diff --git a/changelog.d/fix-mt-salt-cap-joint-return.fixed.md b/changelog.d/fix-mt-salt-cap-joint-return.fixed.md
new file mode 100644
index 00000000000..512c8a641c6
--- /dev/null
+++ b/changelog.d/fix-mt-salt-cap-joint-return.fixed.md
@@ -0,0 +1 @@
+Cap Montana's 2021–2023 state and local tax deduction (Itemized Deductions Schedule, line 5) once for a joint return and at $5,000 in each spouse's column when filing separately on the same form, and include general sales taxes (line 5a) and local income taxes (line 5b).
diff --git a/policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/state_specific_deduction_applies.yaml b/policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/state_specific_deduction_applies.yaml
new file mode 100644
index 00000000000..8b6d63360ab
--- /dev/null
+++ b/policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/state_specific_deduction_applies.yaml
@@ -0,0 +1,16 @@
+description: Montana computes itemized deductions on its own Itemized Deductions Schedule, in lieu of adjusted federal itemized deductions, if this is true.
+metadata:
+  label: Montana state specific itemized deductions apply
+  period: year
+  unit: bool
+  reference:
+    - title: Montana Individual Income Tax Return (Form 2) 2023 | Itemized Deductions Schedule
+      href: https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2/2023_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=7
+    - title: 2024 Montana Form 2 Instructions | Discontinued Worksheets and Schedules
+      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=8
+    - title: 2024 Montana Form 2 Instructions | Line 2 and Worksheet A, Adjusted Federal Itemized Deductions
+      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=15
+
+values:
+  2021-01-01: true
+  2024-01-01: false
diff --git a/policyengine_us/tests/policy/baseline/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax.yaml b/policyengine_us/tests/policy/baseline/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax.yaml
new file mode 100644
index 00000000000..1096df63047
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax.yaml
@@ -0,0 +1,21 @@
+- name: Each person owes 1% of their own Kansas City taxable earnings
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 45
+        mo_kansas_city_earnings_tax_taxable_earnings: 100_000
+      person2:
+        age: 45
+        mo_kansas_city_earnings_tax_taxable_earnings: 50_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MO
+  output:
+    # 1% x 100,000 and 1% x 50,000.
+    mo_kansas_city_earnings_tax_person: [1_000, 500]
+    mo_kansas_city_earnings_tax: 1_500
diff --git a/policyengine_us/tests/policy/baseline/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax.yaml b/policyengine_us/tests/policy/baseline/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax.yaml
new file mode 100644
index 00000000000..dc90c9bc2c9
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax.yaml
@@ -0,0 +1,24 @@
+- name: A person's credit reduces only their own St. Louis earnings tax
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 45
+        mo_st_louis_earnings_tax_taxable_earnings: 100_000
+        mo_st_louis_earnings_tax_credit: 1_200
+      person2:
+        age: 45
+        mo_st_louis_earnings_tax_taxable_earnings: 100_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MO
+  output:
+    # 1% x 100,000 = 1,000 each. Person 1's 1,200 credit takes their tax to
+    # zero and does not reduce person 2's.
+    mo_st_louis_earnings_tax_person: [0, 1_000]
+    mo_st_louis_earnings_tax: 1_000
+    mo_st_louis_earnings_tax_before_credit: 2_000
diff --git a/policyengine_us/tests/policy/baseline/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax.yaml b/policyengine_us/tests/policy/baseline/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax.yaml
new file mode 100644
index 00000000000..f7b94f9bcbb
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax.yaml
@@ -0,0 +1,37 @@
+- name: Each person's Philadelphia wage tax uses their own rate
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 45
+        pa_philadelphia_wage_tax_taxable_wages: 100_000
+        pa_philadelphia_wage_tax_resident: true
+      person2:
+        age: 45
+        pa_philadelphia_wage_tax_taxable_wages: 100_000
+        pa_philadelphia_wage_tax_resident: false
+      person3:
+        age: 45
+        pa_philadelphia_wage_tax_taxable_wages: 40_000
+        pa_philadelphia_wage_tax_resident: true
+        pa_philadelphia_wage_tax_reduced_rate_eligible: true
+    tax_units:
+      tax_unit:
+        members: [person1, person2, person3]
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_code: PA
+  output:
+    # Rates from July 2023: resident 3.75%, nonresident 3.44%, reduced 1.5%.
