CoplayDev/unity-mcp · error · JsonException

Failed to parse 'properties' JSON string. Raw value: {token}

Error message

Failed to parse 'properties' JSON string. Raw value: {token}

What it means

ExtractProperties accepts the 'properties' field either as a JSON object or as a JSON-encoded string. When it is a string, JToken.Parse is attempted; if the string is not valid JSON a JsonException is thrown embedding the raw value.

Source

Thrown at MCPForUnity/Editor/Tools/Animation/ManageAnimation.cs:101

            if (token == null || token.Type == JTokenType.Null)
            {
                return null;
            }

            if (token is JObject obj)
            {
                return obj;
            }

            if (token.Type == JTokenType.String)
            {
                try
                {
                    return JToken.Parse(token.ToString()) as JObject;
                }
                catch (JsonException ex)
                {
                    throw new JsonException(
                        $"Failed to parse 'properties' JSON string. Raw value: {token}",
                        ex);
                }
            }

            return null;
        }

        private static string NormalizeKey(string key, bool allowAliases)
        {
            if (string.IsNullOrEmpty(key))
            {
                return key;
            }
            if (string.Equals(key, "action", StringComparison.OrdinalIgnoreCase))
            {
                return "action";
            }

View on GitHub (pinned to c21bf496bc)

Solutions

1. Pass 'properties' as a JSON object directly, not as a string.
2. If it must be a string, ensure it is valid JSON (quoted keys, proper braces).
3. Validate the payload with a JSON parser before sending.

Example fix

// before
{ "action": "set", "properties": "scale,rotation" }
// after
{ "action": "set", "properties": { "scale": 1.0, "rotation": 0 } }

if (payload.TryGetValue("properties", out var t) && t.Type == JTokenType.String)
{
    if (!IsValidJson(t.ToString()))
        throw new ArgumentException("'properties' string must be valid JSON.");
}
// bool IsValidJson(string s) => try { JToken.Parse(s); return true; } catch { return false; }

// Pass properties as a JSON object, not a string:
// valid:  { "properties": { "scale": 1.0 } }
// valid:  { "properties": "{ \"scale\": 1.0 }" }  (JSON-encoded string)
// invalid:{ "properties": "scale,rotation" }

When it happens

Trigger: An AI assistant or caller sends 'properties' as a plain delimited string ('scale,rotation'), malformed JSON ('{scale: 1}' in some strict parsers), or a truncated JSON fragment; double-encoding produced a non-JSON string.

Common situations: LLM formats the field as CSV instead of JSON; a serialization layer double-encoded the object; manual payload construction with a syntax error.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Color array must have 3 or 4 elements.
• Failed to parse float at index {i}: '{arr[i]}'
• Failed to parse 'properties' JSON string. Raw value: {token}
• set-pixels must be a JSON object
• import_settings must be a JSON object

AI-assisted analysis of CoplayDev/unity-mcp@c21bf496bc (2026-08-13). Data as JSON: /api/errors/b141151325a545fe. Report an issue: GitHub.