Cheapest LLM for Structured Output (May 2026)
Models with reliable JSON schema mode, ranked by output token price. Adherence scores from 2026-05-06 evals.
Structured output — JSON mode, schema mode, response_format, constrained decoding — is the contract that lets you parse an LLM response without a fragile string-extraction step. It is the boundary between an LLM and the rest of your stack: APIs, databases, downstream functions, and validators.
Cheap matters because extraction workloads are volume. You are not generating prose; you are emitting compact objects at scale — receipts, invoices, ticket fields, inventory rows, entities, sentiment labels. Output token count is small, but call count is enormous. A 100x price gap on output translates directly to a 100x bill on a high-volume extraction pipeline.
Ranking — cheapest first
| Model | Input / 1M | Output / 1M | Quality | Notes |
|---|---|---|---|---|
| DeepSeek V4 Flash | $0.14 | $0.28 | 84/100 | JSON mode reliable on flat or 1-level nested schemas. |
| Gemma 4 27B | Free | Free | 76/100 | Self-host. Pair with outlines/instructor for grammar-constrained decoding. |
| Qwen 3.6 Plus | $1.40 | $5.60 | 87/100 | Good JSON adherence; weaker on enum-heavy schemas. |
| DeepSeek V4 Pro | $1.74 | $3.48 | 92/100 | Frontier-class on most schemas; occasional slips on complex unions. |
| Gemini 3.1 Pro | $3.50 | $10.50 | 93/100 | Strong response_schema support; excellent on tabular extraction. |
| GPT-5.5 | $5.00 | $30.00 | 97/100 | OpenAI strict mode is the gold standard. Constrained-grammar enforced. |
| Claude Opus 4.7 | $5.00 | $25.00 | 96/100 | Best for deeply nested or recursive schemas. |
| GPT-5.5 Pro | $30.00 | $180.00 | 98/100 | Marginal lift over GPT-5.5; overkill for most extraction work. |
Cost visualised
Cost per 1M output tokens (lower = cheaper) DeepSeek V4 Flash # $0.28 Qwen 3.6 Plus # $5.60 DeepSeek V4 Pro # $3.48 Gemini 3.1 Pro ## $10.50 GPT-5.5 ####### $30.00 Claude Opus 4.7 ###### $25.00 GPT-5.5 Pro ######################################## $180.00
The winner
DeepSeek V4 Flash
For 90% of structured-output workloads, DeepSeek V4 Flash at $0.14/$0.28 per 1M tokens is the right answer. Schemas with up to two levels of nesting validate cleanly on 96%+ of calls in our evals. Wrap responses in a zod parser, route validation failures to GPT-5.5 strict mode for the residual 4%, and you have a production extraction pipeline at near-zero unit cost. Pick GPT-5.5 from the start only when the schema is guaranteed-strict (financial records, medical codes, legal entity extraction).
Honourable mentions
- GPT-5.5 (strict mode) — best-in-class adherence, schema enforced at decode time. Pay the premium when guaranteed-strict matters.
- Claude Opus 4.7 — pick when schemas are deeply nested or recursively defined. Handles types other models flatten.
- Gemma 4 + outlines / instructor — open-weight grammar-constrained decoding. Quality competitive with paid tiers on flat schemas.
When to upgrade to a frontier model
- Schema validation failure rate exceeds 3% even after retry.
- Your schema has recursive types or polymorphic unions.
- Downstream consumers cannot tolerate any malformed output (writes, payments).
- Enum fields contain 20+ values and the cheap model picks adjacent options.
- Extraction accuracy (not just JSON validity) requires frontier reasoning.
FAQ
What is the cheapest free option for structured output?
Self-host Gemma 4 27B with the outlines or instructor library wrapping decoding. Outlines enforces a JSON-schema-derived grammar at sample time, so the model literally cannot emit invalid tokens. Quality on flat schemas matches paid tiers.
What is the cheapest model with API JSON mode?
DeepSeek V4 Flash at $0.14 in / $0.28 out. JSON mode is reliable for flat schemas and 1-2 levels of nesting. For complex schemas, validate client-side and retry with a stronger model on failure.
What is the cheapest open-weight option?
Gemma 4 27B + outlines is the practical answer. DeepSeek V4 also has open weights and works well with grammar-constrained decoding tools.
What is the cheapest model for production structured output?
For strict-schema production work, GPT-5.5 with strict mode at $5 in / $30 out is hard to beat — it enforces the schema as a decoding constraint, not a soft suggestion. For everyday extraction, DeepSeek V4 Flash plus client-side validation is dramatically cheaper.
What should I watch out for?
JSON mode does not equal schema mode. Many providers will return valid JSON that does not match your schema. Always validate against the actual schema (zod, ajv) and route validation failures to a stronger model rather than retrying on the cheap one.
Related
- AI Model Leaderboard
- Token Cost Calculator
- Per-Million Tokens True Cost
- Cheapest LLM for Function Calling
- Cheapest LLM for Long Context
All prices from official provider pages and OpenRouter as of 2026-05-06. Schema-adherence scores from internal Swfte evals on 1K-sample extraction sets across 32 catalogued models.