Free, Fast, Rust-powered AI Coding Agent – zerostack

zerostack is a free, fast, open-source AI coding agent written in Rust that helps you work on code with multiple model providers, permission modes, sessions, prompts, tools, MCP servers, subagents, loops, and git worktrees.

Many AI coding agents feel heavy because they rely on large JavaScript runtimes, cloud-first workflows, or polished editor layers.

zerostack takes a different route. It focuses on a small binary, low memory use, direct terminal control, and enough safety controls to keep agent work from turning into uncontrolled file edits or repeated tool calls.

The report shows that its source code has approximately 16,000 lines, the binary file size is 26MB, the average memory usage is approximately 16MB, and the peak memory usage is approximately 24MB; the CPU utilization is 0.0% in idle state and approximately 1.5% when the agent is running.

Features

• Runs as a minimal Rust coding agent with a 26MB binary.
• Supports OpenRouter, OpenAI, Anthropic, Gemini, Ollama, and custom providers.
• Standard coding-agent tools for reading, writing, editing, searching, listing files, and running bash commands.
• 5 permission modes for read-only work, guarded edits, standard tasks, restrictive approval, and high-trust operation.
• Stores session history so you can save, load, resume, and compact longer coding conversations.
• A terminal UI with markdown display, scrollback, mouse selection, copy support, and a reasoning visibility toggle.
• Switches task behavior at runtime through prompt modes such as code, plan, review, debug, ask, and review-security.
• Connects MCP servers through an optional compile-time feature.
• Exa search for WebFetch and WebSearch tools.
• Iterative loops for longer coding tasks through the loop system.
• Moves coding tasks across git worktrees through slash commands.
• ACP support through a gated feature for compatible editors such as Zed.
• Persistent Markdown memory through a gated feature.
• Subagents for faster codebase exploration.
• Loads AGENTS.md, CLAUDE.md, and optional ARCHITECTURE.md files as project context.
• Sandboxed bash commands through bubblewrap or zerobox.

Use Cases

• Run a coding agent on a laptop with limited RAM where JS-based agents cause slowdowns.
• Edit local codebases without sending source code to any cloud API by using Ollama.
• Review pull requests locally using the review prompt mode that checks correctness and design.
• Automate refactoring tasks with the loop system that repeats until tests pass.
• Connect Zed editor to zerostack as an ACP agent backend for inline coding assistance.

zerostack vs Claude Code

Claude Code is the safer default when you want a mature AI coding agent with strong product support, smoother onboarding, and a Claude-first workflow. zerostack is the better fit when you want a smaller open-source terminal agent with provider choice, local model support, and more direct control over agent behavior.

Choose Claude Code when you want the most practical daily coding agent with fewer setup decisions. Choose zerostack when you want an open-source CLI agent that can route work through different providers, run local models through Ollama, and keep more of the workflow under developer control.

How to use it

Installation

zerostack requires Cargo and git.

cargo install zerostack

Install with ACP support for editor agent connections.

cargo install zerostack --features acp

Install with persistent memory support.

cargo install zerostack --features memory

Install with experimental multi-threaded subagents.

cargo install zerostack --features multithread

Pre-built binaries are available through GitHub Releases.

Sandbox Setup (OPTIONAL)

zerostack can run bash commands inside an isolated environment through bubblewrap.

Debian and Ubuntu:

apt install bubblewrap

Fedora:

dnf install bubblewrap

Arch:

pacman -S bubblewrap

zerobox can also act as a sandbox backend.

Quick Start

OpenRouter acts as the default provider.

export OPENROUTER_API_KEY="[api_key]"

Start an interactive session with the default code prompt.

zerostack

Start a monochrome terminal UI.

zerostack --no-color

Run one prompt and exit.

zerostack -p "Explain this project"

Continue the last session.

zerostack -c

Select a provider and model.

zerostack --provider openrouter --model deepseek/deepseek-v4-flash

Run the interactive setup assistant inside zerostack.

/prompt autoconfig

Supported AI Providers

Built-in Prompt Modes

Custom prompts belong in this directory:

$XDG_CONFIG_HOME/zerostack/prompts/

Context Files

Disable context file loading with this flag:

zerostack -n

zerostack --no-context-files

Permission Modes

The permission system supports per-tool glob rules and session allowlists. Doom-loop detection warns or blocks repeated identical tool calls after three or more repeats.

