diff --git a/changelog.d/ca-prop3-2030-expiry.added.md b/changelog.d/ca-prop3-2030-expiry.added.md
new file mode 100644
index 00000000000..f6cf7129c2c
--- /dev/null
+++ b/changelog.d/ca-prop3-2030-expiry.added.md
@@ -0,0 +1 @@
+Add California Proposition 3 (2026) as a contributed reform, toggled by gov.contrib.states.ca.prop3.in_effect, that keeps the rates above 9.3% from 2031 onward.
diff --git a/changelog.d/ca-prop3-2030-expiry.fixed.md b/changelog.d/ca-prop3-2030-expiry.fixed.md
new file mode 100644
index 00000000000..6a248088adf
--- /dev/null
+++ b/changelog.d/ca-prop3-2030-expiry.fixed.md
@@ -0,0 +1 @@
+Encode the expiry of California's 10.3%, 11.3% and 12.3% income tax rates after tax year 2030 in all five filing-status schedules (Cal. Const. art. XIII, § 36(f)(2)-(3)).
diff --git a/policyengine_us/parameters/gov/contrib/states/ca/prop3/in_effect.yaml b/policyengine_us/parameters/gov/contrib/states/ca/prop3/in_effect.yaml
new file mode 100644
index 00000000000..2df610071a2
--- /dev/null
+++ b/policyengine_us/parameters/gov/contrib/states/ca/prop3/in_effect.yaml
@@ -0,0 +1,16 @@
+description: California applies the Proposition 3 (2026) permanent extension of the income tax rates above 9.3% in years from 2031 onward in which this is true.
+values:
+  0000-01-01: false
+metadata:
+  unit: bool
+  period: year
+  label: California Proposition 3 (2026) in effect
+  reference:
+    - title: California Proposition 3 (2026), Official Voter Information Guide, Legislative Analyst's analysis, Figure 1
+      href: https://vig.cdn.sos.ca.gov/2026/general/pdf/prop3.pdf#page=2
+    - title: California Proposition 3 (2026), Text of Proposed Law, SEC. 4, amending Cal. Const. art. XIII, § 36(f)(2) (strikes "and before January 1, 2031,")
+      href: https://vig.cdn.sos.ca.gov/2026/general/pdf/prop3-text-proposed-laws.pdf#page=6
+    - title: California Proposition 3 (2026), Text of Proposed Law, SEC. 4, amending Cal. Const. art. XIII, § 36(f)(2)(D), (f)(3) and (f)(3)(D)
+      href: https://vig.cdn.sos.ca.gov/2026/general/pdf/prop3-text-proposed-laws.pdf#page=7
+    - title: Cal. Const. art. XIII, § 36(f)(2)-(3)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CONS&sectionNum=SEC.%2036.&article=XIII
diff --git a/policyengine_us/parameters/gov/states/ca/tax/income/rates/head_of_household.yaml b/policyengine_us/parameters/gov/states/ca/tax/income/rates/head_of_household.yaml
index d30bf8988a7..e52f46c6afc 100644
--- a/policyengine_us/parameters/gov/states/ca/tax/income/rates/head_of_household.yaml
+++ b/policyengine_us/parameters/gov/states/ca/tax/income/rates/head_of_household.yaml
@@ -70,6 +70,9 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.103
+      # Proposition 3 (November 3, 2026 ballot) would repeal this 2031 reversion here and in
+      # the next two brackets; check the certified result and remove these entries if it passes.
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 510_303
@@ -81,6 +84,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.113
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 850_503
@@ -92,6 +96,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.123
+      2031-01-01: 0.093
 
 metadata:
   threshold_unit: currency_USD
@@ -110,6 +115,9 @@ metadata:
       href: https://www.ftb.ca.gov/forms/2024/2024-540-tax-rate-schedules.pdf
     - title: California Income Tax Rate Schedules for 2025
       href: https://www.ftb.ca.gov/forms/2025/2025-540-tax-rate-schedules.pdf
-    - title: Cal. Rev. & Tax. Code § 17041
+    - title: Cal. Const. art. XIII, § 36(f)(3) (rates above 9.3% apply to taxable years before January 1, 2031)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CONS&sectionNum=SEC.%2036.&article=XIII
+    - title: Proposition 55 (2016), Text of Proposed Law, SEC. 4, amending Cal. Const. art. XIII, § 36(f)(3) to end the rates above 9.3% before January 1, 2031
+      href: https://vig.cdn.sos.ca.gov/2016/general/en/pdf/text-proposed-laws.pdf#page=16
+    - title: Cal. Rev. & Tax. Code § 17041(c)(1), (h)
       href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=17041.&lawCode=RTC
-      # Index according to federal tax parameter rules until we add California CPI trend.
diff --git a/policyengine_us/parameters/gov/states/ca/tax/income/rates/joint.yaml b/policyengine_us/parameters/gov/states/ca/tax/income/rates/joint.yaml
index db176716116..1717969d2a2 100644
--- a/policyengine_us/parameters/gov/states/ca/tax/income/rates/joint.yaml
+++ b/policyengine_us/parameters/gov/states/ca/tax/income/rates/joint.yaml
@@ -70,6 +70,9 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.103
+      # Proposition 3 (November 3, 2026 ballot) would repeal this 2031 reversion here and in
+      # the next two brackets; check the certified result and remove these entries if it passes.
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 750_442
@@ -81,6 +84,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.113
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 1_250_738
@@ -92,6 +96,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.123
+      2031-01-01: 0.093
 
 metadata:
   threshold_unit: currency_USD
@@ -110,6 +115,11 @@ metadata:
       href: https://www.ftb.ca.gov/forms/2024/2024-540-tax-rate-schedules.pdf
     - title: California Income Tax Rate Schedules for 2025
       href: https://www.ftb.ca.gov/forms/2025/2025-540-tax-rate-schedules.pdf
-    - title: Cal. Rev. & Tax. Code § 17041
+    - title: Cal. Const. art. XIII, § 36(f)(2) (rates above 9.3% apply to taxable years before January 1, 2031)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CONS&sectionNum=SEC.%2036.&article=XIII
+    - title: Proposition 55 (2016), Text of Proposed Law, SEC. 4, amending Cal. Const. art. XIII, § 36(f)(2) to end the rates above 9.3% before January 1, 2031
+      href: https://vig.cdn.sos.ca.gov/2016/general/en/pdf/text-proposed-laws.pdf#page=16
+    - title: Cal. Rev. & Tax. Code § 17041(a)(1), (h)
       href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=17041.&lawCode=RTC
-      # Index according to federal tax parameter rules until we add California CPI trend.
