diff --git a/changelog.d/tx-ssi-supplement-history.fixed.md b/changelog.d/tx-ssi-supplement-history.fixed.md
new file mode 100644
index 00000000000..00c3204dc5b
--- /dev/null
+++ b/changelog.d/tx-ssi-supplement-history.fixed.md
@@ -0,0 +1 @@
+Backfill the Texas SSI supplement's personal needs allowance at $30 from July 1988 (no supplement before September 1999), so Texas simulations for months from July 1988 through August 1999 no longer fail, and document the supplement's sources from 1999 to 2026.
diff --git a/policyengine_us/parameters/gov/states/tx/hhsc/ssi_state_supplement/personal_needs_allowance.yaml b/policyengine_us/parameters/gov/states/tx/hhsc/ssi_state_supplement/personal_needs_allowance.yaml
index eea60dbf67f..568a2a49625 100644
--- a/policyengine_us/parameters/gov/states/tx/hhsc/ssi_state_supplement/personal_needs_allowance.yaml
+++ b/policyengine_us/parameters/gov/states/tx/hhsc/ssi_state_supplement/personal_needs_allowance.yaml
@@ -1,6 +1,11 @@
 description: Texas provides this amount as the personal needs allowance for Supplemental Security Income recipients in Medicaid-funded long-term care facilities under the Optional State Supplement program.
 
 values:
+  # H-1500 and the 2002 DHS handbook give $30 before September 1999 without a
+  # start date. The modeled backfill starts July 1, 1988, when the $30 federal
+  # minimum in 42 USC 1396a(q)(2) took effect; Texas's own adoption date is
+  # not in these sources.
+  1988-07-01: 30
   1999-09-01: 45
   2001-09-01: 60
   2003-09-01: 45
@@ -16,3 +21,17 @@ metadata:
       href: https://statutes.capitol.texas.gov/Docs/HR/htm/HR.32.htm
     - title: HHSC MEPD Handbook Section H-1500 - Personal Needs Allowance
       href: https://www.hhs.texas.gov/handbooks/medicaid-elderly-people-disabilities-handbook/h-1500-personal-needs-allowance
+    - title: 1 Texas Administrative Code Section 358.381(c)
+      href: https://www.law.cornell.edu/regulations/texas/1-Tex-Admin-Code-SS-358-381
+    - title: 42 USC 1396a(q)(2) - Minimum monthly personal needs allowance
+      href: https://www.law.cornell.edu/uscode/text/42/1396a#q_2
+    - title: Texas DHS Medicaid Eligibility Handbook Revision Notice 02-1 (December 21, 2001), Personal Needs Allowance - Effective September 1, 2001
+      href: https://web.archive.org/web/20020504200936/http://www.dhs.state.tx.us:80/handbooks/archives/meh/02-1/memo.htm
+    - title: H.B. 2292, 78th Texas Legislature (2003), Section 2.207
+      href: https://capitol.texas.gov/tlodocs/78R/billtext/html/HB02292F.htm
+    - title: Texas DHS Medicaid Eligibility Handbook Item 3231, Personal Needs Allowance (Revision 03-7, effective September 1, 2003)
+      href: https://web.archive.org/web/20030910100506/http://www.dhs.state.tx.us:80/handbooks/meh/3000/3231.htm
+    - title: HHSC MEPD Handbook Section H-6000 (Revision 09-4, effective December 1, 2009)
+      href: https://web.archive.org/web/20100126020411/http://www.dads.state.tx.us/handbooks/mepd/H/H-6000.htm
+    - title: H.B. 54, 88th Texas Legislature (2023)
+      href: https://capitol.texas.gov/tlodocs/88R/billtext/html/HB00054F.htm
diff --git a/policyengine_us/tests/core/test_tx_ssi_state_supplement_invariants.py b/policyengine_us/tests/core/test_tx_ssi_state_supplement_invariants.py
new file mode 100644
index 00000000000..440228f83a6
--- /dev/null
+++ b/policyengine_us/tests/core/test_tx_ssi_state_supplement_invariants.py
@@ -0,0 +1,150 @@
+"""Income invariance of the Texas SSI supplement for Medicaid-facility residents.
