I built a JSON repair tool for LLM output — here's why it exists

You ask an LLM for JSON. You get this: json { "name": "test", "valid": True, "items": [1, 2, markdown Three problems in one response: markdown fences the LLM wasn't supposed to add, True instead of true (Python literal), and the response was cut off mid-array. JSON.parse() throws on all three. A linter tells you what's wrong. But you're left fixing it manually. I kept hitting this wall so often that I built a dedicated repair pipeline: AI JSONMedic . Why existing tools don't cut it JSONLint / JSONFormatter — great for valid-ish JSON with one missing comma. Not built for LLM failure modes: they flag errors but don't repair them. jsonrepair (npm) — solid library, handles many cases. AI JSONMedic actually uses it as a last-resort fallback. But it doesn't tell you what it changed, and doesn't handle all the LLM-specific cases we needed. The 14 failure modes we target Through building this, we catalogued how LLMs specifically break JSON: Markdown fences — ` json wrapping the output Trailing commas — [1, 2, 3,] (the model "runs out" of items but adds one more comma) Python literals — True , False , None instead of true , false , null Single quotes — {'key': 'value'} instead of double quotes Smart quotes — "key" (curly quotes from copy-paste) Unclosed brackets — truncated at max_tokens mid-array or mid-object Unclosed strings — "value without closing Concatenated objects — {"a":1}{"b":2} when streaming produces multiple chunks NDJSON — newline-delimited JSON that needs wrapping Python-style comments — # this is a comment inside JSON JavaScript-style comments — // inline or /* block */ Escaped backslashes — \\n instead of \n Duplicate keys — same key appearing twice (ambiguous — we warn, not silently pick) BOM / encoding issues — UTF-8 BOM at start of response What makes the repair pipeline different Each pass targets one failure mode. The order matters — strip fences first, then normalize quotes, then fix commas, then close truncated structures. Each change is tracked. The