2024_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=46
+  - title: HB191 (2021), section 1(9), all nontaxable income
+    href: https://archive.legmt.gov/bills/2021/HB0199/HB0191_X.pdf#page=2
   - title: ARM 42.4.301(2) as in effect 8/20/2004 to 11/9/2017 ("elderly homeowner/renter credit refunds received")
     href: https://rules.mt.gov/api/policy-library-public/collections/aec52c46-128e-4279-9068-8af5d5432d74/policies/c90ea93a-03a2-4605-a0f8-f2f3760198bf/document/788508cc-8418-41c8-926d-3dfb11af3fd6
   - title: ARM 42.4.301(2) as in effect 11/10/2017 to 9/6/2024 ("federal refundable credits received" and "any state refundable credits received")
     href: https://rules.mt.gov/api/policy-library-public/collections/aec52c46-128e-4279-9068-8af5d5432d74/policies/c90ea93a-03a2-4605-a0f8-f2f3760198bf/document/297c7a8d-f6f4-4b70-a7b7-fe1cedee53e7
   - title: ARM 42.4.301(2) as in effect from 9/7/2024 ("federal refundable tax credits received" and "any state refundable tax credits received")
     href: https://rules.mt.gov/api/policy-library-public/collections/aec52c46-128e-4279-9068-8af5d5432d74/policies/c90ea93a-03a2-4605-a0f8-f2f3760198bf/document/768c8344-24ee-4950-9d5e-9d8fc97ec0f7
-  - title: 2017 Montana Form 2EC instructions, Gross Household Income Worksheet, item 4 ("Any federal or state refundable credits including the 2EC credit")
-    href: https://taxsim.nber.org/historical_state_tax_forms/MT/2017/Form%202%202017%20Booklet.pdf#page=65
-  - title: 2023 Montana Form 2 instructions, Elderly Homeowner/Renter Credit Schedule, line 7, refundable credits received
+  - title: 2016 Montana Form 2EC instructions, Part II, disability benefits including SSI payments and workers' compensation, and cash public assistance and relief
+    href: https://taxsim.nber.org/historical_state_tax_forms/MT/2016/Form_2_booklet_2016.pdf#page=74
+  - title: "2017 Montana Form 2EC instructions, Part II, disability benefits including SSI payments and workers' compensation, and cash public assistance and relief; Gross Household Income Worksheet, items 4 and 5 #page 64-65"
+    href: https://taxsim.nber.org/historical_state_tax_forms/MT/2017/Form%202%202017%20Booklet.pdf#page=64
+  - title: "2022 Montana Form 2 instructions, Elderly Homeowner/Renter Credit Schedule, gross household income; line 8, disability benefits including SSI payments and workers' compensation #page 47-48"
+    href: https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2022_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=47
+  - title: "2023 Montana Form 2 instructions, Elderly Homeowner/Renter Credit Schedule, line 7, refundable credits received; line 8, disability benefits including SSI payments and workers' compensation #page 51-52"
     href: https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2023_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=51
-  - title: 2024 Montana Form 2 instructions, Schedule 2EC, line 8, refundable credits received
-    href: https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2024_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=47
-  - title: 2025 Montana Form 2 instructions, Schedule 2EC, line 8, refundable credits received
+  - title: "2024 Montana Form 2 instructions, Schedule 2EC, gross household income; line 8, refundable credits received; line 16, government assistance #page 46-48"
+    href: https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2024_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=46
+  - title: "2025 Montana Form 2 instructions, Schedule 2EC, line 8, refundable credits received; line 16, government assistance #page 45-46"
     href: https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2025_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=45
 
 values:
@@ -43,6 +47,19 @@ values:
     - child_support_received # § 15-30-2337(9)(a)(iv) – support money
     - strike_benefits # § 15-30-2337(9)(a)(v) – nontaxable strike benefits
     - general_assistance # § 15-30-2337(9)(a)(vi) – cash public assistance
+    # Government assistance (Schedule 2EC line 16 from 2024; "disability
+    # benefits including ... SSI payments and workers' compensation" and
+    # "cash public assistance and relief" in earlier instructions). TANF is
+    # held for the SPM unit; tanf_person is each member's per capita share,
+    # so the household sum counts the unit's TANF once. LIEAP and SNAP are
+    # not cash public assistance (2014-2017 Form 2EC instructions).
