kataras/iris · warning

ErrNotSupportedCompression

ErrNotSupportedCompression

Error message

%w: %s

What it means

compress.Writer.GetEncoding returns ErrNotSupportedCompression wrapped with the empty/failed encoding when the Accept-Encoding negotiation finds no usable offer. negotiateAcceptHeader scans the client's Accept-Encoding header against the offers and falls back to IDENTITY; if even that match yields an empty string, the error is returned. It is reached from AcquireCompressResponseWriter in the middleware chain.

Source

Thrown at context/compress.go:58

	// compression algorithms. Check that error with `errors.Is`.
	ErrNotSupportedCompression = errors.New("compress: unsupported compression")
)

// AllEncodings is a slice of default content encodings.
// See `AcquireCompressResponseWriter`.
var AllEncodings = []string{GZIP, DEFLATE, BROTLI, SNAPPY}

// GetEncoding extracts the best available encoding from the request.
func GetEncoding(r *http.Request, offers []string) (string, error) {
	acceptEncoding := r.Header[AcceptEncodingHeaderKey]

	if len(acceptEncoding) == 0 {
		return "", ErrResponseNotCompressed
	}

	encoding := negotiateAcceptHeader(acceptEncoding, offers, IDENTITY)
	if encoding == "" {
		return "", fmt.Errorf("%w: %s", ErrNotSupportedCompression, encoding)
	}

	return encoding, nil
}

type (
	noOpWriter struct{}

	noOpReadCloser struct {
		io.Reader
	}
)

var (
	_ io.ReadCloser = (*noOpReadCloser)(nil)
	_ io.Writer     = (*noOpWriter)(nil)
)

View on GitHub (pinned to 7bedaf55a0)

Solutions

1. Inspect the client's Accept-Encoding header and confirm at least one encoding matches the middleware offers (gzip, deflate, br, snappy, s2).
2. Check errors.Is(err, compress.ErrNotSupportedCompression) and fall through to serving the response uncompressed instead of failing the request.
3. Adjust middleware offers to include the encodings your real clients send, or ensure IDENTITY is an accepted fallback.
4. If a proxy strips or rewrites Accept-Encoding, fix the proxy config or normalize the header server-side.

Example fix

// before
err := w.GetEncoding(header.Get("Accept-Encoding")) // returns ErrNotSupportedCompression
// after
encoding, err := w.GetEncoding(header.Get("Accept-Encoding"))
if errors.Is(err, compress.ErrNotSupportedCompression) {
    // serve identity (uncompressed) response
    next.ServeHTTP(w, r)
    return
}

ae := r.Header.Get("Accept-Encoding")
if ae != "" && !strings.Contains(ae, "gzip") && !strings.Contains(ae, "deflate") && !strings.Contains(ae, "br") {
    // client accepts no offered encoding; skip compress middleware
}

encoding, err := w.GetEncoding(acceptEncoding)
if err != nil {
    if errors.Is(err, compress.ErrNotSupportedCompression) {
        return "", err // caller should serve identity/uncompressed
    }
    return "", err
}

When it happens

Trigger: Calling GetEncoding (via AcquireCompressResponseWriter / the compress middleware) when the client's Accept-Encoding header values cannot be matched to the configured offers and identity negotiation fails. Note the returned 'encoding' interpolated in the message is empty at this point.

Common situations: Exotic or malformed Accept-Encoding headers from proxies, crawlers, or API clients; middleware configured with offers that exclude all client-accepted encodings; version changes where IDENTITY fallback stopped matching a 'identity;q=0' style header.

Related errors

• compress: response will not be compressed
• compress: request is not compressed
• compress: unsupported compression
• empty form field
• precondition failed

AI-assisted analysis of kataras/iris@7bedaf55a0 (2026-08-30). Data as JSON: /api/errors/fc8f19d5c148e793. Report an issue: GitHub.