+    - title: Cal. Rev. & Tax. Code § 17045 (joint and surviving spouse tax is twice the tax on one-half of taxable income)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=17045.&lawCode=RTC
diff --git a/policyengine_us/parameters/gov/states/ca/tax/income/rates/separate.yaml b/policyengine_us/parameters/gov/states/ca/tax/income/rates/separate.yaml
index c5e1ae6154b..310316ae8af 100644
--- a/policyengine_us/parameters/gov/states/ca/tax/income/rates/separate.yaml
+++ b/policyengine_us/parameters/gov/states/ca/tax/income/rates/separate.yaml
@@ -70,6 +70,9 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.103
+      # Proposition 3 (November 3, 2026 ballot) would repeal this 2031 reversion here and in
+      # the next two brackets; check the certified result and remove these entries if it passes.
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 375_221
@@ -81,6 +84,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.113
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 625_369
@@ -92,6 +96,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.123
+      2031-01-01: 0.093
 
 metadata:
   threshold_unit: currency_USD
@@ -110,6 +115,9 @@ metadata:
       href: https://www.ftb.ca.gov/forms/2024/2024-540-tax-rate-schedules.pdf
     - title: California Income Tax Rate Schedules for 2025
       href: https://www.ftb.ca.gov/forms/2025/2025-540-tax-rate-schedules.pdf
-    - title: Cal. Rev. & Tax. Code § 17041
+    - title: Cal. Const. art. XIII, § 36(f)(2) (rates above 9.3% apply to taxable years before January 1, 2031)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CONS&sectionNum=SEC.%2036.&article=XIII
+    - title: Proposition 55 (2016), Text of Proposed Law, SEC. 4, amending Cal. Const. art. XIII, § 36(f)(2) to end the rates above 9.3% before January 1, 2031
+      href: https://vig.cdn.sos.ca.gov/2016/general/en/pdf/text-proposed-laws.pdf#page=16
+    - title: Cal. Rev. & Tax. Code § 17041(a)(1), (h)
       href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=17041.&lawCode=RTC
-      # Index according to federal tax parameter rules until we add California CPI trend.
diff --git a/policyengine_us/parameters/gov/states/ca/tax/income/rates/single.yaml b/policyengine_us/parameters/gov/states/ca/tax/income/rates/single.yaml
index 356ddee849b..38695aeff12 100644
--- a/policyengine_us/parameters/gov/states/ca/tax/income/rates/single.yaml
+++ b/policyengine_us/parameters/gov/states/ca/tax/income/rates/single.yaml
@@ -70,6 +70,9 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.103
+      # Proposition 3 (November 3, 2026 ballot) would repeal this 2031 reversion here and in
+      # the next two brackets; check the certified result and remove these entries if it passes.
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 375_221
@@ -81,6 +84,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.113
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 625_369
@@ -92,6 +96,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.123
+      2031-01-01: 0.093
 
 metadata:
   threshold_unit: currency_USD
@@ -110,6 +115,9 @@ metadata:
       href: https://www.ftb.ca.gov/forms/2024/2024-540-tax-rate-schedules.pdf
     - title: California Income Tax Rate Schedules for 2025
       href: https://www.ftb.ca.gov/forms/2025/2025-540-tax-rate-schedules.pdf
-    - title: Cal. Rev. & Tax. Code § 17041
+    - title: Cal. Const. art. XIII, § 36(f)(2) (rates above 9.3% apply to taxable years before January 1, 2031)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CONS&sectionNum=SEC.%2036.&article=XIII
+    - title: Proposition 55 (2016), Text of Proposed Law, SEC. 4, amending Cal. Const. art. XIII, § 36(f)(2) to end the rates above 9.3% before January 1, 2031
+      href: https://vig.cdn.sos.ca.gov/2016/general/en/pdf/text-proposed-laws.pdf#page=16
+    - title: Cal. Rev. & Tax. Code § 17041(a)(1), (h)
       href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=17041.&lawCode=RTC
-      # Index according to federal tax parameter rules until we add California CPI trend.
diff --git a/policyengine_us/parameters/gov/states/ca/tax/income/rates/surviving_spouse.yaml b/policyengine_us/parameters/gov/states/ca/tax/income/rates/surviving_spouse.yaml
index 1553538f47b..6c8255ac033 100644
--- a/policyengine_us/parameters/gov/states/ca/tax/income/rates/surviving_spouse.yaml
+++ b/policyengine_us/parameters/gov/states/ca/tax/income/rates/surviving_spouse.yaml
@@ -70,6 +70,9 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.103
+      # Proposition 3 (November 3, 2026 ballot) would repeal this 2031 reversion here and in
+      # the next two brackets; check the certified result and remove these entries if it passes.
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 750_442
@@ -81,6 +84,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.113
+      2031-01-01: 0.093
   - threshold:
       values:
         2021-01-01: 1_250_738
@@ -92,6 +96,7 @@ brackets:
         uprating: gov.states.ca.cpi
     rate:
       2021-01-01: 0.123
+      2031-01-01: 0.093
 
 metadata:
   threshold_unit: currency_USD
@@ -110,6 +115,11 @@ metadata:
       href: https://www.ftb.ca.gov/forms/2024/2024-540-tax-rate-schedules.pdf
     - title: California Income Tax Rate Schedules for 2025
       href: https://www.ftb.ca.gov/forms/2025/2025-540-tax-rate-schedules.pdf
-    - title: Cal. Rev. & Tax. Code § 17041
+    - title: Cal. Const. art. XIII, § 36(f)(2) (rates above 9.3% apply to taxable years before January 1, 2031)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CONS&sectionNum=SEC.%2036.&article=XIII
+    - title: Proposition 55 (2016), Text of Proposed Law, SEC. 4, amending Cal. Const. art. XIII, § 36(f)(2) to end the rates above 9.3% before January 1, 2031
+      href: https://vig.cdn.sos.ca.gov/2016/general/en/pdf/text-proposed-laws.pdf#page=16
+    - title: Cal. Rev. & Tax. Code § 17041(a)(1), (h)
       href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=17041.&lawCode=RTC
-      # Index according to federal tax parameter rules until we add California CPI trend.