+    - ssi
+    - tanf_person
+    - workers_compensation
+    # Guaranteed income pilot payments are outside federal AGI here, and
+    # (9)(a) counts all nontaxable income; a government-funded pilot is also
+    # "any other government assistance" (Schedule 2EC line 16).
+    - gi_cash_assistance
     # Refundable credits received (2023 schedule line 7, Schedule 2EC line 8
     # from 2024): Montana's refundable credits other than this credit,
     # and this credit as received during the year (ARM 42.4.301(2)(c)).
@@ -60,6 +77,10 @@ values:
     - child_support_received
     - strike_benefits
     - general_assistance
+    - ssi
+    - tanf_person
+    - workers_compensation
+    - gi_cash_assistance
     - mt_refundable_credits_before_renter_credit
     - mt_elderly_homeowner_or_renter_credit_received
     - mt_elderly_homeowner_or_renter_credit_federal_refundable_credits
@@ -76,6 +97,10 @@ values:
     - child_support_received
     - strike_benefits
     - general_assistance
+    - ssi
+    - tanf_person
+    - workers_compensation
+    - gi_cash_assistance
     - mt_refundable_credits_before_renter_credit
     - mt_elderly_homeowner_or_renter_credit_received
     - mt_elderly_homeowner_or_renter_credit_federal_refundable_credits
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..dac39e3fb98 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
@@ -351,6 +351,304 @@
     mt_elderly_homeowner_or_renter_credit_net_household_income: [870, 0]
     mt_elderly_homeowner_or_renter_credit: [1_130, 0]
 
+# Government assistance in gross household income. § 15-30-2337(9)(a)
+# counts "all nontaxable income, including but not limited to ... (vi) cash
+# public assistance and relief". 2024 and 2025 Schedule 2EC line 16 names
+# SSI payments, workers' compensation "and any other government assistance";
+# the 2014-2023 instructions list "Disability benefits including, but not
+# limited to, veteran's disability, SSI payments and workers' compensation".
+
+- name: 2024 Schedule 2EC worked case, the claimant's SSI counts on line 16
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 15_000
+        ssi: 6_000
+        real_estate_taxes: 1_200
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # Line 5 pensions 15_000; line 16 SSI 6_000; line 18 = 21_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 21_000
+    # Line 19 12_600; line 20 = 8_400; line 21 = 0.039 (8_000 to 8_999);
+    # line 22 = 8_400 x 0.039 = 327.60.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: 327.60
+    # Line 23 = 1_200; line 26 = 1_200; line 27 = 1_200 - 327.60 = 872.40;
+    # line 28 = 872.40 (under 1_150); line 29 = 100%; line 30 = 872.40.
+    # Without the SSI, line 18 = 15_000, line 22 = 2_400 x 0.006 = 14.40 and
+    # the credit would be the 1_150 cap.
+    mt_elderly_homeowner_or_renter_credit: 872.40
+
+
+- name: 2025 Schedule 2EC worked case, a household member's workers' compensation counts on line 17
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 72
+        taxable_pension_income: 18_000
+        real_estate_taxes: 1_500
+      person2:
+        age: 45
+        workers_compensation: 9_000
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # The claimant's adult son lives in the household and files his own
+    # return. Line 5 pensions 18_000; line 17 his workers' compensation, a
+    # line 16 item for another member, 9_000; line 18 = 27_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [18_000, 9_000]
+    # Line 20 = 27_000 - 12_600 = 14_400; line 21 = 0.05; line 22 = 720.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [720, 0]
+    # Line 27 = 1_500 - 720 = 780; line 28 = 780; line 29 = 100%; line 30 =
+    # 780. Without his workers' compensation, line 18 = 18_000, line 22 =
+    # 5_400 x 0.028 = 151.20 and the credit would be the 1_150 cap.
