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

Raise app.Config().BodyLimit (fiber.Config{BodyLimit: ...}) when large bodies are expected — set it on fiber.New(Config{BodyLimit: 32 << 20}).

gofiber/fiber · warning

Request Entity Too Large

Error message

Request Entity Too Large

What it means

The net/http -> fasthttp adaptor (HTTPHandlerFunc / adaptor's handlerFunc) enforces the Fiber app's BodyLimit on inbound request bodies before copying them into the fasthttp request. When the request's declared Content-Length exceeds app.Config().BodyLimit (default 4 * 1024 * 1024 = 4 MiB), the adaptor writes a 413 Request Entity Too Large response and stops processing — it does not panic, and the process keeps serving other requests. A LimitedReader also caps the actual streamed bytes at BodyLimit+1 so a client lying about Content-Length cannot exhaust memory.

Solutions

Raise app.Config().BodyLimit (fiber.Config{BodyLimit: ...}) when large bodies are expected — set it on fiber.New(Config{BodyLimit: 32 << 20}).
Move large-body endpoints (uploads) to streaming or multipart handling rather than buffering the whole body.
Reject or chunk oversized payloads at the reverse proxy / load balancer upstream of the app.

Example fix

// before
app := fiber.New() // BodyLimit defaults to 4 MiB
http.ListenAndServe(":8080", adaptor.HTTPHandler(app, bigUploadHandler))

// after
app := fiber.New(fiber.Config{
    BodyLimit: 64 * 1024 * 1024, // 64 MiB for upload routes
})
http.ListenAndServe(":8080", adaptor.HTTPHandler(app, bigUploadHandler))

Defensive patterns

Strategy: validation

Validate before calling

func bodyLimitFor(route string) int {
    if strings.HasPrefix(route, "/upload") { return 64 * 1024 * 1024 }
    return 4 * 1024 * 1024 // default
}
// app := fiber.New(fiber.Config{BodyLimit: bodyLimitFor("/upload")})
//
// client-side guard before sending:
// if fileSize > int64(app.Config().BodyLimit) { return fmt.Errorf("file too large") }

Type guard

func requestWithinLimit(contentLength int64, bodyLimit int) bool {
    return contentLength <= int64(bodyLimit)
}

Prevention

Set BodyLimit explicitly on fiber.New to match the largest legitimate payload, rather than relying on the 4 MiB default.
Handle large uploads via streaming/multipart instead of buffering the whole body.
Reject oversized payloads at the upstream reverse proxy so they never reach the app.
Remember the adaptor applies Fiber's BodyLimit even though the request arrives through net/http.

When it happens

Trigger: A client POST/PUT whose Content-Length header exceeds the configured BodyLimit reaches a route mounted via adaptor.HTTPHandlerFunc or adaptor.FiberApp wrapped in net/http. Default limit is 4 MiB; a file upload, large JSON payload, or webhook exceeding it triggers the 413.

Common situations: File uploads through an adaptor-mounted handler; a webhook receiver whose payloads grew past 4 MiB; a default-config app receiving legitimate large bodies; migrating a net/http handler to Fiber via the adaptor without raising BodyLimit to match the prior server's limit.

Related errors

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/31e85aa3344ac330. Report an issue: GitHub.

Appendix: source

Thrown at middleware/adaptor/adaptor.go:614

			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

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