caddyserver/caddy · error

encoding %s is duplicated in prefer

Error message

encoding %s is duplicated in prefer

What it means

Validate-time error from encode: the same encoder name appears more than once in the 'prefer' array. The dedup check map detects the second occurrence.

Source

Thrown at modules/caddyhttp/encode/encode.go:152

			if _, ok := enc.writerPools[encName]; ok {
				enc.Prefer = append(enc.Prefer, encName)
			}
		}
	}

	return nil
}

// Validate ensures that enc's configuration is valid.
func (enc *Encode) Validate() error {
	check := make(map[string]bool)
	for _, encName := range enc.Prefer {
		if _, ok := enc.writerPools[encName]; !ok {
			return fmt.Errorf("encoding %s not enabled", encName)
		}

		if _, ok := check[encName]; ok {
			return fmt.Errorf("encoding %s is duplicated in prefer", encName)
		}
		check[encName] = true
	}

	return nil
}

func isEncodeAllowed(h http.Header) bool {
	return !strings.Contains(h.Get("Cache-Control"), "no-transform")
}

func (enc *Encode) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
	if isEncodeAllowed(r.Header) {
		for _, encName := range AcceptedEncodings(r, enc.Prefer) {
			if _, ok := enc.writerPools[encName]; !ok {
				continue // encoding not offered
			}
			w = enc.openResponseWriter(encName, w, r.Method == http.MethodConnect)

View on GitHub (pinned to 50e54ee279)

Solutions

1. Remove duplicate entries from the prefer array
2. If generating config, deduplicate the list before serializing

Example fix

// before
"prefer": ["zstd", "gzip", "zstd"]
// after
"prefer": ["zstd", "gzip"]

// dedupe before writing config
seen := map[string]bool{}
out := enc.Prefer[:0]
for _, p := range enc.Prefer {
    if !seen[p] { seen[p] = true; out = append(out, p) }
}

When it happens

Trigger: {"prefer": ["zstd", "zstd"]} — a repeated entry in the preference list.

Common situations: Config generated by concatenation or copy-paste producing duplicate entries; YAML anchors merging lists with overlap.

Related errors

• encoding %s not enabled
• quality too low; must be >= %d
• quality too high; must be <= %d
• unexpected compression level, use one of '%s', '%s', '%s', '
• parsing listener address: %v

AI-assisted analysis of caddyserver/caddy@50e54ee279 (2026-08-15). Data as JSON: /api/errors/9144064f75d58c6f. Report an issue: GitHub.