gofiber/fiber · error

failed to write string: %w

Error message

failed to write string: %w

What it means

Returned by buildRouteURL (router.go:246) when writing a static (constant) path segment into the reusable bytebufferpool buffer fails during URL construction (e.g. via app.Get route reverse-lookup). The underlying io.Writer error is wrapped with %w so callers can inspect the root cause. In practice a bytebufferpool.ByteBuffer WriteString almost never errors, so seeing this indicates a corrupted/stressed buffer or a custom buffer wrapper.

Source

Thrown at router.go:246

// to ensure consistent URL generation behavior across APIs.
//
// Parameter resolution uses a deterministic three-step lookup:
//  1. Exact key match on segment.ParamName
//  2. Case-insensitive fallback picking the lexicographically-smallest matching key (when !caseSensitive)
//  3. Greedy parameter fallback for wildcard (*) and plus (+) parameters
func buildRouteURL(route *Route, params Map) (string, error) {
	if len(route.routeParser.segs) == 0 {
		return 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

View on GitHub (pinned to 9a4c7e57fe)

Solutions

1. Treat the wrapped error as the source of truth: inspect err via errors.Unwrap/is to find the real cause (OOM, closed buffer, etc.).
2. If using a custom/forked bytebufferpool, restore the upstream version to eliminate the non-standard WriteString failure.
3. Free memory / reduce concurrency if the failure correlates with allocation pressure.

Example fix

// before: ignoring the returned error
url, _ := buildRouteURL(route, params)

// after: surface and log the root cause
url, err := buildRouteURL(route, params)
if err != nil {
    return fmt.Errorf("build url for %s: %w", route.Path, err)
}

url, err := buildRouteURL(route, params)
if err != nil {
    // err already wraps the writer failure; log and degrade
    log.Printf("route url build failed: %v", err)
    return fallbackURL
}

When it happens

Trigger: Calling any route-URL builder (e.g. a helper that calls buildRouteURL with a *Route containing constant segments) where the underlying buffer's WriteString returns a non-nil error. This happens only when the pooled bytebufferpool buffer has been replaced/tampered with or the process is in an OOM/panic-recovery state.

Common situations: Almost never seen in normal operation because bytebufferpool.ByteString cannot fail. Could surface in test harnesses that swap the pool, in fuzzing runs that exhaust memory, or after a shared buffer is closed concurrently by misuse.

Related errors

• min constraint requires an argument
• max constraint requires an argument
• range constraint requires two arguments
• regex constraint requires a pattern argument
• parse constraint arg: %w

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/dfc48efc19c234b6.json. Report an issue: GitHub.