+    pa_philadelphia_wage_tax_person: [3_750, 3_440, 600]
+    pa_philadelphia_wage_tax: 7_790
+
+- name: No Philadelphia wages, no tax
+  period: 2024
+  input:
+    pa_philadelphia_wage_tax_taxable_wages: 0
+    state_code: PA
+  output:
+    pa_philadelphia_wage_tax_person: 0
+    pa_philadelphia_wage_tax: 0
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.yaml
index 4435e64a801..47b6f0f8a7c 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.yaml
@@ -3,7 +3,7 @@
   input:
      charitable_deduction: 800
      investment_interest_expense: 265
-     mt_salt_deduction: 5_000
+     mt_salt_deduction_indiv: 5_000
      mt_medical_expense_deduction_indiv: 1_000
      casualty_loss_deduction: 400
      mt_child_dependent_care_expense_deduction: 123
@@ -17,7 +17,7 @@
   input:
      charitable_deduction: 0
      investment_interest_expense: 1_234
-     mt_salt_deduction: 10_000
+     mt_salt_deduction_indiv: 10_000
      mt_medical_expense_deduction_indiv: 3_000 
      casualty_loss_deduction: 0
      mt_child_dependent_care_expense_deduction: 0
@@ -31,7 +31,7 @@
   input:
      charitable_deduction: 1_000
      mortgage_interest: 34
-     mt_salt_deduction: 1_035
+     mt_salt_deduction_indiv: 1_035
      mt_medical_expense_deduction_indiv: 3_000
      casualty_loss_deduction: 745
      mt_child_dependent_care_expense_deduction: 1_004
@@ -49,20 +49,20 @@
         is_tax_unit_head_or_spouse: true
         mt_misc_deductions: 100
         mt_medical_expense_deduction_indiv: 200
-        mt_salt_deduction: 300
+        mt_salt_deduction_indiv: 300
         mt_federal_income_tax_deduction_for_federal_itemization_indiv: 400
       person2:
         is_tax_unit_head_or_spouse: true
         mt_misc_deductions: 100
         mt_medical_expense_deduction_indiv: 200
-        mt_salt_deduction: 300
+        mt_salt_deduction_indiv: 300
         mt_federal_income_tax_deduction_for_federal_itemization_indiv: 100
       person3:
         is_tax_unit_head_or_spouse: false
         is_tax_unit_dependent: true
         mt_misc_deductions: 100
         mt_medical_expense_deduction_indiv: 12
-        mt_salt_deduction: 200
+        mt_salt_deduction_indiv: 200
         mt_federal_income_tax_deduction_for_federal_itemization_indiv: 100
     tax_units:
       tax_unit:
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_itemized_deductions_indiv.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_itemized_deductions_indiv.yaml
index 2bc390f8ad3..c10c66239d5 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_itemized_deductions_indiv.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_itemized_deductions_indiv.yaml
@@ -3,7 +3,7 @@
   input:
      charitable_deduction: 800
      investment_interest_expense: 265
-     mt_salt_deduction: 5_000
+     mt_salt_deduction_indiv: 5_000
      mt_medical_expense_deduction_indiv: 1_000
      casualty_loss_deduction: 400
      mt_child_dependent_care_expense_deduction: 123
@@ -17,7 +17,7 @@
   input:
      charitable_deduction: 0
      investment_interest_expense: 1_234
-     mt_salt_deduction: 10_000
+     mt_salt_deduction_indiv: 10_000
      mt_medical_expense_deduction_indiv: 3_000 
      casualty_loss_deduction: 0
      mt_child_dependent_care_expense_deduction: 0
@@ -31,7 +31,7 @@
   input:
      charitable_deduction: 1_000
      mortgage_interest: 34
-     mt_salt_deduction: 1_035
+     mt_salt_deduction_indiv: 1_035
      mt_medical_expense_deduction_indiv: 3_000
      casualty_loss_deduction: 745
      mt_child_dependent_care_expense_deduction: 1_004
@@ -49,20 +49,20 @@
         is_tax_unit_head_or_spouse: true
         mt_misc_deductions: 100
         mt_medical_expense_deduction_indiv: 200
-        mt_salt_deduction: 300
+        mt_salt_deduction_indiv: 300
         mt_federal_income_tax_deduction_indiv: 400
       person2:
         is_tax_unit_head_or_spouse: true
         mt_misc_deductions: 100
         mt_medical_expense_deduction_indiv: 200
-        mt_salt_deduction: 300
+        mt_salt_deduction_indiv: 300
         mt_federal_income_tax_deduction_indiv: 100
       person3:
         is_tax_unit_head_or_spouse: false
         is_tax_unit_dependent: true
         mt_misc_deductions: 100
         mt_medical_expense_deduction_indiv: 12
-        mt_salt_deduction: 200
+        mt_salt_deduction_indiv: 200
         mt_federal_income_tax_deduction_indiv: 100
     tax_units:
       tax_unit:
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_salt_deduction.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_salt_deduction.yaml
index 1e3da2a4f1b..51d97882b53 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_salt_deduction.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_salt_deduction.yaml
@@ -35,3 +35,151 @@
   output:
      mt_salt_deduction: 10_000
 
+- name: Test case 5, head of household, higher than cap
+  period: 2022
+  input:
+     filing_status: HEAD_OF_HOUSEHOLD
+     real_estate_taxes: 12_000
+     state_code: MT
+  output:
+     mt_salt_deduction: 10_000
+
+# Form 2, Itemized Deductions Schedule, line 5 (2021-2023): "Add lines 5a
+# through 5d, enter the total here, but not more than $10,000 if your status
+# is single, head of household or married filing jointly". A joint return has
+# one column, so the couple's taxes are added and capped once.
+
+- name: Joint return caps the couple's combined state and local taxes once
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 8_000
+      person2:
+        age: 45
+        real_estate_taxes: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Line 5c: 8,000 + 8,000 = 16,000; line 5: min(16,000, 10,000).
+    mt_salt_deduction: [10_000, 0]
+    # No income, so the federal income tax deduction (line 4) is zero and
+    # line 5 is the only itemized deduction.
+    mt_itemized_deductions_joint: [10_000, 0]
+
+- name: Joint return below the cap deducts both spouses' taxes
+  period: 2022
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 6_000
+      person2:
+        age: 45
+        real_estate_taxes: 2_500
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Line 5c: 6,000 + 2,500 = 8,500, under the 10,000 cap.
+    mt_salt_deduction: [8_500, 0]
+
+- name: Joint return adds general sales taxes before the cap
+  period: 2021
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 9_000
+      person2:
+        age: 45
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        state_sales_tax: 1_500
+        local_sales_tax: 300
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Line 5a: 1,500 + 300 = 1,800; line 5c: 9,000. Line 5: min(10,800, 10,000).
+    mt_salt_deduction: [10_000, 0]
+
+- name: Single return adds general sales taxes below the cap
+  period: 2023
+  input:
+     filing_status: SINGLE
+     real_estate_taxes: 2_000
+     state_sales_tax: 1_500
+     local_sales_tax: 300
+     state_code: MT
+  output:
+     # Line 5a: 1,800; line 5c: 2,000. Line 5: 3,800, under the 10,000 cap.
+     mt_salt_deduction: 3_800
+
+- name: Single return adds another city's earnings tax
+  period: 2023
+  input:
+     filing_status: SINGLE
+     real_estate_taxes: 2_000
+     mo_kansas_city_earnings_tax_taxable_earnings: 100_000
+     state_code: MT
+  output:
+     # Line 5b: Kansas City earnings tax, 1% x 100,000 = 1,000; line 5c:
+     # 2,000. Line 5: 3,000.