+    - title: Cal. Rev. & Tax. Code § 17045 (joint and surviving spouse tax is twice the tax on one-half of taxable income)
+      href: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=17045.&lawCode=RTC
diff --git a/policyengine_us/reforms/reforms.py b/policyengine_us/reforms/reforms.py
index 46b0754e164..61fcd3050de 100644
--- a/policyengine_us/reforms/reforms.py
+++ b/policyengine_us/reforms/reforms.py
@@ -290,6 +290,9 @@
 from .states.ca.ab2591 import (
     create_ca_ab2591_reform,
 )
+from .states.ca.prop3 import (
+    create_ca_prop3_reform,
+)
 from .states.ga.sb520 import (
     create_ga_sb520_reform,
 )
@@ -562,6 +565,7 @@ def create_structural_reforms_from_parameters(parameters, period):
         parameters, period
     )
     ca_ab2591 = create_ca_ab2591_reform(parameters, period)
+    ca_prop3 = create_ca_prop3_reform(parameters, period)
     ga_sb520 = create_ga_sb520_reform(parameters, period)
     hi_hb2306_cdcc = create_hi_hb2306_cdcc_reform(parameters, period)
     nc_eitc = create_nc_eitc_reform(parameters, period)
@@ -712,6 +716,7 @@ def create_structural_reforms_from_parameters(parameters, period):
         tx_rebate,
         al_hb527_overtime_deduction,
         ca_ab2591,
+        ca_prop3,
         ga_sb520,
         hi_hb2306_cdcc,
         nc_eitc,
diff --git a/policyengine_us/reforms/states/ca/__init__.py b/policyengine_us/reforms/states/ca/__init__.py
index 3eb47243a2e..3fa280d85cc 100644
--- a/policyengine_us/reforms/states/ca/__init__.py
+++ b/policyengine_us/reforms/states/ca/__init__.py
@@ -1 +1,2 @@
 from .ab2591 import create_ca_ab2591_reform
+from .prop3 import create_ca_prop3_reform
diff --git a/policyengine_us/reforms/states/ca/prop3/__init__.py b/policyengine_us/reforms/states/ca/prop3/__init__.py
new file mode 100644
index 00000000000..cd68b87c7ee
--- /dev/null
+++ b/policyengine_us/reforms/states/ca/prop3/__init__.py
@@ -0,0 +1,5 @@
+from .ca_prop3_reform import (
+    create_ca_prop3,
+    create_ca_prop3_reform,
+    ca_prop3,
+)
diff --git a/policyengine_us/reforms/states/ca/prop3/ca_prop3_reform.py b/policyengine_us/reforms/states/ca/prop3/ca_prop3_reform.py
new file mode 100644
index 00000000000..b5219999a66
--- /dev/null
+++ b/policyengine_us/reforms/states/ca/prop3/ca_prop3_reform.py
@@ -0,0 +1,134 @@
+import functools
+
+from policyengine_us.model_api import *
+from policyengine_core.parameters import load_parameter_file
+from policyengine_core.periods import instant
+
+# Cal. Const. art. XIII, § 36(f)(2) (single, separate, and through RTC § 17045
+# joint and surviving spouse) and § 36(f)(3) (head of household) apply the
+# rates above 9.3% to taxable years before January 1, 2031. Proposition 3
+# (2026), SEC. 4, strikes that end date, so the rates in effect for 2030 keep
+# applying from 2031 onward.
+LAST_PRE_SUNSET_YEAR = instant("2030-01-01")
+SUNSET = instant("2031-01-01")
+HORIZON_END = instant("2100-12-31")
+FILING_STATUSES = [
+    "single",
+    "joint",
+    "separate",
+    "surviving_spouse",
+    "head_of_household",
+]
+
+
+def _active_windows(in_effect) -> list:
+    """Return the (start, stop) instants within SUNSET..HORIZON_END where the
+    dated values of ``in_effect`` are true.
+
+    ``values_list`` holds the user's dated overrides in reverse chronological
+    order; each value applies from its instant until the next later one.
+    """
+    windows = []
+    later_start = None
+    for value_at_instant in in_effect.values_list:
+        start = instant(value_at_instant.instant_str)
+        if value_at_instant.value:
+            window_start = max(start, SUNSET)
+            window_stop = HORIZON_END
+            if later_start is not None:
+                window_stop = min(later_start.offset(-1, "day"), HORIZON_END)
+            if window_start <= window_stop:
+                windows.append((window_start, window_stop))
+        later_start = start
+    return sorted(windows)
+
+
+@functools.lru_cache(maxsize=None)
+def _enacted_schedule(file_path: str):
+    # The schedule as enacted, read from the file the rates were loaded from,
+    # so the reform can tell the statutory sunset apart from a user's edit.
+    # The YAML must remain readable at this path, including after unpickling
+    # the system on another machine.
+    return load_parameter_file(file_path)
+
+
+def _constant_intervals(start, stop, *parameters) -> list:
+    """Split start..stop into the (start, stop) intervals on which every
+    parameter keeps one value."""
+    cuts = {start}
+    for parameter in parameters:
+        for value_at_instant in parameter.values_list:
+            cut = instant(value_at_instant.instant_str)
+            if start < cut <= stop:
+                cuts.add(cut)
+    cuts = sorted(cuts)
+    stops = [cut.offset(-1, "day") for cut in cuts[1:]] + [stop]
+    return list(zip(cuts, stops))
+
+
+def _restore_pre_sunset_rates(rates, enacted_rates: dict, windows: list) -> None:
+    """Within each window, give each bracket whose enacted rate the sunset
+    changes its 2030 rate, wherever the enacted post-sunset rate applies.
+
+    ``enacted_rates`` maps each filing status to its enacted schedule. Brackets
+    the sunset leaves unchanged keep any edit, including one bounded to 2030,
+    and so do intervals whose rate differs from the enacted rate, which a user
+    has set explicitly. Restored intervals no longer match the enacted rate, so
+    applying the reform again changes nothing.
+    """
+    updates = []
+    for status in FILING_STATUSES:
+        brackets = getattr(rates, status).brackets
+        for bracket, enacted_bracket in zip(brackets, enacted_rates[status].brackets):
+            rate = bracket.rate
+            enacted_rate = enacted_bracket.rate
+            if enacted_rate(SUNSET) == enacted_rate(LAST_PRE_SUNSET_YEAR):
+                continue
+            # The 2030 rate, including any rate the user set for 2030.
+            pre_sunset_rate = rate(LAST_PRE_SUNSET_YEAR)
+            for window_start, window_stop in windows:
+                for start, stop in _constant_intervals(
+                    window_start, window_stop, rate, enacted_rate
+                ):
+                    if rate(start) == enacted_rate(start):
+                        updates.append((rate, start, stop, pre_sunset_rate))
+    # Read every rate before writing any.
+    for rate, start, stop, value in updates:
+        rate.update(start=start, stop=stop, value=value)
+
+
+def create_ca_prop3(respect_in_effect: bool = False) -> Reform:
+    def modify_parameters(parameters):
+        if respect_in_effect:
+            windows = _active_windows(parameters.gov.contrib.states.ca.prop3.in_effect)
+        else:
+            windows = [(SUNSET, HORIZON_END)]
+        rates = parameters.gov.states.ca.tax.income.rates
+        enacted_rates = {
+            status: _enacted_schedule(getattr(rates, status).file_path)
+            for status in FILING_STATUSES
+        }
+        _restore_pre_sunset_rates(rates, enacted_rates, windows)
+        return parameters
+
+    class reform(Reform):
+        def apply(self):
+            self.modify_parameters(modify_parameters)
+
+    return reform
+
+
+def create_ca_prop3_reform(parameters, period, bypass: bool = False):
+    # The bypass instance, used by YAML `reforms:` tests, restores the rates
+    # for 2031-2100 regardless of in_effect. Otherwise the reform applies only
+    # in the years from 2031 onward in which in_effect is true.
