diff --git a/packages/react/test-app/Pages/FormHelper/Data.tsx b/packages/react/test-app/Pages/FormHelper/Data.tsx
index a867ee549..26b408afb 100644
--- a/packages/react/test-app/Pages/FormHelper/Data.tsx
+++ b/packages/react/test-app/Pages/FormHelper/Data.tsx
@@ -1,10 +1,12 @@
 import { useForm, usePage } from '@inertiajs/react'
+import { useEffect, useState } from 'react'
 
 export default ({ errors }: { errors?: { name?: string; handle?: string } }) => {
   const form = useForm({
     name: 'foo',
     handle: 'example',
     remember: false,
+    custom: {},
   })
 
   const page = usePage()
@@ -33,6 +35,23 @@ export default ({ errors }: { errors?: { name?: string; handle?: string } }) =>
     form.setDefaults('name', 'single value')
   }
 
+  const addCustomOtherProp = () => {
+    form.setData((prevData) => ({
+      ...prevData,
+      custom: {
+        ...prevData.custom,
+        other_prop: 'dynamic_value',
+      },
+    }))
+  }
+
+  const [formDataOutput, setFormDataOutput] = useState('')
+
+  // Effect to watch form.data and update formDataOutput
+  useEffect(() => {
+    setFormDataOutput(JSON.stringify(form.data))
+  }, [form.data])
+
   return (
     <div>
       <label>
@@ -69,6 +88,18 @@ export default ({ errors }: { errors?: { name?: string; handle?: string } }) =>
       </label>
       {form.errors.remember && <span className="remember_error">{form.errors.remember}</span>}
 
+      <label>
+        Accept Terms and Conditions
+        <input
+          type="checkbox"
+          id="accept_tos"
+          name="accept_tos"
+          onChange={(e) => form.setData('accept_tos', e.target.checked)}
+          checked={!!form.data.accept_tos}
+        />
+      </label>
+      {form.errors.accept_tos && <span className="accept_tos_error">{form.errors.accept_tos}</span>}
+
       <button onClick={submit} className="submit">
         Submit form
       </button>
@@ -90,7 +121,15 @@ export default ({ errors }: { errors?: { name?: string; handle?: string } }) =>
         Reassign single default
       </button>
 
+      <button onClick={addCustomOtherProp} className="add-custom-other-prop">
+        Add custom.other_prop
+      </button>
+
       <span className="errors-status">Form has {form.hasErrors ? '' : 'no '}errors</span>
+
+      <div id="form-data-output" data-test-id="form-data-output" style={{ display: 'none' }}>
+        {formDataOutput}
+      </div>
     </div>
   )
 }
diff --git a/packages/svelte/src/useForm.ts b/packages/svelte/src/useForm.ts
index 6d8df8095..8ee7fe611 100644
--- a/packages/svelte/src/useForm.ts
+++ b/packages/svelte/src/useForm.ts
@@ -74,8 +74,10 @@ export default function useForm<TForm extends FormDataType<TForm>>(
   let recentlySuccessfulTimeoutId: ReturnType<typeof setTimeout> | null = null
   let transform = (data: TForm) => data as object
 
-  const store = writable<InertiaForm<TForm>>({
-    ...(restored ? restored.data : data),
+  const initialData = restored ? restored.data : cloneDeep(defaults)
+
+  const formObject: InertiaForm<TForm> = {
+    ...initialData,
     isDirty: false,
     errors: (restored ? restored.errors : {}) as FormDataErrors<TForm>,
     hasErrors: false,
@@ -89,7 +91,10 @@ export default function useForm<TForm extends FormDataType<TForm>>(
       })
     },
     data() {
-      return Object.keys(data).reduce((carry, key) => {
+      return (Object.keys(this) as Array<FormDataKeys<TForm>>).reduce((carry, key) => {
+        if (RESERVED_KEYS.includes(key)) {
+          return carry
+        }
         return set(carry, key, get(this, key))
       }, {} as TForm)
     },
@@ -257,9 +262,13 @@ export default function useForm<TForm extends FormDataType<TForm>>(
     cancel() {
       cancelToken?.cancel()
     },
-  } as InertiaForm<TForm>)
+  } as InertiaForm<TForm>
+
+  const store = writable<InertiaForm<TForm>>(formObject)
+
+  const RESERVED_KEYS = Object.keys(formObject).filter((key) => !(key in initialData))
 