+     mt_salt_deduction: 3_000
+
+- name: Joint return adds a spouse's city earnings tax before the cap
+  period: 2022
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 9_500
+      person2:
+        age: 45
+        mo_kansas_city_earnings_tax_taxable_earnings: 100_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Line 5b: 1,000; line 5c: 9,500. Line 5: min(10,500, 10,000).
+    mt_salt_deduction: [10_000, 0]
+
+- name: Joint return adds both spouses' city taxes before the cap
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 4_000
+        mo_kansas_city_earnings_tax_taxable_earnings: 100_000
+      person2:
+        age: 45
+        pa_philadelphia_wage_tax_taxable_wages: 100_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Line 5b: 1,000 (Kansas City) + 3,440 (Philadelphia nonresident 3.44%);
+    # line 5c: 4,000. Line 5: 8,440, under the 10,000 cap.
+    mt_salt_deduction: [8_440, 0]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_salt_deduction_indiv.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_salt_deduction_indiv.yaml
new file mode 100644
index 00000000000..fcb4eb1a0b1
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_salt_deduction_indiv.yaml
@@ -0,0 +1,266 @@
+# Form 2, Itemized Deductions Schedule, line 5 (2021-2023): "Add lines 5a
+# through 5d, enter the total here, but not more than $10,000 if your status
+# is single, head of household or married filing jointly; or $5,000 if you are
+# married filing separately." Spouses filing separately on the same form
+# (filing status 2a) allocate between columns A and B, "with each column
+# constituting its own return" (2023 instructions, PDF p. 12). So each
+# spouse's column is capped at $5,000.
+
+- name: Each spouse's column is capped at the married filing separately amount
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 8_000
+      person2:
+        age: 45
+        real_estate_taxes: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Column A: min(8,000, 5,000); column B: min(8,000, 5,000).
+    mt_salt_deduction_indiv: [5_000, 5_000]
+    # No income, so the federal income tax deduction (line 4) is zero and
+    # line 5 is each column's only itemized deduction.
+    mt_itemized_deductions_indiv: [5_000, 5_000]
+
+- name: A spouse below the cap deducts their own taxes
+  period: 2022
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 8_000
+      person2:
+        age: 45
+        real_estate_taxes: 2_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Column A: min(8,000, 5,000); column B: 2,000.
+    mt_salt_deduction_indiv: [5_000, 2_000]
+
+- name: The couple's general sales taxes are divided equally between the columns
+  period: 2021
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 4_800
+      person2:
+        age: 45
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        state_sales_tax: 1_000
+        local_sales_tax: 0
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Line 5a: 1,000 / 2 = 500 in each column.
+    # Column A: min(4,800 + 500, 5,000) = 5,000; column B: 500.
+    mt_salt_deduction_indiv: [5_000, 500]
+
+- name: A dependent has no column
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 3_000
+      person2:
+        age: 45
+        real_estate_taxes: 1_000
+      person3:
+        age: 20
+        is_tax_unit_dependent: true
+        real_estate_taxes: 1_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2, person3]
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_code: MT
+  output:
+    mt_salt_deduction_indiv: [3_000, 1_000, 0]
+
+- name: A single filer's one column takes the single cap
+  period: 2023
+  input:
+     filing_status: SINGLE
+     real_estate_taxes: 12_000
+     state_code: MT
+  output:
+     # min(12,000, 10,000)
+     mt_salt_deduction_indiv: 10_000
+
+- name: A single filer below the single cap deducts all taxes
+  period: 2022
+  input:
+     filing_status: SINGLE
+     real_estate_taxes: 8_000
+     state_code: MT
+  output:
+     mt_salt_deduction_indiv: 8_000
+
+- name: A married person filing on a separate form takes the separate cap
+  period: 2021
+  input:
+     filing_status: SEPARATE
+     real_estate_taxes: 6_000
+     state_code: MT
+  output:
+     # min(6,000, 5,000)
+     mt_salt_deduction_indiv: 5_000
+
+- name: No separate columns from 2024
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 8_000
+      person2:
+        age: 45
+        real_estate_taxes: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # SB 399 ended filing separately on the same form after 2023.
+    mt_salt_deduction_indiv: [0, 0]
+
+# Line 5b: "Deductions that are attributable to only one spouse must be
+# claimed by that spouse" (2023 instructions, PDF p. 13). A city earnings tax
+# belongs to the spouse who earned the taxed income.
+
+- name: A city earnings tax goes in the column of the spouse who earned it
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 3_000
+        mo_kansas_city_earnings_tax_taxable_earnings: 100_000
+      person2:
+        age: 45
+        real_estate_taxes: 3_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Column A: 3,000 + 1% x 100,000 = 4,000; column B: 3,000.
+    mt_salt_deduction_indiv: [4_000, 3_000]
+
+- name: Both spouses' city earnings taxes, one column at the cap
+  period: 2022
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 4_500
+        mo_kansas_city_earnings_tax_taxable_earnings: 100_000
+      person2:
+        age: 45
+        mo_kansas_city_earnings_tax_taxable_earnings: 50_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Kansas City tax: 1,000 on column A's earnings and 500 on column B's.
+    # Column A: min(4,500 + 1,000, 5,000) = 5,000; column B: 500.
+    mt_salt_deduction_indiv: [5_000, 500]
+
+- name: Each column takes its own spouse's Philadelphia wage tax at that spouse's rate
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 45
+        pa_philadelphia_wage_tax_taxable_wages: 100_000
+      person2:
+        age: 45
+        pa_philadelphia_wage_tax_taxable_wages: 100_000
+        pa_philadelphia_wage_tax_reduced_rate_eligible: true
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Nonresident rate 3.44% x 100,000 = 3,440; reduced rate 1.5% x 100,000
+    # = 1,500. Splitting the 4,940 by wages would give 2,470 each.
