Reading a parse error
The two failures that account for most invalid JSON are a trailing comma after the last element, and a single quote where a double quote belongs. Both are legal in JavaScript object literals and illegal in JSON, which is why they survive code review.
The error message reports the position where parsing failed, which is usually just after the real mistake rather than at it. Look at the character before the reported position first.
Why nesting depth is reported
Deep nesting costs tokens disproportionately: every level adds indentation to every line beneath it, and adds a brace pair of its own. A payload five levels deep spends a large share of its budget on structure.
It also correlates with model errors. Flat structures are read more reliably than deeply nested ones, so a depth figure above three or four is worth questioning on quality grounds as well as cost.
Validating model output, not just input
If you are checking JSON that a model produced rather than JSON you are about to send, use the LLM output validator instead. It strips the prose and code fences models wrap around their answers before parsing, and checks that the keys you require are actually present.