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..4d6a460dcfd
--- /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 `tx_ssi_state_supplement` returns $0 instead of raising `ParameterNotFoundError` for July 1988 through August 1999, 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..0fa05f76148 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 +1,13 @@
 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: Pub. L. 100-203 section
+  # 9119(d)(2), in the statutory note to 42 USC 1396a, applies it to "calendar
+  # quarters beginning on or after July 1, 1988". 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 +23,17 @@ 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/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..4d48d38aed6 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 +35,306 @@
     # 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
+
+- name: Case 28, facility resident with a pension and earnings gets the full supplement.
+  # $30/month pension less the $20 general exclusion is $10 of countable
+  # unearned income; $100/month of earnings less the $65 earned income
+  # exclusion, halved, is $17.50. Countable income is $27.50, so SSI pays
+  # $30 - $27.50 = $2.50/month. The supplement does not depend on income.
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 80
+        pension_income: 360
+        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: 330
+    ssi: 30
+    tx_ssi_state_supplement_eligible: true
+    tx_ssi_state_supplement: 540
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