Claude Sonnet 4.6 vs GPT-5.4 for Structured Output

Structured Output demands strict JSON schema compliance, exact field types/names, deterministic formatting, and reliable handling of nested schemas. Our structured_output benchmark (JSON schema compliance and format adherence) is the primary measure for this task. In our testing GPT-5.4 earned 5/5 while Claude Sonnet 4.6 earned 4/5, reflecting GPT-5.4's superior format fidelity. Supporting signals: GPT-5.4 also scores higher on constrained_rewriting (4 vs Sonnet's 3), which matters when outputs must fit tight character limits or compressed encodings. Claude Sonnet 4.6 scores higher on tool_calling (5 vs 4) and classification (4 vs 3), which helps in workflows that combine API/function calls or require routing decisions alongside structured payloads. Both models are equally strong on long_context and faithfulness (5/5).

1. Strict API responses: For a public API that requires exact, machine-parseable JSON (nested schema, type constraints), choose GPT-5.4 — 5/5 structured_output and rank 1 in our tests ensures fewer schema violations. 2) Multi-step agent that returns structured results plus invokes tools: Choose Claude Sonnet 4.6 when tool selection and argument sequencing matter — Sonnet scores tool_calling 5 vs GPT-5.4's 4, so it better handles function selection and multi-call workflows while still producing near-correct structured payloads (4/5). 3) Tight character budgets or compressed payloads: GPT-5.4's constrained_rewriting 4 vs Sonnet's 3 means GPT-5.4 is likelier to meet hard length constraints without breaking schema. 4) Classification + structured output (routing users to endpoints and returning JSON): Sonnet's classification 4 vs GPT's 3 can reduce misrouting while returning structured data. Supplementary signals: on SWE-bench Verified (Epoch AI) GPT-5.4 scores 76.9% vs Claude Sonnet 4.6 at 75.2%, and on AIME 2025 (Epoch AI) GPT-5.4 scores 95.3% vs Claude Sonnet 4.6 at 85.8% — useful context for reasoning-intensive formatting tasks.

For Structured Output, choose GPT-5.4 if you need strict JSON schema adherence, tight-length conformance, and the highest format fidelity (GPT-5.4: 5/5, rank 1 of 52). Choose Claude Sonnet 4.6 if your workflow pairs structured outputs with heavy tool orchestration or classification responsibilities (Sonnet 4.6: structured_output 4/5, tool_calling 5/5, classification 4/5) and you can tolerate occasional schema edge cases.