# typelevel/cats-effect

The pure asynchronous runtime for Scala

Repository: https://github.com/typelevel/cats-effect
Canonical: https://ross.abutalabs.com/products/cats-effect
Homepage: https://typelevel.org/cats-effect/
Language: Scala
License: Apache-2.0
License Family: permissive
Topics: effects, scala, fp, asynchronous, typeclasses
Last push: 2026-08-25T01:23:27+00:00

## Health v2 (maintenance only)
Score: 99/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 99, release rhythm 99, longevity 100
- inputs: {"age_days": 3434, "days_push": 9, "days_rel": 10, "gap_med": 22, "n_releases_24m": 10}
- flags: none
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 2236, forks 577 (observed 2026-08-28T04:06:29.376838+00:00)

## What it is
Cats Effect is a high-performance, purely functional asynchronous runtime for Scala built around the IO monad and a typeclass hierarchy defining functional effect systems. It provides lightweight fibers, resource safety, cancellation, and concurrency primitives that power a large ecosystem of streaming, HTTP, and database libraries in the Typelevel ecosystem.

## Use cases
- build highly concurrent Scala applications with lightweight fibers
- manage resources safely with automatic acquisition and release
- capture and compose asynchronous side effects in a pure functional style
- write testable effectful Scala programs using typeclasses
- run web services handling tens of thousands of requests per second
- implement producer-consumer patterns with queues and semaphores
- trace and debug effectful program execution in production

## When to choose
- you are writing Scala applications in a purely functional style
- you need a mature, resource-safe concurrency runtime with lightweight fibers
- you want to interoperate with the Typelevel ecosystem (fs2, http4s, doobie)
- you need composable asynchronous code with cancellation and backpressure

## When to avoid
- your team is unfamiliar with functional programming concepts like monads and typeclasses
- you are maintaining a legacy Scala 2 codebase pinned to the end-of-life 2.x series
- you prefer direct imperative threading models like plain Futures or OS threads
- you are working outside the JVM/Scala ecosystem

## Facets
- artifact type: library
- maturity: stable
- function: concurrency, middleware, sdk
- domain: programming-languages, developer-tools, backend
- platform: jvm, cross-platform
- tags: scala, functional-programming, effect-system, io-monad, typeclasses, fibers, typelevel, runtime, resource-safety, asynchronous, javascript

## Member repositories
- typelevel/cats-effect (main) score 99

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:06:29.376838+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T02:44:18.182280+00:00, confidence not recorded.
  - readme: https://github.com/typelevel/cats-effect (fetched 2026-08-28T04:06:29.376838+00:00, sha 7402238d1d0b)
  - homepage: https://typelevel.org/cats-effect/ (fetched 2026-08-29T10:24:50.446594+00:00, sha 7868b6eb3d2c)
  - site_page: https://typelevel.org/cats-effect/docs/getting-started (fetched 2026-08-29T10:24:50.449061+00:00, sha 7ceb3d4862a7)
  - site_page: https://typelevel.org/cats-effect/docs/tutorial (fetched 2026-08-29T10:24:50.451271+00:00, sha 614592ab710c)
  - site_page: https://typelevel.org/cats-effect/docs/typeclasses (fetched 2026-08-29T10:24:50.454910+00:00, sha 1b2e61bf122d)
  - site_page: https://typelevel.org/cats-effect/docs/schedulers (fetched 2026-08-29T10:24:50.456945+00:00, sha 008a8009faaa)
- Data as of 2026-08-30T08:39:29.467469+00:00.