gohugoio/hugo · error

failed to detect target data serialization format

Error message

failed to detect target data serialization format

What it means

Thrown by transform.Remarshal when the target format argument is not one of the recognized serialization formats (tpl/transform/remarshal.go:81-86, via toFormatMark). The format is lowercased and trimmed, then matched; anything other than json/yaml/toml fails.

Solutions

1. Use one of: json, yaml, or toml.
2. Check for typos and stray whitespace in the format argument.
3. If the format comes from a variable, validate it against the allowed set.

Example fix

// before
{{ transform.Remarshal "ymal" .Data }}
// after
{{ transform.Remarshal "yaml" .Data }}

{{/* Only json, yaml, toml allowed */}}
{{ transform.Remarshal "yaml" .Data }}

When it happens

Trigger: Calling {{ transform.Remarshal "xml" .Data }} or {{ transform.Remarshal "JSON5" .Data }} (case aside, unrecognized names) or with a typo like {{ transform.Remarshal "ymal" .Data }}.

Common situations: Typos in the format string; assuming an unsupported format like XML/INI/CSV is supported; trailing spaces or newlines in the format variable.

Related errors

• failed to detect format from content
• first argument must be a map
• must provide at least one argument
• transform.Highlight: expects at most 3 arguments
• unmarshal takes 1 or 2 arguments

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/5e5106b91f09e2ad. Report an issue: GitHub.

Appendix: source

Thrown at tpl/transform/remarshal.go:86

	for k, v := range m {
		switch t := v.(type) {
		case map[string]any:
			applyMarshalTypes(t)
		case float64:
			i := int64(t)
			if t == float64(i) {
				m[k] = i
			}
		}
	}
}

func toFormatMark(format string) (metadecoders.Format, error) {
	if f := metadecoders.FormatFromString(format); f != "" {
		return f, nil
	}

	return "", errors.New("failed to detect target data serialization format")
}

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