diff --git a/app/components/form/fields/DiskSizeField.tsx b/app/components/form/fields/DiskSizeField.tsx
deleted file mode 100644
index 44cce80ff..000000000
--- a/app/components/form/fields/DiskSizeField.tsx
+++ /dev/null
@@ -1,59 +0,0 @@
-/*
- * This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, you can obtain one at https://mozilla.org/MPL/2.0/.
- *
- * Copyright Oxide Computer Company
- */
-import type { FieldPathByValue, FieldValues, ValidateResult } from 'react-hook-form'
-
-import { NumberField } from './NumberField'
-import type { TextFieldProps } from './TextField'
-
-interface DiskSizeProps<
-  TFieldValues extends FieldValues,
-  TName extends FieldPathByValue<TFieldValues, number>,
-> extends Omit<TextFieldProps<TFieldValues, TName>, 'min' | 'max' | 'validate'> {
-  // replace max and min with our own because original max/min allow string
-  min?: number
-  /** Undefined means no client-side limit (e.g., for local disks) */
-  max: number | undefined
-  validate?(diskSizeGiB: number): ValidateResult
-}
-
-export function DiskSizeField<
-  TFieldValues extends FieldValues,
-  TName extends FieldPathByValue<TFieldValues, number>,
->({
-  required = true,
-  name,
-  min = 1,
-  max,
-  validate,
-  ...props
-}: DiskSizeProps<TFieldValues, TName>) {
-  return (
-    <NumberField
-      units="GiB"
-      required={required}
-      name={name}
-      min={min}
-      max={max}
-      validate={(diskSizeGiB) => {
-        // Run a number of default validators
-        if (Number.isNaN(diskSizeGiB)) {
-          return 'Disk size is required'
-        }
-        if (diskSizeGiB < min) {
-          return `Must be at least ${min} GiB`
-        }
-        if (max !== undefined && diskSizeGiB > max) {
-          return `Can be at most ${max} GiB`
-        }
-        // Run any additional validators passed in from the callsite
-        return validate?.(diskSizeGiB)
-      }}
-      {...props}
-    />
-  )
-}
diff --git a/app/components/form/fields/ListboxField.tsx b/app/components/form/fields/ListboxField.tsx
index b4947038b..3b647eebf 100644
--- a/app/components/form/fields/ListboxField.tsx
+++ b/app/components/form/fields/ListboxField.tsx
@@ -11,6 +11,7 @@ import {
   type Control,
   type FieldPath,
   type FieldValues,
+  type RegisterOptions,
 } from 'react-hook-form'
 
 import { Listbox, type ListboxItem } from '~/ui/lib/Listbox'
@@ -37,6 +38,8 @@ export type ListboxFieldProps<
   isLoading?: boolean
   noItemsPlaceholder?: string
   hideOptionalTag?: boolean
+  /** Other fields to re-validate when this one changes */
+  deps?: RegisterOptions<TFieldValues, TName>['deps']
 }
 
 export function ListboxField<
@@ -57,10 +60,11 @@ export function ListboxField<
   noItemsPlaceholder,
   hideOptionalTag,
   hideLabel,
+  deps,
 }: ListboxFieldProps<TFieldValues, TName>) {
   // TODO: recreate this logic
   //   validate: (v) => (required && !v ? `${name} is required` : undefined),
-  const { field, fieldState } = useController({ name, control, rules: { required } })
+  const { field, fieldState } = useController({ name, control, rules: { required, deps } })
   return (
     <div className={cn('max-w-lg', className)}>
       <Listbox
diff --git a/app/components/form/fields/NumberField.tsx b/app/components/form/fields/NumberField.tsx
index a8bea7201..9d919a402 100644
--- a/app/components/form/fields/NumberField.tsx
+++ b/app/components/form/fields/NumberField.tsx
@@ -44,7 +44,14 @@ export function NumberField<
         )}
       </div>
       {/* passing the generated id is very important for a11y */}
-      <NumberFieldInner name={name} id={id} label={label} required={required} {...props} />
+      <NumberFieldInner
+        name={name}
+        id={id}
+        label={label}
+        units={units}
+        required={required}
+        {...props}
+      />
     </div>
   )
 }
@@ -72,6 +79,7 @@ export const NumberFieldInner = <
   disabled,
   max,
   min = 0,
+  units,
 }: TextFieldProps<TFieldValues, TName>) => {
   const generatedId = useId()
   const id = idProp || generatedId
@@ -85,9 +93,16 @@ export const NumberFieldInner = <
     rules: {
       required,
       deps,
-      // it seems we need special logic to enforce required on NaN
+      // RHF's required rule doesn't catch NaN, and its min/max rules don't
+      // know about units, so we do all three here. The input itself no longer
+      // clamps, so this is what stops out-of-range values.
       validate(value, values) {
-        if (required && Number.isNaN(value)) return `${label} is required`
+        if (Number.isNaN(value)) return required ? `${label} is required` : undefined
+        const suffix = units ? ` ${units}` : ''
+        if (min !== undefined && value < Number(min))
+          return `Must be at least ${min}${suffix}`
+        if (max !== undefined && value > Number(max))
+          return `Can be at most ${max}${suffix}`
         return validate?.(value, values)
       },
     },
diff --git a/app/forms/disk-create.tsx b/app/forms/disk-create.tsx
index 262ab7b3a..6371eda76 100644
--- a/app/forms/disk-create.tsx
+++ b/app/forms/disk-create.tsx
@@ -24,10 +24,10 @@ import {
 
