Why is Anthropic not returning valid JSON?

Common reasons Anthropic outputs “almost JSON” instead of valid JSON:

1. You didn’t force strict JSON mode

   • If you ask for JSON in plain text, the model may add markdown fences, comments, or trailing text.
   • Use a response format / tool schema if your API wrapper supports it.

2. The prompt is ambiguous

   • If examples include prose, explanations, or code fences, the model may imitate that.
   • Be explicit: “Return only raw JSON. No markdown. No code fences. No extra text.”

3. The schema is underspecified or conflicting

   • If you say “include optional fields” but also require all fields, the model may improvise.
   • Keep field names, types, and nesting very clear.

4. The model is trying to be helpful

   • It may add natural-language clarifications before/after the JSON unless constrained.

5. Streaming/parsing issues

   • Sometimes the model output is fine, but your client reads partial chunks or concatenates them incorrectly.

6. Post-processing is altering the output

   • Middleware, logging, or templating may insert text around the JSON.

7. The output is valid JSON in spirit, but not strict JSON

   • Common issues:
     • trailing commas
     • single quotes instead of double quotes
     • unescaped newlines in strings
     • comments
     • NaN/Infinity

How to fix it:

• Use a strict JSON response mechanism if available.
• Add a system prompt like:
  • “You must output only valid JSON, with no markdown or extra text.”
• Provide a concrete schema and one minimal example.
• Validate on the client side and retry if parsing fails.
• Lower temperature if the model is being creative.

If you want, paste:

1. your exact prompt,
2. a sample response,
3. and the API call code,

and I can tell you exactly why it’s breaking.