best API for structured output from LLMs

If by structured output you mean reliably getting JSON / typed data from an LLM, the best API depends on what you want to optimize for:

Best overall

OpenAI API with structured outputs / JSON schema support

• Strong reliability for valid JSON
• Can enforce a schema
• Good tool/function-calling ecosystem
• Easy to integrate

Best for flexibility and open models

Anthropic API

• Very good at following formatting instructions
• Strong for tool use and complex reasoning
• Less “schema-enforced” than OpenAI in many workflows, but still solid

Best if you want open-source / self-hosted

vLLM + guided decoding / schema constraints

• Use with models like Llama, Qwen, Mistral
• More control and lower cost at scale
• Reliability depends heavily on model quality and the decoding library

Best if you want to enforce strict JSON from any model

Use a schema-constrained generation layer:

• Outlines
• Instructor
• Guidance
• Jsonformer
• lm-format-enforcer

These sit on top of an LLM API or local model and help ensure valid structured output.

---

My practical recommendation

• Need highest reliability with least engineering? → OpenAI structured outputs
• Need strong reasoning + tool use? → Anthropic
• Need self-hosted / cheaper at scale? → vLLM + Outlines/Instructor

---

If you want a simple decision rule

Choose based on your priority:

1. Schema correctness → OpenAI
2. Reasoning quality → Anthropic
3. Cost/control → Local model + constrained decoding
4. Fastest integration → OpenAI or Anthropic

If you want, I can also give you a ranked comparison table of OpenAI vs Anthropic vs Gemini vs local options specifically for JSON/schema output.