d2lang/d2 · error

unsupported content encoding %q

Error message

unsupported content encoding %q

What it means

decodeContentEncoding supports only gzip, br (brotli), and deflate. Any other Content-Encoding value (or an unrecognized token) hits the default branch and returns this error. The bundler cannot decode the response body, so the image fetch fails.

Source

Thrown at lib/imgbundler/imgbundler.go:270

func decodeContentEncoding(buf []byte, contentEncoding string) ([]byte, error) {
	encodings := strings.Split(contentEncoding, ",")
	for i := len(encodings) - 1; i >= 0; i-- {
		encoding := strings.TrimSpace(strings.ToLower(encodings[i]))
		if encoding == "" || encoding == "identity" {
			continue
		}

		var err error
		switch encoding {
		case "gzip", "x-gzip":
			buf, err = gunzip(buf)
		case "br":
			buf, err = readDecoded(brotli.NewReader(bytes.NewReader(buf)))
		case "deflate":
			buf, err = inflate(buf)
		default:
			return nil, fmt.Errorf("unsupported content encoding %q", encoding)
		}
		if err != nil {
			return nil, err
		}
	}
	return buf, nil
}

func gunzip(buf []byte) ([]byte, error) {
	r, err := gzip.NewReader(bytes.NewReader(buf))
	if err != nil {
		return nil, err
	}
	defer r.Close()
	return readDecoded(r)
}

func inflate(buf []byte) ([]byte, error) {

View on GitHub (pinned to 0d69dca6f5)

Solutions

1. Configure the origin/CDN to serve gzip, br, or deflate (or identity) for images — images rarely need zstd
2. Strip Accept-Encoding for zstd in any client/transport config you control so the server falls back to a supported encoding
3. If you control the library, add a case for the encoding (e.g. zstd) in decodeContentEncoding
4. Work around by fetching the asset without compression (Accept-Encoding: identity)

Example fix

// before
req.Header.Set("Accept-Encoding", "gzip, deflate, br, zstd")
// after
req.Header.Set("Accept-Encoding", "gzip, deflate, br")

allowed := map[string]bool{"gzip": true, "br": true, "deflate": true, "": true}
req.Header.Set("Accept-Encoding", "gzip, deflate, br") // never advertise zstd/compress

if _, _, err := httpGet(ctx, client, href); err != nil {
	if strings.Contains(err.Error(), "unsupported content encoding") {
		// refetch with Accept-Encoding: identity
	}
}

When it happens

Trigger: Server advertises an encoding outside the supported set — e.g. zstd, compress, or a nonstandard token like 'x-gzip' — in the Content-Encoding header of an image response.

Common situations: Modern origins negotiating zstd (supported by browsers but not this client), proxies injecting 'compress', or misconfigured servers emitting unusual encoding tokens.

Related errors

• failed to decode %q response for %s: %w
• expected status 200 but got %d %s
• no color stops in gradient
• cannot read a directory
• %s is not a supported format. Supported formats are: %s

AI-assisted analysis of d2lang/d2@0d69dca6f5 (2026-08-31). Data as JSON: /api/errors/9a6fbb6be1beed20. Report an issue: GitHub.