+    mt_salt_deduction_indiv: [3_440, 1_500]
+
+- name: A St. Louis credit reduces only its own spouse's column
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 45
+        mo_st_louis_earnings_tax_taxable_earnings: 100_000
+        mo_st_louis_earnings_tax_credit: 1_000
+      person2:
+        age: 45
+        mo_st_louis_earnings_tax_taxable_earnings: 100_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Column A: 1% x 100,000 - 1,000 credit = 0; column B: 1,000.
+    mt_salt_deduction_indiv: [0, 1_000]
+
+- name: Two cities' taxes, one column at the cap
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 45
+        real_estate_taxes: 4_000
+        mo_kansas_city_earnings_tax_taxable_earnings: 100_000
+      person2:
+        age: 45
+        pa_philadelphia_wage_tax_taxable_wages: 100_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Column A: min(4,000 + 1% x 100,000, 5,000) = 5,000.
+    # Column B: 3.44% x 100,000 = 3,440.
+    mt_salt_deduction_indiv: [5_000, 3_440]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/test_mt_salt_deduction_invariants.py b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/test_mt_salt_deduction_invariants.py
new file mode 100644
index 00000000000..8a008587b55
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/test_mt_salt_deduction_invariants.py
@@ -0,0 +1,327 @@
+"""Invariants for Montana's state and local tax deduction, 2021-2023.
+
+Form 2, Itemized Deductions Schedule, line 5 adds lines 5a (general sales
+taxes), 5b (local income taxes), 5c (real estate taxes) and 5d, and caps the
+total at $10,000, or $5,000 if married filing separately. A joint return has
+one column, so its cap applies once to the couple's combined taxes
+(mt_salt_deduction, on the head). Spouses filing separately on the same form
+(status 2a) each have their own column, capped at $5,000
+(mt_salt_deduction_indiv). From 2024 the schedule is gone and this change
+leaves the earlier per-person amounts as they were.
+
+The YAML files hold the worked cases. This file checks what they cannot: many
+mixed tax units (single, head of household, separate, joint, joint with
+dependents, some with sales taxes or Kansas City and St. Louis earnings taxes)
+in one vectorized simulation, and how results move when inputs change. For
+every tax unit:
+
+1. Differential: both variables equal an independent numpy calculation of
+   line 5, with the caps taken from the forms rather than from parameters.
+2. Bounds and order: each value lies between zero and its cap, and a couple's
+   two separate columns never deduct more than their joint return.
+3. Monotone: raising any member's real estate taxes never lowers the joint
+   return's or any column's deduction.
+4. Head swap: swapping which spouse is the head moves each column with its
+   spouse and leaves the joint return's deduction unchanged.
+5. From 2024 the per-person amounts equal min(real estate taxes, federal cap)
+   for the tax unit's filing status, as before this change.
+"""
+
+import numpy as np
+import pytest
+from hypothesis import HealthCheck, given, settings
+from hypothesis import strategies as st
+
+from policyengine_us import Simulation
+
+TOLERANCE = 0.01  # dollars
+
+# Form 2 (2021-2023), Itemized Deductions Schedule, line 5.
+FORM_CAP = {"SINGLE": 10_000, "HEAD_OF_HOUSEHOLD": 10_000, "JOINT": 10_000}
+FORM_CAP["SEPARATE"] = 5_000
+SCHEDULE_YEARS = [2021, 2022, 2023]
+# Kansas City and St. Louis earnings tax rates: local income taxes with no
+# residence test, on each person's own earnings in the city.
+KANSAS_CITY_RATE = 0.01
+ST_LOUIS_RATE = 0.01
+# 26 USC 164(b)(6) and (7)(A): $10,000 ($5,000 separate) through 2024, and
+# $40,400 ($20,200 separate) for 2026.
+FEDERAL_CAP = {
+    2024: {"SINGLE": 10_000, "HEAD_OF_HOUSEHOLD": 10_000, "JOINT": 10_000},
+    2026: {"SINGLE": 40_400, "HEAD_OF_HOUSEHOLD": 40_400, "JOINT": 40_400},
+}
+FEDERAL_CAP[2024]["SEPARATE"] = 5_000
+FEDERAL_CAP[2026]["SEPARATE"] = 20_200
+KINDS = ["single", "hoh", "separate", "joint", "joint_dependents"]
+STATUS = {
+    "single": "SINGLE",
+    "hoh": "HEAD_OF_HOUSEHOLD",
+    "separate": "SEPARATE",
+    "joint": "JOINT",
+    "joint_dependents": "JOINT",
+}
+
+# Mostly amounts near the caps, plus zeros and large amounts.
