nodejs/node · warning · NotSupportedError

UND_ERR_NOT_SUPPORTED

UND_ERR_NOT_SUPPORTED

Error message

Not supported error

What it means

Thrown as NotSupportedError (code UND_ERR_NOT_SUPPORTED) by the fetch Body/Response .formData() method in undici's readable.js. Parsing multipart/form-data and application/x-www-form-urlencoded bodies into FormData is intentionally not implemented (the source marks it TODO). Calling .formData() on any Response always throws, regardless of the Content-Type.

Source

Thrown at deps/undici/src/lib/api/readable.js:227

  /**
   * Consumes and returns the body as an ArrayBuffer.
   *
   * @see https://fetch.spec.whatwg.org/#dom-body-arraybuffer
   * @returns {Promise<ArrayBuffer>}
   */
  arrayBuffer () {
    return consume(this, 'arrayBuffer')
  }

  /**
   * Not implemented
   *
   * @see https://fetch.spec.whatwg.org/#dom-body-formdata
   * @throws {NotSupportedError}
   */
  async formData () {
    // TODO: Implement.
    throw new NotSupportedError()
  }

  /**
   * Returns true if the body is not null and the body has been consumed.
   * Otherwise, returns false.
   *
   * @see https://fetch.spec.whatwg.org/#dom-body-bodyused
   * @readonly
   * @returns {boolean}
   */
  get bodyUsed () {
    return util.isDisturbed(this)
  }

  /**
   * @see https://fetch.spec.whatwg.org/#dom-body-body
   * @readonly
   * @returns {ReadableStream}

View on GitHub (pinned to 1b2de5e052)

Solutions

1. Parse the body manually: for multipart use a parser like busboy/formdata-node/undici's own multipart utility; for urlencoded use new URLSearchParams(await res.text()).
2. If you control the server, have it return JSON instead of form-encoded bodies.
3. Upgrade undici — check release notes; if formData support landed, update to that version. Until then treat formData() as unavailable.

Example fix

// before
const form = await response.formData()

// after (urlencoded)
const params = new URLSearchParams(await response.text())
const value = params.get('field')

// or (multipart) use a dedicated parser such as formdata-node/busboy

async function readFormData(response) {
  const ct = response.headers.get('content-type') || ''
  const text = await response.text()
  if (ct.includes('application/x-www-form-urlencoded')) {
    return new URLSearchParams(text)
  }
  // multipart: delegate to busboy/formdata-node
  throw new Error('multipart formData parsing not supported by undici; use busboy')
}

function isFormDataSupported() { return false } // undici formData() is unimplemented

try {
  const form = await response.formData()
} catch (e) {
  if (e.code === 'UND_ERR_NOT_SUPPORTED') {
    // fall back to manual URLSearchParams or a multipart parser
  } else throw e
}

When it happens

Trigger: Calling const form = await response.formData() on an undici fetch Response, even when the body is multipart/form-data. This is a hard unimplemented stub, not a runtime/content condition.

Common situations: Porting browser fetch code that uses response.formData() to Node/undici; handling file upload responses; parsing webhook payloads that are form-encoded.

Related errors

• UND_ERR_REQ_CONTENT_LENGTH_MISMATCH
• UND_ERR_REQ_CONTENT_LENGTH_MISMATCH
• UND_ERR_INVALID_ARG
• UND_ERR_INVALID_ARG
• UND_ERR_SOCKET

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/3d33f4ea7b3bc3f5. Report an issue: GitHub.