+    mt_elderly_homeowner_or_renter_credit: [780, 0]
+
+
+- name: 2024 Schedule 2EC worked case, a household member's guaranteed income payments count on line 17
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 16_000
+        real_estate_taxes: 1_200
+      person2:
+        age: 35
+        gi_cash_assistance: 6_000
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # The claimant's daughter files her own return and receives 6_000 from a
+    # guaranteed income pilot, which is outside federal AGI. § 15-30-2337(9)(a)
+    # counts "all nontaxable income", and line 16 counts "any other
+    # government assistance". Line 5 = 16_000; line 17 = 6_000; line 18 =
+    # 22_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [16_000, 6_000]
+    # Line 20 = 9_400; line 21 = 0.042; line 22 = 394.80.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [394.80, 0]
+    # Line 27 = 1_200 - 394.80 = 805.20; line 28 = 805.20; line 29 = 100%;
+    # line 30 = 805.20. Without the payments, line 18 = 16_000, line 22 =
+    # 3_400 x 0.016 = 54.40 and the credit 1_145.60.
+    mt_elderly_homeowner_or_renter_credit: [805.20, 0]
+
+
+- name: 2024 Schedule 2EC worked case, TANF in a shared SPM unit counts once across two returns
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 1_500
+      person2:
+        age: 30
+      person3:
+        age: 5
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2, person3]
+    spm_units:
+      spm_unit:
+        members: [person1, person2, person3]
+        tanf: 4_800
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_name: MT
+  output:
+    # The claimant's daughter and granddaughter live with her; the daughter
+    # files her own return and receives 4_800 of TANF. Line 5 pensions
+    # 20_000; line 17 the TANF 4_800; line 18 = 24_800. The model holds TANF
+    # for the SPM unit and counts each member's per capita share
+    # (tanf_person, 1_600 each), so the household total counts it once.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [21_600, 1_600, 1_600]
+    # Line 20 = 24_800 - 12_600 = 12_200; line 21 = 0.05; line 22 = 610.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [610, 0, 0]
+    # Line 27 = 1_500 - 610 = 890; line 28 = 890; line 29 = 100%; line 30 =
+    # 890. Without the TANF, line 18 = 20_000, line 22 = 7_400 x 0.035 = 259
+    # and the credit would be the 1_150 cap. Counting the whole 4_800 for
+    # each of the three members would give line 18 = 34_400 and a credit of
+    # 1_500 - 1_090 = 410.
+    mt_elderly_homeowner_or_renter_credit: [890, 0, 0]
+
+
+- name: 2024 Schedule 2EC worked case, Montana TANF paid to a separate SPM unit counts on line 17
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 1_500
+      person2:
+        age: 30
+      person3:
+        age: 5
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2, person3]
+    spm_units:
+      spm_unit1:
+        members: [person1]
+      spm_unit2:
+        members: [person2, person3]
+        mt_tanf: 4_800
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_name: MT
+  output:
+    # The same household with the daughter's family in its own SPM unit and
+    # the payment entered as Montana TANF (ARM 37.78), which tanf sums with
+    # the other states' programs. Line 17 = 4_800; line 18 = 24_800.
+    tanf: [0, 4_800]
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 2_400, 2_400]
+    # Line 22 = (24_800 - 12_600) x 0.05 = 610; line 27 = 890; line 30 = 890.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [610, 0, 0]
+    mt_elderly_homeowner_or_renter_credit: [890, 0, 0]
+
+
+- name: 2023 Elderly Homeowner/Renter Credit Schedule worked case, workers' compensation and a spouse's SSI count on line 8
+  period: 2023
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 66
+        taxable_pension_income: 14_000
+        workers_compensation: 6_000
+        real_estate_taxes: 1_000
+      person2:
+        age: 64
+        ssi: 5_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Joint return. Line 1 federal AGI 14_000; line 8 "Disability benefits
+    # including, but not limited to, veteran's disability, SSI payments, and
+    # workers' compensation": 6_000 + 5_000 = 11_000; line 10 = 25_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 5_000]
+    # Line 11 12_600; line 12 = 12_400; line 13 = 0.05; line 14 = 620.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [620, 0]
+    # Line 15 = 1_000; line 18 = 1_000; line 19 = 1_000 - 620 = 380; line 20
+    # = 380; line 21 = 100%; line 22 = 380. Without either amount, line 10 =
+    # 14_000, line 12 = 1_400 at a 0 rate and the credit would be 1_000.
+    mt_elderly_homeowner_or_renter_credit: [380, 0]
+
+
+- name: 2017 Form 2EC worked case, a household member's SSI counts in gross household income
+  period: 2017
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 15_000
+        real_estate_taxes: 1_000
+      person2:
+        age: 40
+        # 2017 federal benefit rate: 735 a month.
