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L4 Computational Law Plugin for Claude Code

A Claude Code plugin for writing L4 rules-as-code — a statically-typed, pure-functional programming language for computational law. Encode legislation, contracts, policies, and regulations as executable, type-checked rules, then deploy them as live REST APIs, MCP tools, and WebMCP agents via Legalese Cloud.

Features

  • L4 authoring skill — expert guidance for formalising legal text as typed decision functions and regulative rules (MUST / MAY / SHANT / DO, with deadlines, HENCE / LEST consequences, and BREACH semantics).
  • Deploy to Legalese Cloud — annotate functions with @export and @desc, deploy from the VS Code extension's Deploy tab, and your rules are instantly available as:
    • REST endpoints (POST /deployments/{id}/functions/{fn}/evaluation)
    • MCP JSON-RPC 2.0 tools for LLM agent tool-use (POST /deployments/{id}/.mcp)
    • WebMCP tools for browser AI agents (<script src="/.webmcp/embed.js">)
    • OpenAPI 3.0 schemas (GET /deployments/{id}/openapi.json)
    • Query planning for interactive questionnaires that only ask inputs that still matter
    • Audit-grade execution traces (?trace=full&graphviz=true on any evaluation)
  • Reference documentation — curated deep-dives on regulative rules, builtin libraries, and traps a general-purpose LLM won't get right, all discoverable from the skill.

Installation

Via Marketplace (recommended)

  1. Add the Legalese marketplace: 
    /plugin marketplace add legalese/l4-ide
  2. Install the plugin: 
    /plugin install l4-computational-law@legalese

Once installed, Claude Code auto-loads the writing-l4-rules skill whenever the plugin is active.

Usage

The skill activates when you:

  • Ask to formalise legal text, contracts, policies, or regulations
  • Work with .l4 files
  • Ask about computational law, obligations/deadlines, or deploying L4 rules as APIs or agent tools

Example prompts

  • "Formalise this insurance policy clause in L4"
  • "Draft an L4 rule for a 30-day payment obligation with breach reparation"
  • "Model this statute as L4 with #TRACE test cases"
  • "Export this decision function and deploy it to Legalese Cloud"
  • "Turn this eligibility checklist into an MCP tool my agent can call"

What is L4?

L4 is a statically-typed, pure-functional language designed for legal drafting:

  • Layout-sensitive like Python
  • Haskell-style algebraic data types for modelling legal ontologies
  • Backtick identifiers that read like prose: `the applicant qualifies for benefits`
  • Regulative rules with deadlines and reparations: MUST … WITHIN 30 … HENCE … LEST BREACH
  • @export / @desc annotations that publish typed functions to REST, MCP, and OpenAPI with LLM-friendly parameter descriptions

Canonical documentation lives at https://legalese.com/l4/.

Example: an exported decision function

@desc Given the day of the week, a flag about whether it's a public holiday, and prevailing weather conditions, calculate the cost of parking.
@export default Calculate the cost of parking
GIVEN
  day_of_week       IS A NUMBER  @desc the day of the week (1 = Monday, ..., 7 = Sunday)
  is_public_holiday IS A BOOLEAN @desc whether it is a public holiday
  current_weather   IS A STRING  @desc Current weather conditions. Possible values are: fair, rain, snow
GIVETH A NUMBER
DECIDE `Parking cost` IS ...

The @desc on each parameter flows into the generated OpenAPI schema and the MCP tool's inputSchema, so downstream LLMs know exactly how to construct a valid call. See the full working file at .claude/skills/writing-l4-rules/assets/example-parking.l4.

Deployment with Legalese Cloud

Legalese Cloud is the managed hosting service for L4 rule bundles. One deployment gives you every interface at once — REST, MCP, WebMCP, OpenAPI, query planning, and traces — all backed by the same type-checked source.

Each organisation gets a subdomain (https://{org-slug}.legalese.cloud), OAuth-protected endpoints, handled compilation, and one-click deployment from the VS Code extension's Deploy tab. Discovery starts at:

https://{org-slug}.legalese.cloud/.well-known/oauth-protected-resource

You can also self-host with jl4-service — the API surface is identical, so client code is portable between self-hosted and managed deployments.

Validation

Validate .l4 files locally with the l4 CLI:

# Fast typecheck (use for CI / pre-commit)
l4 check path/to/file.l4

# Full typecheck + evaluate #EVAL directives
l4 run path/to/file.l4

# From a Haskell checkout (no installed binary)
cabal run l4 -- run path/to/file.l4

A wrapper script ships inside the skill at .claude/skills/writing-l4-rules/scripts/validate.sh.

Documentation

Inside the plugin (.claude/skills/writing-l4-rules/):

External:

Support

License

Apache 2.0

Credits

Developed by Legalese Pte. Ltd. in Singapore