I'm building a feature that returns JSON from AI, what API should I use

If you want the model to reliably return JSON, use an API that supports structured output / JSON mode rather than asking the model to “please output JSON” in plain text.

Best choice

Use an API with one of these features:

• Structured Outputs: you provide a JSON Schema, and the model must match it
• JSON mode: the model is constrained to valid JSON, but less strict than schema validation

Recommended approach

If your use case has a defined shape, prefer:

1. Structured Outputs with a schema
2. Fallback to JSON mode only if you just need valid JSON text

Example use cases

• {"name": "...", "age": 42}
• arrays of objects
• classification results
• extraction tasks
• tool/API responses

Why this matters

Plain prompting can still produce:

• invalid JSON
• extra commentary
• missing fields
• wrong types

Structured output reduces those failures a lot.

If you’re using OpenAI

Use the API’s Responses API with structured outputs / JSON schema support.

Example pattern:

• define your schema
• send it in the request
• parse the response as JSON

If you’re using another provider

Look for terms like:

• “JSON mode”
• “schema mode”
• “structured output”
• “function calling” / “tool calling”

Those are usually the right features.

My recommendation

• Need strict machine-readable JSON? Use Structured Outputs
• Need just valid JSON, flexible shape? Use JSON mode
• Need the model to decide among actions or fill fields? Use tool/function calling

If you want, I can show you:

1. the best OpenAI API call for JSON output, or
2. examples in Python / JavaScript / TypeScript.