+    if bypass:
+        return create_ca_prop3()
+
+    if _active_windows(parameters.gov.contrib.states.ca.prop3.in_effect):
+        return create_ca_prop3(respect_in_effect=True)
+    else:
+        return None
+
+
+ca_prop3 = create_ca_prop3_reform(None, None, bypass=True)
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/alternative_minimum_tax/ca_amt.yaml b/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/alternative_minimum_tax/ca_amt.yaml
index 181d814a34b..7674c753f4c 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/alternative_minimum_tax/ca_amt.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/alternative_minimum_tax/ca_amt.yaml
@@ -53,6 +53,24 @@
   output:
     ca_amt: 0
 
+- name: The 2031 rate expiry exposes AMT for a single filer with $1 million of taxable income
+  period: 2031
+  absolute_error_margin: 1
+  input:
+    state_code: CA
+    filing_status: SINGLE
+    ca_taxable_income: 1_000_000
+    ca_amti: 1_400_000
+    ca_amt_exemption: 0
+    ca_non_refundable_credits: 0
+  output:
+    # Regular tax falls to 88,942.90 once the rates above 9.3% expire (see
+    # ca_income_tax_before_credits.yaml). Tentative minimum tax is .07 x 1,400,000 = 98,000,
+    # so AMT is 98,000 - 88,942.90 = 9,057.10 and total tax is 98,000.
+    ca_income_tax_before_credits: 88_942.90
+    ca_amt: 9_057.10
+    ca_income_tax_before_refundable_credits: 98_000
+
 
 - name: Excess employee deductions do not create California AMT below the exemption
   # Schedule P line 15 uses -99,000, not the zero on Form 540 line 19.
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/ca_income_tax_before_credits.yaml b/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/ca_income_tax_before_credits.yaml
index 56b2bf12ede..c53e439957b 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/ca_income_tax_before_credits.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/ca_income_tax_before_credits.yaml
@@ -11,7 +11,7 @@
     ca_income_tax_before_credits: 464.61
 
 
-- name: Single filing, income in 3rd tax bracket 
+- name: Head of household with $1 million in 2021 reaches the 12.3% bracket
   period: 2021
   absolute_error_margin: 1
   input:
@@ -21,3 +21,89 @@
   output:
   # 82_046.3 + (12.30% of (1_000_000 - 850_503)) = 100_434.131
     ca_income_tax_before_credits: 100_434.131
+
+# 2030-2031 cases: thresholds are the FTB 2025 schedules uprated by gov.states.ca.cpi,
+# a factor of f2030 = 193 / 172.797 for 2030 and f2031 = 196.85 / 172.797 for 2031.
+# Single and separate tax at the 9.3% threshold is 3,201.97f (head of household 3,616.45f).
+# Joint and surviving spouse tax is twice the single tax on half the income (RTC § 17045).
+# The higher rates apply before January 1, 2031 (Cal. Const. art. XIII, § 36(f)(2)-(3)).
+# These are also the reform-off controls for contrib/states/ca/prop3/ca_prop3.yaml.
+
+- name: Single filer with $1 million in 2030 pays the 10.3%, 11.3% and 12.3% rates
+  period: 2030
+  absolute_error_margin: 1
+  input:
+    ca_taxable_income: 1_000_000
+    filing_status: SINGLE
+    state_code: CA
+  output:
+    # 3,201.97f + .093 x (1,000,000 - 72,724f) + .01 x (1,000,000 - 371,479f)
+    # + .01 x (1,000,000 - 445,771f) + .01 x (1,000,000 - 742,953f), f = f2030
+    ca_income_tax_before_credits: 101_596.07
+
+- name: Single filer with $1 million in 2031 pays 9.3% on all income above the 9.3% threshold
+  period: 2031
+  absolute_error_margin: 1
+  input:
+    ca_taxable_income: 1_000_000
+    filing_status: SINGLE
+    state_code: CA
+  output:
+    # 3,201.97f + .093 x (1,000,000 - 72,724f), f = f2031; the 9.3% threshold is 82,847.04.
+    ca_income_tax_before_credits: 88_942.90
+
+- name: Joint filers with $2 million in 2031 pay 9.3% on all income above the 9.3% threshold
+  period: 2031
+  absolute_error_margin: 1
+  input:
+    ca_taxable_income: 2_000_000
+    filing_status: JOINT
+    state_code: CA
+  output:
+    # 2 x 88,942.90, the single tax on $1 million.
+    ca_income_tax_before_credits: 177_885.81
+
+- name: Separate filer with $1 million in 2031 pays 9.3% on all income above the 9.3% threshold
+  period: 2031
+  absolute_error_margin: 1
+  input:
+    ca_taxable_income: 1_000_000
+    filing_status: SEPARATE
+    state_code: CA
+  output:
+    # Same schedule as single: 3,201.97f + .093 x (1,000,000 - 72,724f), f = f2031.
+    ca_income_tax_before_credits: 88_942.90
+
+- name: Surviving spouse with $2 million in 2031 pays 9.3% on all income above the 9.3% threshold
+  period: 2031
+  absolute_error_margin: 1
+  input:
+    ca_taxable_income: 2_000_000
+    filing_status: SURVIVING_SPOUSE
+    state_code: CA
+  output:
+    # Treated as a joint return (RTC § 17045): 2 x 88,942.90.
+    ca_income_tax_before_credits: 177_885.81
+
+- name: Head of household with $1.5 million in 2031 pays 9.3% on all income above the 9.3% threshold
+  period: 2031
+  absolute_error_margin: 1
+  input:
+    ca_taxable_income: 1_500_000
+    filing_status: HEAD_OF_HOUSEHOLD
+    state_code: CA
+  output:
+    # 3,616.45f + .093 x (1,500,000 - 98,990f), f = f2031.
+    ca_income_tax_before_credits: 133_132.32
+
+- name: Head of household with $1.5 million in 2030 pays the 10.3%, 11.3% and 12.3% rates
+  period: 2030
+  absolute_error_margin: 1
+  input:
+    ca_taxable_income: 1_500_000
+    filing_status: HEAD_OF_HOUSEHOLD
+    state_code: CA
+  output:
+    # 3,616.45f + .093 x (1,500,000 - 98,990f) + .01 x (1,500,000 - 505,208f)
+    # + .01 x (1,500,000 - 606,251f) + .01 x (1,500,000 - 1,010,417f), f = f2030
+    ca_income_tax_before_credits: 154_557.25
diff --git a/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/ca_income_tax_before_refundable_credits.yaml b/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/ca_income_tax_before_refundable_credits.yaml
index 09fac0645db..25e778aa531 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/ca_income_tax_before_refundable_credits.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/ca/tax/income/ca_income_tax_before_refundable_credits.yaml
@@ -33,6 +33,20 @@
     # (50000 - 1000) + 2000 + 5000 = 56000
     ca_income_tax_before_refundable_credits: 56_000
 
+- name: The 1% mental health services tax stacks on the 2031 regular tax
+  period: 2031
+  absolute_error_margin: 1
+  input:
+    state_code: CA
+    filing_status: SINGLE
+    ca_taxable_income: 2_000_000
+    ca_non_refundable_credits: 0
+    ca_amt: 0
+  output:
+    # Regular tax 3,201.97f + .093 x (2,000,000 - 72,724f) = 181,942.90 (f = 196.85 / 172.797),
+    # plus mental health services tax .01 x (2,000,000 - 1,000,000) = 10,000.
