gofiber/fiber · critical

%s: invalid handler #%d (%T)\n

Error message

%s: invalid handler #%d (%T)\n

What it means

Panicked by collectHandlers (adapter.go:275) when one of the handler arguments passed to a route registration (app.Get/Post/Add/etc.) cannot be converted to a Fiber Handler via toFiberHandler. The panic message includes the context (which API was called), the 0-based argument index, and the Go type (%T) of the offending argument, so you can pinpoint the bad handler.

Source

Thrown at adapter.go:275

	adapted := fasthttpadaptor.NewFastHTTPHandler(handler)

	return func(c Ctx) error {
		adapted(c.RequestCtx())
		return nil
	}
}

// collectHandlers converts a slice of handler arguments to Fiber handlers.
// The context string is used to provide informative panic messages when an
// unsupported handler type is encountered.
func collectHandlers(context string, args ...any) []Handler {
	handlers := make([]Handler, 0, len(args))

	for i, arg := range args {
		handler, ok := toFiberHandler(arg)

		if !ok {
			panic(fmt.Sprintf("%s: invalid handler #%d (%T)\n", context, i, arg))
		}
		handlers = append(handlers, handler)
	}

	return handlers
}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Check the panic message for the index and %T — the offending argument's type tells you what's wrong.
2. Ensure each handler matches a supported signature: func(fiber.Ctx) error is the canonical Fiber handler.
3. If wrapping handlers, return the exact signature Fiber expects.
4. Run go vet / staticcheck to catch signature mismatches at build time.

Example fix

// before: wrong signature (no error return)
app.Get("/", func(c fiber.Ctx) {})

// after: canonical Fiber handler
app.Get("/", func(c fiber.Ctx) error {
    return c.SendString("ok")
})

func isFiberHandler(h any) bool {
    switch h.(type) {
    case fiber.Handler, func(fiber.Ctx),
        func(fiber.Req, fiber.Res) error, func(fiber.Req, fiber.Res),
        func(http.ResponseWriter, *http.Request), http.Handler, http.HandlerFunc,
        fasthttp.RequestHandler, func(*fasthttp.RequestCtx) error:
        return true
    }
    return false
}

When it happens

Trigger: Passing a value to app.Get(path, X) where X is not one of the ~17 supported handler signatures (fiber.Handler, func(Ctx), Express-style funcs, net/http handlers, fasthttp handlers). E.g. passing a struct, an int, a func with the wrong signature, or a wrapped interface.

Common situations: Refactor changing a handler signature; passing a method value with wrong receiver type; typo'd import using a different Ctx type; passing middleware that returns the wrong type; copy-paste between frameworks.

Related errors

• use: invalid handler %v\n
• route handler 'fn' cannot be nil
• fiber: Config.RegexHandler return type must support MatchStr
• fiber: Config.RegexHandler must not return nil
• fiber: Config.RegexHandler must be a non-nil function

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/78086661bb89631f.json. Report an issue: GitHub.