+
+1 TAC 358.381(c)(2): "For a person receiving the reduced SSI federal benefit
+rate, HHSC issues a supplement to give the person a PNA at the minimum level
+set by the HHSC executive commissioner." HHSC MEPD Handbook H-6000 states it as
+a fixed amount: "HHSC supplements the reduced SSI payment standard by $45 per
+month". No Texas source subtracts the recipient's own income from it.
+
+Checked for single residents of a Medicaid facility who are paid SSI on the
+reduced standard (a positive federal payment), with generated pensions,
+earnings and ages, in June 2011 and June 2024:
+
+1. Income invariance: each such eligible resident gets exactly PNA - $30 a
+   month, whatever their income.
+2. Federal identity: federal SSI plus countable income equals the $30 reduced
+   standard.
+3. Differential: PNA - SSI - countable income, the reading of SSA's
+   description that subtracts the recipient's income, equals the supplement.
+4. Bounds: 0 <= supplement <= PNA - $30 for every resident.
+
+Residents with no federal payment (countable income at or above $30) are left
+to the eligibility tests: the sources do not settle whether Texas pays one
+whose countable income is exactly $30.
+
+Why Python: YAML checks fixed expected outputs for listed households. These
+properties compare three computed variables against each other for generated
+residents, which YAML cannot express. The dated amounts are in
+tx_ssi_state_supplement.yaml.
+"""
+
+import numpy as np
+from hypothesis import HealthCheck, given, settings
+from hypothesis import strategies as st
+
+from policyengine_us import Simulation
+
+PERIODS = ("2011-06", "2024-06")
+TOLERANCE = 0.01
+FACILITY_INPUTS = dict(
+    is_in_medicaid_facility=True,
+    ssi_lives_in_medical_treatment_facility=True,
+    ssi_medicaid_pays_majority_of_care=True,
+)
+
+
+def _situation(residents, years):
+    situation = {
+        group: {}
+        for group in (
+            "people",
+            "tax_units",
+            "marital_units",
+            "spm_units",
+            "families",
+            "households",
+        )
+    }
+    for index, (age, pension, earnings) in enumerate(residents):
+        name = f"resident{index}"
+        person = {
+            "age": {year: age for year in years},
+            "pension_income": {year: 12 * pension for year in years},
+            "employment_income": {year: 12 * earnings for year in years},
+        }
+        for variable, value in FACILITY_INPUTS.items():
+            person[variable] = {year: value for year in years}
+        situation["people"][name] = person
+        for group in ("tax_units", "marital_units", "spm_units", "families"):
+            situation[group][f"{group}_{index}"] = {"members": [name]}
+        situation["households"][f"household_{index}"] = {
+            "members": [name],
+            "state_code": {year: "TX" for year in years},
+        }
+    return situation
+
+
+def _check(residents):
+    years = sorted({int(period[:4]) for period in PERIODS})
+    simulation = Simulation(situation=_situation(residents, years))
+    paid = []
+    for period in PERIODS:
+        parameters = simulation.tax_benefit_system.parameters(period)
+        pna = (
+            parameters.gov.states.tx.hhsc.ssi_state_supplement.personal_needs_allowance
+        )
+        reduced = parameters.gov.ssa.ssi.amount.institutional.individual
+        eligible = simulation.calculate("tx_ssi_state_supplement_eligible", period)
+        supplement = simulation.calculate("tx_ssi_state_supplement", period)
+        ssi = simulation.calculate("ssi", period)
+        countable = simulation.calculate("ssi_countable_income", period)
+        recipient = eligible & (ssi > 0)
+        paid.append(recipient)
+
+        # 4. Bounds.
+        assert (supplement >= 0).all()
+        assert (supplement <= pna - reduced + TOLERANCE).all()
+        # 1. Income invariance.
+        np.testing.assert_allclose(supplement[recipient], pna - reduced, atol=TOLERANCE)
+        # 2. Federal SSI plus countable income is the reduced standard.
+        np.testing.assert_allclose(
+            (ssi + countable)[recipient], reduced, atol=TOLERANCE
+        )
+        # 3. Agreement with "allowance less federal SSI less countable income".
+        np.testing.assert_allclose(
+            supplement[recipient],
+            (pna - ssi - countable)[recipient],
+            atol=TOLERANCE,
+        )
+    return np.any(paid, axis=0)
+
+
+residents = st.tuples(
+    st.integers(min_value=65, max_value=95),
+    # Monthly pension: $0-$150, so countable unearned income spans the $30
+    # reduced standard.
