LAIN-mcp

Rust MCP server that gives AI coding agents architectural awareness — persistent knowledge graph, blast radius analysis, co-change detection via git, and local semantic search. No API keys, runs entirely on-premise.

GitHub

LAIN-mcp

LAIN builds a map of how all the code in your project connects — what calls what, what depends on what, which files tend to change together. Then it lets your AI coding assistant ask questions about that map. So instead of the AI just looking at one file and guessing, it can ask "if I change this function, what else breaks?" and get a real answer. It plugs into any AI agent that supports MCP and runs in the background while you work.

Screenshot 2026-04-29 at 9 18 15 PM

TL,DR:

# One-line install (interactive - will ask you to configure and add to PATH)
curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | bash

# After install: reload your shell (or open a new terminal)
source ~/.zshrc   # or ~/.bashrc

# Or non-interactive (skips prompts, auto-adds to PATH)
curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | \
  bash /dev/stdin --workspace . --transport both --yes

What is Lain?

Lain is a persistent code-intelligence MCP server. It builds a queryable knowledge graph of your codebase — symbols and their relationships extracted via LSP and tree-sitter, augmented with git co-change history and optional semantic embeddings — and exposes that graph through MCP tools. The value over LSP-only or RAG-based approaches is cross-file structural reasoning: blast radius for proposed changes, transitive dependency traces, anchor identification, co-change correlation, and contextual build failure decoration so agents can reason about callers rather than just the failing line. Written in Rust, persists across sessions, stays fresh during editing via a file watcher that updates a volatile overlay layered on top of the static graph.

Installation

Quick Install (recommended - interactive)

curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | bash

The installer will ask you to configure:

  • Workspace path
  • MCP transport mode (stdio, http, or both)
  • HTTP port (if using http/both)
  • Target agent (auto-detects Claude Code, Cursor, Windsurf, Cline)
  • Whether to download the ONNX model for semantic search

After you confirm your settings, it will:

  1. Download and install LAIN to ~/.local/lain
  2. Optionally download the ONNX model (~120MB)
  3. Run lain init with your configuration
  4. Add LAIN to your agent's settings

Non-interactive install (with options):

# Install with specific workspace and download ONNX model for semantic search
curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | \
  bash /dev/stdin --workspace . --transport both --download-model --yes

# Install for specific agent
curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | \
  bash /dev/stdin --agent cursor --yes

# See all options
curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | \
  bash /dev/stdin --help

Install options:

OptionDescriptionDefault
--workspace PATHWorkspace path for LAIN.
--transport MODEMCP transport: stdio, http, bothstdio
--port PORTHTTP port for MCP server9999
--agent AGENTTarget agent: auto, claude, cursor, windsurf, clineauto
--embedding-model PATHPath to ONNX embedding model-
--download-modelDownload default ONNX model (all-MiniLM-L6-v2.onnx, ~120MB)-
-y, --yesSkip all confirmation prompts-

After installation:

# Reload your shell (the installer adds to ~/.zshrc or ~/.bashrc automatically)
source ~/.zshrc   # or ~/.bashrc, then open a new terminal

# Verify installation
lain --version

# Query the graph
lain query "find Function | limit 5"

Homebrew

brew tap spuentesp/lain https://github.com/spuentesp/lain
brew install lain

# Initialize
lain init

Pre-built Binary

Download the latest release for your platform from GitHub releases, then:

# Make executable
chmod +x lain

# Run directly
./lain --workspace /path/to/your/project --transport stdio

Build from Source

# Clone the repo
git clone https://github.com/spuentesp/lain.git
cd lain

# Build (requires Rust 1.75+)
cargo build --release

# Binary will be at ./target/release/lain

Quick Start

1. Install LAIN

curl -fsSL https://raw.githubusercontent.com/spuentesp/lain/main/install.sh | bash

2. Initialize for Claude Code (or other agents)

# Auto-detect agent (Claude Code, Cursor, Windsurf, Cline)
lain init

# Or specify agent explicitly
lain init --agent claude

3. Run

# Standard mode (for Claude Code)
lain --workspace /path/to/project --transport stdio

# With HTTP diagnostics (web UI at http://localhost:9999)
lain --workspace /path/to/project --transport both --port 9999

# With semantic search (requires ONNX model)
lain --workspace /path/to/project --embedding-model ~/.local/lain/models/all-MiniLM-L6-v2.onnx