+        ssi: 8_820
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # The claimant's adult son receives SSI. Gross Household Income
+    # Worksheet: line 6 pensions 15_000; line 7 income of other household
+    # members, his SSI 8_820 ("Disability benefits including ... SSI
+    # payments"); line 8 = 23_820, Form 2EC line 1.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [15_000, 8_820]
+    # Line 2 6_300; line 3 = 17_520; line 4 = 0.05; line 5 = 876.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [876, 0]
+    # Line 6 = 1_000; line 9 = 1_000; line 10 = 1_000 - 876 = 124; line 11 =
+    # 124 (under 1_000); line 12 = 100%; line 13 = 124. Without his SSI, line
+    # 1 = 15_000, line 5 = 8_700 x 0.039 = 339.30 and the credit 660.70.
+    mt_elderly_homeowner_or_renter_credit: [124, 0]
+
+
+- name: 2016 Form 2EC worked case, cash public assistance received by another household member counts
+  period: 2016
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 12_000
+        real_estate_taxes: 900
+      person2:
+        age: 30
+      person3:
+        age: 5
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2, person3]
+    spm_units:
+      spm_unit1:
+        members: [person1]
+      spm_unit2:
+        members: [person2, person3]
+        tanf: 5_400
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_name: MT
+  output:
+    # Gross Household Income Worksheet: line 8 pensions 12_000; the
+    # daughter's 5_400 of TANF, cash public assistance (line 7), as income of
+    # another household member (line 9); line 10 = 17_400, Form 2EC line 1.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [12_000, 2_700, 2_700]
+    # Line 2 6_300; line 3 = 11_100; line 4 = 0.048; line 5 = 532.80.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [532.80, 0, 0]
+    # Line 6 = 900; line 10 = 900 - 532.80 = 367.20; line 11 = 367.20; line
+    # 12 = 100%; line 13 = 367.20. Without the TANF, line 1 = 12_000, line 5
+    # = 5_700 x 0.028 = 159.60 and the credit 740.40.
+    mt_elderly_homeowner_or_renter_credit: [367.20, 0, 0]
+
 - name: 2024 Schedule 2EC worked case, a dependent's wages move the credit into the 40 percent band
   period: 2024
   absolute_error_margin: 0.01
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..442caea56f2 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
@@ -8,6 +8,9 @@
         child_support_received: 3_000
         strike_benefits: 2_000
         general_assistance: 1_000
+        # These amounts would make the model compute SSI, which also counts;
+        # its own cases are in the government assistance test file.
+        ssi: 0
     tax_units:
       tax_unit:
         members: [person1]
@@ -86,6 +89,29 @@
     # income: (20_000 - 12_600) * 0.035 = 259, allocated to the head.
     mt_elderly_homeowner_or_renter_credit_net_household_income: [259, 0]
 
+- name: SNAP is not cash public assistance and does not count
+  period: 2017
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 15_000
+    spm_units:
+      spm_unit:
+        members: [person1]
+        snap: 1_800
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # 2017 Form 2EC instructions: "Cash, public assistance and relief - Do
+    # not include LIEAP (Low Income Energy Assistance Program) or SNAP".
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 15_000
+
 - name: A dependent's wages count in gross household income
   period: 2024
   input:
@@ -117,6 +143,8 @@
     people:
       person1:
         age: 70
+        # No SSI for the claimant, so the case isolates the dependent's income.
+        ssi: 0
       person2:
         age: 17
         employment_income: 25_000
@@ -144,6 +172,8 @@
     people:
       person1:
         age: 70
+        # No SSI for the claimant, so the case isolates the dependent's income.
+        ssi: 0
       person2:
         age: 16
         employment_income: 10_000
diff --git a/policyengine_us/tests/test_mt_elderly_credit_dependent_deductions.py b/policyengine_us/tests/test_mt_elderly_credit_dependent_deductions.py
index 90736451eca..819ce2ce5ff 100644
--- a/policyengine_us/tests/test_mt_elderly_credit_dependent_deductions.py
+++ b/policyengine_us/tests/test_mt_elderly_credit_dependent_deductions.py
@@ -31,6 +31,8 @@ def build_situation(cases, year):
                 "age": {year: 70},
                 "taxable_pension_income": {year: case["pension"]},
                 "early_withdrawal_penalty": {year: case["parent_penalty"]},
+                # No computed SSI; it has its own tests (government assistance).