+    ca_income_tax_before_refundable_credits: 191_942.90
+
 # PolicyBench scenario_099, snapshot 20260501, tax year 2026. Exact inputs
 # copied from scenario_json via the PolicyBench household converter.
 # FTB Schedule CA (540), Section C, line 11, restores the spouse's $337.50
diff --git a/policyengine_us/tests/policy/contrib/states/ca/prop3/ca_prop3.yaml b/policyengine_us/tests/policy/contrib/states/ca/prop3/ca_prop3.yaml
new file mode 100644
index 00000000000..324fa8ecc43
--- /dev/null
+++ b/policyengine_us/tests/policy/contrib/states/ca/prop3/ca_prop3.yaml
@@ -0,0 +1,157 @@
+# California Proposition 3 (2026) keeps the 10.3%, 11.3% and 12.3% rates after 2030.
+#
+# The `reforms:` key applies the bypass instance of the reform (`ca_prop3`), which restores
+# the rates for 2031-2100 whatever gov.contrib.states.ca.prop3.in_effect says, so these cases
+# do not set the toggle. The app path, which reads the dated toggle, is tested in
+# test_ca_prop3_reform.py. The reform-off controls are the 2030 and 2031 cases in
+# baseline/gov/states/ca/tax/income/ca_income_tax_before_credits.yaml.
+#
+# Expected values apply the 2031 thresholds by hand: the FTB 2025 schedules uprated by
+# f = 196.85 / 172.797. The baseline single and separate 2031 tax is
+# 3,201.97f + .093 x (income - 72,724f); Prop 3 adds .01 x (income - t) above each of
+# t = 371,479f, 445,771f and 742,953f. Head of household uses 3,616.45f, 98,990f and
+# t = 505,208f, 606,251f and 1,010,417f. Joint and surviving spouse tax is twice the single
+# tax on half the income (RTC § 17045).
+
+- name: Prop 3 keeps the higher rates for a single filer with $1 million in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 1_000_000
+    filing_status: SINGLE
+    state_code: CA
+  output:
+    # Baseline 2031 is 88,942.90; Prop 3 adds 1% on 84,633.30, 2% on 338,549.15 and 3% on 153,629.41.
+    ca_income_tax_before_credits: 101_169.10
+
+- name: Prop 3 keeps the higher rates for a separate filer with $1 million in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 1_000_000
+    filing_status: SEPARATE
+    state_code: CA
+  output:
+    # Same schedule as single: 88,942.90 + 12,226.20.
+    ca_income_tax_before_credits: 101_169.10
+
+- name: Prop 3 keeps the higher rates for joint filers with $2 million in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 2_000_000
+    filing_status: JOINT
+    state_code: CA
+  output:
+    # 2 x 101,169.10, the single tax on $1 million.
+    ca_income_tax_before_credits: 202_338.20
+
+- name: Prop 3 keeps the higher rates for a surviving spouse with $2 million in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 2_000_000
+    filing_status: SURVIVING_SPOUSE
+    state_code: CA
+  output:
+    # Treated as a joint return: 2 x 101,169.10.
+    ca_income_tax_before_credits: 202_338.20
+
+- name: Prop 3 keeps the higher rates for a head of household with $1.5 million in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 1_500_000
+    filing_status: HEAD_OF_HOUSEHOLD
+    state_code: CA
+  output:
+    # Baseline 133,132.32 + .01 x (1,500,000 - 575,531.95) + .01 x (1,500,000 - 690,639.94)
+    # + .01 x (1,500,000 - 1,151,065.04) = 153,959.95.
+    ca_income_tax_before_credits: 153_959.95
+
+- name: Prop 3 applies only the 10.3% rate inside the first surcharge bracket in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 450_000
+    filing_status: SINGLE
+    state_code: CA
+  output:
+    # Baseline 37,792.90 + .01 x (450,000 - 423,188.14) = 38,061.02.
+    ca_income_tax_before_credits: 38_061.02
+
+- name: Prop 3 applies the 10.3% and 11.3% rates inside the second surcharge bracket in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 600_000
+    filing_status: SINGLE
+    state_code: CA
+  output:
+    # Baseline 51,742.90 + .01 x (600,000 - 423,188.14) + .01 x (600,000 - 507,821.44) = 54,432.81.
+    ca_income_tax_before_credits: 54_432.81
+
+- name: Prop 3 does not change tax below the 10.3% threshold in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 300_000
+    filing_status: SINGLE
+    state_code: CA
+  output:
+    # 3,201.97f + .093 x (300,000 - 72,724f), the same as the baseline.
+    ca_income_tax_before_credits: 23_842.90
+
+- name: Prop 3 does not change 2030 tax, when the higher rates already apply
+  period: 2030
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 1_000_000
+    filing_status: SINGLE
+    state_code: CA
+  output:
+    # Same as the 2030 baseline case.
+    ca_income_tax_before_credits: 101_596.07
+
+- name: Prop 3 removes the AMT that the 2031 rate expiry exposes
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 1_000_000
+    filing_status: SINGLE
+    state_code: CA
+    ca_amti: 1_400_000
+    ca_amt_exemption: 0
+    ca_non_refundable_credits: 0
+  output:
+    # Tentative minimum tax .07 x 1,400,000 = 98,000 is below the 101,169.10 regular tax,
+    # so AMT is 0 (the baseline AMT is 9,057.10; see alternative_minimum_tax/ca_amt.yaml).
+    ca_income_tax_before_credits: 101_169.10
+    ca_amt: 0
+    ca_income_tax_before_refundable_credits: 101_169.10
+
+- name: Prop 3 regular tax and the 1% mental health services tax stack for a single filer with $2 million in 2031
+  period: 2031
+  absolute_error_margin: 1
+  reforms: policyengine_us.reforms.states.ca.prop3.ca_prop3
+  input:
+    ca_taxable_income: 2_000_000
+    filing_status: SINGLE
+    state_code: CA
+    ca_non_refundable_credits: 0
+    ca_amt: 0
+  output:
+    # Regular tax 181,942.90 + .01 x (2,000,000 - 423,188.14) + .01 x (2,000,000 - 507,821.44)
+    # + .01 x (2,000,000 - 846,370.59) = 224,169.10, plus MHST .01 x 1,000,000 = 10,000.
+    ca_income_tax_before_credits: 224_169.10
+    ca_income_tax_before_refundable_credits: 234_169.10
diff --git a/policyengine_us/tests/policy/contrib/states/ca/prop3/test_ca_prop3_reform.py b/policyengine_us/tests/policy/contrib/states/ca/prop3/test_ca_prop3_reform.py
new file mode 100644
index 00000000000..690337e58af
--- /dev/null
+++ b/policyengine_us/tests/policy/contrib/states/ca/prop3/test_ca_prop3_reform.py
@@ -0,0 +1,479 @@
+"""Tests for the 2031 expiry of California's rates above 9.3% and for the
+Proposition 3 (2026) contributed reform that would repeal it.
