gofiber/fiber · warning

failed to write string

Error message

failed to write string: %w

What it means

Returned by buildRouteURL() (Route.URL / GetRouteURL) when writing a constant route segment to a bytebufferpool.Buffer fails. bytebufferpool.Buffer.WriteString only errors if the underlying write exceeds MaxInt32 bytes — functionally unreachable for URL construction. Encountering this error indicates memory corruption or an absurdly large constant route path.

Solutions

1. Treat as a fatal/panic condition — if WriteString on a pooled buffer errors, memory integrity is suspect; restart the process.
2. Audit route registrations for pathological constant paths (e.g., a path built from an unbounded external string).
3. Ensure no code retains bytebufferpool.Buffers past their Put() (double-use).

// Bound constant route paths at registration time.
func registerRoute(app *fiber.App, path string) {
    if len(path) > 4096 { panic("route path too long: " + path) }
    app.Get(path, /* ... */)
}

if url, err := route.URL(params); err != nil {
    // Practically unreachable; treat as fatal.
    log.Panicf("route.URL constant write failed: %v", err)
} else {
    _ = url
}

When it happens

Trigger: Calling route.URL(params) or res.GetRouteURL(name, params) where the route has constant segments (not parameters), and buf.WriteString(segment.Const) returns an error. Realistically never fires under normal operation.

Common situations: This is a defensive guard. If it appears, suspect a corrupted bytebufferpool state from unsafe concurrent reuse of a pooled buffer, or a route registered with a multi-gigabyte constant path (misconfiguration).

Related errors

• add: invalid http method
• betweenLen constraint requires two arguments
• [CORS] Invalid origin format after normalization:
• datetime constraint requires a layout argument
• Domain pattern cannot be empty

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/dfc48efc19c234b6. Report an issue: GitHub.

Appendix: source

Thrown at router.go:263

	// "/internal " — reachable through "/internal%20" under UnescapePath — reads
	// as "/internal", a different route. No URL names either.
	normalized := urlnorm.AsBrowserReads(route.Path)
	if normalized != route.Path || urlnorm.LeadingSlashes(normalized) > 1 {
		return "", ErrRouteNotRepresentable
	}

	if len(route.routeParser.segs) == 0 {
		return urlnorm.RootedPath(route.Path), nil
	}

	buf := bytebufferpool.Get()
	defer bytebufferpool.Put(buf)

	for _, segment := range route.routeParser.segs {
		if !segment.IsParam {
			_, err := buf.WriteString(segment.Const)
			if err != nil {
				return "", fmt.Errorf("failed to write string: %w", err)
			}
			continue
		}

		var (
			val   any
			found bool
		)

		// Prefer an exact parameter name match
		if val, found = params[segment.ParamName]; !found && !route.caseSensitive {
			// Fall back to a case-insensitive match using a deterministic winner
			var matchedKey string
			foundMatch := false
			for key := range params {
				if utils.EqualFold(key, segment.ParamName) && (!foundMatch || key < matchedKey) {
					matchedKey = key
					foundMatch = true

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