Claude Haiku 4.5 vs Gemini 2.5 Flash for Structured Output

What Structured Output demands: JSON schema compliance and strict format adherence; deterministic field ordering, exact key names and types, predictable error handling, and stable behavior when prompted repeatedly. Important capabilities: high faithfulness (staying strictly to the schema), reliable structured_output/response_format support, strong tool_calling when outputs feed downstream functions, and good constrained_rewriting for tight character limits. In our testing both models score 4/5 on the Structured Output benchmark and share the same task rank (26 of 52). Supporting signals: Claude Haiku 4.5 shows stronger faithfulness (5 vs 4) and better classification (4 vs 3) in our tests — useful for correct field typing and routing. Gemini 2.5 Flash scores higher on constrained_rewriting (4 vs 3) and safety_calibration (4 vs 2), and it is materially cheaper on output ($2.50 vs $5.00 per mTok). Both models expose structured_outputs and response_format controls and scored 5/5 on tool_calling, so both can integrate into function-driven pipelines. Context sizes are ample for schema-heavy tasks (Haiku 4.5: 200,000 tokens; Gemini 2.5 Flash: 1,048,576 tokens).

Where Claude Haiku 4.5 shines: - High-integrity API payload generation (faithfulness 5 vs 4): fewer schema deviations for billing, identity, or regulatory payloads. - Schema-first data pipelines that rely on accurate classification/routing (classification 4 vs 3). - Large-context schema assemblies where downstream tools expect exact keys (tool_calling 5 for both). Where Gemini 2.5 Flash shines: - Cost-sensitive bulk generation (output cost $2.50 vs $5.00/mTok) for high-volume structured exports. - Tight character/byte-limited formats (constrained_rewriting 4 vs 3) such as compact CSV-like JSON or embedded JSON in single-line logs. - Safety-sensitive outputs where a higher safety_calibration score (4 vs 2) reduces the chance of producing disallowed content inside structured fields. Concrete numbers to ground choices: both models score 4/5 on Structured Output in our testing; choose Claude for stronger schema fidelity (faithfulness 5 vs 4) and routing; choose Gemini for cheaper per-output cost and better constrained rewriting.

For Structured Output, choose Claude Haiku 4.5 if you need the highest fidelity to schema and more reliable classification/routing (faithfulness 5 vs 4; classification 4 vs 3). Choose Gemini 2.5 Flash if per-output cost and tighter constrained rewriting matter more (output $2.50 vs $5.00/mTok; constrained_rewriting 4 vs 3).