pi-lean-ctx

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

• Default: embedded MCP bridge ON (persistent session cache → unchanged re-reads cost ~13 tokens), additive mode (Pi builtins preserved)
• Opt out: LEAN_CTX_PI_ENABLE_MCP=0 (or "enableMcp": false) forces the one-shot CLI path, which cannot cache across calls
• 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

Config file

If you only use lean-ctx through Pi, keep every setting in one file instead of env vars — ~/.pi/agent/extensions/pi-lean-ctx/config.json:

{
  "mode": "replace",
  "enableMcp": true,
  "binary": "/opt/lean-ctx/bin/lean-ctx",
  "env": { "LEAN_CTX_COMPRESSION": "aggressive" }
}

mode → LEAN_CTX_PI_MODE, enableMcp → LEAN_CTX_PI_ENABLE_MCP, binary → LEAN_CTX_BIN. The env map is forwarded to every lean-ctx subprocess, so it can override ~/.lean-ctx/config.toml engine settings. Explicit env vars still win over the file; the file wins over defaults.

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.

Embedded MCP bridge (session cache + advanced tools)

On by default, pi-lean-ctx spawns the lean-ctx binary as an MCP server (JSON-RPC over stdio). This persistent process holds the session cache: ctx_read (every mode, including line ranges) is routed through the bridge, so an unchanged re-read costs ~13 tokens instead of the full file and the read registers as a real CEP session (counted by lean-ctx gain). The bridge also discovers the server's advanced tools (ctx_edit, ctx_overview, ctx_graph, …), filters out those already exposed as ctx_ CLI tools, and registers the rest as native Pi tools.

The bridge wins over ~/.pi/agent/mcp.json: a lean-ctx entry there (written by lean-ctx init --agent pi) does not disable the embedded bridge, because Pi has no native MCP support and that entry only does anything if you separately run pi-mcp-adapter. /lean-ctx warns about possible duplicates only when the adapter is genuinely running. If the bridge can't start, the CLI path keeps working — only the cache and advanced tools are unavailable.

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.

Disabling the bridge (optional)

The bridge is on by default. To force the one-shot CLI path (no cross-call cache), set an environment variable and restart Pi:

export LEAN_CTX_PI_ENABLE_MCP=0
pi

…or set "enableMcp": false in ~/.pi/agent/extensions/pi-lean-ctx/config.json.

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. binary in ~/.pi/agent/extensions/pi-lean-ctx/config.json
3. ~/.cargo/bin/lean-ctx
4. ~/.local/bin/lean-ctx (Linux) or %APPDATA%\Local\lean-ctx\lean-ctx.exe (Windows)
5. /usr/local/bin/lean-ctx (macOS/Linux)
6. 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 Context Layer for AI coding agents
• GitHub
• Discord