+    st.integers(min_value=0, max_value=150),
+    # Monthly earnings: $0-$250.
+    st.integers(min_value=0, max_value=250),
+)
+
+# 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=5,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow, HealthCheck.data_too_large],
+)
+
+
+@settings(**SETTINGS)
+@given(st.lists(residents, min_size=10, max_size=30))
+def test_generated_residents(population):
+    _check(population)
+
+
+def test_seeded_residents():
+    # Pensions below, at and above the $50 that puts countable income at the
+    # $30 standard, earnings with and without a pension, and no income.
+    population = [
+        (age, pension, earnings)
+        for age in (66, 85)
+        for pension in (0, 20, 40, 49, 50, 51, 120)
+        for earnings in (0, 100, 145, 200)
+    ]
+    paid = _check(population)
+    # The grid reaches residents paid SSI with and without other income, and
+    # residents with no federal payment.
+    assert paid.sum() >= 10
+    assert (~paid).any()
diff --git a/policyengine_us/tests/policy/baseline/gov/states/tx/hhsc/ssi_state_supplement/tx_ssi_state_supplement.yaml b/policyengine_us/tests/policy/baseline/gov/states/tx/hhsc/ssi_state_supplement/tx_ssi_state_supplement.yaml
index 12f2413e89b..cd093cbbe83 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/tx/hhsc/ssi_state_supplement/tx_ssi_state_supplement.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/tx/hhsc/ssi_state_supplement/tx_ssi_state_supplement.yaml
@@ -1,6 +1,14 @@
 # Unit tests for Texas SSI State Supplement benefit amount
-# Supplement = max(0, (PNA - Federal institutional SSI rate) * 12)
-# For 2024: ($75 - $30) * 12 = $45/month * 12 = $540/year
+# Monthly supplement = max(0, PNA - federal reduced SSI benefit of $30).
+# Annual periods sum the months, so years with a September 1 change mix
+# two amounts.
+# PNA history (HHSC MEPD Handbook H-1500):
+#   $30 before Sept. 1, 1999 (no supplement)
+#   $45 Sept. 1, 1999 - Aug. 31, 2001 ($15 supplement)
+#   $60 Sept. 1, 2001 - Aug. 31, 2003 ($30 supplement)
+#   $45 Sept. 1, 2003 - Dec. 31, 2005 ($15 supplement)
+#   $60 Jan. 1, 2006 - Dec. 31, 2023 ($30 supplement)
+#   $75 from Jan. 1, 2024 ($45 supplement)
 # Reference: Texas Human Resources Code Section 32.024(w)
 
 - name: Case 1, eligible individual receives full supplement.
@@ -27,3 +35,275 @@
     # PNA = $60/month (2006-2023), Federal reduced SSI = $30/month
     # Supplement = ($60 - $30) * 12 = $360/year
     tx_ssi_state_supplement: 360
+
+- name: Case 4, 1998, before the supplement began.
+  period: 1998
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # H-1500: before Sept. 1, 1999 the PNA was $30, equal to the reduced
+    # federal benefit, so there was no supplement.
+    tx_ssi_state_supplement: 0
+
+- name: Case 5, 1999, supplement starts September 1.
+  period: 1999
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # Jan.-Aug.: $30 - $30 = $0; Sept.-Dec.: $45 - $30 = $15
+    # 4 * $15 = $60/year
+    tx_ssi_state_supplement: 60
+
+- name: Case 6, 2000, $45 PNA all year.
+  period: 2000
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # SSA, State Assistance Programs for SSI Recipients, January 2002 (Texas):
+    # combined federal and state $45, state-supplemented portion $15.
+    # $15 * 12 = $180/year
+    tx_ssi_state_supplement: 180
+
+- name: Case 7, 2001, supplement rises to $30 on September 1.
+  period: 2001
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # DHS Revision Notice 02-1: PNA $60 effective Sept. 1, 2001; "the state
+    # will supplement their incomes by $30 per month".
+    # 8 * $15 + 4 * $30 = $240/year
+    tx_ssi_state_supplement: 240
+
+- name: Case 8, 2002, $60 PNA all year.
+  period: 2002
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # DHS Medicaid Eligibility Handbook Item 3231 (40 TAC 15.450(h)):
+    # "the state supplements their incomes by $30 per month".