-  store.subscribe((form) => {
+  store.subscribe((form: InertiaForm<TForm>) => {
     if (form.isDirty === isEqual(form.data(), defaults)) {
       form.setStore('isDirty', !form.isDirty)
     }
diff --git a/packages/svelte/test-app/Pages/FormHelper/Data.svelte b/packages/svelte/test-app/Pages/FormHelper/Data.svelte
index 81f827a0b..776280969 100644
--- a/packages/svelte/test-app/Pages/FormHelper/Data.svelte
+++ b/packages/svelte/test-app/Pages/FormHelper/Data.svelte
@@ -5,6 +5,7 @@
     name: 'foo',
     handle: 'example',
     remember: false,
+    custom: {},
   })
 
   const submit = () => {
@@ -33,6 +34,22 @@
   const reassignSingle = () => {
     $form.defaults('name', 'single value')
   }
+
+  // New functions for dynamic properties
+  const addAcceptTos = () => {
+    $form.accept_tos = true // Add root-level dynamic property
+  }
+
+  const addCustomOtherProp = () => {
+    $form.custom.other_prop = 'dynamic_value' // Add nested dynamic property
+  }
+
+  // Reactive property to hold the stringified form.data() output
+  import { writable } from 'svelte/store'
+  const formDataOutput = writable('')
+
+  // Watch $form.data() and update formDataOutput
+  $: formDataOutput.set(JSON.stringify($form.data()))
 </script>
 
 <div>
@@ -58,6 +75,14 @@
     <span class="remember_error">{$form.errors.remember}</span>
   {/if}
 
+  <label>
+    Accept Terms and Conditions
+    <input type="checkbox" id="accept_tos" name="accept_tos" on:change={() => ($form.accept_tos = true)} />
+  </label>
+  {#if $form.errors.accept_tos}
+    <span class="accept_tos_error">{$form.errors.accept_tos}</span>
+  {/if}
+
   <button on:click={submit} class="submit">Submit form</button>
 
   <button on:click={resetAll} class="reset">Reset all data</button>
@@ -67,5 +92,10 @@
   <button on:click={reassignObject} class="reassign-object">Reassign default values</button>
   <button on:click={reassignSingle} class="reassign-single">Reassign single default</button>
 
+  <button on:click={addCustomOtherProp} class="add-custom-other-prop">Add custom.other_prop</button>
+
   <span class="errors-status">Form has {$form.hasErrors ? '' : 'no '}errors</span>
+
+  <!-- Hidden div to display form.data() output for Playwright -->
+  <div id="form-data-output" data-test-id="form-data-output" style="display: none">{$formDataOutput}</div>
 </div>
diff --git a/packages/vue3/src/useForm.ts b/packages/vue3/src/useForm.ts
index bbb0f58e5..b6c107547 100644
--- a/packages/vue3/src/useForm.ts
+++ b/packages/vue3/src/useForm.ts
@@ -63,8 +63,10 @@ export default function useForm<TForm extends FormDataType<TForm>>(
   let recentlySuccessfulTimeoutId = null
   let transform = (data) => data
 
+  const initialData = restored ? restored.data : cloneDeep(defaults)
+
   const form = reactive({
-    ...(restored ? restored.data : cloneDeep(defaults)),
+    ...initialData,
     isDirty: false,
     errors: (restored ? restored.errors : {}) as FormDataErrors<TForm>,
     hasErrors: false,
@@ -73,7 +75,10 @@ export default function useForm<TForm extends FormDataType<TForm>>(
     wasSuccessful: false,
     recentlySuccessful: false,
     data() {
-      return (Object.keys(defaults) as Array<FormDataKeys<TForm>>).reduce((carry, key) => {
+      return (Object.keys(this) as Array<FormDataKeys<TForm>>).reduce((carry, key) => {
+        if (RESERVED_KEYS.includes(key)) {
+          return carry
+        }
         return set(carry, key, get(this, key))
       }, {} as Partial<TForm>) as TForm
     },
@@ -258,6 +263,8 @@ export default function useForm<TForm extends FormDataType<TForm>>(
     },
   })
 
