decode icon URL for stable diagram hash: missing value

Error message

decode icon URL for stable diagram hash: missing value

What it means

This error comes from the internal parser that scans the raw JSON of an icon URL shape while building a stable diagram hash (hash must be identical across serialization variants). It skips whitespace after a key and, if the JSON ends before any value appears, it reports that the value is missing. It guards the hashing pipeline from truncated or hand-mangled raw JSON.

Source

Thrown at d2target/d2target.go:344

		return nil, err
	}
	out.Write(b[lastWrite:])
	return out.Bytes(), nil
}

func hashJSONValueStart(b []byte, keyEnd int) (int, error) {
	i := keyEnd
	for i < len(b) && (b[i] == ' ' || b[i] == '\t' || b[i] == '\r' || b[i] == '\n') {
		i++
	}
	if i < len(b) && b[i] == ':' {
		i++
	}
	for i < len(b) && (b[i] == ' ' || b[i] == '\t' || b[i] == '\r' || b[i] == '\n') {
		i++
	}
	if i >= len(b) {
		return 0, fmt.Errorf("decode icon URL for stable diagram hash: missing value")
	}
	return i, nil
}

func stableHashURL(raw json.RawMessage) ([]byte, error) {
	fields := make(map[string]json.RawMessage, len(legacyURLFieldOrder))
	if err := json.Unmarshal(raw, &fields); err != nil {
		return nil, fmt.Errorf("decode icon URL for stable diagram hash: %w", err)
	}
	if len(fields) != len(legacyURLFieldOrder) {
		return nil, fmt.Errorf("decode icon URL for stable diagram hash: got %d fields, want %d", len(fields), len(legacyURLFieldOrder))
	}

	var out bytes.Buffer
	out.WriteByte('{')
	for i, name := range legacyURLFieldOrder {
		value, ok := fields[name]
		if !ok {

View on GitHub (pinned to 0d69dca6f5)

Solutions

Inspect the raw icon URL JSON passed into hashing and make sure each key is followed by a complete value
Regenerate the diagram JSON from the library instead of editing it by hand
Validate the JSON parses with encoding/json before feeding it to stable hashing
Check for code that trims or truncates serialized payloads before hashing

Example fix

// before
raw := []byte(`{"val":"`)
// after
raw := []byte(`{"val":"https://example.com/icon.svg"}`)

Defensive patterns

Strategy: validation

Validate before calling

raw := iconURLRawJSON
if !json.Valid(raw) {
    return fmt.Errorf("icon URL raw JSON is not valid JSON: %s", raw)
}
var probe map[string]json.RawMessage
if err := json.Unmarshal(raw, &probe); err != nil {
    return err
}
if len(probe) != len(legacyURLFieldOrder) {
    return fmt.Errorf("icon URL JSON has %d fields, want %d", len(probe), len(legacyURLFieldOrder))
}

Type guard

func isValidIconURLJSON(raw json.RawMessage) bool {
    if !json.Valid(raw) {
        return false
    }
    var probe map[string]json.RawMessage
    return json.Unmarshal(raw, &probe) == nil && len(probe) == len(legacyURLFieldOrder)
}

Try / catch

hashed, err := stableHashURL(raw)
if err != nil {
    log.Printf("icon URL hash skipped, malformed JSON %q: %v", raw, err)
    return err
}

Prevention

Always produce icon URL JSON via d2's own marshaling, never by hand
Run json.Valid on any raw JSON before hashing
Never truncate serialized payloads before passing them downstream

When it happens

Trigger: Calling the stable-hash code path with a raw JSON icon URL payload that ends right after a key/colon, i.e. truncated JSON with no value token before EOF.

Common situations: Manually constructed or partially written JSON for icon nearShape/shape URLs, external tools truncating output, or version drift where a serializer emits incomplete objects into the hash input.

Related errors

AI-assisted analysis of d2lang/d2@0d69dca6f5 (2026-08-31). Data as JSON: /api/errors/0f52d4af85b90931. Report an issue: GitHub.