caddyserver/caddy · error

encoder does not specify an Accept-Encoding value

Error message

encoder does not specify an Accept-Encoding value

What it means

Returned by Encode.addEncoding when an Encoding implementation's AcceptEncoding() returns an empty string. The Accept-Encoding token is used as the pool key and for client negotiation, so an empty value makes the encoder unusable.

Source

Thrown at modules/caddyhttp/encode/encode.go:206

		}
	}

	err := next.ServeHTTP(w, r)
	// If there was an error, disable encoding completely
	// This prevents corruption when handle_errors processes the response
	if err != nil {
		if ew, ok := w.(*responseWriter); ok {
			ew.disabled = true
		}
	}

	return err
}

func (enc *Encode) addEncoding(e Encoding) error {
	ae := e.AcceptEncoding()
	if ae == "" {
		return fmt.Errorf("encoder does not specify an Accept-Encoding value")
	}
	if _, ok := enc.writerPools[ae]; ok {
		return fmt.Errorf("encoder already added: %s", ae)
	}
	if enc.writerPools == nil {
		enc.writerPools = make(map[string]*sync.Pool)
	}
	enc.writerPools[ae] = &sync.Pool{
		New: func() any {
			return e.NewEncoder()
		},
	}
	return nil
}

// openResponseWriter creates a new response writer that may (or may not)
// encode the response with encodingName. The returned response writer MUST
// be closed after the handler completes.

View on GitHub (pinned to 50e54ee279)

Solutions

1. Implement AcceptEncoding() to return a valid, unique token like "br" or "myzip"
2. Register the token with IANA-style naming and keep it consistent with what clients send

Example fix

// before
func (E) AcceptEncoding() string { return "" }
// after
func (E) AcceptEncoding() string { return "myzip" }

// compile-time: your encoder satisfies the full contract
var _ encode.Encoding = (*MyEncoding)(nil)

When it happens

Trigger: Any custom encoder module whose AcceptEncoding() method returns "" being loaded during encode provisioning.

Common situations: Plugin authors implementing encode.Encoding but forgetting or stubbing AcceptEncoding().

Related errors

• adding encoding %s: %v
• encoder already added: %s
• loading encoder modules: %v
• encoding %s not enabled
• encoding %s is duplicated in prefer

AI-assisted analysis of caddyserver/caddy@50e54ee279 (2026-08-15). Data as JSON: /api/errors/eecb110f4130a1b8. Report an issue: GitHub.