4. Verify

# Check health and LSP status
curl -s -X POST http://localhost:9999/mcp -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"get_health","arguments":{}},"id":1}'

# Query the graph directly
lain query "find Function | limit 5"

Key Features

Query Language (query_graph)

JSON-based ops array for flexible graph traversals:

{
  "ops": [
    { "op": "find", "type": "Function" },
    { "op": "connect", "edge": "Calls", "depth": { "min": 1, "max": 3 } },
    { "op": "filter", "label": "test" },
    { "op": "semantic_filter", "like": "error handling", "threshold": 0.35 },
    { "op": "limit", "count": 10 }
  ]
}

Available ops: find, connect, filter, semantic_filter, group, sort, limit

Dependency Intelligence

  • get_call_chain — Shortest path between two functions
  • get_blast_radius — Everything affected by a change
  • trace_dependency — What a symbol depends on
  • get_coupling_radar — Files that change together

Architectural Analysis

  • find_anchors — Most-called, most-stable symbols (architectural pillars)
  • list_entry_points — Find main(), route handlers, app initialization
  • get_context_depth — How far from an entry point (abstraction layers)
  • explore_architecture — High-level tree of modules and files

Search

  • semantic_search — Find code by meaning, not just names (uses local ONNX embeddings)

Code Health

  • find_dead_code — Potentially unreachable code (filters trait defaults, common names)
  • suggest_refactor_targets — High-coupling, low-stability nodes

Build Integration

Lain enriches build failures with architectural context:

  • run_build — Build with Rust/Go/JS/Python toolchain error parsing
  • run_tests — Tests with error enrichment
  • run_clippy — cargo clippy with context

Requirements

RequirementDetails
Rust1.75 or newer
GitRequired for co-change analysis
ONNX ModelOptional — for semantic search

Optional: Semantic Search

For semantic_search to work, you need an ONNX embedding model. The easiest way to set this up is using the provided install script:

./scripts/install.sh

Alternatively, you can set it up manually:

# Create model directory
mkdir -p .lain/models

# Download all-MiniLM-L6-v2 (or any compatible model)
# Model produces 384-dim embeddings
curl -L https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2/resolve/main/onnx/model.onnx -o .lain/models/model.onnx
curl -L https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2/resolve/main/tokenizer.json -o .lain/models/tokenizer.json

Set the model path:

export LAIN_EMBEDDING_MODEL=$PWD/.lain/models/model.onnx
# or
./lain --embedding-model ./.lain/models/model.onnx ...

Without the model, semantic_search returns "unavailable" but all other features work.

MCP Transport Modes

ModeCommandUse Case
stdio--transport stdioClaude Code, MCP clients
http--transport http --port 9999Web diagnostics dashboard
both--transport both --port 9999Both stdio + diagnostics

Troubleshooting

LSP servers not ready?

# Install missing language servers
curl -X POST http://localhost:9999/mcp \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"install_language_server","arguments":{"language":"rust"}},"id":2}'

Graph stale?

# Sync to current git HEAD
curl -X POST http://localhost:9999/mcp \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"sync_state","arguments":{}},"id":3}'

View all available tools:

curl -s -X POST http://localhost:9999/mcp \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"get_agent_strategy","arguments":{}},"id":4}'

A/B Testing Results

A simple A/B test was run on the asciinema_fix_pty_bug (a small fork i made from https://github.com/asciinema/asciinema.git ) across 5 passes, 4 times using a script. Median numbers are reported.

Metricwith_lainwithout_lain
Pass rate5/5 (100%)5/5 (100%)
Median duration39.3s54.1s
Median tokens in35,48841,731

Key observations:

  • Both conditions passed 100% — the bug fix worked in both conditions, with variation per run.
  • with_lain used fewer input tokens (~35k vs ~42k median), a difference of ~7k tokens per run.

About the bug: The failing test (pty::tests::spawn_extra_env on macOS) stems from handle_child() setting env vars via env::set_var() before execvp(). The shell's interpretation of echo -n $VAR varies across platforms — sometimes -n is treated as a literal argument. The fix: use printf "%s" "$ASCIINEMA_TEST_FOO" instead, portable across all Unix-like systems.

This was a test I did for A/B comparison — not a rigorous evaluation.

License

MIT — Copyright (c) 2026 spuentesp

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