karatelabs/karate · error

Invalid escape '\ ' in JSON string

Error message

Invalid escape '\${esc}' in JSON string

What it means

Thrown by JsonParser.parseString when a backslash inside a JSON string is followed by a character that is not a valid JSON escape ('"', '\\', '/', 'b', 'f', 'n', 'r', 't', 'u'). RFC 8259 defines a closed set of escape sequences; anything else is invalid.

Solutions

1. Replace the invalid escape with the correct JSON escape — for a literal backslash use '\\'.
2. Remove the backslash if the character should be literal (e.g. "it's" needs no escape in JSON).
3. Convert non-JSON escapes: '\x41' → '\u0041'; drop '\'' → "'".
4. Serialize data with a JSON library instead of writing escaped strings by hand.

Example fix

// before
'{"re": "a\x41b"}'
// after
'{"re": "a\\x41b"}' // or '{"re": "a\u0041b"}'

// JSON allows only " \ / b f n r t u after backslash
private static final java.util.regex.Pattern BAD_ESCAPE =
    java.util.regex.Pattern.compile("\\\\[^\"\\\\/bfnrtu]");
static void assertNoBadEscapes(String json) {
    java.util.regex.Matcher m = BAD_ESCAPE.matcher(json);
    if (m.find()) throw new IllegalArgumentException("Invalid escape at index " + m.start() + ": " + m.group());
}

try {
    return Json.parse(raw);
} catch (JsonSyntaxException e) {
    if (e.getMessage().startsWith("Invalid escape")) {
        throw new IllegalArgumentException("Unsupported escape in JSON string: use \\\\ for a literal backslash", e);
    }
    throw e;
}

When it happens

Trigger: Parsing JSON strings containing escapes like '\x41', '\' (single-quote escape, valid in JS but not JSON), or a bare '\' before a normal letter such as '\p' — common when JavaScript/other-language string literals are pasted into JSON.

Common situations: Serializing regex patterns or Windows paths without doubling backslashes, using JS-style escapes ('\'', '\xNN') in JSON, template output that didn't escape properly, hand-editing escaped strings.

Understand the failure class

Background: JSON parse error: "Unexpected token" / "not valid JSON" / "failed to parse" — what JSON parsers are really complaining about — this error's family across 45 libraries.

Related errors

• Unexpected end of JSON input in string escape
• Expected ':' after object key
• Expected string key in object
• Invalid control character in JSON string
• Invalid literal — expected 'false'

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/a073d660dce9a088. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsonParser.java:224

                            sb.append('\b');
                            break;
                        case 'f':
                            sb.append('\f');
                            break;
                        case 'n':
                            sb.append('\n');
                            break;
                        case 'r':
                            sb.append('\r');
                            break;
                        case 't':
                            sb.append('\t');
                            break;
                        case 'u':
                            sb.append(parseHex4());
                            break;
                        default:
                            throw syntaxError("Invalid escape '\\" + esc + "' in JSON string");
                    }
                    continue;
                }
                if (c < 0x20) {
                    throw syntaxError("Invalid control character in JSON string");
                }
                sb.append(c);
                pos++;
            }
            throw syntaxError("Unterminated JSON string");
        }

        private char parseHex4() {
            if (pos + 4 > len) {
                throw syntaxError("Invalid \\u escape in JSON string");
            }
            int v = 0;
            for (int i = 0; i < 4; i++) {

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