+                "ssi": {year: 0},
                 "is_tax_unit_dependent": {year: False},
                 "mt_refundable_credits_before_renter_credit": {year: 0},
                 "mt_elderly_homeowner_or_renter_credit_received": {year: 0},
diff --git a/policyengine_us/tests/test_mt_elderly_credit_government_assistance.py b/policyengine_us/tests/test_mt_elderly_credit_government_assistance.py
new file mode 100644
index 00000000000..aaf5fa438f5
--- /dev/null
+++ b/policyengine_us/tests/test_mt_elderly_credit_government_assistance.py
@@ -0,0 +1,323 @@
+"""Property tests: government assistance in the Montana elderly
+homeowner/renter credit.
+
+MCA 15-30-2337(9)(a) counts "all nontaxable income, including but not
+limited to ... (vi) cash public assistance and relief", and (4) counts "all
+income received by all individuals of a household". The 2024 Schedule 2EC
+line 16 names "supplemental security income (SSI) payments, worker's
+compensation, and any other government assistance"; line 17 carries the same
+items for the other members of the household.
+
+The model holds SSI, workers' compensation and guaranteed income payments
+per person and TANF per SPM unit, which it spreads over the unit's members
+(tanf_person). For a claimant aged 62 or older (taxable pension, SSI,
+workers' compensation, property tax, rent) living with another adult (SSI,
+workers' compensation, guaranteed income payments) and a child, whose SPM
+unit receives TANF, in four arrangements:
+
+  A. one return, the other adult and the child the claimant's dependents, and
+     one SPM unit;
+  B. two returns (the claimant's, and the other adult's with the child) and
+     one SPM unit;
+  C. two returns and two SPM units, the TANF going to the other adult's;
+  D. as C, with more of one kind of assistance;
+
+1. The household's gross household income is the pension plus every
+   member's SSI, workers' compensation and guaranteed income payments plus
+   the TANF, once, in A, B and C.
+2. So the claimant's credit is the same in A, B and C, and nobody else
+   claims.
+3. The credit equals an independent reading of Schedule 2EC lines 18 to 30,
+   is between zero and the cap, and is zero at $45,000 or more (line 29).
+4. More assistance never raises the credit (D against C).
+
+No one has earned income, so no refundable credit enters line 8. The model
+computes in single precision, so comparisons allow one cent or eight float32
+spacings at the household's total income, whichever is larger.
+
+YAML cases (mt_elderly_homeowner_or_renter_credit.yaml) pin fixed worked
+examples. This file adds what they cannot: randomized amounts across
+arrangements that must agree, an independent Schedule 2EC reference for the
+credit, and monotonicity. Each example is one small Simulation on the shared
+tax-benefit system.
+"""
+
+import numpy as np
+import pytest
+
+from policyengine_us import Simulation
+
+YEAR = 2024
+CAP = 1_150
+INCOME_LIMIT = 45_000
+TOLERANCE = 0.01
+ARRANGEMENTS = "ABCD"
+# 2024 Schedule 2EC, line 19.
+EXCLUSION = 12_600
+# Household Income Reduction Table (line 21): lower bounds and rates.
+REDUCTION_TABLE = [
+    (0, 0),
+    (2_000, 0.006),
+    (3_000, 0.016),
+    (4_000, 0.024),
+    (5_000, 0.028),
+    (6_000, 0.032),
+    (7_000, 0.035),
+    (8_000, 0.039),
+    (9_000, 0.042),
+    (10_000, 0.045),
+    (11_000, 0.048),
+    (12_000, 0.05),
+]
+# Credit Multiplier Table (line 29): lower bounds of line 18 and percentages.
+MULTIPLIER_TABLE = [
+    (0, 1),
+    (35_000, 0.4),
+    (37_501, 0.3),
+    (40_001, 0.2),
+    (42_501, 0.1),
+    (45_000, 0),
+]
+EXTRA_KINDS = [
+    "claimant_ssi",
+    "member_workers_compensation",
+    "member_gi_cash_assistance",
+    "tanf",
+]
+
+
+def amounts(c, arrangement):
+    """The case's assistance amounts, with D's extra amount added."""
