Validate JSON online and find the invalid character

Double quoted keys and strings, no trailing commas, no comments, and no special values such as NaN or undefined. Anything JavaScript accepts as an object literal but JSON does not will fail here, which is usually the surprise.

This checks syntax: whether the document parses at all. The schema validator checks structured data for search engines against schema.org types. Use this one for a config file or an API response, that one for JSON-LD markup on a page.

Then the syntax is fine and the shape is not. The API is expecting particular fields, types or value ranges, which is schema validation rather than syntax validation. Check the API documentation for the required fields.

No. It validates syntax only, and reports the top level type and size of what it parsed. For JSON Schema validation you need a tool that takes the schema as a second input.

No. Validation uses the browser JSON parser and nothing leaves the page, so pasting a production payload is safe.