CASL
Isomorphic authorization library for JavaScript/TypeScript. CASL defines what a user CAN do using ability rules — 'user can read Post', 'user cannot delete Post if not owner'. Works in both frontend (hide UI elements) and backend (enforce API access). Supports Role-Based (RBAC), Attribute-Based (ABAC), and subject-based access control. Integrates with Prisma, Mongoose, AngularJS, React, and Vue.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
In-process authorization — always enforce server-side; never rely on client-side checks for security. Rule definition in code means permissions are version-controlled. MIT licensed.
⚡ Reliability
Best When
You need fine-grained, isomorphic authorization with JavaScript/TypeScript that works in both your frontend and backend with the same rules.
Avoid When
You need centralized external policy management, audit logging, or a standardized policy language — use Cerbos or Open Policy Agent for those requirements.
Use Cases
- • Define user permissions for agent systems — 'agent can execute X if authorized for namespace Y' — with unified frontend/backend enforcement
- • Implement RBAC in Node.js APIs where different user roles (admin, editor, viewer) have different CRUD permissions per resource type
- • Hide UI components based on user permissions in React using CASL's React integration (Can component, useAbility hook)
- • Filter database queries based on user permissions using CASL's Prisma/Mongoose integration — automatically scope queries to accessible records
- • Build multi-tenant authorization where users can only access their organization's data using attribute-based conditions
Not For
- • Simple role checks where a basic if/else or enum comparison is sufficient
- • External policy services — CASL is in-process authorization; use Open Policy Agent or Cerbos for external policy engines
- • Complex policy languages — CASL rules are defined in JavaScript, not a separate policy language like Rego (OPA)
Interface
Authentication
No authentication — authorization library. Integrates with existing auth sessions to define abilities based on authenticated user.
Pricing
MIT-licensed open source library.
Agent Metadata
Known Gotchas
- ⚠ CASL ability rules are evaluated in order — more specific rules override general rules; rule ordering matters and affects which rules take precedence
- ⚠ Ability rules must be rebuilt when user permissions change — rules are defined at object creation time; changing user role requires creating a new AbilityBuilder with new rules
- ⚠ Condition fields in CASL rules ({ author: user.id }) are compared against subject attributes — the subject object must have the matching fields populated for conditions to work
- ⚠ Frontend CASL integration (Can component) is for UX only — always enforce permissions server-side; never rely on frontend CASL for security
- ⚠ CASL v6 changed the API for defining abilities — v5 and v6 code is not compatible; major version upgrades require ability definition rewrites
- ⚠ Database integration (casl-prisma, casl-mongoose) generates query filters but requires explicit application — CASL doesn't automatically filter queries; call accessibleBy() explicitly
Alternatives
Full Evaluation Report
Detailed scoring breakdown, competitive positioning, security analysis, and improvement recommendations for CASL.
Scores are editorial opinions as of 2026-03-06.