+    a = {
+        "claimant_ssi": c["claimant_ssi"],
+        "claimant_workers_compensation": c["claimant_workers_compensation"],
+        "member_ssi": c["member_ssi"],
+        "member_workers_compensation": c["member_workers_compensation"],
+        "member_gi_cash_assistance": c["member_gi_cash_assistance"],
+        "tanf": c["tanf"],
+    }
+    if arrangement == "D":
+        a[c["extra_kind"]] += c["extra"]
+    return a
+
+
+def build_situation(cases):
+    people, tax_units, marital_units, spm_units, families, households = (
+        {},
+        {},
+        {},
+        {},
+        {},
+        {},
+    )
+    for i, c in enumerate(cases):
+        for arrangement in ARRANGEMENTS:
+            key = f"{arrangement}{i}"
+            a = amounts(c, arrangement)
+            claimant, member, child = (
+                f"claimant_{key}",
+                f"member_{key}",
+                f"child_{key}",
+            )
+            people[claimant] = {
+                "age": {YEAR: c["claimant_age"]},
+                "taxable_pension_income": {YEAR: c["pension"]},
+                "ssi": {YEAR: a["claimant_ssi"]},
+                "workers_compensation": {YEAR: a["claimant_workers_compensation"]},
+                "real_estate_taxes": {YEAR: c["property_tax"]},
+                "rent": {YEAR: c["rent"]},
+                "is_tax_unit_dependent": {YEAR: False},
+            }
+            people[member] = {
+                "age": {YEAR: c["member_age"]},
+                "ssi": {YEAR: a["member_ssi"]},
+                "workers_compensation": {YEAR: a["member_workers_compensation"]},
+                "gi_cash_assistance": {YEAR: a["member_gi_cash_assistance"]},
+                "is_tax_unit_dependent": {YEAR: arrangement == "A"},
+            }
+            people[child] = {
+                "age": {YEAR: c["child_age"]},
+                "is_tax_unit_dependent": {YEAR: True},
+            }
+            if arrangement == "A":
+                tax_units[f"tax_unit_{key}"] = {"members": [claimant, member, child]}
+            else:
+                tax_units[f"tax_unit_{key}"] = {"members": [claimant]}
+                tax_units[f"tax_unit_{key}_member"] = {"members": [member, child]}
+            if arrangement in "AB":
+                spm_units[f"spm_unit_{key}"] = {
+                    "members": [claimant, member, child],
+                    "tanf": {YEAR: a["tanf"]},
+                }
+            else:
+                spm_units[f"spm_unit_{key}"] = {
+                    "members": [claimant],
+                    "tanf": {YEAR: 0},
+                }
+                spm_units[f"spm_unit_{key}_member"] = {
+                    "members": [member, child],
+                    "tanf": {YEAR: a["tanf"]},
+                }
+            for person in (claimant, member, child):
+                marital_units[f"marital_unit_{person}"] = {"members": [person]}
+            families[f"family_{key}"] = {"members": [claimant, member, child]}
+            households[f"household_{key}"] = {
+                "members": [claimant, member, child],
+                "state_code": {YEAR: "MT"},
+            }
+    return {
+        "people": people,
+        "tax_units": tax_units,
+        "marital_units": marital_units,
+        "spm_units": spm_units,
+        "families": families,
+        "households": households,
+    }
+
+
+def calculate(cases):
+    """Household totals of gross household income and the credit, and the
+    other members' credits, as arrays indexed [arrangement, case]."""
+    simulation = Simulation(situation=build_situation(cases))
+    n, k = len(cases), len(ARRANGEMENTS)
+
+    def by_household(variable):
+        values = simulation.calculate(variable, YEAR, map_to="household")
+        return np.asarray(values).reshape(n, k).T
+
+    person_credit = np.asarray(
+        simulation.calculate("mt_elderly_homeowner_or_renter_credit", YEAR)
+    ).reshape(n, k, 3)
+    return {
+        "gross_household_income": by_household(
+            "mt_elderly_homeowner_or_renter_credit_gross_household_income"
+        ),
+        "credit": by_household("mt_elderly_homeowner_or_renter_credit"),
+        # The other adult's and the child's credits.
