Malli

What hackers pick instead

• PydanticPython library that validates data against type-annotated models, popular with FastAPI and AI output. Critics call it slow, heavy and prone to breaking changes.62recommend
• ZodPopular TypeScript schema library that validates untrusted data at boundaries and infers static types. Critics cite added dependency weight, inferred-type quirks, and the reported 4.6.6 malware compromise.58recommend
• clojure.specBuilt into Clojure, spec lets people express structural guarantees more expressively than an ordinary type system. Critics say it mostly retrofits static typing onto a dynamic language.18recommend
• Guardrails AIDevelopers recommend it for forcing AI agents to pass tests and build checks, and for being simpler with less code. It is free software.5recommend

Every quote

The 14 most detailed of 16 comments. Each links to the original.

1. The LLM has been pretty good at falling back to looking at the schema if tests fail because of validation issues.

   Recommends · May 2026 · in “ClojureScript Gets Async/Await”

2. you can easily make those structural guarantees in a way that is more expressive than an ordinary type system

   Recommends · Nov 2025

3. Tacking on malli (runtime type checking) or spec (types/contracts) helps LLMs avoid type problems altogether

   Recommends · May 2026 · in “ClojureScript Gets Async/Await”

4. there's always something that needs paranoia, or it's handy to generate example data

   Recommends · Jun 2026 · in “My thoughts after using Clojure for about a month”

5. If you want to calcify something and add robustness, use clojure.spec or Malli.

   Recommends · Apr 2026 · in “Bringing Clojure programming to Enterprise (2021)”

6. validating the value properly rather than just checking for presence

   Recommends · Jul 2026 · in “Clojure 1.13 adds support for checked keys”

7. When need some rigidity - Spec/Malli are great.

   Recommends · Nov 2025 · in “Experiment: Making TypeScript immutable-by-default”

8. generated tests based on Specs or Malli

   Recommends · Apr 2026 · in “Clojure: Transducers”

9. Theyre more feature-rich than types

   Recommends · Mar 2026 · in “Show HN: I built a self-hosted course platform in Clojure”

10. There are some _very good_ examples of (data-) DSLs provided by libraries

    Recommends · Apr 2026 · in “Bringing Clojure programming to Enterprise (2021)”

11. you might throw schemas (eg. Malli) at pre 1.13

    Recommends · Jul 2026 · in “Clojure 1.13 adds support for checked keys”

12. Case in point: spec vs. schema vs. malli.

    Mixed · Apr 2026 · in “A Perfectable Programming Language”

13. generate a json schema with malli

    Mixed · Sep 2026 · in “Using jq to format JSON on the clipboard”

14. malli is infinitely better

    Recommends · May 2026