Error log: XContentParseException – The malformed JSON

Error log: This is a low-level error that means the JSON you sent in your request body is broken.

JSON

None
{

  "error": {

    "root_cause": [

      {

        "type": "x_content_parse_exception",

        "reason": "[1:30] [my_index] 

                   Unexpected character ('\"' (code 34)): 

                   was expecting a colon to separate field name and value",

        "line": 1,

        "col": 30

      }

    ],

    "type": "x_content_parse_exception",

    // ...

  },

  "status": 400

}

Why… is this happening? This is not an OpenSearch logic error; it’s a JSON syntax error. The OpenSearch parser (called XContent) tried to read your request body but failed because the JSON itself is invalid.

This is different from ParsingException (Blog #14), which is for valid JSON but invalid query logic. This error means the JSON itself is malformed.

The error message is extremely helpful:

• [1:30]: The error is on line: 1, column: 30.
• Unexpected character ('"'): It found a " character.
• was expecting a colon It wanted a : character.

This means you probably wrote something like {"my_field" "my_value"} instead of {"my_field": "my_value"}.

Other common causes:

• A trailing comma after the last item in a list or object.
• A missing comma between items.
• Using single quotes (‘) instead of the required double quotes (“).

Best Practice:

1. Use a JSON linter: This is the fastest fix. Copy the entire JSON request body and paste it into a “JSON Linter” or “JSON Validator” website. It will instantly highlight the syntax error.
2. Read the error message: The line, col, and reason tell you exactly where to look and what’s wrong.
3. Use dashboards dev tools: When building queries, use the Dev Tools console. It has built-in JSON validation and will show a red X next to the line with a syntax error before you even send the request.
4. Use a JSON library: If you are building JSON in your application, do not build it by concatenating strings. Use a proper library (like gson, jackson, json.dumps) to serialize your objects. This guarantees valid JSON every time.

What else can I do? Still can’t see the typo? It happens to everyone. Paste your JSON request into the OpenSearch community forums, and a fresh pair of eyes will likely spot it immediately. For direct help, contact us in The OpenSearch Slack Channel in #General.