typelevel/cats-effect

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The pure asynchronous runtime for Scala

Stars: 2.2k
Forks: 576
Open beginner issues: 0
Indexed issues: 198
Avg merge: 2d 11h
Merged PRs (30d): 18

Dominant language: Scala
License: Apache-2.0
Last GitHub push: Sep 18, 2026
Latest indexed: Sep 19, 2026
Contributing guide: Contributing guide
Code of conduct: Code of conduct
Beginner labels: good first issue help wanted

Beginner friendly Everything

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198 open issues indexed

`IO#syncStep` drops `uncancelable` and `onCancel` from the returned `IO`, so `Resource` release is skipped Open

Difficulty 4/5 3-5 days Newbie friendliness 68/100

typelevel/cats-effect#4687 · 2 comments · Opened Sep 18, 2026
`LocalQueue.enqueueBatch` spins forever on a dangling steal tag, and the queue becomes permanently unstealable Open

:beetle: bug

Difficulty 5/5 Over a week Newbie friendliness 35/100

typelevel/cats-effect#4674 · 1 comment · Opened Aug 27, 2026
TracingPlatform performs undefined behavior in JS that makes it unusable in Wasm Open

:beetle: bug

Difficulty 3/5 1-2 days Newbie friendliness 55/100

typelevel/cats-effect#4644 · 8 comments · Opened Jul 26, 2026
MapAtomicCell Open

Difficulty 4/5 3-5 days Newbie friendliness 52/100

typelevel/cats-effect#4635 · 1 comment · Opened Jul 12, 2026
Integrated `Selector`: a ready I/O event can go undelivered for seconds while worker threads sit idle Open

:beetle: bug

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4628 · 19 comments · Opened Jun 29, 2026
`Resource.eval` extra cancellation point Open

:beetle: bug

Difficulty 4/5 3-5 days Newbie friendliness 52/100

typelevel/cats-effect#4627 · Opened Jun 28, 2026
Add `Fiber#joinOrCancel` Open

:mushroom: enhancement

Difficulty 5/5 Over a week Newbie friendliness 42/100

typelevel/cats-effect#4620 · 36 comments · 1 reaction · Opened Jun 21, 2026
Converge on a single nomenclature for fiber blocking in docs Open

:books: docs good first issue

Difficulty 2/5 1-3 hours Newbie friendliness 74/100

typelevel/cats-effect#4597 · 3 reactions · Opened May 5, 2026
Cirrus CI shutdown Open

:gear: infrastructure

typelevel/cats-effect#4590 · 4 comments · 1 assignee · Opened Apr 15, 2026
Queue.take.timeout(...) loses elements on cancelation Open

:beetle: bug

Difficulty 4/5 3-5 days Newbie friendliness 48/100

typelevel/cats-effect#4571 · 4 comments · Opened Mar 19, 2026
Flaky `DispatcherSpec` on JS Open

Difficulty 3/5 1-2 days Newbie friendliness 45/100

typelevel/cats-effect#4541 · Opened Dec 30, 2025
SIGPIPE during tests on scala-native Open

:beetle: bug

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4527 · Opened Nov 23, 2025
`TestContext` on scala-native is not threadsafe Open

:beetle: bug

Difficulty 3/5 1-2 days Newbie friendliness 48/100

typelevel/cats-effect#4524 · 6 comments · Opened Nov 18, 2025
Fork-with-finalizer method on `GenSpawn` Open

:mushroom: enhancement

Difficulty 5/5 Over a week Newbie friendliness 25/100

typelevel/cats-effect#4517 · 2 comments · 1 reaction · Opened Oct 21, 2025
`OutOfMemoryError` not propagated when `IO` originates from `CompletableFuture` Open

:beetle: bug good first issue

Difficulty 4/5 3-5 days Newbie friendliness 48/100

typelevel/cats-effect#4505 · 16 comments · 2 reactions · Opened Oct 13, 2025
`Queue#mapK` violates atomicity guarantees Open

:beetle: bug

Difficulty 3/5 1-2 days Newbie friendliness 45/100

typelevel/cats-effect#4483 · 6 comments · Opened Sep 5, 2025
Scala Native + Windows? Open

:books: docs

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4473 · 1 comment · Opened Aug 21, 2025
IOAppSuite failure on JDK 25 Open

Difficulty 3/5 1-2 days Newbie friendliness 35/100

typelevel/cats-effect#4471 · 2 comments · 1 reaction · Opened Aug 17, 2025
`AsyncSuite` failure on macos-14 Open

Difficulty 4/5 3-5 days Newbie friendliness 30/100

typelevel/cats-effect#4469 · Opened Aug 11, 2025
BoundedAsyncQueue performs poorly on Scala Native Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4465 · 2 comments · Opened Aug 3, 2025
Can we implement some flavor of `interruptible` on Native using signals? Open

:mushroom: enhancement

Difficulty 5/5 Over a week Newbie friendliness 25/100

typelevel/cats-effect#4463 · 1 comment · Opened Jul 30, 2025
macOS CI build hung again Open

