Production AI agent migrated to GPT-5.6: 2.2x faster, 27% cheaper
Original: Migrating a production AI agent to GPT-5.6: 2.2x faster, 27% cheaper
Why This Matters
The switch highlights how model-specific behaviors create hidden migration complexity even with universal SDKs.
Ploy has migrated its production AI agent from Claude Opus 4.8 to OpenAI's GPT-5.6 Sol, reporting builds completing 2.2x faster and at 27% lower cost. The switch came after four months of no model surpassing Claude Opus — until GPT-5.6.
AI website builder Ploy has switched its production agent's default model from Claude Opus 4.8 to GPT-5.6 Sol, the flagship tier of OpenAI's newly released model family. The company reports builds completing in less than half the previous wall-clock time, at 27% lower cost, with quality scores at or above the incumbent model.
Ploy's agent handles complex end-to-end tasks: planning pages, reading codebases, writing components, generating imagery, and self-evaluating screenshots. The team tested every frontier release against Claude Opus over four months — including Opus 4.7 and 4.8 — and found no challenger until GPT-5.6.
Despite using Vercel's AI SDK (a universal LLM SDK), the migration required significant engineering work. Key differences discovered between models included: tool argument formatting, prompt caching behavior, and reasoning replay between turns. GPT-5.6 also favors parallel tool calls, which initially overwhelmed budget limits sized for Opus's sequential style.
The team identified that roughly one-third of initial eval failures were caused by harness assumptions tuned to Opus, not actual model weaknesses. A silent default minScore threshold of 1.0 also caused GPT-5.6 to 'fail' cases it scored 0.98 on. The post documents a four-step migration process: fix the eval harness, then tool schemas, caching, and reasoning replay.