Ploy has migrated its production AI agent from Claude Opus 4.8 to OpenAI GPT-5.6 Sol. In head-to-head testing the agent completed website builds 2.2 times faster and at 27 percent lower cost than the previous model. The migration required updating the evaluation harness, tool schema, prompt caching configuration, and reasoning replay logic. The original harness was tuned to Claude's sequential tool usage and did not account for GPT-5.6's parallel call pattern, causing a portion of initial failures. The tool schema was adjusted so optional parameters were marked nullable before being stripped before execution, eliminating erroneous file reads. Prompt caching was reconfigured to use a workspace-scoped key, raising first‑call cache hit rates from near zero to about 84 percent and reducing uncached token volume by 28 percent. Reasoning replay was made self‑contained by setting the SDK option to store false, preventing server‑side reference errors. After these changes GPT-5.6 Sol became the default model for Ploy workspaces and is available for free trial on the company's website.