diff --git a/sdks/nodejs-client/src/client/base.ts b/sdks/nodejs-client/src/client/base.ts
index 3f465d2946e..7081100597b 100644
--- a/sdks/nodejs-client/src/client/base.ts
+++ b/sdks/nodejs-client/src/client/base.ts
@@ -26,8 +26,12 @@ const toConfig = (init: string | DifyClientConfig, baseUrl?: string): DifyClient
   return init
 }
 
-const appendUserToFormData = (form: SdkFormData, user: string): void => {
-  form.append('user', user)
+const setUserOnFormData = (form: SdkFormData, user: string): void => {
+  if ('set' in form && typeof form.set === 'function') {
+    form.set('user', user)
+  } else {
+    form.append('user', user)
+  }
 }
 
 export class DifyClient {
@@ -178,7 +182,7 @@ export class DifyClient {
       throw new FileUploadError('FormData is required for file uploads')
     }
     ensureNonEmptyString(user, 'user')
-    appendUserToFormData(form, user)
+    setUserOnFormData(form, user)
     return this.http.request({
       method: 'POST',
       path: '/files/upload',
@@ -205,7 +209,7 @@ export class DifyClient {
       throw new FileUploadError('FormData is required for audio uploads')
     }
     ensureNonEmptyString(user, 'user')
-    appendUserToFormData(form, user)
+    setUserOnFormData(form, user)
     return this.http.request({
       method: 'POST',
       path: '/audio-to-text',
diff --git a/sdks/nodejs-client/tests/form-user.integration.test.ts b/sdks/nodejs-client/tests/form-user.integration.test.ts
new file mode 100644
index 00000000000..e57315da531
--- /dev/null
+++ b/sdks/nodejs-client/tests/form-user.integration.test.ts
@@ -0,0 +1,88 @@
+import type { AddressInfo } from 'node:net'
+import { createServer } from 'node:http'
+import { afterAll, beforeAll, describe, expect, it } from 'vite-plus/test'
+import { DifyClient } from '../src/client/base'
+
+describe('upload user fields', () => {
+  const server = createServer((req, res) => {
+    void (async () => {
+      const chunks: Buffer[] = []
+      for await (const chunk of req as AsyncIterable<Uint8Array>) {
+        chunks.push(Buffer.from(chunk))
+      }
+      const request = new Request('http://localhost', {
+        method: 'POST',
+        headers: { 'content-type': req.headers['content-type'] ?? '' },
+        body: new Uint8Array(Buffer.concat(chunks)),
+      })
+      const form = await request.formData()
+      const file = form.get('file') as File
+      res.writeHead(200, { 'content-type': 'application/json' })
+      res.end(
+        JSON.stringify({
+          users: form.getAll('user'),
+          fileName: file.name,
+          fileText: await file.text(),
+          extra: form.get('extra'),
+        }),
+      )
+    })().catch(() => {
+      res.writeHead(500)
+      res.end()
+    })
+  })
+
+  let client: DifyClient
+  beforeAll(async () => {
+    await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve))
+    const address = server.address() as AddressInfo
+    client = new DifyClient({
+      apiKey: 'test',
+      baseUrl: `http://127.0.0.1:${address.port}`,
+      maxRetries: 0,
+    })
+  })
+  afterAll(async () => {
+    await new Promise<void>((resolve, reject) => {
+      server.close((error) => (error ? reject(error) : resolve()))
+    })
+  })
+
+  const makeForm = () => {
+    const form = new FormData()
+    form.append('file', new Blob(['payload']), 'sample.txt')
+    form.append('extra', 'preserved')
+    return form
+  }
+
+  it.each(['fileUpload', 'audioToText'] as const)(
+    '%s replaces existing user fields',
+    async (method) => {
+      const form = makeForm()
+      form.append('user', 'stale-user')
+      form.append('user', 'another-user')
+
+      const response = await client[method](form, 'current-user')
+
+      expect(response.data).toEqual({
+        users: ['current-user'],
+        fileName: 'sample.txt',
+        fileText: 'payload',
+        extra: 'preserved',
+      })
+    },
+  )
+
+  it.each(['fileUpload', 'audioToText'] as const)(
+    '%s can reuse native FormData for a new user',
+    async (method) => {
+      const form = makeForm()
+      const first = await client[method](form, 'first-user')
+      expect(first.data.users).toEqual(['first-user'])
+
+      const second = await client[method](form, 'second-user')
+      expect(second.data.users).toEqual(['second-user'])
+      expect(second.data.fileText).toBe('payload')
+    },
+  )
+})