Migrating a production AI agent to GPT-5.6: 2.2x faster, 27% cheaper

What happened

Ploy.ai documented migrating a production UI-generation agent from Claude Opus 4.8 to GPT-5.6 Sol, achieving 2.2x faster builds (3m 42s vs. 8m) at 27% lower cost per build.

Context and impact

The write-up surfaces key behavioral differences in GPT-5.6 relevant to anyone considering a migration. GPT-5.6 invents plausible values for unused tool-call parameters, causing 52-64% of file reads to return empty until schemas were restructured with nullable fields. GPT-5.6's workspace-scoped caching also differs from Anthropic's org-wide cache, initially making it appear 50% more expensive.

Details

• Build time: 3m 42s (GPT-5.6 Sol) vs. 8m (Claude Opus 4.8) — 2.2x improvement
• Cost savings: 27% lower per build
• Pitfall 1: GPT-5.6 invents values for unused tool params => 52-64% empty file reads
• Fix: restructure schemas with nullable fields
• Pitfall 2: workspace-scoped caching (OpenAI) vs. org-wide (Anthropic) — expect different behavior