diff --git a/docs/guide/pages-routing/actions-and-forms.md b/docs/guide/pages-routing/actions-and-forms.md
index adb9619bf..90220f4f5 100644
--- a/docs/guide/pages-routing/actions-and-forms.md
+++ b/docs/guide/pages-routing/actions-and-forms.md
@@ -443,6 +443,8 @@ export const action = defineHandler(async (c) => {
 });
 ```
 
+`withValidator({ body })` accepts the same upload. Nested names such as `profile[name]`, `tags[0]`, and `tags[]` are objects and arrays again. A multipart schema has to coerce the rest: booleans arrive as `'1'` or `'0'`, numbers and dates as strings, and `null` as `''`.
+
 Pick the storage key on the server and allow only the file types you expect. See [Serving Files](../storage.md#serving-files) to send them back safely.
 
 ## Choosing a Primitive
diff --git a/packages/void/src/runtime/validator.ts b/packages/void/src/runtime/validator.ts
index 6a1c34424..f51ccd7ac 100644
--- a/packages/void/src/runtime/validator.ts
+++ b/packages/void/src/runtime/validator.ts
@@ -45,6 +45,172 @@ async function validateSlot(
   return { ok: true, value: result.value };
 }
 
+type FormEntry = string | File;
+
+const MAX_FORM_DEPTH = 32;
+const MAX_FORM_NODES = 10_000;
+const MAX_FORM_INDEX = 10_000;
+const DANGEROUS_KEYS = new Set(['__proto__', 'constructor', 'prototype']);
+
+function isFormContentType(contentType: string | undefined): boolean {
+  const mediaType = contentType?.split(';', 1)[0]?.trim().toLowerCase();
+  return mediaType === 'multipart/form-data' || mediaType === 'application/x-www-form-urlencoded';
+}
+
+/**
+ * `objectToFormData` writes nested fields as `a[b]`, array items as `a[0]`,
+ * and file lists as `a[]`. Split one of those keys into path segments.
+ * A key that is not entirely bracket-encoded stays a single literal segment.
+ */
+function formKeySegments(key: string): Array<string> {
+  const bracket = key.indexOf('[');
+  if (bracket <= 0 || !key.endsWith(']')) {
+    return [key];
+  }
+
+  const segments = [key.slice(0, bracket)];
+  const rest = key.slice(bracket);
+  const pattern = /\[([^[\]]*)\]/g;
+  let consumed = 0;
+  let match: RegExpExecArray | null;
+  while ((match = pattern.exec(rest)) !== null) {
+    if (match.index !== consumed) {
+      return [key];
+    }
+    segments.push(match[1] ?? '');
+    consumed = pattern.lastIndex;
+    if (segments.length > MAX_FORM_DEPTH) {
+      throw new Error('Form field is nested too deeply');
+    }
+  }
+  if (consumed !== rest.length) {
+    return [key];
+  }
+  return segments;
+}
+
+function arrayIndex(segment: string): number | null {
+  if (segment !== '0' && !/^[1-9]\d*$/.test(segment)) {
+    return null;
+  }
+  if (segment.length > 5) {
+    throw new Error('Form field index is out of range');
+  }
+  const index = Number(segment);
+  if (index > MAX_FORM_INDEX) {
+    throw new Error('Form field index is out of range');
+  }
+  return index;
+}
+
+function isContainerRecord(value: unknown): value is Record<string, unknown> {
+  return (
+    typeof value === 'object' && value !== null && !Array.isArray(value) && !(value instanceof Blob)
+  );
+}
+
+function own(record: Record<string, unknown>, key: string): unknown {
+  if (DANGEROUS_KEYS.has(key)) {
+    return Object.getOwnPropertyDescriptor(record, key)?.value;
+  }
+  return record[key];
+}
+
+function setOwn(record: Record<string, unknown>, key: string, value: unknown): void {
+  if (DANGEROUS_KEYS.has(key)) {
+    Object.defineProperty(record, key, {
+      value,
+      writable: true,
+      enumerable: true,
+      configurable: true,
+    });
+    return;
+  }
+  record[key] = value;
+}
+
+/**
+ * Rebuild the object `objectToFormData` flattened. Numeric segments and `[]`
+ * become arrays. Scalars are left as the client encoded them.
