Gemini 3.1 Pro Preview Custom Tools is positioned as a preview variant within Google's Gemini 3.1 Pro family, with its defining trait being first-class support for user-defined tools and APIs. Rather than treating tool use as an afterthought, the variant is explicitly designed to let the model orchestrate external services, trigger workflows, and fetch live data through developer-supplied functions, then combine those calls with Gemini's general reasoning and multimodal understanding. This makes the model a natural fit for production agents and assistants that need to ground language-model outputs in real systems, where the LLM plans an action, invokes a tool, and continues the response based on the returned result.

In practical terms, the model accepts text, image, video, audio, and PDF inputs while producing text-only output, and it exposes controls such as temperature adjustment, reasoning-focused behavior, and structured tool calling that developers can shape per request. Its large context window and high output ceiling support extended multi-turn conversations, long document analysis, and complex tool-call chains without aggressive truncation. Teams building domain-specific copilots, retrieval pipelines, or automation flows benefit most, since the model is tuned for controlled, tool-mediated behavior rather than purely open-ended chat.