gohugoio/hugo · error

must be all lower case and no spaces

Error message

must be all lower case and no spaces

What it means

`paths.ValidateIdentifier` enforces Hugo's basic path-normalization rule: an identifier must already equal its normalized form (lowercase, no spaces). It's used to validate user-supplied keys such as output-format, media-type, or segment/config identifiers so that internal path-based lookups, which normalize aggressively, can't silently mismatch the configured name.

Source

Thrown at common/paths/pathparser.go:1092

func HasExt(p string) bool {
	for i := len(p) - 1; i >= 0; i-- {
		if p[i] == '.' {
			return true
		}
		if p[i] == '/' {
			return false
		}
	}
	return false
}

// ValidateIdentifier returns true if the given string is a valid identifier according
// to Hugo's basic path normalization rules.
func ValidateIdentifier(s string) error {
	if s == NormalizePathStringBasic(s) {
		return nil
	}
	return fmt.Errorf("must be all lower case and no spaces")
}

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Rename the identifier in your config to all lowercase with no spaces (use hyphens if separation is needed), matching `NormalizePathStringBasic` output.
2. Update every reference to the old identifier (front matter `outputs`, templates) to the new lowercase name.

Example fix

# before (hugo.toml)
[outputFormats."My Format"]
# after
[outputFormats.myformat]

// Validate identifiers (e.g. custom path segments/tokens) before passing to Hugo:
func validSegment(s string) bool {
    return s == strings.ToLower(s) && !strings.ContainsAny(s, " \t")
}

When it happens

Trigger: Providing an identifier in site configuration that contains uppercase letters or spaces where Hugo calls `ValidateIdentifier` — e.g. defining custom output format or media type names, cache/segment keys, or similar config identifiers like `name = "My Format"` instead of `myformat`.

Common situations: Copying human-readable titles into config `name` fields; migrating configs where older Hugo versions were lenient; camelCase identifiers like `ampPages` used where a lowercase key is required.

Related errors

• redirects must have either From or FromRe set
• failed to create config from result: %w
• failed to create config: %w
• failed to decode %q: %w
• language name cannot be empty

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/a2a649c016fc347d.json. Report an issue: GitHub ↗.