+    # $30 * 12 = $360/year
+    tx_ssi_state_supplement: 360
+
+- name: Case 9, 2003, supplement falls to $15 on September 1.
+  period: 2003
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # H.B. 2292 (2003) Section 2.207 and Item 3231 (Revision 03-7): PNA $45
+    # from Sept. 1, 2003.
+    # 8 * $30 + 4 * $15 = $300/year
+    tx_ssi_state_supplement: 300
+
+- name: Case 10, 2005, $45 PNA all year.
+  period: 2005
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # SSA January 2005 (Texas): combined $45, state $15.
+    # $15 * 12 = $180/year
+    tx_ssi_state_supplement: 180
+
+- name: Case 11, 2006, PNA restored to $60 on January 1.
+  period: 2006
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # SSA January 2006 (Texas): combined $60, state $30.
+    # $30 * 12 = $360/year
+    tx_ssi_state_supplement: 360
+
+- name: Case 12, 2011, $30 supplement despite SSA's $60 listing.
+  period: 2011
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # SSA's January 2007-2011 Texas tables list a $60 state supplement
+    # ($90 combined). HHSC's own handbook for the same years (MEPD H-6000,
+    # Revision 09-4 of Dec. 2009 and Revision 11-2 of June 2011) says "HHSC
+    # supplements the reduced SSI payment standard by $30 per month so that
+    # SSI persons also have a $60 personal needs allowance."
+    # $30 * 12 = $360/year
+    tx_ssi_state_supplement: 360
+
+- name: Case 13, 2026, $75 PNA.
+  period: 2026
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # H-6000 (Revision 26-1): "HHSC supplements the reduced SSI payment
+    # standard by $45 per month".
+    # $45 * 12 = $540/year
+    tx_ssi_state_supplement: 540
+
+- name: Case 14, facility resident with a pension gets the full supplement.
+  # A $40/month pension less the $20 general exclusion is $20 of countable
+  # income, so SSI pays $30 - $20 = $10/month. The supplement is still $45:
+  # H-6000 states it as a fixed amount on the reduced standard, and no Texas
+  # source reduces it by the recipient's income.
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 80
+        pension_income: 480
+        is_in_medicaid_facility: true
+        ssi_lives_in_medical_treatment_facility: true
+        ssi_medicaid_pays_majority_of_care: true
+    tax_units:
+      tax_unit:
+        members: [person1]
+    spm_units:
+      spm_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: TX
+  output:
+    ssi_countable_income: 240
+    ssi: 120
+    tx_ssi_state_supplement_eligible: true
+    # ($75 - $30) * 12 = $540/year
+    tx_ssi_state_supplement: 540
+
+- name: Case 15, facility resident with earnings gets the full supplement.
+  # $100/month of earnings less the $20 general and $65 earned income
+  # exclusions, halved, is $7.50 of countable income, so SSI pays
+  # $30 - $7.50 = $22.50/month.
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        employment_income: 1_200
+        is_in_medicaid_facility: true
+        ssi_lives_in_medical_treatment_facility: true
+        ssi_medicaid_pays_majority_of_care: true
+    tax_units:
+      tax_unit:
+        members: [person1]
+    spm_units:
+      spm_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: TX
+  output:
+    ssi_countable_income: 90
+    ssi: 270
+    tx_ssi_state_supplement_eligible: true
+    tx_ssi_state_supplement: 540
+
+- name: Case 16, 2011 facility resident with no other income.
+  # Federal SSI is the $30 reduced benefit; HHSC adds $30 for a $60 PNA.
+  period: 2011
+  input:
+    people:
+      person1:
+        age: 80
+        is_in_medicaid_facility: true
+        ssi_lives_in_medical_treatment_facility: true
+        ssi_medicaid_pays_majority_of_care: true
+    tax_units:
+      tax_unit:
+        members: [person1]
+    spm_units:
+      spm_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: TX
+  output:
+    ssi: 360
+    tx_ssi_state_supplement_eligible: true
+    tx_ssi_state_supplement: 360
+
+- name: Case 17, 1988-07.
+  period: 1988-07
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # First modeled month: PNA $30 equals the reduced federal benefit.
+    tx_ssi_state_supplement: 0
+
+- name: Case 18, 1999-08.
