diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+# effectful-2.6.1.0 (2025-08-30)
+* Add `MonadError`, `MonadReader`, `MonadState` and `MonadWriter` instances for
+  `Eff` for compatibility with existing code.
+* Re-export `writeTMVar` from `stm-2.5.1.0` in `Effectful.Concurrent.STM`.
+* Add `cancelMany` to `Effectful.Concurrent.Async`.
+
 # effectful-core-2.6.0.0 (2025-06-13)
 * Adjust `generalBracket` with `base >= 4.21` to make use of the new exception
   annotation mechanism.
diff --git a/bench/FileSizes.hs b/bench/FileSizes.hs
--- a/bench/FileSizes.hs
+++ b/bench/FileSizes.hs
@@ -1,6 +1,5 @@
 {-# LANGUAGE BlockArguments #-}
 {-# LANGUAGE CPP #-}
-{-# LANGUAGE UndecidableInstances #-}
 module FileSizes where
 
 import Control.Exception
diff --git a/effectful.cabal b/effectful.cabal
--- a/effectful.cabal
+++ b/effectful.cabal
@@ -1,7 +1,7 @@
 cabal-version:      3.0
 build-type:         Simple
 name:               effectful
-version:            2.6.0.0
+version:            2.6.1.0
 license:            BSD-3-Clause
 license-file:       LICENSE
 category:           Control
@@ -22,7 +22,7 @@
   CHANGELOG.md
   README.md
 
-tested-with: GHC == { 8.10.7, 9.0.2, 9.2.8, 9.4.8, 9.6.7, 9.8.4, 9.10.2, 9.12.2 }
+tested-with: GHC == { 8.10.7, 9.0.2, 9.2.8, 9.4.8, 9.6.7, 9.8.4, 9.10.2, 9.12.2, 9.14.1 }
 
 bug-reports:   https://github.com/haskell-effectful/effectful/issues
 source-repository head
@@ -66,19 +66,20 @@
                         TypeApplications
                         TypeFamilies
                         TypeOperators
+                        UndecidableInstances
 
 library
     import:         language
 
     build-depends:    base                >= 4.14      && < 5
-                    , async               >= 2.2.2
+                    , async               >= 2.2.5
                     , bytestring          >= 0.10
                     , directory           >= 1.3.2
-                    , effectful-core      >= 2.6.0.0   && < 2.6.1.0
+                    , effectful-core      >= 2.6.1.0   && < 2.6.2.0
                     , process             >= 1.6.9
                     , strict-mutable-base >= 1.1.0.0
                     , time                >= 1.9.2
-                    , stm                 >= 2.5.0.0
+                    , stm                 >= 2.5.1.0
                     , unliftio            >= 0.2.20
 
     hs-source-dirs:  src
@@ -188,10 +189,10 @@
        if impl(ghc < 9.9)
           build-depends: freer-simple >= 1.2.1.2
 
-       if impl(ghc < 9.13)
+       if impl(ghc < 9.15)
           build-depends: fused-effects >= 1.1.2.3
 
-       if impl(ghc < 9.13)
+       if impl(ghc < 9.15)
           build-depends: polysemy >= 1.9.2.0
 
     build-depends:    base
diff --git a/src/Effectful/Concurrent/Async.hs b/src/Effectful/Concurrent/Async.hs
--- a/src/Effectful/Concurrent/Async.hs
+++ b/src/Effectful/Concurrent/Async.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE UndecidableInstances #-}
 -- | Lifted "Control.Concurrent.Async".
 module Effectful.Concurrent.Async
   ( -- * Effect
@@ -18,7 +17,8 @@
 
     -- ** Querying 'Async's
   , wait, poll, waitCatch, A.asyncThreadId
-  , cancel, uninterruptibleCancel, cancelWith, A.AsyncCancelled(..)
+  , cancel, cancelMany, uninterruptibleCancel, cancelWith
+  , A.AsyncCancelled(..)
   , A.compareAsyncs
 
     -- ** High-level utilities
@@ -173,6 +173,12 @@
 -- | Lifted 'A.cancel'.
 cancel :: Concurrent :> es => Async a -> Eff es ()
 cancel = unsafeEff_ . A.cancel
+
+-- | Lifted 'A.cancelMany'.
+--
+-- @since 2.6.1.0
+cancelMany :: Concurrent :> es => [Async a] -> Eff es ()
+cancelMany = unsafeEff_ . A.cancelMany
 
 -- | Lifted 'A.cancelWith'.
 cancelWith :: (Exception e, Concurrent :> es) => Async a -> e -> Eff es ()
diff --git a/src/Effectful/Concurrent/STM.hs b/src/Effectful/Concurrent/STM.hs
--- a/src/Effectful/Concurrent/STM.hs
+++ b/src/Effectful/Concurrent/STM.hs
@@ -37,6 +37,7 @@
   , STM.takeTMVar
   , STM.putTMVar
   , STM.readTMVar
+  , STM.writeTMVar
   , STM.tryReadTMVar
   , STM.swapTMVar
   , STM.tryTakeTMVar