pi-lean-ctx

Pi Coding Agent extension that provides ctx_-prefixed tools backed by lean-ctx for 60–90% token savings.

• Default: CLI-only, additive mode (no MCP required, Pi builtins preserved)
• Optional: enable MCP tools (LEAN_CTX_PI_ENABLE_MCP=1) or run lean-ctx init --agent pi --mode mcp
• Optional: replace mode (LEAN_CTX_PI_MODE=replace) disables Pi builtins

Tool Mode

By default, pi-lean-ctx runs in additive mode: Pi's built-in tools (read, bash, ls, find, grep) remain available alongside the ctx_* tools. Agents can use either set.

To switch to replace mode (disables Pi builtins, only ctx_* tools available):

export LEAN_CTX_PI_MODE=replace

What it does

ctx_ Tools (CLI-backed)

Adds ctx_-prefixed tools alongside Pi's builtins (or replaces them in replace mode):

Pi's edit and write builtins remain unchanged.

Direct lean-ctx CLI tool

The lean_ctx tool runs lean-ctx directly (no nested compression). Use it for commands like:

• lean_ctx overview
• lean_ctx session …
• lean_ctx knowledge …
• lean_ctx gain / lean_ctx stats
• lean_ctx index …

Optional MCP Tools (Embedded Bridge)

By default, pi-lean-ctx does not start an MCP server. If enabled, it spawns lean-ctx as an MCP server and registers advanced tools directly in Pi:

If you don't want MCP: keep it disabled and use the ctx_ CLI tools + lean_ctx tool only.

Install

# 1. Install lean-ctx (if not already installed)
cargo install lean-ctx
# or: brew tap yvgude/lean-ctx && brew install lean-ctx

# 2. Install the Pi package
pi install npm:pi-lean-ctx

# 3. Restart Pi

Or use the automated setup:

lean-ctx init --agent pi

How it works

ctx_ tools (CLI-backed)

These tools invoke the lean-ctx binary via CLI with LEAN_CTX_COMPRESS=1. The built-in tools they replace (read, bash, ls, find, grep) are disabled via pi.setActiveTools() so only the ctx_ versions are available to the LLM.

Optional MCP bridge (all other tools)

If you enable the MCP bridge, pi-lean-ctx spawns the lean-ctx binary as an MCP server (JSON-RPC over stdio). It discovers available tools via list_tools, filters out those already covered by ctx_ CLI tools, and registers the rest as native Pi tools.

If lean-ctx is already configured as an MCP server via pi-mcp-adapter in ~/.pi/agent/mcp.json, the embedded bridge is skipped to avoid duplicate tools.

Automatic reconnection

If the MCP server process crashes, the bridge automatically reconnects (up to 3 attempts with exponential backoff). If reconnection fails, CLI-based tools continue working normally — only the advanced MCP tools become unavailable.

Enabling MCP (optional)

Set an environment variable and restart Pi:

export LEAN_CTX_PI_ENABLE_MCP=1
pi

Or configure MCP via lean-ctx init:

lean-ctx init --agent pi --mode mcp

pi-mcp-adapter compatibility

If you prefer using pi-mcp-adapter to manage your MCP servers, lean-ctx integrates automatically:

# Option A: lean-ctx writes the config for you
lean-ctx init --agent pi

# Option B: Manual configuration in ~/.pi/agent/mcp.json

{
  "mcpServers": {
    "lean-ctx": {
      "command": "/path/to/lean-ctx",
      "lifecycle": "lazy",
      "directTools": true
    }
  }
}

When pi-mcp-adapter manages the lean-ctx MCP server, pi-lean-ctx detects this and only registers its CLI-based tool overrides, leaving MCP tool management to the adapter.

Binary Resolution

The extension locates the lean-ctx binary in this order:

1. LEAN_CTX_BIN environment variable
2. ~/.cargo/bin/lean-ctx
3. ~/.local/bin/lean-ctx (Linux) or %APPDATA%\Local\lean-ctx\lean-ctx.exe (Windows)
4. /usr/local/bin/lean-ctx (macOS/Linux)
5. lean-ctx on PATH

Smart Read Modes

The ctx_read tool automatically selects the optimal lean-ctx mode:

Slash Command

Use /lean-ctx in Pi to check:

• Which binary is being used
• MCP bridge status (disabled / embedded / adapter)
• Active ctx_ tool names

Disabling specific tools

To disable specific MCP tools, configure disabled_tools in ~/.lean-ctx/config.toml:

disabled_tools = ["ctx_graph", "ctx_benchmark"]

Or via environment variable:

LEAN_CTX_DISABLED_TOOLS=ctx_graph,ctx_benchmark pi

Links

• lean-ctx — The Cognitive Filter for AI Engineering
• GitHub
• Discord