gohugoio/hugo · error

invalid output format configuration; wrong type for media ty

Error message

invalid output format configuration; wrong type for media type, expected string (e.g. text/html), got %T

What it means

Raised by the same decode hook (output/config.go:124) when an output format's mediaType value is neither an already-resolved media.Type nor a string. The hook only knows how to look up string values like "text/html"; a map, number, list, or other structure is a malformed configuration and is rejected with the offending Go type.

Source

Thrown at output/config.go:124

						continue
					}
					if strings.EqualFold(keyStr, "mediaType") {
						// If mediaType is a string, look it up and replace it
						// in the map.
						vv := dataVal.MapIndex(key)
						vvi := vv.Interface()

						switch vviv := vvi.(type) {
						case media.Type:
						// OK
						case string:
							mediaType, found := mediaTypes.GetByType(vviv)
							if !found {
								return c, fmt.Errorf("media type %q not found", vviv)
							}
							dataVal.SetMapIndex(key, reflect.ValueOf(mediaType))
						default:
							return nil, fmt.Errorf("invalid output format configuration; wrong type for media type, expected string (e.g. text/html), got %T", vvi)
						}
					}
				}
			}
			return c, nil
		},
	}

	decoder, err := mapstructure.NewDecoder(config)
	if err != nil {
		return err
	}

	if err = decoder.Decode(input); err != nil {
		return fmt.Errorf("failed to decode output format configuration: %w", err)
	}

	return nil

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Set mediaType to a plain string such as "text/html" in the output format definition.
2. Move any suffix/structural configuration into a [mediaTypes.'<type>'] block and reference it by name from the output format.
3. Validate generated config (YAML/JSON) to ensure mediaType serializes as a scalar string.

Example fix

# before (yaml)
outputFormats:
  MyFormat:
    mediaType:
      type: text/html

# after
outputFormats:
  MyFormat:
    mediaType: text/html

// In outputFormats config, mediaType must be a plain string:
[outputFormats.myformat]
  mediaType = 'text/html'   # correct
# mediaType = { type = 'text/html' }  # wrong: table/map, will fail

func validMediaTypeValue(v any) bool { _, ok := v.(string); return ok }

When it happens

Trigger: Decoding [outputFormats] config where mediaType is given as a table/map (e.g. an inline object with type/suffixes fields), a list, or a number instead of a plain "type/subtype" string.

Common situations: Confusing the [mediaTypes] definition shape (which is a table with suffixes) with the outputFormats reference (which must be a string); YAML config where mediaType is accidentally nested as a mapping; programmatic config generation emitting an object instead of a string.

Related errors

• failed to resolve output formats %v: %w
• media type %q not found
• failed to decode output format configuration: %w
• unknown output format %q for kind %q
• unknown default output format %q

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/fad4182b78b85c1a.json. Report an issue: GitHub ↗.