The model identifier to pass to the AI agent (for example claude-sonnet-4, gpt-5.5, gemini-2.5-pro, grok-4.5). For Codex, Claude Code, and OpenCode, append a recognized reasoning effort with a colon, such as gpt-5.6-sol:xhigh. Antigravity CLI and Grok Build accept low, medium, and high, such as grok-4.5:high. Other colon suffixes remain part of the model identifier. Use a double colon to keep a recognized suffix literal, such as provider/model::high for model provider/model:high. The separate Reasoning effort field overrides the shorthand. Leave empty to use the agent's default model.