+taxes = st.one_of(
+    st.just(0.0),
+    st.integers(1, 6_000).map(float),
+    st.integers(6_000, 30_000).map(float),
+)
+sales = st.one_of(st.just(0.0), st.integers(1, 3_000).map(float))
+increases = st.one_of(st.just(0.0), st.integers(1, 5_000).map(float))
+city_earnings = st.one_of(st.just(0.0), st.integers(1, 300_000).map(float))
+city_credits = st.one_of(st.just(0.0), st.integers(1, 3_000).map(float))
+
+
+@st.composite
+def people(draw):
+    return {
+        "real_estate_taxes": draw(taxes),
+        "increase": draw(increases),
+        "kansas_city_earnings": draw(city_earnings),
+        "st_louis_earnings": draw(city_earnings),
+        "st_louis_credit": draw(city_credits),
+    }
+
+
+@st.composite
+def tax_units(draw):
+    kind = draw(st.sampled_from(KINDS))
+    n_dependents = draw(st.integers(1, 2)) if kind in ("hoh", "joint_dependents") else 0
+    return {
+        "kind": kind,
+        "head": draw(people()),
+        "spouse": draw(people()) if kind.startswith("joint") else None,
+        "dependents": [draw(people()) for _ in range(n_dependents)],
+        "state_sales_tax": draw(sales),
+        "local_sales_tax": draw(sales),
+    }
+
+
+SEED = 20261007
+
+
+def _seeded_units(n=150):
+    rng = np.random.default_rng(SEED)
+
+    def person():
+        band = rng.choice(3)
+        return {
+            "real_estate_taxes": float(
+                [0, rng.integers(1, 6_001), rng.integers(6_000, 30_001)][band]
+            ),
+            "increase": float(rng.integers(0, 5_001)) if rng.random() < 0.7 else 0.0,
+            "kansas_city_earnings": float(rng.integers(1, 300_001))
+            if rng.random() < 0.2
+            else 0.0,
+            "st_louis_earnings": float(rng.integers(1, 300_001))
+            if rng.random() < 0.2
+            else 0.0,
+            "st_louis_credit": float(rng.integers(1, 3_001))
+            if rng.random() < 0.3
+            else 0.0,
+        }
+
+    units = []
+    for _ in range(n):
+        kind = str(rng.choice(KINDS))
+        n_dependents = (
+            int(rng.integers(1, 3)) if kind in ("hoh", "joint_dependents") else 0
+        )
+        units.append(
+            {
+                "kind": kind,
+                "head": person(),
+                "spouse": person() if kind.startswith("joint") else None,
+                "dependents": [person() for _ in range(n_dependents)],
+                "state_sales_tax": float(rng.integers(0, 3_001))
+                if rng.random() < 0.3
+                else 0.0,
+                "local_sales_tax": float(rng.integers(0, 601))
+                if rng.random() < 0.3
+                else 0.0,
+            }
+        )
+    return units
+
+
+def _situation(units, year, *, raised=False, swapped=False):
+    people, groups = {}, {"tax_units": {}, "households": {}, "marital_units": {}}
+
+    def add(name, values, role):
+        people[name] = {
+            "age": 20 if role == "dependent" else 45,
+            "is_tax_unit_head": role == "head",
+            "is_tax_unit_spouse": role == "spouse",
+            "is_tax_unit_dependent": role == "dependent",
+            "real_estate_taxes": values["real_estate_taxes"]
+            + raised * values["increase"],
+            "mo_kansas_city_earnings_tax_taxable_earnings": values[
+                "kansas_city_earnings"
+            ],
+            "mo_st_louis_earnings_tax_taxable_earnings": values["st_louis_earnings"],
+            "mo_st_louis_earnings_tax_credit": values["st_louis_credit"],
+        }
+
+    for i, unit in enumerate(units):
+        first, second = f"first_{i}", f"second_{i}"
+        couple = unit["spouse"] is not None
+        # With swapped, the second spouse is the head.
+        add(first, unit["head"], "spouse" if couple and swapped else "head")
+        members, marital = [first], [first]
+        if couple:
+            add(second, unit["spouse"], "head" if swapped else "spouse")
+            members.append(second)
+            marital.append(second)
+        groups["marital_units"][f"couple_{i}"] = {"members": marital}
+        for j, dependent in enumerate(unit["dependents"]):
+            name = f"dependent_{i}_{j}"
+            add(name, dependent, "dependent")
+            members.append(name)
+            groups["marital_units"][f"single_{i}_{j}"] = {"members": [name]}
+        groups["tax_units"][f"tax_unit_{i}"] = {
+            "members": members,
+            "filing_status": STATUS[unit["kind"]],
+            "state_sales_tax": unit["state_sales_tax"],
+            "local_sales_tax": unit["local_sales_tax"],
+        }
+        groups["households"][f"household_{i}"] = {
+            "members": members,
+            "state_code": "MT",
+        }
+    situation = {"people": people, **groups}
+    for entities in situation.values():
+        for values in entities.values():
+            for key, value in values.items():
+                if key != "members":
+                    values[key] = {year: value}
+    return situation
+
+
+def _run(units, year, **kwargs):
+    sim = Simulation(situation=_situation(units, year, **kwargs))
+    out = {
+        name: np.asarray(sim.calculate(name, year), dtype=float)
+        for name in [
+            "mt_salt_deduction",
+            "mt_salt_deduction_indiv",
+            "real_estate_taxes",
+            "mo_kansas_city_earnings_tax_taxable_earnings",
+            "mo_st_louis_earnings_tax_taxable_earnings",
+            "mo_st_louis_earnings_tax_credit",
+        ]
+    }
+    out["unit"] = sim.populations["tax_unit"].members_entity_id
+    out["head"] = np.asarray(sim.calculate("is_tax_unit_head", year))
+    out["filer"] = np.asarray(sim.calculate("is_tax_unit_head_or_spouse", year))
+    return out
+
+
+def _unit_sum(run, values):
+    return np.bincount(run["unit"], weights=values)
+
+
+def _check_schedule_year(units, year):
+    base = _run(units, year)
+    unit, head, filer = base["unit"], base["head"], base["filer"]
+    status = np.array([STATUS[u["kind"]] for u in units])
+    married = status == "JOINT"
+    sales = np.array([u["state_sales_tax"] + u["local_sales_tax"] for u in units])
+    joint_cap = np.array([FORM_CAP[s] for s in status])
+    column_cap = np.where(married, FORM_CAP["SEPARATE"], joint_cap)
+
+    # 1. Differential against numpy. Each person owes city tax on their own
+    # earnings there, less their own credits; a column takes its own
+    # spouse's tax.
+    city_tax = KANSAS_CITY_RATE * base[
+        "mo_kansas_city_earnings_tax_taxable_earnings"
+    ] + np.maximum(
+        0,
+        ST_LOUIS_RATE * base["mo_st_louis_earnings_tax_taxable_earnings"]
+        - base["mo_st_louis_earnings_tax_credit"],
+    )
+    total = _unit_sum(base, base["real_estate_taxes"] + city_tax) + sales
+    np.testing.assert_allclose(
+        base["mt_salt_deduction"],
+        head * np.minimum(total, joint_cap)[unit],
+        atol=TOLERANCE,
+    )
+    share = np.where(married, 0.5, 1.0)[unit]
+    own = base["real_estate_taxes"] + share * sales[unit] + city_tax
+    np.testing.assert_allclose(
+        base["mt_salt_deduction_indiv"],
+        filer * np.minimum(own, column_cap[unit]),
+        atol=TOLERANCE,
+    )
+
+    # 2. Bounds and order.