The full bypass flag removes permission checks.

zerostack --dangerously-skip-permissions

Available zerostack Commands

Input Queue Commands

Plain text entered during an active run gets queued. Slash commands, dot commands, and bang commands do not enter the queue during an active run.

Session Management

Sessions live in this directory:

$XDG_DATA_HOME/zerostack/sessions/

Continue the most recent session.

zerostack -c

Browse and select a previous session.

zerostack -r

Load a specific session.

zerostack --session <id>

Memory

Memory requires the memory feature.

cargo install zerostack --features memory

Memory files live in this directory:

$XDG_DATA_HOME/zerostack/agent/memory/

The memory system uses a global MEMORY.md file plus per-project daily logs, scratchpad files, and notes. zerostack injects relevant memory into the system prompt at session start.

Parallel Agent

The parallel mode creates and manages a temporary git worktree.

zerostack --parallel

Each parallel agent works in its own temporary git worktree. zerostack merges and removes the worktree when the agent exits.

Loop System

Start a loop from the chat UI.

/loop Implement the user authentication system

Stop the active loop.

/loop stop

Check loop status.

/loop status

Run a headless loop from the CLI.

zerostack --loop --loop-prompt "Refactor the API" --loop-max 10 --loop-run "cargo test"

The default loop plan file is:

LOOP_PLAN.md

Git Worktree Commands

ACP Support

ACP support requires the acp feature.

cargo install zerostack --features acp

Start zerostack in ACP stdio mode.

zerostack --acp

Start zerostack in ACP TCP mode.

zerostack --acp --acp-host 0.0.0.0 --acp-port 7243

ACP server configuration can live in this file:

~/.local/share/zerostack/config.json

Example ACP configuration:

{
  "acp_servers": {
    "my-editor": {
      "host": "127.0.0.1",
      "port": 7243
    }
  }
}

ACP mode still requires a configured provider, model, and API key.

Configuration

The config file handles accepted keys, provider aliases, permission rules, and MCP server configuration.

Run this command inside zerostack for guided setup:

/prompt autoconfig

Custom providers can use a custom base URL and API key environment variable through the config file.

Alternatives and Related Resources

• Free AI Tools for Developers: Browse more developer-focused AI tools, coding assistants, CLI tools, and automation utilities.
• Best Free CLI AI Coding Agents: Compare more AI coding agents for terminal, repository, and autonomous coding workflows.
• The Ultimate Claude Code Resource List: Explore Claude Code agents, skills, plugins, guides, and related developer resources.
• Best & Open-source MCP Servers: Find MCP servers that extend AI assistants with external tools and structured context.
• SmallCode: Compare zerostack with a local AI coding agent focused on smaller local LLMs.
• Pi Coding Agent: Compare zerostack with the coding agent project that influenced its design.

Pros

• Small Rust binary.
• Low memory footprint.
• Multiple model providers.
• Ollama support.
• Granular permission modes.
• MCP support.
• Prompt mode switching.
• Git worktree workflow.
• ACP support.
• Plain Markdown memory.

Cons

• Cargo setup required.
• API keys required.
• Memory needs feature build.
• ACP needs feature build.
• Terminal workflow only.

FAQs

Q: Is zerostack free?
A: zerostack is free and open source under the GPL-3.0-only license.

Q: Does zerostack require a sign-up?
A: zerostack does not use a web signup flow, but cloud model providers require API keys or account credentials.

Q: Can zerostack run with local models?
A: zerostack supports Ollama, so you can use local models when your hardware and model setup can handle the coding task.

Q: Is zerostack a Claude Code replacement?
A: zerostack can replace parts of a terminal AI coding workflow when you want multi-provider control, local model support, prompt modes, permissions, and worktree tools, but it does not copy Claude Code’s hosted product experience.

Q: Does zerostack send my code to the cloud?
A: Not when you use Ollama or another local provider. With cloud providers like OpenAI or Anthropic, your prompts and context go to those APIs. Use the --read-only or --restrictive permission modes to limit what the agent can access.

Q: What is the difference between standard and yolo permission modes?
A: Standard mode auto-allows safe commands like ls, cat, and git log within the current directory. Yolo mode skips most confirmations but still prompts for destructive bash commands like rm or dd.