+  period: 1999-08
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # Last month before the supplement began.
+    tx_ssi_state_supplement: 0
+
+- name: Case 19, 1999-09.
+  period: 1999-09
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # 25 TexReg 558: $15 supplement retroactive to Sept. 1, 1999.
+    tx_ssi_state_supplement: 15
+
+- name: Case 20, 2001-08.
+  period: 2001-08
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # Last month of the $45 PNA.
+    tx_ssi_state_supplement: 15
+
+- name: Case 21, 2001-09.
+  period: 2001-09
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # DHS Revision Notice 02-1: $30 effective Sept. 1, 2001.
+    tx_ssi_state_supplement: 30
+
+- name: Case 22, 2003-08.
+  period: 2003-08
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # Last month of the $60 PNA.
+    tx_ssi_state_supplement: 30
+
+- name: Case 23, 2003-09.
+  period: 2003-09
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # H.B. 2292 Section 2.207: $45 PNA from Sept. 1, 2003.
+    tx_ssi_state_supplement: 15
+
+- name: Case 24, 2005-12.
+  period: 2005-12
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # Last month of the $45 PNA.
+    tx_ssi_state_supplement: 15
+
+- name: Case 25, 2006-01.
+  period: 2006-01
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # PNA restored to $60 on Jan. 1, 2006.
+    tx_ssi_state_supplement: 30
+
+- name: Case 26, 2023-12.
+  period: 2023-12
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # Last month of the $60 PNA.
+    tx_ssi_state_supplement: 30
+
+- name: Case 27, 2024-01.
+  period: 2024-01
+  input:
+    tx_ssi_state_supplement_eligible: true
+  output:
+    # IL 2023-45: $45 from January 2024.
+    tx_ssi_state_supplement: 45
diff --git a/policyengine_us/variables/gov/states/tx/hhsc/ssi_state_supplement/tx_ssi_state_supplement.py b/policyengine_us/variables/gov/states/tx/hhsc/ssi_state_supplement/tx_ssi_state_supplement.py
index e1d858ee45e..ef6f2e4c6fa 100644
--- a/policyengine_us/variables/gov/states/tx/hhsc/ssi_state_supplement/tx_ssi_state_supplement.py
+++ b/policyengine_us/variables/gov/states/tx/hhsc/ssi_state_supplement/tx_ssi_state_supplement.py
@@ -10,11 +10,25 @@ class tx_ssi_state_supplement(Variable):
     defined_for = "tx_ssi_state_supplement_eligible"
     reference = (
         "https://statutes.capitol.texas.gov/Docs/HR/htm/HR.32.htm",
+        "https://www.hhs.texas.gov/handbooks/medicaid-elderly-people-disabilities-handbook/h-1500-personal-needs-allowance",
         "https://www.hhs.texas.gov/handbooks/medicaid-elderly-people-disabilities-handbook/h-6000-co-payment-ssi-cases",
+        "https://www.law.cornell.edu/regulations/texas/1-Tex-Admin-Code-SS-358-381",
     )
 
     def formula(person, period, parameters):
-        # Per 42 USC 1382(e)(1)(B) and Texas HR Code 32.024(w)
+        # 1 TAC 358.381(c)(2): "For a person receiving the reduced SSI federal
+        # benefit rate, HHSC issues a supplement to give the person a PNA at
+        # the minimum level set by the HHSC executive commissioner." MEPD
+        # Handbook H-6000: "HHSC supplements the reduced SSI payment standard
+        # by $45 per month to ensure SSI recipients also have a $75 personal
+        # needs allowance", i.e. the allowance (Texas HR Code 32.024(w)) less
+        # the $30 reduced federal benefit (42 USC 1382(e)(1)(B)).
+        # H-6000 states the supplement as a fixed amount on the reduced
+        # standard and, separately, has workers check SSI recipients for other
+        # income and calculate a co-payment; no Texas source subtracts the
+        # recipient's income from the supplement. For an individual paid SSI
+        # on the reduced standard, federal SSI plus countable income equals
+        # the $30 standard, so the allowance less both is the same amount.
         p = parameters(period).gov.states.tx.hhsc.ssi_state_supplement
         pna = p.personal_needs_allowance
         federal_reduced = parameters(period).gov.ssa.ssi.amount.institutional.individual