invalid output format configuration; wrong type for media…

Error message

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

What it means

Thrown in the mapstructure decode hook (output/config.go:124) when a custom output format's 'mediaType' field is present but is neither a string nor an already-resolved media.Type. The %T shows the offending type. Hugo expects mediaType to be a string like "text/html".

Solutions

Set mediaType to a plain string: mediaType = "text/html".
If the value comes from a template or script, ensure it is serialized as a string scalar.
Run hugo config to inspect the resolved value and confirm its type.

Example fix

# before
[outputFormats.MYFORMAT]
mediaType = ["text/html"]

# after
[outputFormats.MYFORMAT]
mediaType = "text/html"

Defensive patterns

Strategy: type-guard

Validate before calling

// mediaType must be a string scalar before decode.
func ensureMediaTypeIsString(v any) error {
    if _, ok := v.(string); !ok {
        return fmt.Errorf("mediaType must be string, got %T", v)
    }
    return nil
}

Type guard

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

Try / catch

if err := output.DecodeConfig(mediaTypes, in); err != nil {
    // Distinguish type errors from missing media types in the message.
    return fmt.Errorf("outputFormats decode: %w", err)
}

Prevention

Quote mediaType values: mediaType = "text/html".
Avoid list/scalar values for mediaType.
Lint config for type consistency.

When it happens

Trigger: Setting mediaType to a number (mediaType = 123), a boolean, an array (mediaType = ["text/html"]), or a nested map; WeaklyTypedInput is on but does not cover the type mismatch for this specific non-string scalar.

Common situations: TOML/YAML config where mediaType is accidentally quoted as a list or left as a bare number; tooling that injects mediaType from a variable of the wrong type; migrating config and dropping the surrounding quotes incorrectly.

Related errors

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/fad4182b78b85c1a. Report an issue: GitHub.

Appendix: 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 52c9bd7908)