diff --git a/Gemfile.lock b/Gemfile.lock
index 2d3f618..3402f94 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -465,6 +465,7 @@ PLATFORMS
   arm-linux-musl
   arm64-darwin-23
   arm64-darwin-24
+  arm64-darwin-25
   x86_64-linux
   x86_64-linux-gnu
   x86_64-linux-musl
diff --git a/app/javascript/controllers/one_time_amount_controller.js b/app/javascript/controllers/one_time_amount_controller.js
new file mode 100644
index 0000000..a635c20
--- /dev/null
+++ b/app/javascript/controllers/one_time_amount_controller.js
@@ -0,0 +1,33 @@
+import { Controller } from "@hotwired/stimulus"
+
+const currency = new Intl.NumberFormat("en-US", {
+  style: "currency",
+  currency: "USD",
+  maximumFractionDigits: 0,
+})
+
+// Keeps a one-time giving amount within the scenario's remaining giving budget.
+// Sets a custom-validity message so the browser blocks submission (via native
+// constraint validation, regardless of number-input support) and shows a
+// friendly error without closing the modal.
+export default class extends Controller {
+  static values = { max: Number }
+
+  connect() {
+    this.element.addEventListener("input", this.validate)
+    this.validate()
+  }
+
+  disconnect() {
+    this.element.removeEventListener("input", this.validate)
+  }
+
+  validate = () => {
+    const overMaxValue = this.element.value !== "" && Number(this.element.value) > this.maxValue
+    this.element.setCustomValidity(overMaxValue ? this.message : "")
+  }
+
+  get message() {
+    return `Enter an amount of ${currency.format(this.maxValue)} or less — that's your remaining one-time giving budget.`
+  }
+}
\ No newline at end of file
diff --git a/app/models/allocation/one_time.rb b/app/models/allocation/one_time.rb
index 2df868a..d81eee1 100644
--- a/app/models/allocation/one_time.rb
+++ b/app/models/allocation/one_time.rb
@@ -19,14 +19,36 @@ def share_percentage
     (amount.to_i / total.to_f * 100).round
   end
 
+  # The most this allocation can be set to while staying within the scenario's
+  # total giving budget. Returns nil when no budget is set (the server imposes
+  # no cap there either). Never drops below the allocation's own amount so
+  # pre-existing over-allocated data stays editable. Used by the view helper so
+  # the slider cap and the server validator stay in sync.
+  def max_amount(scenario = self.scenario)
+    remaining = budget_remaining(scenario)
+    return if remaining.nil?
+
+    [ remaining, amount.to_i ].max
+  end
+
   private
 
   def within_total_giving_amount
     return if amount.blank? || scenario&.total_giving_amount.blank?
 
-    others = scenario.one_time_allocations.where.not(id: id).sum(:amount)
-    if others + amount > scenario.total_giving_amount
+    remaining = budget_remaining(scenario)
+    return if remaining.nil?
+
+    if amount > remaining
+      others = scenario.total_giving_amount - remaining
       errors.add(:amount, "would bring one-time giving to #{others + amount}, over the total giving amount of #{scenario.total_giving_amount.to_i}")
     end
   end
+
+  def budget_remaining(scenario = self.scenario)
+    return nil if scenario.blank? || scenario.total_giving_amount.blank?
+
+    others = scenario.one_time_allocations.where.not(id: id).sum(:amount)
+    scenario.total_giving_amount - others
+  end
 end
diff --git a/app/views/scenarios/_allocation_modal.html.erb b/app/views/scenarios/_allocation_modal.html.erb
index b8e094f..3dfd9b8 100644
--- a/app/views/scenarios/_allocation_modal.html.erb
+++ b/app/views/scenarios/_allocation_modal.html.erb
@@ -26,13 +26,14 @@
               class: "allocation-slider block mt-2" %>
       </div>
     <% else %>
+      <% max_amount = allocation.max_amount(scenario) if klass == Allocation::OneTime %>
       <div class="mt-6">
         <%= label_tag "allocation_amount_#{suffix}", "Amount", class: "block text-sm text-ink-soft" %>
         <div class="relative mt-1">
           <span class="absolute left-3 top-1/2 -translate-y-1/2 text-ink-faint">$</span>
-          <%= number_field_tag "allocation[amount]", allocation.amount, id: "allocation_amount_#{suffix}", min: 0, step: "1", required: true, placeholder: "0",
+          <%= number_field_tag "allocation[amount]", allocation.amount, id: "allocation_amount_#{suffix}", min: 0, max: max_amount, step: "1", required: true, placeholder: "0",
                 inputmode: "numeric",
-                data: { controller: "integer-input", action: "input->integer-input#transform" },
+                data: max_amount.present? ? { controller: "integer-input one-time-amount", action: "input->integer-input#transform", one_time_amount_max_value: max_amount } : { controller: "integer-input", action: "input->integer-input#transform" },
                 class: "block w-full rounded-md border border-line bg-surface pl-7 pr-3 py-2 shadow-sm focus:border-accent focus:outline-none focus:ring-2 focus:ring-accent/10" %>
         </div>
       </div>
diff --git a/test/controllers/allocations_controller_test.rb b/test/controllers/allocations_controller_test.rb
index 7900eae..9ebd078 100644
--- a/test/controllers/allocations_controller_test.rb
+++ b/test/controllers/allocations_controller_test.rb
@@ -56,6 +56,14 @@ class AllocationsControllerTest < ActionDispatch::IntegrationTest
     assert_match Allocation::GreatestCommunityNeed::DESCRIPTION, response.body
   end
 
+  test "caps the one-time amount field at the remaining giving budget" do
+    # scenario total is 10000 and education_grant fixture already allocates 5000.
+    get scenario_url(@scenario)
+    assert_response :success
+    assert_match %r{id="allocation_amount_one_time"[^>]*max="5000"}, response.body
+    assert_match %r{data-one-time-amount-max-value="5000"}, response.body
+  end
+
   test "rejects a duplicate Greatest Community Need allocation" do
     # one_arlington already has the greatest_need fixture allocation.
     assert_no_difference -> { @scenario.allocations.count } do
diff --git a/test/models/allocation/one_time_test.rb b/test/models/allocation/one_time_test.rb
index 8b6e232..89f9212 100644
--- a/test/models/allocation/one_time_test.rb
+++ b/test/models/allocation/one_time_test.rb
@@ -9,4 +9,34 @@ class Allocation::OneTimeTest < ActiveSupport::TestCase
     allocation = Allocation::OneTime.new(amount: 500, scenario: scenarios(:two_boston))
     assert_equal 0, allocation.share_percentage
   end
+
+  test "max_amount is the remaining budget for a new allocation" do
+    scenario = scenarios(:one_arlington)
+    allocation = Allocation::OneTime.new
+
+    assert_equal 5000, allocation.max_amount(scenario)
+  end
+
+  test "max_amount excludes the allocation's own amount when editing" do
+    scenario = scenarios(:one_arlington)
+    allocation = allocations(:education_grant)
+
+    assert_equal 10000, allocation.max_amount(scenario)
+  end
+
+  test "max_amount never drops below the allocation's own amount" do
+    scenario = scenarios(:one_arlington)
+    scenario.update!(total_giving_amount: 4000)
+    allocation = allocations(:education_grant)
+
+    assert_equal 5000, allocation.max_amount(scenario)
+  end
+
+  test "max_amount is nil when no total giving amount is set" do
+    scenario = scenarios(:two_boston)
+    scenario.update!(total_giving_amount: nil)
+    allocation = Allocation::OneTime.new
+
+    assert_nil allocation.max_amount(scenario)
+  end
 end