L4 Rules-as-code Language Support and Tools

Language support for L4 rules-as-code with a built-in Legalese AI chat, MCP for AI agents like Claude Code and Copilot, WebMCP for browser AI, syntax highlighting, IntelliSense, CLI, decision graph visualizations, result inspection, and Legalese Cloud deployment.

Documentation: https://legalese.com/l4

Features

Legalese AI: Built-in chat tab for composing and iterating on L4 rules with an assistant that knows L4 and can drive the fs / LSP / MCP tool set directly.
MCP Server (Model Context Protocol): Expose deployed L4 rules as tools that any AI agent can discover and call
- Built-in MCP proxy server registers automatically with VS Code
- Legalese AI (in-editor) and any MCP-speaking agent (Cursor, Copilot, Claude Code, third parties) can discover and invoke your L4 functions as structured tools
- Scoped access — expose all rules org-wide or restrict to specific deployments
- Authenticated via Legalese Cloud or API key for self-hosted setups
WebMCP — Browser AI Integration: Let browser-based AI agents call your L4 rules from any web page
- Embed a single <script> tag to register L4 functions with navigator.modelContext
- Browser AI agents discover and call your rules without a backend integration
- Configurable scope, tool registration mode, and authentication via data attributes
Cloud Deploy: Deploy L4 rules as REST APIs from the sidebar
- Deploy Preview tab shows exported functions with parameter schemas
- Breaking change detection when updating existing deployments
- Deployments tab lists active deployments with full function metadata
- Supports Legalese Cloud (browser login) and self-hosted jl4-service (API key)
Syntax Highlighting: Full support for L4 syntax with proper color coding
Language Server Protocol (LSP): Advanced language features including:
- IntelliSense and autocompletion
- Error detection and diagnostics
- Code navigation
- Symbol search
Ladder Diagram Visualization: Interactive decision graph of L4 rules as ladder diagrams
- Real-time updates as you edit your code
- Visual representation of logical relationships
- Interactive exploration of rule structures
Result Inspector: Live evaluation results for #EVAL, #EVALTRACE, #CHECK, and #ASSERT directives
- Click "Track result" above a directive to add it to the inspector
- Results update automatically as you edit your code
- Collapsible sections grouped by file
- Syntax-highlighted output

What is L4?

L4 is a domain-specific language designed for expressing legal logic and rules in a formal, computable way. It allows legal professionals and developers to:

Write legal rules in a structured, unambiguous format
Visualize complex legal logic through ladder diagrams
Validate legal reasoning through formal methods
Deploy rules as REST APIs for integration with other systems
Expose rules to AI agents via MCP and browser AI via WebMCP

Getting Started

Install the Extension: Install this extension from the VS Code marketplace
Open an L4 File: Create or open a file with the .l4 extension
Configure the Language Server: The extension requires the jl4-lsp language server

The L4 sidebar is accessible via the L4 icon in the activity bar. It has three tabs:

Result Inspector

Open an L4 file with #EVAL, #CHECK, or #ASSERT directives
Click "Track result" in the CodeLens above a directive
The sidebar switches to the Result Inspector tab and shows the evaluation result
Results update live as you edit code

Deploy Preview

Shows exported functions from the active L4 file with their parameter schemas, types, and descriptions
Updates automatically as you edit
Click "Deploy" to start the deployment flow

Deployments

Lists active deployments with their functions (sourced from the service's OpenAPI spec)
Expand a deployment to see its function schemas
Undeploy with confirmation

Using the L4 Decision Graph

Open an L4 file containing rules
Look for the "Show decision graph" codelens above L4 rules with boolean returns
Click on the codelens to open the ladder diagram visualization
The diagram will update automatically as you edit your code

MCP — AI Agent Integration

Once you deploy L4 rules, they are automatically available as MCP tools that AI agents can discover and call.

Local MCP Proxy (VS Code)

The extension starts a local MCP proxy server automatically. Legalese AI (built in), along with any external MCP-speaking agent (Cursor, GitHub Copilot, Claude Code, third-party clients), can connect to it to discover and invoke your deployed L4 functions.

The proxy registers with VS Code's built-in MCP system. The default port is 19415 (configurable via jl4.mcpPort). The "Add L4 Tools to Claude Code" dropdown action in the sidebar wires the same MCP endpoint plus the writing-l4-rules skill into a local Claude Code installation on demand.

