gohugoio/hugo · error

media type %q not found

Error message

media type %q not found

What it means

Raised in the output-format decode hook (output/config.go:120) when an output format's mediaType field names a media type that isn't registered. Hugo resolves the string (e.g. "text/html") against the site's configured media types via mediaTypes.GetByType; if there's no match, the output format cannot be wired to a MIME type and config decoding fails.

Source

Thrown at output/config.go:120

				for _, key := range dataVal.MapKeys() {
					keyStr, ok := key.Interface().(string)
					if !ok {
						// Not a string key
						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 {

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Declare the custom media type in [mediaTypes] before referencing it: add a [mediaTypes.'<type/subtype>'] block with its suffixes.
2. Fix typos — mediaType must exactly match a known type like text/html, application/json, text/calendar.
3. When copying an outputFormats definition from another project, copy its mediaTypes definition too.
4. Run 'hugo config' to inspect the effective mediaTypes list your build knows about.

Example fix

# before
[outputFormats.MyFeed]
mediaType = "application/x-myfeed"

# after
[mediaTypes.'application/x-myfeed']
suffixes = ["myfeed"]

[outputFormats.MyFeed]
mediaType = "application/x-myfeed"

// Every mediaType named in outputFormats must exist in mediaTypes config
[mediaTypes.'application/vnd.api+json']
  suffixes = ['json']
[outputFormats.api]
  mediaType = 'application/vnd.api+json'

When it happens

Trigger: Decoding [outputFormats] configuration where mediaType is a string not present in the (built-in plus [mediaTypes]-defined) media type set — including custom types used before being declared, or lookups happening in a context where the custom [mediaTypes] block wasn't merged.

Common situations: Typos like "text/htm" or "application/josn"; defining a custom output format with mediaType = "application/x-foo" without a matching [mediaTypes.'application/x-foo'] block; copying output-format snippets between sites without also copying the mediaTypes definition; suffix-style values ("json") where a full type/subtype is required.

Related errors

• failed to resolve output formats %v: %w
• invalid output format configuration; wrong type for media ty
• 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/f0bec5f6f63afd96.json. Report an issue: GitHub ↗.