 import { CheckboxField } from '~/components/form/fields/CheckboxField'
 import { DescriptionField } from '~/components/form/fields/DescriptionField'
-import { DiskSizeField } from '~/components/form/fields/DiskSizeField'
 import { toImageComboboxItem } from '~/components/form/fields/ImageSelectField'
 import { ListboxField } from '~/components/form/fields/ListboxField'
 import { NameField } from '~/components/form/fields/NameField'
+import { NumberField } from '~/components/form/fields/NumberField'
 import { RadioField } from '~/components/form/fields/RadioField'
 import { SideModalForm } from '~/components/form/SideModalForm'
 import { HL } from '~/components/HL'
@@ -207,9 +207,12 @@ export function CreateDiskSideModalForm({
         }}
       />
       <DescriptionField name="description" control={form.control} />
-      <DiskSizeField
+      <NumberField
         name="size"
         control={form.control}
+        units="GiB"
+        required
+        min={1}
         // Local disk size is only capped by server capacity
         max={match(diskBackend)
           .with({ type: 'local' }, () => undefined)
@@ -242,7 +245,12 @@ const DiskBackendField = ({
 }) => {
   const {
     field: { value: diskBackend, onChange },
-  } = useController({ control, name: 'diskBackend' })
+  } = useController({
+    control,
+    name: 'diskBackend',
+    // Switching disk type changes the size limit.
+    rules: { deps: 'size' },
+  })
   // react-hook-form types onChange as (...event: any[]) => void
   // https://github.com/react-hook-form/react-hook-form/issues/10466
   const setDiskBackend: (value: DiskBackendForm) => void = onChange
diff --git a/app/forms/external-subnet-create.tsx b/app/forms/external-subnet-create.tsx
index 404cde2cf..3b2830ac6 100644
--- a/app/forms/external-subnet-create.tsx
+++ b/app/forms/external-subnet-create.tsx
@@ -132,6 +132,8 @@ export default function CreateExternalSubnetSideModalForm() {
             items={pools.items.map(toPoolItem)}
             required
             description="Subnet pool to allocate from"
+            // the pool's IP version sets the prefix length max
+            deps="prefixLength"
           />
           <NumberField
             name="prefixLength"
diff --git a/app/forms/instance-create.tsx b/app/forms/instance-create.tsx
index e838cdf20..211ee7aa3 100644
--- a/app/forms/instance-create.tsx
+++ b/app/forms/instance-create.tsx
@@ -49,7 +49,6 @@ import { DocsPopover } from '~/components/DocsPopover'
 import { CheckboxField } from '~/components/form/fields/CheckboxField'
 import { ComboboxField } from '~/components/form/fields/ComboboxField'
 import { DescriptionField } from '~/components/form/fields/DescriptionField'
-import { DiskSizeField } from '~/components/form/fields/DiskSizeField'
 import {
   DisksTableField,
   type DiskTableItem,
@@ -487,11 +486,13 @@ export default function CreateInstanceForm() {
   const bootDiskSizeAndName = (
     <>
       <div key="divider1" className="my-6! content-['a']" />
-      <DiskSizeField
+      <NumberField
         key="diskSizeField"
         label="Disk size"
         name="bootDiskSize"
         control={control}
+        units="GiB"
+        required
         min={imageSizeGiB || 1}
         // Max size applies: this disk can only be distributed
         max={MAX_DISK_SIZE_GiB}
diff --git a/test/e2e/external-subnets.e2e.ts b/test/e2e/external-subnets.e2e.ts
index 32284d4b4..67957eef7 100644
--- a/test/e2e/external-subnets.e2e.ts
+++ b/test/e2e/external-subnets.e2e.ts
@@ -393,6 +393,14 @@ test('create form prefix length max changes with pool IP version', async ({ page
   // Switch back to v4 — value should clamp back to 32
   await selectOption(page, 'Subnet pool', v4Pool)
   await expect(prefixLen).toHaveValue('32')
+
+  // The clamp is display-only: the form still holds 64, so submit must be
+  // blocked with a message rather than sending an out-of-range prefix
+  await page.getByRole('textbox', { name: 'Name' }).fill('too-long')
+  await page.getByRole('button', { name: 'Create external subnet' }).click()
+  const dialog = page.getByRole('dialog', { name: 'Create external subnet' })
+  await expect(dialog.getByText('Can be at most 32')).toBeVisible()
+  await expect(dialog).toBeVisible()
 })
 
 test('create form toggles between auto and explicit fields', async ({ page }) => {