+        "others_credit": person_credit[:, :, 1:].transpose(1, 0, 2),
+    }
+
+
+def expected_gross_household_income(c, arrangement):
+    """2024 Schedule 2EC line 18: line 5 pension and line 16 assistance for
+    the claimant, line 17 the other members' assistance and guaranteed income
+    payments, the TANF once."""
+    return c["pension"] + sum(amounts(c, arrangement).values())
+
+
+def lookup(table, amount):
+    rate = 0
+    for lower, value in table:
+        if amount >= lower:
+            rate = value
+    return rate
+
+
+def expected_credit(c, gross_household_income):
+    """2024 Schedule 2EC lines 19 to 30 for a claimant who owns or rents."""
+    line_20 = max(gross_household_income - EXCLUSION, 0)
+    line_22 = line_20 * lookup(REDUCTION_TABLE, line_20)
+    line_26 = c["property_tax"] + 0.15 * c["rent"]
+    line_27 = max(line_26 - line_22, 0)
+    line_28 = min(line_27, CAP)
+    return line_28 * lookup(MULTIPLIER_TABLE, gross_household_income)
+
+
+def tolerance(amount):
+    return max(TOLERANCE, 8 * float(np.spacing(np.float32(abs(amount)))))
+
+
+def assert_properties(cases):
+    result = calculate(cases)
+    ghi, credit = result["gross_household_income"], result["credit"]
+    for i, c in enumerate(cases):
+        for j, arrangement in enumerate(ARRANGEMENTS):
+            expected = expected_gross_household_income(c, arrangement)
+            tol = tolerance(expected)
+            # 1. Every member's assistance counts, the TANF once.
+            assert ghi[j, i] == pytest.approx(expected, abs=tol), (arrangement, c)
+            # 3. The schedule, the bounds and the income limit.
+            assert credit[j, i] == pytest.approx(
+                expected_credit(c, expected), abs=tol
+            ), (arrangement, c)
+            assert -tol <= credit[j, i] <= CAP + tol, (arrangement, c)
+            if expected >= INCOME_LIMIT:
+                assert credit[j, i] == 0, (arrangement, c)
+        # 2. The arrangement does not change the credit, and only the
+        # claimant claims.
+        tol = tolerance(expected_gross_household_income(c, "D"))
+        assert credit[1, i] == pytest.approx(credit[0, i], abs=tol), c
+        assert credit[2, i] == pytest.approx(credit[0, i], abs=tol), c
+        assert np.all(result["others_credit"][:, i] == 0), c
+        # 4. More assistance never raises the credit.
+        assert credit[3, i] <= credit[2, i] + tol, c
+
+
+def test_worked_example():
+    """2024 Schedule 2EC: line 5 pension 20,000; line 16 the claimant's SSI
+    3,000; line 17 the other adult's SSI 2,000 and workers' compensation
+    1,000 and the TANF 1,800; line 18 = 27,800; line 22 = (27,800 - 12,600)
+    x 0.05 = 760; line 27 = 1,500 - 760 = 740; line 30 = 740 in A, B and C.
+    D adds 8,000 of TANF: line 18 = 35,800; line 22 = 1,160; line 27 = 340;
+    line 29 = 40%; line 30 = 136."""
+    case = {
+        "claimant_age": 70,
+        "pension": 20_000,
+        "claimant_ssi": 3_000,
+        "claimant_workers_compensation": 0,
+        "property_tax": 1_500,
+        "rent": 0,
+        "member_age": 35,
+        "member_ssi": 2_000,
+        "member_workers_compensation": 1_000,
+        "member_gi_cash_assistance": 0,
+        "child_age": 6,
+        "tanf": 1_800,
+        "extra_kind": "tanf",
+        "extra": 8_000,
+    }
+    result = calculate([case])
+    assert result["gross_household_income"][:, 0] == pytest.approx(
+        [27_800, 27_800, 27_800, 35_800], abs=TOLERANCE
+    )
+    assert result["credit"][:, 0] == pytest.approx([740, 740, 740, 136], abs=TOLERANCE)
+    assert_properties([case])
+
+
+try:
+    import hypothesis
+    import hypothesis.strategies as st
+except ImportError:  # pragma: no cover
+    hypothesis = None
+
+if hypothesis is not None:
+    # Each example builds a Simulation; on a loaded runner input generation
+    # can trip the too_slow health check, which says nothing about the model.
