The wrapping problem
You asked for JSON. The model returned "Here is the result you asked for:" followed by a fenced code block followed by "Let me know if you need anything else!". Your parser fails on the first character.
This extracts the JSON from that wrapper, so you can see whether the structure underneath was actually correct — which it usually is. The failure is presentation, not content.
Checking the contract
Parsing is not enough. Valid JSON with a missing key breaks your application just as thoroughly as malformed JSON, and does so further downstream where it is harder to diagnose.
List the keys your code depends on and the validator reports both what is missing and what arrived unexpectedly. Unexpected keys are worth attention too — they often mean the model is answering a slightly different question than you think.
Fixing the cause rather than the symptom
If you regularly need to strip prose before parsing, the prompt is the problem. Tighten the output-format section: state the exact shape, name the keys, and say explicitly that the response must contain nothing but the JSON object.
Structured output modes, where the provider offers them, remove the problem entirely by constraining generation to a schema. That is strictly better than repairing output, and it is usually a one-line change.