+ */
+function formRecordToObject(
+  record: Record<string, FormEntry | Array<FormEntry>>,
+): Record<string, unknown> {
+  const root: Record<string, unknown> = {};
+  let nodes = 0;
+
+  const createContainer = (asArray: boolean): Record<string, unknown> | Array<unknown> => {
+    if (++nodes > MAX_FORM_NODES) {
+      throw new Error('Form field has too many nested values');
+    }
+    return asArray ? [] : {};
+  };
+
+  const read = (parent: Record<string, unknown> | Array<unknown>, segment: string): unknown => {
+    if (Array.isArray(parent)) {
+      return segment === '' ? undefined : parent[Number(segment)];
+    }
+    return own(parent, segment);
+  };
+
+  const write = (
+    parent: Record<string, unknown> | Array<unknown>,
+    segment: string,
+    value: unknown,
+  ): void => {
+    if (Array.isArray(parent)) {
+      if (segment === '') {
+        parent.push(value);
+        return;
+      }
+      const index = arrayIndex(segment);
+      if (index === null) {
+        throw new Error('Form field shape is inconsistent');
+      }
+      parent[index] = value;
+      return;
+    }
+    setOwn(parent, segment, value);
+  };
+
+  const assign = (segments: Array<string>, value: FormEntry): void => {
+    let parent: Record<string, unknown> | Array<unknown> = root;
+    for (let i = 0; i < segments.length - 1; i++) {
+      const segment = segments[i] as string;
+      const next = segments[i + 1] as string;
+      const nextIsArray = next === '' || arrayIndex(next) !== null;
+      const child = read(parent, segment);
+      const childIsContainer = nextIsArray ? Array.isArray(child) : isContainerRecord(child);
+      const container = childIsContainer
+        ? (child as Record<string, unknown> | Array<unknown>)
+        : createContainer(nextIsArray);
+      if (!childIsContainer) {
+        write(parent, segment, container);
+      }
+      parent = container;
+    }
+    write(parent, segments[segments.length - 1] as string, value);
+  };
+
+  for (const [key, raw] of Object.entries(record)) {
+    const segments = formKeySegments(key);
+    const values = Array.isArray(raw) ? raw : [raw];
+    for (const value of values) {
+      assign(segments, value);
+    }
+  }
+  return root;
+}
+
+async function readBody(c: CloudContext): Promise<unknown> {
+  if (!isFormContentType(c.req.header('content-type'))) {
+    return c.req.json();
+  }
+  // `[]` keys are arrays at runtime even when the declared return type is not.
+  const parsed = (await c.req.parseBody()) as Record<string, FormEntry | Array<FormEntry>>;
+  return formRecordToObject(parsed);
+}
+
 export async function runValidation<V extends ValidatorSlots>(
   validators: V,
   c: CloudContext,
@@ -82,12 +248,18 @@ export async function runValidation<V extends ValidatorSlots>(
 
   if (validators.body) {
     let raw: unknown;
+    const formBody = isFormContentType(c.req.header('content-type'));
     try {
-      raw = await c.req.json();
+      // Multipart and URL-encoded bodies are what `useForm` sends for files.
+      // JSON stays on `c.req.json()` so scalar types there are unchanged.
+      raw = await readBody(c);
     } catch {
       return {
         ok: false,
-        response: Response.json({ error: 'Invalid JSON body' }, { status: 400 }),
+        response: Response.json(
+          { error: formBody ? 'Invalid form body' : 'Invalid JSON body' },
+          { status: 400 },
+        ),
       };
     }
     const result = await validateSlot(validators.body, raw);
diff --git a/packages/void/test/unit/define-handler.test.ts b/packages/void/test/unit/define-handler.test.ts
index 9edc6ba7b..14b3e5105 100644
--- a/packages/void/test/unit/define-handler.test.ts
+++ b/packages/void/test/unit/define-handler.test.ts
@@ -1,6 +1,7 @@
 import { Hono } from 'hono';
 import type { MiddlewareHandler } from 'hono';
 import { describe, expect, it } from 'vite-plus/test';
+import { objectToFormData } from '../../src/pages/navigation-engine.ts';
 import {
   defineHandler,
   defineMiddleware,
@@ -350,4 +351,176 @@ describe('defineHandler.withValidator', () => {
       body: { name: 'Bob' },
     });
   });
+
+  it('passes a multipart file to the schema and the handler', async () => {
+    const photo = new File(['hello'], 'photo.png', { type: 'image/png' });
+    let seenBySchema: unknown;
+    const handler = defineHandler.withValidator({
+      body: {
+        '~standard': {
+          version: 1,
+          vendor: 'test',
+          validate(value: unknown) {
+            seenBySchema = value;
+            const uploaded = (value as { photo?: unknown }).photo;
+            if (!(uploaded instanceof File)) {
+              return { issues: [{ message: 'expected a file', path: ['photo'] }] };
+            }
+            return { value };
+          },
+        },
+      },
+    })((_c, { body }) => {
+      const received = body as { title: string; photo: File };
+      return {
+        title: received.title,
+        name: received.photo.name,
+        type: received.photo.type,
+        size: received.photo.size,
+      };
+    });
+
+    const app = createValidatedApp('post', '/test', handler);