+    SLOW = [hypothesis.HealthCheck.too_slow]
+
+    def amount(high):
+        return st.one_of(st.just(0), st.integers(min_value=1, max_value=high))
+
+    @st.composite
+    def case(draw):
+        return {
+            "claimant_age": draw(st.integers(min_value=62, max_value=90)),
+            "pension": draw(amount(40_000)),
+            "claimant_ssi": draw(amount(12_000)),
+            "claimant_workers_compensation": draw(amount(15_000)),
+            "property_tax": draw(st.integers(min_value=0, max_value=4_000)),
+            "rent": draw(amount(12_000)),
+            "member_age": draw(st.integers(min_value=18, max_value=61)),
+            "member_ssi": draw(amount(12_000)),
+            "member_workers_compensation": draw(amount(15_000)),
+            "member_gi_cash_assistance": draw(amount(12_000)),
+            "child_age": draw(st.integers(min_value=0, max_value=17)),
+            "tanf": draw(amount(10_000)),
+            "extra_kind": draw(st.sampled_from(EXTRA_KINDS)),
+            "extra": draw(st.integers(min_value=1, max_value=20_000)),
+        }
+
+    @hypothesis.settings(max_examples=15, deadline=None, suppress_health_check=SLOW)
+    @hypothesis.given(st.lists(case(), min_size=1, max_size=8))
+    def test_properties(cases):
+        assert_properties(cases)
diff --git a/policyengine_us/tests/test_mt_elderly_credit_household_members.py b/policyengine_us/tests/test_mt_elderly_credit_household_members.py
index 2d551fb39fd..bb0ef7d06ec 100644
--- a/policyengine_us/tests/test_mt_elderly_credit_household_members.py
+++ b/policyengine_us/tests/test_mt_elderly_credit_household_members.py
@@ -79,6 +79,9 @@ def build_situation(cases):
                 "social_security_retirement": {YEAR: c["claimant_ss"]},
                 "real_estate_taxes": {YEAR: c["property_tax"]},
                 "rent": {YEAR: c["rent"]},
+                # The model would compute SSI for a claimant with little
+                # income; SSI has its own tests (government assistance).
+                "ssi": {YEAR: 0},
             }
             people[member] = {
                 "age": {YEAR: c["member_age"]},
diff --git a/policyengine_us/tests/test_mt_elderly_credit_one_claimant.py b/policyengine_us/tests/test_mt_elderly_credit_one_claimant.py
index d1dc874a300..ae224c8dea5 100644
--- a/policyengine_us/tests/test_mt_elderly_credit_one_claimant.py
+++ b/policyengine_us/tests/test_mt_elderly_credit_one_claimant.py
@@ -68,6 +68,10 @@ def build_situation(households):
                         "taxable_pension_income": {YEAR: adult["pension"]},
                         "taxable_interest_income": {YEAR: adult["interest"]},
                         "social_security_retirement": {YEAR: adult["social_security"]},
+                        # The model would compute SSI for an adult with
+                        # little income; SSI has its own tests (government
+                        # assistance), so the reference leaves it out.
+                        "ssi": {YEAR: 0},
                         "is_tax_unit_dependent": {YEAR: False},
                     }
                     unit_members.append(person)
diff --git a/policyengine_us/tests/test_mt_elderly_credit_rebate_receipts.py b/policyengine_us/tests/test_mt_elderly_credit_rebate_receipts.py
index 9f44d62671b..09470be0d6f 100644
--- a/policyengine_us/tests/test_mt_elderly_credit_rebate_receipts.py
+++ b/policyengine_us/tests/test_mt_elderly_credit_rebate_receipts.py
@@ -35,6 +35,8 @@ def build_situation(cases, year, with_receipts):
                     },
                     "mt_refundable_credits_before_renter_credit": {year: 0},
                     "mt_elderly_homeowner_or_renter_credit_received": {year: 0},
+                    # No computed SSI; it has its own tests.
+                    "ssi": {year: 0},
                 }
                 if arrangement == "C":
                     people[name][