tau

README

tau

A coding agent for your terminal. tau is a Go reimplementation of Pi — same architecture, same feature surface, same minimalist philosophy — shipped as a single static binary with no runtime dependencies.

Status: early development. tau is being built in phases toward parity with Pi v0.82.1. The interactive agent works on Anthropic models; other providers, session trees, and the extension subprocess protocol are still landing.

tau does not ask before it acts. It edits files and runs shell commands without a confirmation prompt. Run it on a clean git tree or in a scratch directory until approval gating lands.

Goals

• Single binary. No Node, no npm. brew install or download one file.
• Pi parity. The agent loop, session trees, providers, extensions, themes, skills, and modes you know from Pi, in idiomatic Go.
• Easy migration. tau reads your existing ~/.pi state (sessions, settings, auth, models) and imports it. Existing Pi TypeScript extensions run against tau via a host shim.
• Libraries first. ai, agent, session, tools, and extension are importable Go packages, not just internals of the CLI.

Install

brew install ihavespoons/tap/tau
# or
go install github.com/ihavespoons/tau/cmd/tau@latest

Use

tau login                  # Claude Pro/Max OAuth, or an API key with -k
tau                        # interactive session in the current directory
tau -c                     # …continuing the most recent one
tau -p "fix the failing test"    # one-shot, non-interactive
tau -p --mode json "…"     # JSONL events, for scripts and CI

Tools: read, write, edit, bash, grep, find, ls. Search runs on ripgrep and fd, found on your PATH or downloaded into ~/.tau/agent/bin on first use — set TAU_OFFLINE=1 to refuse the download instead.

Inside a session: Enter sends, Alt+Enter adds a newline, Esc stops the agent, Ctrl+P cycles models, Ctrl+T cycles thinking level, and /help lists the commands. Typing while the agent is working steers it — the message is delivered at the next turn boundary instead of starting a competing run.

The transcript is printed into your terminal's scrollback rather than an alternate screen, so scrolling, selection, and search keep working, and session length costs nothing to render.

Other providers

37 providers and 1,153 models ship compiled in — Anthropic, OpenAI, OpenRouter, Google, Groq, Cerebras, xAI, DeepSeek, Mistral, Together, Fireworks, Moonshot, Z.ai, MiniMax, NVIDIA, Bedrock, Vertex, Azure, Copilot, the Vercel and Cloudflare gateways, and the rest. Set the provider's API key environment variable and its models appear in tau models, ready to select.

The catalog is generated from models.dev and diffed field-by-field against Pi's own, so the per-model quirks come along with it: which thinking levels a model accepts, which token field it wants, where its pricing tier changes, whether it needs cache markers, which wire it actually speaks.

Nine wire APIs are live: Anthropic messages, OpenAI chat-completions, OpenAI responses, Azure OpenAI, Gemini, Vertex AI, Mistral, Bedrock ConverseStream, and the ChatGPT Codex backend — between them, every provider in the catalog except Pi's own server protocol. A model on a wire tau cannot talk to is still listed and selectable, and says so by name rather than pretending not to exist.

Three providers have a login rather than a key:

tau login                    # Anthropic — Claude Pro/Max
tau login github-copilot     # device flow
tau login openai-codex       # ChatGPT subscription

Two authenticate from the ambient cloud environment instead. Vertex uses Application Default Credentials, so gcloud auth application-default login is enough; set GOOGLE_CLOUD_PROJECT and GOOGLE_CLOUD_LOCATION. Bedrock uses the standard AWS chain — AWS_PROFILE, environment keys, SSO, or an instance role — and honours AWS_REGION, inference-profile ARNs, and AWS_BEARER_TOKEN_BEDROCK for Bedrock API keys. Everything else takes an API key from the environment.

Anything not listed that speaks /v1/chat/completions — vLLM, llama.cpp, LiteLLM, Ollama, a private gateway — can be declared in ~/.tau/agent/models.json and used immediately:

{
  "providers": {
    "openrouter": {
      "name": "OpenRouter",
      "baseUrl": "https://openrouter.ai/api/v1",
      // A "$VAR" reference is read from the environment, so the key
      // never sits in a file.
      "apiKey": "$OPENROUTER_API_KEY",
      "models": [
        {
          "id": "anthropic/claude-sonnet-4.5",
          "reasoning": true,
          "input": ["text", "image"],
          "contextWindow": 1000000,
          "maxTokens": 64000,
          "cost": { "input": 3.0, "output": 15.0, "cacheRead": 0.3, "cacheWrite": 3.75 }
        }
      ]
    }
  }
}

Then tau --model openrouter/anthropic/claude-sonnet-4.5, or set defaultModel in settings. Omitting api assumes openai-completions, which is what nearly every endpoint speaks; the per-provider quirks are detected from the provider id and base URL.

Qualify a model with its provider — anthropic/claude-sonnet-5, not claude-sonnet-5. A bare id is matched loosely across the whole catalog, and a dozen providers resell the same models.

To refresh the compiled catalogs after an upstream change: go run ./cmd/tau-genmodels -refresh.

MCP

tau speaks the Model Context Protocol through a bundled extension. Configure servers in ~/.tau/agent/mcp.json, using the same mcpServers shape other MCP hosts read:

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]
    },
    "example": {
      "url": "https://example.com/mcp",
      "headers": { "Authorization": "Bearer …" }
    }
  }
}

A project may add servers in .tau/mcp.json, but only in a directory you have trusted — otherwise cloning a repository would be enough to make tau launch a process. Run /mcp to see what connected.

Everything MCP does goes through the public extension package, which is the point: it is the acceptance test for that API, not a privileged core feature.

License

MIT. tau is derived from Pi, MIT © 2025 Mario Zechner — see THIRD-PARTY-NOTICES.md.