+    joint = _unit_sum(base, base["mt_salt_deduction"])
+    columns = _unit_sum(base, base["mt_salt_deduction_indiv"])
+    assert (base["mt_salt_deduction"] >= 0).all()
+    assert (base["mt_salt_deduction_indiv"] >= 0).all()
+    assert (joint <= joint_cap + TOLERANCE).all()
+    assert (base["mt_salt_deduction_indiv"] <= column_cap[unit] + TOLERANCE).all()
+    assert (columns[married] <= joint[married] + TOLERANCE).all()
+
+    # 3. Monotone in real estate taxes.
+    raised = _run(units, year, raised=True)
+    for name in ["mt_salt_deduction", "mt_salt_deduction_indiv"]:
+        assert (raised[name] >= base[name] - TOLERANCE).all(), name
+
+    # 4. Head swap.
+    swapped = _run(units, year, swapped=True)
+    np.testing.assert_allclose(
+        _unit_sum(swapped, swapped["mt_salt_deduction"]), joint, atol=TOLERANCE
+    )
+    np.testing.assert_allclose(
+        swapped["mt_salt_deduction_indiv"],
+        base["mt_salt_deduction_indiv"],
+        atol=TOLERANCE,
+    )
+
+
+def _check_unchanged_year(units, year):
+    base = _run(units, year)
+    status = np.array([STATUS[u["kind"]] for u in units])
+    cap = np.array([FEDERAL_CAP[year][s] for s in status])[base["unit"]]
+    np.testing.assert_allclose(
+        base["mt_salt_deduction"],
+        np.minimum(base["real_estate_taxes"], cap),
+        atol=TOLERANCE,
+    )
+    # Spouses no longer file separately on the same form.
+    assert (base["mt_salt_deduction_indiv"] == 0).all()
+
+
+# A batch's cost is mostly per-variable overhead, so each example is a large
+# batch and there are few examples.
+SETTINGS = dict(
+    max_examples=6,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow, HealthCheck.data_too_large],
+)
+
+
+@settings(**SETTINGS)
+@given(st.sampled_from(SCHEDULE_YEARS), st.lists(tax_units(), min_size=5, max_size=30))
+def test_mt_salt_deduction_schedule_invariants(year, units):
+    _check_schedule_year(units, year)
+
+
+@settings(**SETTINGS)
+@given(
+    st.sampled_from(sorted(FEDERAL_CAP)), st.lists(tax_units(), min_size=5, max_size=30)
+)
+def test_mt_salt_deduction_unchanged_from_2024(year, units):
+    _check_unchanged_year(units, year)
+
+
+@pytest.mark.parametrize("year", SCHEDULE_YEARS)
+def test_seeded_population_schedule_years(year):
+    _check_schedule_year(_seeded_units(), year)
+
+
+@pytest.mark.parametrize("year", sorted(FEDERAL_CAP))
+def test_seeded_population_unchanged_from_2024(year):
+    _check_unchanged_year(_seeded_units(), year)
diff --git a/policyengine_us/variables/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax.py b/policyengine_us/variables/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax.py
index 220f8254fd2..a31e9c6523a 100644
--- a/policyengine_us/variables/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax.py
+++ b/policyengine_us/variables/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax.py
@@ -11,11 +11,4 @@ class mo_kansas_city_earnings_tax(Variable):
     definition_period = YEAR
     unit = USD
     reference = "https://www.kcmo.gov/city-hall/departments/finance/earnings-tax"
-
-    def formula(tax_unit, period, parameters):
-        person = tax_unit.members
-        p = parameters(period).gov.local.mo.kansas_city.tax.income
-        taxable_earnings = person(
-            "mo_kansas_city_earnings_tax_taxable_earnings", period
-        )
-        return tax_unit.sum(taxable_earnings * p.rate)
+    adds = ["mo_kansas_city_earnings_tax_person"]
diff --git a/policyengine_us/variables/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax_person.py b/policyengine_us/variables/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax_person.py
new file mode 100644
index 00000000000..99ff1759eda
--- /dev/null
+++ b/policyengine_us/variables/gov/local/mo/kansas_city/tax/income/mo_kansas_city_earnings_tax_person.py
@@ -0,0 +1,18 @@
+from policyengine_us.model_api import *
+
+
+class mo_kansas_city_earnings_tax_person(Variable):
+    value_type = float
+    entity = Person
+    label = "Kansas City earnings tax for each person"
+    documentation = "Kansas City earnings tax on the person's own taxable earnings."
+    definition_period = YEAR
+    unit = USD
+    reference = "https://www.kcmo.gov/city-hall/departments/finance/earnings-tax"
+
+    def formula(person, period, parameters):
+        p = parameters(period).gov.local.mo.kansas_city.tax.income
+        taxable_earnings = person(
+            "mo_kansas_city_earnings_tax_taxable_earnings", period
+        )
+        return taxable_earnings * p.rate
diff --git a/policyengine_us/variables/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax.py b/policyengine_us/variables/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax.py
index 1c31987db47..42bbf173fb7 100644
--- a/policyengine_us/variables/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax.py
+++ b/policyengine_us/variables/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax.py
@@ -14,11 +14,4 @@ class mo_st_louis_earnings_tax(Variable):
         "https://www.stlouis-mo.gov/government/departments/collector/"
         "earnings-tax/file-earnings-tax.cfm"
     )
-
-    def formula(tax_unit, period, parameters):
-        person = tax_unit.members
-        p = parameters(period).gov.local.mo.st_louis.tax.income
-        taxable_earnings = person("mo_st_louis_earnings_tax_taxable_earnings", period)
-        credits = person("mo_st_louis_earnings_tax_credit", period)
-        person_tax = taxable_earnings * p.rate
-        return tax_unit.sum(max_(0, person_tax - credits))
+    adds = ["mo_st_louis_earnings_tax_person"]
diff --git a/policyengine_us/variables/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax_person.py b/policyengine_us/variables/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax_person.py
new file mode 100644
index 00000000000..8a2b5aa7f5d
--- /dev/null
+++ b/policyengine_us/variables/gov/local/mo/st_louis/tax/income/mo_st_louis_earnings_tax_person.py
@@ -0,0 +1,23 @@
+from policyengine_us.model_api import *
+
+
+class mo_st_louis_earnings_tax_person(Variable):
+    value_type = float
+    entity = Person
+    label = "St. Louis earnings tax for each person"
+    documentation = (
+        "St. Louis earnings tax on the person's own taxable earnings, after "
+        "the person's optional credits supplied as inputs."
