gohugoio/hugo · error

npm pack: failed to marshal JSON: %w

Error message

npm pack: failed to marshal JSON: %w

What it means

Hugo's `hugo mod npm pack` command merges package.json files from the project and its Hugo Modules dependencies, then writes the result with a JSON encoder. This error wraps a failure of `json.Encoder.Encode` in `writeJSON` (modules/npm/package_builder.go:362-371), meaning the assembled package data could not be serialized to JSON before writing package.json or package.hugo.json.

Source

Thrown at modules/npm/package_builder.go:368

}

func detectIndent(data []byte) string {
	for line := range bytes.SplitSeq(data, []byte("\n")) {
		trimmed := bytes.TrimLeft(line, " \t")
		if len(trimmed) < len(line) && len(trimmed) > 0 && trimmed[0] == '"' {
			return string(line[:len(line)-len(trimmed)])
		}
	}
	return "  "
}

func writeJSON(fs afero.Fs, filename string, v any) error {
	buf := new(bytes.Buffer)
	encoder := json.NewEncoder(buf)
	encoder.SetEscapeHTML(false)
	encoder.SetIndent("", strings.Repeat(" ", 2))
	if err := encoder.Encode(v); err != nil {
		return fmt.Errorf("npm pack: failed to marshal JSON: %w", err)
	}
	return afero.WriteFile(fs, filename, buf.Bytes(), 0o666)
}

func toStringSlice(v any) []string {
	if v == nil {
		return nil
	}
	if s, ok := v.([]any); ok {
		var out []string
		for _, item := range s {
			if str, ok := item.(string); ok {
				out = append(out, str)
			}
		}
		return out
	}
	return nil

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Validate the project's package.json and package.hugo.json with a JSON linter (e.g. `jq . package.hugo.json`) and fix malformed values.
2. Check package.hugo.json files in imported modules/themes (under _vendor or the module cache) for invalid values and update or replace the offending module.
3. Run `hugo mod clean` and `hugo mod get -u` to refresh the module cache, then retry `hugo mod npm pack`.
4. If it persists, minimize the merged inputs to isolate which file introduces the unmarshalable value and report it upstream.

// package.hugo.json must be valid JSON before build
jq . package.hugo.json && jq . package.json

When it happens

Trigger: Running `hugo mod npm pack` where the merged package data contains a value Go's encoding/json cannot marshal — e.g. a NaN/Inf float, a cyclic structure, or a non-string map key introduced from a malformed package.hugo.json in the project or an imported module.

Common situations: A theme or module ships a package.hugo.json with unusual values that survive decoding but fail re-encoding; hand-edited package.hugo.json files; module version upgrades that change the dependency metadata being merged. In practice this is rare because JSON-decoded data normally round-trips cleanly.

Related errors

• unsupported format: %q
• must not be run from the file system root
• failed to detect format from content
• too many arguments to jsonify
• unmarshal failed: %w

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/af9ca3678dff6dda.json. Report an issue: GitHub ↗.