+
+Cal. Const. art. XIII, § 36(f)(2) (single and separate, and through RTC
+§ 17045 joint and surviving spouse) and § 36(f)(3) (head of household) apply
+the 10.3%, 11.3% and 12.3% rates to taxable years before January 1, 2031.
+Proposition 3, SEC. 4, strikes that end date. The YAML cases check dollar
+amounts, which depend on CPI projections; these tests check properties that
+do not:
+
+- the rate ladder of every filing status in 2030, 2031, 2035 and 2100, under
+  the baseline and under the reform;
+- for any year and income, the reform changes no threshold and no rate at or
+  below 9.3%, adds exactly 1% above each of the three surcharge thresholds
+  from 2031 and nothing before, and keeps separate tax equal to single tax and
+  joint and surviving spouse tax at twice the single tax on half the income;
+- the app path (``Reform.from_dict`` on ``in_effect``) activates the reform
+  from a 2031 start and applies it only in the years ``in_effect`` is true;
+- the dated windows the reform reads from ``in_effect`` cover exactly the
+  instants from 2031 through 2100 at which ``in_effect`` is true;
+- the reform carries each bracket's 2030 rate forward, including a rate the
+  user set for 2030; and
+- whatever the user edits, the reform changes only the brackets the enacted
+  sunset changes, only inside the active windows and only where the enacted
+  post-sunset rate still applies, so it keeps explicit future rates, does not
+  extend edits to other brackets, gives the same rates on both construction
+  paths and changes nothing when applied again.
+"""
+
+import functools
+
+import numpy as np
+import pytest
+from hypothesis import HealthCheck, given, settings, strategies as st
+from policyengine_core.parameters import ParameterNode
+from policyengine_core.periods import instant
+from policyengine_core.reforms import Reform
+
+from policyengine_us import CountryTaxBenefitSystem, Simulation
+from policyengine_us.reforms.states.ca.prop3 import (
+    ca_prop3,
+    create_ca_prop3_reform,
+)
+from policyengine_us.reforms.states.ca.prop3.ca_prop3_reform import (
+    LAST_PRE_SUNSET_YEAR,
+    SUNSET,
+    _active_windows,
+    _enacted_schedule,
+    _restore_pre_sunset_rates,
+)
+from policyengine_us.system import system as baseline_system
+
+IN_EFFECT = "gov.contrib.states.ca.prop3.in_effect"
+STATUSES = ["single", "separate", "joint", "surviving_spouse", "head_of_household"]
+# RTC § 17041(a)(1) and (c)(1) rates, then the § 36(f)(2)-(3) surcharge rates.
+BASE_RATES = [0.01, 0.02, 0.04, 0.06, 0.08, 0.093]
+PRE_SUNSET_RATES = BASE_RATES + [0.103, 0.113, 0.123]
+POST_SUNSET_RATES = BASE_RATES + [0.093, 0.093, 0.093]
+SURCHARGE_BRACKETS = [6, 7, 8]
+INCOME = 1_000_000
+
+# Input generation can be descheduled on shared hosts; keep every example and
+# assertion without treating that wall-clock delay as a test failure.
+# Deterministic examples keep CI failures reproducible.
+PROPERTY_SETTINGS = dict(
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow, HealthCheck.data_too_large],
+)
+
+
+@functools.lru_cache(maxsize=None)
+def _bypass_system():
+    # The instance YAML `reforms:` tests use; it ignores in_effect.
+    return CountryTaxBenefitSystem(reform=ca_prop3)
+
+
+def _schedule(system, status, year):
+    return getattr(
+        system.parameters(f"{year}-01-01").gov.states.ca.tax.income.rates, status
+    )
+
+
+def _rates(system, status, year):
+    return list(_schedule(system, status, year).rates)
+
+
+def _tax(system, status, year, income):
+    return float(_schedule(system, status, year).calc(np.array([float(income)]))[0])
+
+
+def _situation(years):
+    years = [str(year) for year in years]
+    return {
+        "people": {"head": {"age": {year: 45 for year in years}}},
+        "tax_units": {
+            "tax_unit": {
+                "members": ["head"],
+                "filing_status": {year: "SINGLE" for year in years},
+                "ca_taxable_income": {year: INCOME for year in years},
+            }
+        },
+        "households": {
+            "household": {
+                "members": ["head"],
+                "state_code": {year: "CA" for year in years},
+            }
+        },
+    }
+
+
+def _app_simulation(in_effect_values, years, other_values=None):
+    # Mirrors how the app applies the toggle: a dated Reform.from_dict, with the
+    # default start_instant.
+    reform = Reform.from_dict(
+        {IN_EFFECT: in_effect_values, **(other_values or {})},
+        country_id="us",
+    )
+    return Simulation(situation=_situation(years), reform=reform)
+
+
+def _assert_active_years(simulation, years, active_years):
+    for year in years:
+        # Before 2031 the baseline already applies the higher rates.
+        keeps_higher_rates = year < 2031 or year in active_years
+        expected_rates = PRE_SUNSET_RATES if keeps_higher_rates else POST_SUNSET_RATES
+        expected_system = _bypass_system() if keeps_higher_rates else baseline_system
+        for status in STATUSES:
+            assert _rates(simulation.tax_benefit_system, status, year) == (
+                pytest.approx(expected_rates)
+            ), (status, year)
+        tax = simulation.calculate("ca_income_tax_before_credits", year)[0]
+        if year >= 2031:
+            # The reform raises tax on $1 million in every year from 2031, so
+            # the two references differ and the comparison identifies the
+            # regime.
+            assert _tax(_bypass_system(), "single", year, INCOME) > _tax(
+                baseline_system, "single", year, INCOME
+            )
+        assert tax == pytest.approx(
+            _tax(expected_system, "single", year, INCOME), abs=0.01
+        ), year
+
+
+@pytest.mark.parametrize("status", STATUSES)
+def test_ca_prop3_rates_all_statuses_and_years(status):
+    for year in [2030, 2031, 2035, 2100]:
+        expected_baseline = PRE_SUNSET_RATES if year < 2031 else POST_SUNSET_RATES
+        assert _rates(baseline_system, status, year) == pytest.approx(
+            expected_baseline
+        ), year
+        # Proposition 3 keeps the 2030 rates in every year through 2100.
+        assert _rates(_bypass_system(), status, year) == pytest.approx(
+            PRE_SUNSET_RATES
+        ), year
+
+
+@settings(max_examples=300, **PROPERTY_SETTINGS)
+@given(
+    year=st.integers(min_value=2025, max_value=2040),
+    income=st.floats(min_value=0, max_value=50_000_000, allow_nan=False),
+)
+def test_ca_prop3_preserves_thresholds_lower_rates_and_joint_scaling(year, income):
+    reformed_system = _bypass_system()
+    for status in STATUSES:
+        baseline = _schedule(baseline_system, status, year)
+        reformed = _schedule(reformed_system, status, year)
+        # Proposition 3 changes no threshold and no rate at or below 9.3%.