Remote MCP Endpoints

Deployed rules are also accessible via MCP endpoints on the service:

POST /.mcp — all deployments (org-wide)
POST /{deployment-id}/.mcp — scoped to one deployment
GET /.well-known/mcp — MCP discovery metadata

Example MCP client config (e.g. for Claude Desktop):

{
  "mcpServers": {
    "l4-rules": {
      "url": "https://your-service/.mcp",
      "headers": {
        "Authorization": "Bearer sk_..."
      }
    }
  }
}

WebMCP — Browser AI Integration

WebMCP lets browser-based AI agents (like Claude on the web) discover and call your L4 rules directly from any web page.

Add this script tag to your page:

<script src="https://your-service/.webmcp/embed.js"></script>

The script registers your L4 functions with the browser's navigator.modelContext API. Browser AI agents can then see and call them as tools.

Configuration

Use data attributes to customize behavior:

<script
  src="https://your-service/.webmcp/embed.js"
  data-scope="insurance-premium/*,safe-valuation/effective-sale-price"
  data-tools="auto"
  data-api-key="sk_..."
></script>

Attribute	Description
`data-scope`	Filter by deployment/function (e.g. `deploy/*` for all functions, `deploy/fn` for a specific one)
`data-tools`	Registration mode: `auto` (default), `discovery`, `direct`, or `all`
`data-api-key`	API key for authenticated access

Cloud Deploy Setup

Legalese Cloud

No configuration needed. Click "Sign in with Legalese Cloud" in the sidebar footer to authenticate via browser.

Legalese Cloud is a service provided by Legalese Pte. Ltd. More info at https://legalese.cloud

Self-hosted jl4-service

Configure the service URL and API key in VS Code settings:

{
  "jl4.serviceUrl": "http://localhost:8080",
  "jl4.serviceApiKey": "sk_..."
}

L4 CLI

The extension ships with the l4 command-line tool for running L4 files, checking rules, and other workflows outside the editor.

Installing

Run L4: Install L4 CLI from the command palette, or accept the prompt the first time you open an L4 project. This installs the bundled l4 binary onto your PATH:

macOS / Linux: symlinks the binary into ~/.local/bin/l4. If ~/.local/bin isn't on your PATH, the extension shows the export PATH="$HOME/.local/bin:$PATH" snippet to add to your ~/.zshrc or ~/.bashrc.
Windows: copies the exe into %LOCALAPPDATA%\Programs\l4\l4.exe and appends that directory to your user PATH via setx. Restart open terminals for the change to take effect.

The installer is safe to re-run — each invocation refreshes the symlink or file, so upgrades propagate when you update the extension.

Requirements

The CLI is bundled only in platform-specific builds of the extension (macOS ARM64/x64, Linux x64, Windows x64). If you installed the universal build, install l4 manually via cabal install exe:l4 --overwrite-policy=always.

Usage

Once installed, run l4 --help in a new terminal to see available subcommands.

Example L4 Code

DECLARE Person
  HAS name    IS A STRING
      age     IS A NUMBER
      country IS A STRING

GIVEN p IS A Person
GIVETH A BOOLEAN
DECIDE `is adult` p IF p's age >= 18

GIVEN p IS A Person
GIVETH A BOOLEAN
DECIDE `can vote` p IF
  `is adult` p
  AND p's country = "UK"

Requirements

VS Code 1.94.0 or higher
jl4-lsp language server (see setup instructions above)

Configuration

jl4.serverExecutablePath: Path to the jl4-lsp executable
jl4.trace.server: Enable server communication tracing (off/messages/verbose)
jl4.serviceUrl: URL of the jl4-service for deploying rules (leave empty for Legalese Cloud)
jl4.serviceApiKey: API key for authenticating with a self-hosted jl4-service
jl4.mcpPort: Port for the local MCP proxy server (default: 19415)

Custom Language Server Setup

Platform-specific versions (recommended): If you installed a platform-specific version of the extension (e.g., for macOS ARM64, Windows x64, etc.), the jl4-lsp language server is bundled and ready to use—no additional setup required!

Universal version: If you installed the universal extension, or if the bundled binary is not available for your platform, install the language server:

cabal install exe:jl4-lsp --overwrite-policy=always

Alternatively, specify the path manually in VS Code settings:

{
  "jl4.serverExecutablePath": "/path/to/jl4-lsp"
}

License

Apache 2.0 - See license file for details.

L4 Rules-as-code

Legalese

L4 Rules-as-code Language Support and Tools

Features

What is L4?

Getting Started

Using the Sidebar

Result Inspector

Deploy Preview

Deployments

Using the L4 Decision Graph

MCP — AI Agent Integration

Local MCP Proxy (VS Code)

Remote MCP Endpoints

WebMCP — Browser AI Integration

Configuration

Cloud Deploy Setup

Legalese Cloud

Self-hosted jl4-service

L4 CLI

Installing

Requirements

Usage

Example L4 Code

Requirements

Configuration

Custom Language Server Setup

License