Can axios serialize FormData consistently across browser and Node.js without manual header management?

Direct answer

No. Axios does not serialize FormData consistently across browser and Node.js without manual header management. In a browser, the underlying XHR or fetch transport detects native FormData and emits multipart/form-data with a generated boundary. In Node.js, the default axios HTTP adapter does not inspect FormData and does not add that header. You must supply it yourself, typically with the form-data package and its getHeaders() method. Axios 1.x does not expose a documented config flag that closes this gap.

Confirmed behavior and likely explanation

• Confirmed: Browser transports set the multipart Content-Type boundary automatically; Node.js axios does not.
• Confirmed: No axios config flag auto-adds multipart headers in Node; external normalization is still required in axios 1.x.
• Likely explanation: Axios delegates multipart encoding to the runtime transport. Node's HTTP module has no native awareness of FormData, so axios leaves the header unset unless the caller attaches it.
• Uncertain, mark for review: How axios treats Node 18+ global FormData versus the form-data package, and whether future axios versions change default handling.

Recommended normalization: request interceptor

A request interceptor in a single shared module is more maintainable than a custom adapter. It keeps axios transforms and response handling intact while normalizing headers before dispatch. Do not set content-type manually in the browser; let the transport generate the boundary.

import axios from 'axios';

const isNode = typeof process !== 'undefined' && process.versions != null && process.versions.node != null;

const instance = axios.create();

instance.interceptors.request.use((config) => {
  const data = config.data;
  const headers = config.headers || {};

  if (data && typeof data.getHeaders === 'function' && isNode && !headers['content-type']) {
    Object.assign(headers, data.getHeaders());
    config.headers = headers;
  }

  return config;
});

export default instance;

Pass a form-data instance as data in Node. In the browser, pass native FormData and omit content-type so the transport sets the boundary.

Server-free verification

1. Create a custom capturing axios adapter that resolves immediately and stores config.headers and config.data.
2. Run the same logical payload through the shared instance in Node and in a browser test with request interception.
3. Parse both multipart bodies with the same parser and compare field names, values, and file names.
4. Do not assert identical raw bytes or boundary strings; boundaries differ by runtime and implementation.

Missing diagnostic that changes the recommendation

Does your Node target use the form-data package or Node 18+ global FormData? This determines whether getHeaders() is available and how boundaries are generated. Confirm the runtime before shipping the interceptor, and re-check after any axios major-version upgrade.

Review needed: verify this against your installed axios and Node versions before publication.