+    )
+    definition_period = YEAR
+    unit = USD
+    reference = (
+        "https://www.stlouis-mo.gov/government/departments/collector/"
+        "earnings-tax/file-earnings-tax.cfm"
+    )
+
+    def formula(person, period, parameters):
+        p = parameters(period).gov.local.mo.st_louis.tax.income
+        taxable_earnings = person("mo_st_louis_earnings_tax_taxable_earnings", period)
+        credits = person("mo_st_louis_earnings_tax_credit", period)
+        return max_(0, taxable_earnings * p.rate - credits)
diff --git a/policyengine_us/variables/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax.py b/policyengine_us/variables/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax.py
index 62e37e476a6..f74a1154d72 100644
--- a/policyengine_us/variables/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax.py
+++ b/policyengine_us/variables/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax.py
@@ -15,18 +15,4 @@ class pa_philadelphia_wage_tax(Variable):
         "https://www.phila.gov/services/payments-assistance-taxes/taxes/"
         "business-taxes/business-taxes-by-type/wage-tax-employers/"
     )
-
-    def formula(tax_unit, period, parameters):
-        person = tax_unit.members
-        taxable_wages = person("pa_philadelphia_wage_tax_taxable_wages", period)
-        if not np.any(taxable_wages):
-            return 0
-
-        p = parameters(period).gov.local.pa.philadelphia.tax.income
-        resident = person("pa_philadelphia_wage_tax_resident", period)
-        reduced_rate_eligible = person(
-            "pa_philadelphia_wage_tax_reduced_rate_eligible", period
-        )
-        standard_rate = where(resident, p.resident_rate, p.nonresident_rate)
-        rate = where(reduced_rate_eligible, p.reduced_rate, standard_rate)
-        return tax_unit.sum(taxable_wages * rate)
+    adds = ["pa_philadelphia_wage_tax_person"]
diff --git a/policyengine_us/variables/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax_person.py b/policyengine_us/variables/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax_person.py
new file mode 100644
index 00000000000..3b2018fae7c
--- /dev/null
+++ b/policyengine_us/variables/gov/local/pa/philadelphia/tax/income/pa_philadelphia_wage_tax_person.py
@@ -0,0 +1,31 @@
+from policyengine_us.model_api import *
+
+
+class pa_philadelphia_wage_tax_person(Variable):
+    value_type = float
+    entity = Person
+    label = "Philadelphia wage tax for each person"
+    documentation = (
+        "Philadelphia employee wage or earnings tax on the person's own "
+        "taxable wages, at the rate for the person's resident status."
+    )
+    definition_period = YEAR
+    unit = USD
+    reference = (
+        "https://www.phila.gov/services/payments-assistance-taxes/taxes/"
+        "business-taxes/business-taxes-by-type/wage-tax-employers/"
+    )
+
+    def formula(person, period, parameters):
+        taxable_wages = person("pa_philadelphia_wage_tax_taxable_wages", period)
+        if not np.any(taxable_wages):
+            return 0
+
+        p = parameters(period).gov.local.pa.philadelphia.tax.income
+        resident = person("pa_philadelphia_wage_tax_resident", period)
+        reduced_rate_eligible = person(
+            "pa_philadelphia_wage_tax_reduced_rate_eligible", period
+        )
+        standard_rate = where(resident, p.resident_rate, p.nonresident_rate)
+        rate = where(reduced_rate_eligible, p.reduced_rate, standard_rate)
+        return taxable_wages * rate
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.py
index 5d1cb0c5153..2757859ca05 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.py
@@ -30,7 +30,7 @@ def formula(person, period, parameters):
             [
                 "mt_misc_deductions",
                 "mt_medical_expense_deduction_indiv",
-                "mt_salt_deduction",
+                "mt_salt_deduction_indiv",
                 "mt_federal_income_tax_deduction_for_federal_itemization_indiv",
             ],
         )
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_indiv.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_indiv.py
index 7b31319e265..ba313b8e88a 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_indiv.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_indiv.py
@@ -32,7 +32,7 @@ def formula(person, period, parameters):
             [
                 "mt_misc_deductions",
                 "mt_medical_expense_deduction_indiv",
-                "mt_salt_deduction",
+                "mt_salt_deduction_indiv",
                 "mt_federal_income_tax_deduction_indiv",
             ],
         )
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/salt/mt_salt_deduction.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/salt/mt_salt_deduction.py
index e5d5221dbf0..00c82008e1c 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/salt/mt_salt_deduction.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/salt/mt_salt_deduction.py
@@ -7,19 +7,53 @@ class mt_salt_deduction(Variable):
     label = "Montana state and local tax deduction"
     unit = USD
     definition_period = YEAR
+    documentation = (
+        "Itemized Deductions Schedule, line 5, for a return with one column: "
+        "a single, head of household or joint return, or a married person "
+        "filing on a separate form. Through 2023 the return's total is "
+        "assigned to the tax unit head."
