gofiber/fiber · critical

client panic: %v

Error message

client panic: %v

What it means

Produced in core.execFunc (client/core.go:77) by a recover() inside the goroutine that drives fasthttp's client.Do/DoRedirects. Any panic raised while copying the request, performing the dial, or building the Response is captured and delivered to the caller as a normal error on the errChan, so a panic never crashes the whole process. Because the panic value is formatted with %v, the message is whatever r the panic carried.

Source

Thrown at client/core.go:77

		defer releaseResponseChan(respChan)

		reqv := fasthttp.AcquireRequest()
		defer fasthttp.ReleaseRequest(reqv)

		respv := fasthttp.AcquireResponse()
		defer func() {
			if respv != nil {
				fasthttp.ReleaseResponse(respv)
			}
		}()

		var resp *Response
		defer func() {
			if r := recover(); r != nil {
				if resp != nil {
					ReleaseResponse(resp)
				}
				errChan <- fmt.Errorf("client panic: %v", r)
			}
		}()

		c.req.RawRequest.CopyTo(reqv)
		if bodyStream := c.req.RawRequest.BodyStream(); bodyStream != nil {
			reqv.SetBodyStream(bodyStream, c.req.RawRequest.Header.ContentLength())
		}

		var err error
		if cfg != nil {
			// Use an exponential backoff retry strategy.
			err = retry.NewExponentialBackoff(*cfg).Retry(func() error {
				if c.req.maxRedirects > 0 && (string(reqv.Header.Method()) == fiber.MethodGet || string(reqv.Header.Method()) == fiber.MethodHead || string(reqv.Header.Method()) == fiber.MethodQuery) {
					return c.client.DoRedirects(reqv, respv, c.req.maxRedirects)
				}
				return c.client.Do(reqv, respv)
			})
		} else {

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Inspect the formatted panic value (often a runtime error string) to find the originating stack — enable a debugger or wrap the call to log r.
2. Ensure each Request is used by exactly one goroutine and not reused after being sent.
3. Validate/replace custom marshalers and hooks so they return errors instead of panicking.
4. Upgrade fasthttp/Fiber to pick up panic fixes if the trace points into library internals.

Example fix

// before — request shared across goroutines, fasthttp panics
for _, u := range urls {
    go func() { resp, _ := core.execute(ctx, client, sharedReq); _ = resp }()
}

// after — each goroutine gets its own acquired request
for _, u := range urls {
    go func(u string) {
        req := AcquireRequest()
        defer ReleaseRequest(req)
        req.SetURL(u)
        resp, err := core.execute(ctx, client, req)
        if err != nil { log.Printf("request failed/panicked: %v", err) }
    }(u)
}

resp, err := core.execute(ctx, client, req)
if err != nil {
    if strings.Contains(err.Error(), "client panic") {
        // a panic was recovered inside the client; log r and fail/restart
        log.Printf("recovered client panic: %v", err)
    }
    return err
}

When it happens

Trigger: A nil-pointer dereference or index-out-of-range inside fasthttp while executing the request; a panic in a user-supplied retry callback body, a hook, or a custom marshaler invoked during request building; concurrent misuse of a Request/Response that fasthttp does not tolerate (e.g. reusing an acquired request from two goroutines).

Common situations: Sharing a single *Request across goroutines; a bodyStream that panics on read; a custom JSON/XML marshaler that panics on an unsupported type; a fasthttp version regression that panics on a malformed header value; retry logic whose callback closure captures a nil client.

Related errors

• sse: handler panic: %v
• fasthttp.Client must not be nil
• runtime.Goexit() called in handler or server panic
• proxy: nil client override passed to Do/Forward
• failed to type-assert to *Middleware

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/a6eae34fde4cf5ad.json. Report an issue: GitHub.