What Is Gemini CLI: Google's Open-Source Terminal Agent

Gemini CLI is Google's open-source (Apache-2.0) AI agent for enterprise and API-key terminal workflows: you state a goal, and it runs an agent loop — calling a Gemini model, reading and editing files, running shell commands, and searching the web — pausing for your approval before changes. Individual Google AI Pro, Ultra, and free-tier accounts moved to Antigravity CLI on June 18, 2026. This guide explains the maintained Gemini CLI path and the migration boundary.

The agentic loop

The source is two packages. packages/cli is the terminal frontend you type into; packages/core is the backend that talks to the Gemini API, manages tools, and processes each request. A turn runs as a loop: core assembles context (history + tool definitions + discovered GEMINI.md files) and sends it to the model; the model replies with an answer or a tool call (read file, run command, search); core executes the tool and returns the result; the model decides the next step. This repeats until the task is done.

Two mechanisms keep the loop safe and durable. Mutating tools require confirmation — before a file write (write_file, replace) or a shell command (run_shell_command), the CLI shows the diff or exact command and waits; read-only tools (grep_search, read_file) run silently. And when a conversation nears the model's token limit, core auto-compresses history (documented as "lossless in terms of information conveyed") so long tasks do not run out of room.

Authenticating — three backends with different access

The model access, quota, and billing path come from one of three backends. Organization-managed Google sign-in uses the browser flow, an AI Studio key uses GEMINI_API_KEY, and Vertex AI combines project/location with one credential method plus a selection in /auth. The same gemini binary serves all three, but available models, limits, billing, and compliance can differ.

Models, defaults, and Auto routing

Gemini CLI defaults to Auto routing. Gemini 3 participates when it is enabled and available to the selected authentication route; the unpaid API-key tier remains Flash-only. The current recommended Pro model ID is gemini-3.1-pro-preview — 1,048,576 input / 65,536 output tokens, knowledge cutoff January 2025, reasoning ("Thinking") supported. You rarely need to type a model ID: use /model to inspect the available Auto and Manual choices, or launch one directly with -m, e.g. gemini -m gemini-3.1-pro-preview or gemini -m gemini-2.5-flash.

Auto routing chooses among the models your account can currently use. When a selected Pro model hits quota or availability trouble, the CLI offers a fallback rather than silently changing the session: Gemini 3 Pro can fall back to 2.5 Pro, then 2.5 Pro to 2.5 Flash. If model identity matters, inspect Manual in /model or pin a live ID with -m, and confirm the model in the session report.