Why does valid-looking JSON fail to parse?

Separate JSON from JavaScript

JSON keys and strings use double quotes. The trailing comma below fails strict parsing, as do comments and single quotes. Run it in the validator, inspect the location, then remove the last comma.

{
  "name": "demo",
  "enabled": true,
}

PlainKit reports where parsing detects a problem. The cause may be on the previous line, such as a missing closing quote.

Valid syntax can still lose meaning

{
  "id": 9007199254740993,
  "role": "reader",
  "role": "admin"
}

This example has a large integer and a duplicate key. PlainKit formatting preserves numeric text and warns; conversion and comparison reject ambiguous inputs. A normal JavaScript parse-and-stringify round trip can change the identifier.

Agree on string representations for identifiers and exact integer amounts. Monetary values also need explicit units and decimal conventions; quoting them alone does not validate the business meaning.

Treat each JSONL line as one record

{"id":"001","status":"ready"}
{"id":"002","status":"queued"}

• Select JSONL mode and keep each record on one line.
• Fix failing lines, then review duplicate-key and precision warnings.
• Validate the downloaded result again in the system that will consume it.