Difficulty 4/5 3-5 days Newbie friendliness 25/100

typelevel/cats-effect#4456 · Opened Jul 23, 2025
`SupervisorSuite` timeout on Windows Open

Difficulty 4/5 3-5 days Newbie friendliness 25/100

typelevel/cats-effect#4445 · 6 comments · Opened Jul 17, 2025
Add different effect init for `Random`/`SecureRandom` Open

Difficulty 5/5 Over a week Newbie friendliness 35/100

typelevel/cats-effect#4410 · 4 comments · Opened May 22, 2025
Add thread name to IO.debug Open

Difficulty 2/5 1-3 hours Newbie friendliness 55/100

typelevel/cats-effect#4409 · 2 comments · Opened May 22, 2025
Cats-effect 3 IO docs are missing Open

Difficulty 3/5 1-2 days Newbie friendliness 45/100

typelevel/cats-effect#4404 · Opened May 13, 2025
UUIDGen Implicit Prioritization Issue on Scala 2.12 Open

typelevel/cats-effect#4397 · 1 comment · 1 assignee · Opened May 1, 2025
Proposal: Retries Open

Difficulty 5/5 Over a week Newbie friendliness 25/100

typelevel/cats-effect#4396 · 6 comments · 4 reactions · Opened May 1, 2025
Add Scala 3.6 to build coverage Open

:gear: infrastructure

Difficulty 2/5 1-3 hours Newbie friendliness 48/100

typelevel/cats-effect#4387 · 2 comments · Opened Apr 18, 2025
Change the `Example` app on Native Open

Difficulty 4/5 3-5 days Newbie friendliness 25/100

typelevel/cats-effect#4351 · 3 comments · Opened Apr 4, 2025
Optimize `SyncRef` on Native Open

typelevel/cats-effect#4350 · 3 comments · 1 assignee · Opened Apr 4, 2025
Optimize `LocalQueue` on Native Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4349 · 1 comment · Opened Apr 4, 2025
DRY the constant definitions Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4348 · 2 comments · 1 reaction · Opened Apr 4, 2025
Optimize `KqueueSystem` on Native Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4347 · Opened Apr 4, 2025
Optimize `EpollSystem` on Native Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4346 · 2 comments · Opened Apr 4, 2025
Implement `TimerHeap#Node` on Native without `@safePublish` Open

Difficulty 4/5 3-5 days Newbie friendliness 25/100

typelevel/cats-effect#4345 · Opened Apr 4, 2025
Update submodule in site for 3.6 Open

:books: docs

Difficulty 2/5 1-3 hours Newbie friendliness 25/100

typelevel/cats-effect#4342 · Opened Apr 3, 2025
Find a way to DRY up the `IOApp`s Open

typelevel/cats-effect#4333 · 4 comments · 1 assignee · Opened Mar 30, 2025
Consider implementing non-daemon thread warning on SN Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4332 · Opened Mar 30, 2025
Implement blocked thread detection on SN Open

Difficulty 5/5 Over a week Newbie friendliness 25/100

typelevel/cats-effect#4330 · Opened Mar 30, 2025
Remove TODO in `IOFiber.readBarrier()` Open

Difficulty 2/5 1-3 hours Newbie friendliness 42/100

typelevel/cats-effect#4321 · 3 comments · Opened Mar 25, 2025
`AsyncSuite.fromFutureCancelable` timeout Open

Difficulty 3/5 1-2 days Newbie friendliness 35/100

typelevel/cats-effect#4305 · 3 comments · Opened Mar 17, 2025
`cats.effect.std.SupervisorSuite.async - restart / cancel race` timeout Open

:beetle: bug

Difficulty 4/5 3-5 days Newbie friendliness 25/100

typelevel/cats-effect#4293 · 1 comment · Opened Mar 5, 2025
Add metrics to track batches vs singletons in external queue Open

:mushroom: enhancement

typelevel/cats-effect#4269 · 8 comments · 1 assignee · Opened Feb 8, 2025
`parTraverseN` fairness Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4262 · 3 comments · Opened Jan 30, 2025
use `java.lang.ClassValue` instead of `IO.tag` Open

Difficulty 3/5 1-2 days Newbie friendliness 35/100

typelevel/cats-effect#4260 · 2 comments · 3 reactions · Opened Jan 29, 2025
Add parallelismFactorHint to Spawn[F] Open

Difficulty 5/5 Over a week Newbie friendliness 25/100

typelevel/cats-effect#4199 · 4 comments · Opened Dec 12, 2024
`WorkerThread#blockOn` implementation note is incorrect Open

Difficulty 2/5 1-3 hours Newbie friendliness 35/100

typelevel/cats-effect#4193 · 2 comments · Opened Dec 6, 2024
Update site diagrams to use mermaid Open

:books: docs

Difficulty 2/5 1-3 hours Newbie friendliness 35/100

typelevel/cats-effect#4132 · 2 comments · 1 reaction · Opened Sep 9, 2024
TestControl + Dispatcher == deadlock Open

Difficulty 4/5 3-5 days Newbie friendliness 35/100

typelevel/cats-effect#4104 · 13 comments · Opened Jul 18, 2024

typelevel/cats-effect

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