+  const RESERVED_KEYS = Object.keys(form).filter((key) => !(key in initialData))
+
   watch(
     form,
     (newValue) => {
diff --git a/packages/vue3/test-app/Pages/FormHelper/DataDynamic.vue b/packages/vue3/test-app/Pages/FormHelper/DataDynamic.vue
new file mode 100644
index 000000000..36a862cbb
--- /dev/null
+++ b/packages/vue3/test-app/Pages/FormHelper/DataDynamic.vue
@@ -0,0 +1,54 @@
+<script setup>
+import { useForm, usePage } from '@inertiajs/vue3'
+import { reactive, watch } from 'vue'
+
+const form = useForm({
+  name: 'foo',
+  handle: 'example',
+  remember: false,
+  custom: {},
+})
+
+const page = usePage()
+
+const submit = () => {
+  form.post(page.url)
+}
+
+const addCustomOtherProp = () => {
+  form.custom.other_prop = 'dynamic_value' // Add nested dynamic property
+}
+
+const formDataOutput = reactive({
+  json: '',
+})
+watch(
+  () => form.data(),
+  (newData) => {
+    formDataOutput.json = JSON.stringify(newData)
+  },
+  { deep: true, immediate: true },
+)
+</script>
+
+<template>
+  <div>
+    <label>
+      Full Name
+      <input type="text" id="name" name="name" v-model="form.name" />
+    </label>
+    <span class="name_error" v-if="form.errors.name">{{ form.errors.name }}</span>
+
+    <label>
+      Accept Terms and Conditions
+      <input type="checkbox" id="accept_tos" name="accept_tos" v-model="form.accept_tos" />
+    </label>
+    <span class="accept_tos_error" v-if="form.errors.accept_tos">{{ form.errors.accept_tos }}</span>
+
+    <button @click="submit" class="submit">Submit form</button>
+
+    <button @click="addCustomOtherProp" class="add-custom-other-prop">Add custom.other_prop</button>
+
+    <div id="form-data-output" data-test-id="form-data-output" style="display: none">{{ formDataOutput.json }}</div>
+  </div>
+</template>
diff --git a/tests/app/server.js b/tests/app/server.js
index 27efa3255..2cea7a432 100644
--- a/tests/app/server.js
+++ b/tests/app/server.js
@@ -157,6 +157,13 @@ app.post('/form-helper/data', (req, res) =>
   }),
 )
 
+app.post('/form-helper/data-dynamic', (req, res) =>
+  inertia.render(req, res, {
+    component: 'FormHelper/DataDynamic',
+    props: {},
+  }),
+)
+
 app.get('/form-helper/nested', (req, res) =>
   inertia.render(req, res, {
     component: 'FormHelper/Nested',
diff --git a/tests/form-component.spec.ts b/tests/form-component.spec.ts
index 3a2f25c4e..82589e839 100644
--- a/tests/form-component.spec.ts
+++ b/tests/form-component.spec.ts
@@ -109,6 +109,56 @@ test.describe('Form Component', () => {
     })
   })
 
+  test.describe('Dynamic Properties', () => {
+    test.beforeEach(async ({ page }) => {
+      pageLoads.watch(page)
+      await page.goto('/form-helper/data-dynamic')
+    })
+
+    test('initial data() output contains only initial properties', async ({ page }) => {
+      const formDataOutput = await page.locator('#form-data-output').innerText()
+      const data = JSON.parse(formDataOutput)
+
+      expect(data).toEqual({
+        name: 'foo',
+        handle: 'example',
+        remember: false,
+        custom: {},
+      })
+    })
+
+    test('data() output includes root-level dynamic property', async ({ page }) => {
+      await page.check('input[name="accept_tos"]')
+
+      const formDataOutput = await page.locator('#form-data-output').innerText()
+      const data = JSON.parse(formDataOutput)
+
+      expect(data).toEqual({
+        name: 'foo',
+        handle: 'example',
+        remember: false,
+        custom: {},
+        accept_tos: true,
+      })
+    })
+
+    test('data() output includes nested dynamic property', async ({ page }) => {
+      await page.getByRole('button', { name: 'Add custom.other_prop' }).click()
+
+      const formDataOutput = await page.locator('#form-data-output').innerText()
+      const data = JSON.parse(formDataOutput)
+
+      expect(data).toEqual({
+        name: 'foo',
+        handle: 'example',
+        remember: false,
+        custom: {
+          other_prop: 'dynamic_value',
+        },
+      })
+    })
+  })
+
   test.describe('Headers', () => {
     test.beforeEach(async ({ page }) => {
       pageLoads.watch(page)