+    )
     reference = (
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2/2021_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=7",
         "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2/2022_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=7",
-        "https://law.justia.com/codes/montana/2022/title-15/chapter-30/part-21/section-15-30-2131/",
-        # Montana Code Annotated MT Code § 15-30-2131 (2022) (1)(a)
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2/2023_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=7",
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2021_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=32",
+        "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=34",
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2023_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=37",
+        "https://mca.legmt.gov/bills/2022/mca/title_0150/chapter_0300/part_0210/section_0310/0150-0300-0210-0310.html",
+        # Former MCA 15-30-2131(1)(a)(ii) (2021): items under IRC 161, except
+        # state income tax paid. The cap is 26 USC 164(b)(6).
     )
     defined_for = StateCode.MT
 
     def formula(person, period, parameters):
-        # Not included the SALT tax, only real estate taxes
-        p = parameters(period).gov.irs.deductions
+        p = parameters(period).gov.irs.deductions.itemized.salt_and_real_estate
+        p_mt = parameters(period).gov.states.mt.tax.income.deductions.itemized
         filing_status = person.tax_unit("filing_status", period)
+        cap = p.cap[filing_status]
+        if p_mt.state_specific_deduction_applies:
+            # Line 5 adds lines 5a through 5d for the whole return and caps
+            # the total once: "not more than $10,000 if your status is
+            # single, head of household or married filing jointly; or $5,000
+            # if you are married filing separately".
+            # 5a: general state and local sales taxes.
+            # 5b: local income taxes. No Montana locality levies one, but
+            #     a resident can owe another city's earnings or wage tax.
+            # 5c: real estate taxes.
+            # 5d: value-based personal property taxes, not modeled.
+            taxes = add(
+                person.tax_unit,
+                period,
+                [
+                    "real_estate_taxes",
+                    "state_sales_tax",
+                    "local_sales_tax",
+                    "local_income_tax",
+                ],
+            )
+            is_head = person("is_tax_unit_head", period)
+            return is_head * min_(taxes, cap)
+        # From 2024 the schedule is discontinued. This keeps the earlier
+        # per-person amounts until the 2024 Form 2 line 2 is modeled.
         real_estate_tax = person("real_estate_taxes", period)
-        return min_(
-            real_estate_tax,
-            p.itemized.salt_and_real_estate.cap[filing_status],
-        )
+        return min_(real_estate_tax, cap)
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/salt/mt_salt_deduction_indiv.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/salt/mt_salt_deduction_indiv.py
new file mode 100644
index 00000000000..3054cfc2aae
--- /dev/null
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/salt/mt_salt_deduction_indiv.py
@@ -0,0 +1,68 @@
+from policyengine_us.model_api import *
+
+
+class mt_salt_deduction_indiv(Variable):
+    value_type = float
+    entity = Person
+    label = "Montana state and local tax deduction when married couples are filing separately"
+    unit = USD
+    definition_period = YEAR
+    documentation = (
+        "Itemized Deductions Schedule, line 5, in each spouse's column when "
+        "a married couple files separately on the same form (filing status "
+        "2a). Each column is a separate return, capped at the married filing "
+        "separately amount."
+    )
+    reference = (
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2/2021_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=7",
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2/2022_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=7",
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2/2023_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=7",
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2023_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=12",
+        # 2023 instructions, "How to allocate income and deductions when
+        # filing separately": "each column constituting its own return".
+        "https://revenuefiles.mt.gov/files/Forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2023_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=37",
+        "https://mca.legmt.gov/bills/2022/mca/title_0150/chapter_0300/part_0210/section_0310/0150-0300-0210-0310.html",
+        # Former MCA 15-30-2131(1)(a)(ii) (2021). The cap is 26 USC 164(b)(6).
+    )
+    defined_for = "mt_married_filing_separately_on_same_return_eligible"
+
+    def formula(person, period, parameters):
+        p = parameters(period).gov.irs.deductions.itemized.salt_and_real_estate
+        p_mt = parameters(period).gov.states.mt.tax.income.deductions.itemized
+        filing_status = person.tax_unit("filing_status", period)
+        # A married couple's columns are each "married filing separately":
+        # "not more than ... $5,000 if you are married filing separately".
+        # Other filers have one column at their own status's cap.
+        married = filing_status == filing_status.possible_values.JOINT
+        cap = where(married, p.cap.SEPARATE, p.cap[filing_status])
+        # Line 5c: each spouse's own real estate taxes.
+        real_estate_tax = person("real_estate_taxes", period)
+        # Line 5a: PolicyEngine estimates sales taxes for the tax unit only.
+        # Jointly paid expenses "can be allocated to either spouse in any
+        # proportional amount", so a couple divides them equally.
+        sales_tax = add(person.tax_unit, period, ["state_sales_tax", "local_sales_tax"])
+        sales_tax_share = where(married, p_mt.spouse_allocation_rate, 1)
+        # Line 5b: "Deductions that are attributable to only one spouse must
+        # be claimed by that spouse." The local income taxes PolicyEngine
+        # levies on a Montana household are city taxes on each person's own
+        # earnings there (Kansas City, St. Louis, Philadelphia), so each
+        # column takes that person's own tax. Any other local income tax of
+        # the tax unit is divided like the sales taxes.
+        own_local_tax = add(
+            person,
+            period,
+            [
+                "mo_kansas_city_earnings_tax_person",
+                "mo_st_louis_earnings_tax_person",
+                "pa_philadelphia_wage_tax_person",
+            ],
+        )
+        local_income_tax = person.tax_unit("local_income_tax", period)
+        other_local_tax = max_(0, local_income_tax - person.tax_unit.sum(own_local_tax))
+        taxes = (
+            real_estate_tax
+            + sales_tax_share * (sales_tax + other_local_tax)
+            + own_local_tax
+        )
+        head_or_spouse = person("is_tax_unit_head_or_spouse", period)
+        return head_or_spouse * min_(taxes, cap)