+        assert list(reformed.thresholds) == list(baseline.thresholds)
+        assert list(reformed.rates)[:6] == list(baseline.rates)[:6]
+        # From 2031 it adds 1% above each surcharge threshold; before, nothing.
+        thresholds = list(baseline.thresholds)
+        expected_change = (
+            sum(0.01 * max(income - thresholds[i], 0) for i in SURCHARGE_BRACKETS)
+            if year >= 2031
+            else 0
+        )
+        change = _tax(reformed_system, status, year, income) - _tax(
+            baseline_system, status, year, income
+        )
+        assert change >= 0
+        assert change == pytest.approx(expected_change, rel=1e-9, abs=1e-6)
+    for system in [baseline_system, reformed_system]:
+        single = _tax(system, "single", year, income)
+        # RTC § 17041(a)(1) applies the single schedule to separate returns.
+        assert _tax(system, "separate", year, income) == pytest.approx(
+            single, rel=1e-9, abs=1e-6
+        )
+        # RTC § 17045: joint tax is twice the tax on half the income, and a
+        # surviving spouse return is treated as a joint return.
+        for status in ["joint", "surviving_spouse"]:
+            assert _tax(system, status, year, 2 * income) == pytest.approx(
+                2 * single, rel=1e-9, abs=1e-6
+            )
+
+
+def test_ca_prop3_app_activation_from_2031():
+    years = [2030, 2031, 2035]
+    simulation = _app_simulation({"2031-01-01.2100-12-31": True}, years)
+    _assert_active_years(simulation, years, active_years={2031, 2035})
+    assert simulation.calculate("ca_income_tax_before_credits", 2031)[
+        0
+    ] == pytest.approx(101_169.10, abs=1)
+    # With the toggle off, 2031 tax is the baseline.
+    baseline = Simulation(situation=_situation(years))
+    assert baseline.calculate("ca_income_tax_before_credits", 2031)[0] == pytest.approx(
+        88_942.90, abs=1
+    )
+
+
+@pytest.mark.parametrize(
+    "in_effect_values, active_years",
+    [
+        # True only in 2026, before the sunset: nothing to restore.
+        ({"2026-01-01.2026-12-31": True}, set()),
+        # A delayed start leaves 2031-2032 at the baseline.
+        ({"2033-01-01.2100-12-31": True}, {2033, 2034, 2035}),
+        # True, then false in 2033-2034, then true again.
+        (
+            {"2031-01-01.2032-12-31": True, "2035-01-01.2100-12-31": True},
+            {2031, 2032, 2035},
+        ),
+    ],
+)
+def test_ca_prop3_respects_delayed_and_bounded_activation(
+    in_effect_values, active_years
+):
+    years = [2030, 2031, 2032, 2033, 2034, 2035]
+    simulation = _app_simulation(in_effect_values, years)
+    _assert_active_years(simulation, years, active_years)
+    reform = create_ca_prop3_reform(
+        simulation.tax_benefit_system.parameters, "2024-01-01"
+    )
+    assert (reform is None) == (not active_years)
+
+
+def test_ca_prop3_copies_2030_user_rates():
+    simulation = _app_simulation(
+        {"2031-01-01.2100-12-31": True},
+        [2030, 2031, 2035],
+        other_values={
+            "gov.states.ca.tax.income.rates.single[6].rate": {
+                "2030-01-01.2030-12-31": 0.104
+            }
+        },
+    )
+    rates = simulation.tax_benefit_system.parameters.gov.states.ca.tax.income.rates
+    for year in [2030, 2031, 2035]:
+        assert rates.single.brackets[6].rate(f"{year}-01-01") == pytest.approx(0.104)
+        # Other schedules keep the statutory 10.3%.
+        assert rates.separate.brackets[6].rate(f"{year}-01-01") == pytest.approx(0.103)
+    tax_2031 = simulation.calculate("ca_income_tax_before_credits", 2031)[0]
+    single_2031 = _schedule(_bypass_system(), "single", 2031)
+    bracket_6_income = single_2031.thresholds[7] - single_2031.thresholds[6]
+    assert tax_2031 == pytest.approx(
+        _tax(_bypass_system(), "single", 2031, INCOME) + 0.001 * bracket_6_income,
+        abs=0.01,
+    )
+
+
+def test_ca_prop3_enacted_schedules_match_baseline_rates():
+    # The reform reads each enacted schedule from the file its rates were
+    # loaded from. The parameter pipeline does not change rates, so the file
+    # and the baseline agree in every year.
+    rates = baseline_system.parameters.gov.states.ca.tax.income.rates
+    for status in STATUSES:
+        schedule = getattr(rates, status)
+        enacted = _enacted_schedule(schedule.file_path)
+        assert len(enacted.brackets) == len(schedule.brackets)
+        for year in range(2021, 2102):
+            date = f"{year}-01-01"
+            assert [bracket.rate(date) for bracket in enacted.brackets] == [
+                bracket.rate(date) for bracket in schedule.brackets
+            ], (status, year)
+
+
+def test_ca_prop3_keeps_user_edits_on_both_construction_paths():
+    rate = "gov.states.ca.tax.income.rates.{}.rate"
+    reform = Reform.from_dict(
+        {
+            IN_EFFECT: {"2033-01-01.2100-12-31": True},
+            # A 2030 surcharge rate is carried into the active years.
+            rate.format("single[6]"): {"2030-01-01.2030-12-31": 0.104},
+            # A 2031-only rate equal to the 2030 rate: the sunset still applies
+            # in 2032, and the reform removes it from 2033.
+            rate.format("separate[6]"): {"2031-01-01.2031-12-31": 0.103},
+            # A 2030-only edit to a bracket the sunset leaves alone is not
+            # extended.
+            rate.format("joint[1]"): {"2030-01-01.2030-12-31": 0.021},
+            # Explicit rates inside the active years are kept.
+            rate.format("head_of_household[7]"): {"2033-01-01.2033-12-31": 0.12},
+            rate.format("head_of_household[8]"): {"2035-01-01.2100-12-31": 0.15},
+        },
+        country_id="us",
+    )
+    expected = {
+        ("single", 6): {
+            2030: 0.104,
+            2031: 0.093,
+            2032: 0.093,
+            2033: 0.104,
+            2036: 0.104,
+        },
+        ("separate", 6): {
+            2030: 0.103,
+            2031: 0.103,
+            2032: 0.093,
+            2033: 0.103,
+            2036: 0.103,
+        },
+        ("joint", 1): {2030: 0.021, 2031: 0.02, 2033: 0.02, 2036: 0.02},
+        ("head_of_household", 7): {2032: 0.093, 2033: 0.12, 2034: 0.113, 2036: 0.113},
+        ("head_of_household", 8): {
+            2032: 0.093,
+            2033: 0.123,
+            2034: 0.123,
+            2035: 0.15,
+            2036: 0.15,
+        },
+        ("surviving_spouse", 8): {2030: 0.123, 2032: 0.093, 2033: 0.123, 2036: 0.123},
+    }
+    simulation = Simulation(situation=_situation([2031, 2033]), reform=reform)
+    # A bare system applies the structural reform once; Simulation applies it
+    # again after replaying the user reform.
