Claude Haiku 4.5 vs Gemini 2.5 Flash for Constrained Rewriting

What Constrained Rewriting demands: precise compression inside hard character limits while preserving meaning, required structure, and safe content. Key capabilities: constrained_rewriting accuracy, faithfulness to the source, structured_output/format adherence, long_context handling when source text is large, and safety_calibration when content touches sensitive material. In our testing the primary signal is the constrained_rewriting task score: Gemini 2.5 Flash = 4/5, Claude Haiku 4.5 = 3/5. Supporting signals: both models score 4/5 on structured_output and 5/5 on long_context, so format and long inputs are handled well by both. Where they differ: Claude scores higher on faithfulness (5 vs 4) and strategic_analysis (5 vs 3), indicating stronger literal fidelity and nuanced tradeoffs; Gemini scores higher on safety_calibration (4 vs 2), which matters when rewrites must refuse or sanitize risky content. Rankings reflect that gap: Gemini is rank 6 of 52 for this task, Haiku is rank 31 of 52 in our tests.

1. Social media ad copy under a 280-character limit: Gemini 2.5 Flash (4/5) is the better pick for reliably compressing copy to fit exact limits while preserving intent and safety checks. 2) Legal clause condensation where literal accuracy matters: Claude Haiku 4.5 (faithfulness 5/5) is preferable when small wording changes can alter meaning; expect slightly lower raw compression performance (3/5) but stronger fidelity. 3) Bulk-document compression from long source files: both models handle long_context (5/5), but Gemini's better constrained_rewriting score and lower output cost ($2.50 vs $5.00 per mTok) make it more cost-effective at scale. 4) Safety-sensitive rewrites (e.g., removing PII or harmful content): Gemini's safety_calibration 4/5 vs Haiku's 2/5 suggests fewer risky outputs in our tests. 5) When you need agentic planning or strategy to rephrase for multiple personas, Claude's strategic_analysis 5/5 and persona_consistency 5/5 can help keep tone and nuance intact even if compression is a bit weaker.

For Constrained Rewriting, choose Claude Haiku 4.5 if you need the highest faithfulness and strategic nuance (faithfulness 5/5, strategic_analysis 5/5) even at higher output cost ($5.00 per mTok). Choose Gemini 2.5 Flash if you prioritize tighter compression performance (4/5 vs 3/5), safer automatic sanitization (safety_calibration 4/5), rank advantage (6 vs 31), and lower cost (input $0.30 / output $2.50 per mTok).