+    const res = await app.request(
+      new Request('http://localhost/test', {
+        method: 'POST',
+        body: objectToFormData({ title: 'Sunset', photo }),
+      }),
+    );
+
+    expect(res.status).toBe(200);
+    expect(await res.json()).toEqual({
+      title: 'Sunset',
+      name: 'photo.png',
+      type: 'image/png',
+      size: photo.size,
+    });
+    const parsed = seenBySchema as { title: string; photo: File };
+    expect(parsed.title).toBe('Sunset');
+    expect(parsed.photo).toBeInstanceOf(File);
+    expect(parsed.photo.name).toBe('photo.png');
+    expect(await parsed.photo.text()).toBe('hello');
+  });
+
+  it('restores nested bracket keys from form bodies', async () => {
+    const handler = defineHandler.withValidator({
+      body: passSchema(),
+    })((_c, { body }) => ({ received: body }));
+    const app = createValidatedApp('post', '/test', handler);
+    const takenAt = new Date('2020-01-02T03:04:05.000Z');
+    const multipart = objectToFormData({
+      profile: { name: 'Ada' },
+      tags: ['work', 'urgent'],
+      album: { photos: [{ name: 'a.png' }] },
+      published: true,
+      count: 2,
+      takenAt,
+      note: null,
+    });
+    multipart.append('photos[]', 'one');
+    multipart.append('photos[]', 'two');
+    multipart.append('__proto__[polluted]', 'yes');
+
+    const multipartRes = await app.request(
+      new Request('http://localhost/test', { method: 'POST', body: multipart }),
+    );
+    expect(multipartRes.status).toBe(200);
+    expect(await multipartRes.json()).toEqual({
+      received: {
+        profile: { name: 'Ada' },
+        tags: ['work', 'urgent'],
+        album: { photos: [{ name: 'a.png' }] },
+        published: '1',
+        count: '2',
+        takenAt: '2020-01-02T03:04:05.000Z',
+        note: '',
+        photos: ['one', 'two'],
+        ['__proto__']: { polluted: 'yes' },
+      },
+    });
+    expect('polluted' in Object.prototype).toBe(false);
+
+    const params = new URLSearchParams();
+    params.append('profile[name]', 'Ada');
+    params.append('tags[0]', 'work');
+    params.append('tags[1]', 'urgent');
+    params.append('photos[]', 'one');
+    params.append('photos[]', 'two');
+    const urlencodedRes = await app.request(
+      new Request('http://localhost/test', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+        body: params.toString(),
+      }),
+    );
+    expect(urlencodedRes.status).toBe(200);
+    expect(await urlencodedRes.json()).toEqual({
+      received: {
+        profile: { name: 'Ada' },
+        tags: ['work', 'urgent'],
+        photos: ['one', 'two'],
+      },
+    });
+  });
+
+  it('keeps JSON body validation unchanged', async () => {
+    const handler = defineHandler.withValidator({
+      body: passSchema(),
+    })((_c, { body }) => ({ received: body }));
+    const app = createValidatedApp('post', '/test', handler);
+
+    const res = await app.request(
+      new Request('http://localhost/test', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({
+          profile: { name: 'Ada' },
+          tags: ['work'],
+          published: true,
+          count: 2,
+          note: null,
+        }),
+      }),
+    );
+
+    expect(res.status).toBe(200);
+    expect(await res.json()).toEqual({
+      received: {
+        profile: { name: 'Ada' },
+        tags: ['work'],
+        published: true,
+        count: 2,
+        note: null,
+      },
+    });
+  });
+
+  it('does not report a bad form body as invalid JSON', async () => {
+    const handler = defineHandler.withValidator({
+      body: failSchema('title is required'),
+    })(() => ({ ok: true }));
+    const app = createValidatedApp('post', '/test', handler);
+
+    const malformed = await app.request(
+      new Request('http://localhost/test', {
+        method: 'POST',
+        headers: { 'Content-Type': 'multipart/form-data; boundary=----bad' },
+        body: 'not a multipart body',
+      }),
+    );
+    expect(malformed.status).toBe(400);
+    expect(((await malformed.json()) as { error: string }).error).toBe('Invalid form body');
+
+    const invalid = await app.request(
+      new Request('http://localhost/test', {
+        method: 'POST',
+        body: objectToFormData({ title: '' }),
+      }),
+    );
+    expect(invalid.status).toBe(400);
+    const json = (await invalid.json()) as ValidationIssueBody;
+    expect(json.error).toBe('Validation failed');
+    expect(json.issues[0]?.slot).toBe('body');
+    expect(json.issues[0]?.issues[0]?.message).toBe('title is required');
+  });
 });
diff --git a/packages/void/test/unit/validator.test.ts b/packages/void/test/unit/validator.test.ts
index 5e46daf0f..80e2fd51b 100644
--- a/packages/void/test/unit/validator.test.ts
+++ b/packages/void/test/unit/validator.test.ts
@@ -25,7 +25,12 @@ function failSchema(issues: Array<StandardSchemaV1.Issue>): StandardSchemaV1 {
 }
 
 function mockContext(body?: unknown): CloudContext {
-  return { req: { json: async () => body } } as unknown as CloudContext;
+  return {
+    req: {
+      json: async () => body,
+      header: () => undefined,
+    },
+  } as unknown as CloudContext;
 }
 
 describe('runValidation', () => {