+    for system in [
+        CountryTaxBenefitSystem(reform=reform),
+        simulation.tax_benefit_system,
+    ]:
+        rates = system.parameters.gov.states.ca.tax.income.rates
+        for (status, index), by_year in expected.items():
+            bracket_rate = getattr(rates, status).brackets[index].rate
+            for year, value in by_year.items():
+                assert bracket_rate(f"{year}-01-01") == pytest.approx(value), (
+                    status,
+                    index,
+                    year,
+                )
+    # 2031 tax is the baseline; 2033 tax uses the carried 10.4% rate.
+    assert simulation.calculate("ca_income_tax_before_credits", 2031)[
+        0
+    ] == pytest.approx(_tax(baseline_system, "single", 2031, INCOME), abs=0.01)
+    single_2033 = _schedule(_bypass_system(), "single", 2033)
+    bracket_6_income = single_2033.thresholds[7] - single_2033.thresholds[6]
+    assert simulation.calculate("ca_income_tax_before_credits", 2033)[
+        0
+    ] == pytest.approx(
+        _tax(_bypass_system(), "single", 2033, INCOME) + 0.001 * bracket_6_income,
+        abs=0.01,
+    )
+
+
+def _around(date):
+    return {date.offset(-1, "day"), date, date.offset(1, "day")}
+
+
+def _toggle(updates):
+    """Build an in_effect parameter from dated updates; also return the dates
+    at which its value can change."""
+    node = ParameterNode("prop3", data={"in_effect": {"values": {"0000-01-01": False}}})
+    in_effect = node.in_effect
+    boundaries = set()
+    for start_year, start_month, length, value in updates:
+        start = instant(f"{start_year}-{start_month:02d}-01")
+        stop = None if length is None else instant(f"{start_year + length}-12-31")
+        in_effect.update(start=start, stop=stop, value=value)
+        boundaries |= _around(start)
+        if stop is not None:
+            boundaries |= _around(stop)
+    return in_effect, boundaries
+
+
+# Dated toggle schedules: (start year, start month, end year or None, value).
+UPDATES = st.lists(
+    st.tuples(
+        st.integers(min_value=2020, max_value=2105),
+        st.sampled_from([1, 7]),
+        st.one_of(st.none(), st.integers(min_value=0, max_value=30)),
+        st.booleans(),
+    ),
+    max_size=5,
+)
+
+
+@settings(max_examples=300, **PROPERTY_SETTINGS)
+@given(updates=UPDATES)
+def test_ca_prop3_windows_match_dated_in_effect(updates):
+    in_effect, boundaries = _toggle(updates)
+    windows = _active_windows(in_effect)
+    # Windows are sorted, disjoint and inside 2031-01-01 through 2100-12-31.
+    for (start, stop), (next_start, _) in zip(windows, windows[1:]):
+        assert stop < next_start
+    for start, stop in windows:
+        assert instant("2031-01-01") <= start <= stop <= instant("2100-12-31")
+    # They cover exactly the dates from 2031 through 2100 at which the toggle
+    # is true, checked on the 1st of January and July of each year and on
+    # each side of every date at which the toggle or the horizon changes.
+    probes = {
+        instant(f"{year}-{month:02d}-01")
+        for year in range(2025, 2106)
+        for month in [1, 7]
+    }
+    probes |= boundaries | _around(SUNSET) | _around(instant("2101-01-01"))
+    for date in sorted(probes):
+        covered = any(start <= date <= stop for start, stop in windows)
+        expected = SUNSET <= date <= instant("2100-12-31") and bool(in_effect(date))
+        assert covered == expected, (date, windows)
+
+
+# User edits to bracket rates: (status, bracket, start year, length in years
+# or None for open-ended, rate).
+EDITS = st.lists(
+    st.tuples(
+        st.sampled_from(STATUSES),
+        st.integers(min_value=0, max_value=8),
+        st.integers(min_value=2028, max_value=2040),
+        st.one_of(st.none(), st.integers(min_value=0, max_value=5)),
+        st.sampled_from([0.02, 0.021, 0.093, 0.103, 0.104, 0.113, 0.15]),
+    ),
+    max_size=6,
+)
+
+
+@settings(max_examples=150, **PROPERTY_SETTINGS)
+@given(edits=EDITS, updates=UPDATES)
+def test_ca_prop3_restores_only_the_enacted_sunset(edits, updates):
+    baseline_rates = baseline_system.parameters.gov.states.ca.tax.income.rates
+    # A detached copy, so the edits stay out of the shared baseline tree.
+    rates = baseline_rates.clone()
+    rates.parent = None
+    probes = _around(LAST_PRE_SUNSET_YEAR) | _around(SUNSET)
+    probes |= _around(instant("2101-01-01"))
+    for status, index, start_year, length, value in edits:
+        start = instant(f"{start_year}-01-01")
+        stop = None if length is None else instant(f"{start_year + length}-12-31")
+        getattr(rates, status).brackets[index].rate.update(
+            start=start, stop=stop, value=value
+        )
+        probes |= _around(start)
+        if stop is not None:
+            probes |= _around(stop)
+    edited = rates.clone()
+    in_effect, boundaries = _toggle(updates)
+    windows = _active_windows(in_effect)
+    probes |= boundaries
+    enacted = {
+        status: _enacted_schedule(getattr(rates, status).file_path)
+        for status in STATUSES
+    }
+    _restore_pre_sunset_rates(rates, enacted, windows)
+    restored = rates.clone()
+    _restore_pre_sunset_rates(rates, enacted, windows)
+    # Every interval on which the edits, windows and enacted rates are constant
+    # contains a probe, since the probes include both sides of every change.
+    probes = sorted(date for date in probes if date >= LAST_PRE_SUNSET_YEAR)
+    for status in STATUSES:
+        for index in range(9):
+            baseline_rate = getattr(baseline_rates, status).brackets[index].rate
+            edited_rate = getattr(edited, status).brackets[index].rate
+            changed_by_sunset = baseline_rate(SUNSET) != baseline_rate(
+                LAST_PRE_SUNSET_YEAR
+            )
+            assert changed_by_sunset == (index in SURCHARGE_BRACKETS)
+            for date in probes:
+                in_window = any(start <= date <= stop for start, stop in windows)
+                if (
+                    changed_by_sunset
+                    and in_window
+                    and edited_rate(date) == baseline_rate(date)
+                ):
+                    expected = edited_rate(LAST_PRE_SUNSET_YEAR)
+                else:
+                    expected = edited_rate(date)
+                assert getattr(restored, status).brackets[index].rate(date) == (
+                    expected
+                ), (status, index, date)
+                # Applying the reform again changes nothing.
+                assert getattr(rates, status).brackets[index].rate(date) == (
+                    expected
+                ), (status, index, date)