What causes "failed to decompress: %w" in d2lang/d2?

Verify the input was compressed with brotli (CompressBrotli) and not gzip/zlib/base64-encoded again

failed to decompress: %w

Error message

failed to decompress: %w

What it means

DecompressBrotli feeds compressed bytes through a brotli reader and returns this wrapped error if io.ReadAll fails mid-stream. It indicates the input is not valid brotli data or the stream is truncated/corrupt.

Source

Thrown at lib/compression/brotli.go:17

// d2 uses compression for compressing large static assets when its built into WASM for d2.js
package compression

import (
	"bytes"
	"fmt"
	"io"

	"github.com/andybalholm/brotli"
)

func DecompressBrotli(compressed []byte) (string, error) {
	reader := brotli.NewReader(bytes.NewReader(compressed))

	decompressed, err := io.ReadAll(reader)
	if err != nil {
		return "", fmt.Errorf("failed to decompress: %w", err)
	}

	return string(decompressed), nil
}

View on GitHub (pinned to 0d69dca6f5)

Solutions

Verify the input was compressed with brotli (CompressBrotli) and not gzip/zlib/base64-encoded again
Check the payload wasn't truncated (compare lengths/hashes with the source)
Test round-trip: compress with the same lib then decompress to isolate corruption
Wrap the call so callers see the underlying brotli error via %w for diagnosis

Defensive patterns

Strategy: validation

Validate before calling

if len(compressed) == 0 {
    return errors.New("empty payload, not brotli data")
}
if compressed[0] != 0x1b && (compressed[0]&0x01) == 0 {
    return fmt.Errorf("first byte %#x does not look like brotli", compressed[0])
}

Type guard

func looksLikeBrotli(b []byte) bool {
    // brotli streams start with WBITS; simple sanity: non-empty and not ASCII text
    return len(b) > 0 && b[0] != '<' && b[0] != '{' && b[0] != 'H'
}

Try / catch

out, err := compression.DecompressBrotli(data)
if err != nil {
    var brErr brotli.Error
    if errors.As(err, &brErr) {
        log.Printf("brotli corruption: %v", brErr)
    }
    return err
}

Prevention

Round-trip test payloads with CompressBrotli/DecompressBrotli at write time
Do not re-encode brotli bytes (e.g. base64) without decoding before decompression
Store payloads with integrity checks (length/hash) to detect truncation

When it happens

Trigger: Calling DecompressBrotli with bytes that are not brotli-compressed (plain text, gzip, base64 still encoded), or a truncated brotli stream.

Common situations: Decoding embedded/compressed diagram links where the payload was double-encoded, corrupted in transit/storage, or produced by a different compression algorithm.

Related errors

AI-assisted analysis of d2lang/d2@0d69dca6f5 (2026-08-31). Data as JSON: /api/errors/5ed4bc1ff9100056. Report an issue: GitHub.