caddyserver/caddy · warning

malformed Content-Type

Error message

malformed Content-Type

What it means

Returned when the parsed media type contains no '/' after strings.Cut, meaning the adapter name cannot be extracted. In practice this is nearly unreachable because mime.ParseMediaType already rejects slash-less types, so it exists as a defensive check. If seen, it indicates a type that passed MIME parsing but has no subtype component.

Source

Thrown at caddyconfig/load.go:201

	}

	ct, _, err := mime.ParseMediaType(contentType)
	if err != nil {
		return nil, nil, caddy.APIError{
			HTTPStatus: http.StatusBadRequest,
			Err:        fmt.Errorf("invalid Content-Type: %v", err),
		}
	}

	// if already JSON, no need to adapt
	if strings.HasSuffix(ct, "/json") {
		return body, nil, nil
	}

	// adapter name should be suffix of MIME type
	_, adapterName, slashFound := strings.Cut(ct, "/")
	if !slashFound {
		return nil, nil, fmt.Errorf("malformed Content-Type")
	}

	cfgAdapter := GetAdapter(adapterName)
	if cfgAdapter == nil {
		return nil, nil, fmt.Errorf("unrecognized config adapter '%s'", adapterName)
	}

	result, warnings, err := cfgAdapter.Adapt(body, nil)
	if err != nil {
		return nil, nil, fmt.Errorf("adapting config using %s adapter: %v", adapterName, err)
	}

	return result, warnings, nil
}

var bufPool = sync.Pool{
	New: func() any {
		return new(bytes.Buffer)

View on GitHub (pinned to 50e54ee279)

Solutions

1. Set a standard type/subtype Content-Type such as text/caddyfile or application/json
2. Omit Content-Type to send raw JSON and bypass adaptation entirely

// require a slash in the media type before calling the API
mt, _, err := mime.ParseMediaType(ct)
if err != nil || !strings.Contains(mt, "/") {
    return fmt.Errorf("Content-Type must be type/subtype, got %q", ct)
}

When it happens

Trigger: A Content-Type value that mime.ParseMediaType accepts yet contains no slash — essentially only possible via exotic or future MIME parser quirks; practically never observed in the wild.

Common situations: Almost none; occasionally reproduced by fuzzing or by clients that send non-standard header values that slip through lenient intermediaries.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• invalid Content-Type: %v
• unrecognized config adapter '%s'
• no metrics registry found
• provisioning admin router module %s: %v
• loading identity issuer modules: %s

AI-assisted analysis of caddyserver/caddy@50e54ee279 (2026-08-15). Data as JSON: /api/errors/eb8fd16b9942f45b. Report an issue: GitHub.