Gemini 3.1 Pro Preview Custom Tools is a variant within the Gemini 3.1 Pro lineup that has been retuned for tool use inside agentic systems. Rather than broadening the base model's general capabilities, the variant focuses on function-calling reliability, pushing the model toward registered custom functions instead of a generic bash tool whenever a more specific registered tool fits the task. This makes it especially attractive for coding agents and multi-tool orchestrations where choosing the right tool on each step is more important than raw capability breadth, and it retains the multimodal understanding of the underlying Gemini 3.1 Pro family across text, images, video, audio, and documents.
In practice, the model accepts text, audio, image, video, and file inputs and produces text output, operating with a 1,048,576-token that quick-info value window and a maximum output on the order of the cataloged API limit tokens, which is well suited to long documents and multi-turn agent loops. Strengths highlighted by catalog evidence include reasoning, tool calling, and structured outputs, with the variant positioning itself as a good fit for image understanding, long-document analysis, structured response generation, and deep analysis scenarios that involve chaining tools. Because the core change is targeted steering of tool selection rather than a wholesale redesign, teams already invested in Gemini 3.1 Pro can adopt it as a drop-in option for agentic pipelines that need more consistent and predictable function-calling behavior.
