What causes "Request Entity Too Large" in gofiber/fiber?

Raise BodyLimit for upload routes: fiber.New(fiber.Config{BodyLimit: 50 * 1024 * 1024}).

Request Entity Too Large

Error message

Request Entity Too Large

What it means

Returns HTTP 413 'Request Entity Too Large' from middleware/adaptor/adaptor.go:499 inside the net/http -> fasthttp adaptor when the incoming request's declared Content-Length exceeds maxBodySize. maxBodySize is derived from the Fiber app's BodyLimit (default DefaultBodyLimit = 4 MiB, app.go:613). The same 413 is re-sent at line 526 if the streamed bytes exceed the limit despite a missing/lying Content-Length.

Source

Thrown at middleware/adaptor/adaptor.go:499

			remoteAddr = nil // Fallback to nil
		}
		pctx.conn.remoteAddr = remoteAddr

		// Init2 mirrors fasthttp's RequestCtx.Init, but with a no-op
		// connection instead of fasthttp's fakeAddrer, whose Write panics.
		// Interim responses (e.g. SendEarlyHints' 103) are then silently
		// discarded instead of panicking; the final response still reaches
		// the client through the ResponseWriter copy-back below. Init2 only
		// touches connection metadata and buffer-retention flags, so the
		// request is built directly into fctx.Request afterwards — the same
		// order fasthttp's Init uses, minus its full request copy.
		fctx.Init2(&pctx.conn, disabledLogger, true)
		req := &fctx.Request

		// Convert net/http -> fasthttp request with size limit
		if r.Body != nil {
			if r.ContentLength > maxBodySize {
				http.Error(w, utils.StatusMessage(fiber.StatusRequestEntityTooLarge), fiber.StatusRequestEntityTooLarge)
				return
			}

			var n int64
			// http.NoBody never yields any bytes, so skip the copy machinery
			// entirely for the (very common) bodyless request.
			if r.Body != http.NoBody {
				limit := maxBodySize
				if limit < math.MaxInt64 {
					limit++
				}
				// The LimitedReader lives in the pooled ctx and the copy
				// buffer comes from the shared pool: io.Copy would otherwise
				// allocate a fresh 32 KiB buffer on every single request.
				pctx.lr.R = r.Body
				pctx.lr.N = limit

				var err error

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Raise BodyLimit for upload routes: fiber.New(fiber.Config{BodyLimit: 50 * 1024 * 1024}).
Have the client chunk large uploads or stream to a dedicated upload endpoint with a higher limit.
Reject oversized requests at the reverse proxy (nginx client_max_body_size) before they reach the app.

Example fix

// before
app := fiber.New() // BodyLimit defaults to 4 MiB
// client POSTs a 10 MiB file -> 413

// after
app := fiber.New(fiber.Config{
    BodyLimit: 64 * 1024 * 1024, // 64 MiB for upload routes
})

Defensive patterns

Strategy: validation

Validate before calling

// Validate client-side before sending.
const limit = 4 * 1024 * 1024 // match BodyLimit
if r.ContentLength > limit {
    return fmt.Errorf("body too large: %d bytes", r.ContentLength)
}
// or raise the server limit:
app := fiber.New(fiber.Config{BodyLimit: 64 * 1024 * 1024})

Prevention

Set BodyLimit high enough for the largest legitimate upload on that route.
Enforce a size cap at the reverse proxy (e.g. nginx client_max_body_size).
Have clients pre-flight sizes and chunk large uploads.

When it happens

Trigger: Serving a Fiber app behind net/http via the adaptor and POSTing/PUTting a body larger than BodyLimit with a Content-Length header that exceeds it; chunked uploads that cross the limit mid-stream. This is a normal HTTP response, not a Go panic/error to the caller.

Common situations: File/image uploads exceeding the 4 MiB default; JSON batches; clients that did not pre-flight the size; default BodyLimit left in place for an upload endpoint.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/31e85aa3344ac330.json. Report an issue: GitHub.