diff --git a/src/Std/Internal/Async.lean b/src/Std/Internal/Async.lean
index 1d7954fd16..214f36aafd 100644
--- a/src/Std/Internal/Async.lean
+++ b/src/Std/Internal/Async.lean
@@ -7,6 +7,7 @@ module
 
 prelude
 public import Std.Internal.Async.Basic
+public import Std.Internal.Async.ContextAsync
 public import Std.Internal.Async.Timer
 public import Std.Internal.Async.TCP
 public import Std.Internal.Async.UDP
diff --git a/src/Std/Internal/Async/ContextAsync.lean b/src/Std/Internal/Async/ContextAsync.lean
new file mode 100644
index 0000000000..bde6733470
--- /dev/null
+++ b/src/Std/Internal/Async/ContextAsync.lean
@@ -0,0 +1,273 @@
+/-
+Copyright (c) 2025 Lean FRO, LLC. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Sofia Rodrigues
+-/
+module
+
+prelude
+public import Std.Time
+public import Std.Internal.UV
+public import Std.Internal.Async.Basic
+public import Std.Internal.Async.Timer
+public import Std.Sync.CancellationContext
+
+public section
+
+/-!
+This module contains the implementation of `ContextAsync`, a monad for asynchronous computations with
+cooperative cancellation support that must be explicitly checked for and cancelled explicitly.
+-/
+
+namespace Std
+namespace Internal
+namespace IO
+namespace Async
+
+/--
+An asynchronous computation with cooperative cancellation support via a `CancellationContext`. `ContextAsync α`
+is equivalent to `ReaderT CancellationContext Async α`, providing a `CancellationContext` value to async
+computations.
+-/
+abbrev ContextAsync (α : Type) := ReaderT CancellationContext Async α
+
+namespace ContextAsync
+
+/--
+Runs a `ContextAsync` computation with a given context. See also `ContextAsync.run` for running with a new
+context that automatically cancels after execution.
+-/
+@[inline]
+protected def runIn (ctx : CancellationContext) (x : ContextAsync α) : Async α :=
+  x ctx
+
+/--
+Runs a `ContextAsync` computation with a new context that cancels after the execution of the computation.
+See also `ContextAsync.runIn` for running with an existing context.
+-/
+@[inline]
+protected def run (x : ContextAsync α) : Async α := do
+  let ctx ← CancellationContext.new
+  x ctx <* ctx.cancel .cancel
+
+/--
+Returns the current context for inspection or to pass to other functions.
+-/
+@[inline]
+def getContext : ContextAsync CancellationContext :=
+  fun ctx => pure ctx
+
+/--
+Checks if the current context is cancelled. Returns `true` if the context (or any ancestor) has been cancelled.
+Long-running operations should periodically check this and exit gracefully when cancelled.
+-/
+@[inline]
+def isCancelled : ContextAsync Bool := do
+  let ctx ← getContext
+  ctx.isCancelled
+
+/--
+Gets the cancellation reason if the context is cancelled. Returns `some reason` if cancelled, `none` otherwise,
+allowing you to distinguish between different cancellation types.
+-/
+@[inline]
+def getCancellationReason : ContextAsync (Option CancellationReason) := do
+  let ctx ← getContext
+  ctx.getCancellationReason
+
+/--
+Cancels the current context with the given reason, cascading to all child contexts.
+Cancellation is cooperative, operations must explicitly check `isCancelled` or use `awaitCancellation` to respond.
+-/
+@[inline]
+def cancel (reason : CancellationReason) : ContextAsync Unit := do
+  let ctx ← getContext
+  ctx.cancel reason
+
+/--
+Returns a selector that completes when the current context is cancelled.
+-/
+@[inline]
+def doneSelector : ContextAsync (Selector Unit) := do
+  let ctx ← getContext
+  return ctx.doneSelector
+
+/--
+Waits for the current context to be cancelled.
+-/
+@[inline]
+def awaitCancellation : ContextAsync Unit := do
+  let ctx ← getContext
+  let task ← ctx.done
+  await task
+
+/--
+Runs two computations concurrently and returns both results. Each computation runs in its own child context;
+if either fails or is cancelled, both are cancelled immediately and the exception is propagated.
+-/
+@[inline, specialize]
+def concurrently (x : ContextAsync α) (y : ContextAsync β)
+    (prio := Task.Priority.default) : ContextAsync (α × β) := do
+
+  let ctx ← getContext
+  let concurrentCtx ← ctx.fork
+
+  let childCtx1 ← concurrentCtx.fork
+  let childCtx2 ← concurrentCtx.fork
+
+  let result ← Async.concurrently
+    (try x childCtx1 catch err => do concurrentCtx.cancel .cancel; throw err finally childCtx1.cancel .cancel)
+    (try y childCtx2 catch err => do concurrentCtx.cancel .cancel; throw err finally childCtx2.cancel .cancel)
+    prio
+
+  concurrentCtx.cancel .cancel
+  return result
+
+/--
+Runs two computations concurrently and returns the result of the first to complete. Each computation runs
+in its own child context; when either completes, the other is cancelled immediately.
+-/
+@[inline, specialize]
+def race [Inhabited α] (x : ContextAsync α) (y : ContextAsync α)
+    (prio := Task.Priority.default) : ContextAsync α := do
+  let parent ← getContext
+  let ctx1 ← CancellationContext.fork parent
+  let ctx2 ← CancellationContext.fork parent
+
+  let task1 ← async (x ctx1) prio
+  let task2 ← async (y ctx2) prio
+
+  let result ← Async.race
+    (await task1 <* ctx2.cancel .cancel)
+    (await task2 <* ctx1.cancel .cancel)
+    prio
+
+  pure result
+
+/--
+Runs all computations concurrently and collects results in the same order. Each runs in its own child context;
+if any computation fails, all others are cancelled and the exception is propagated.
+-/
+@[inline, specialize]
+def concurrentlyAll (xs : Array (ContextAsync α))
+    (prio := Task.Priority.default) : ContextAsync (Array α) := do
+  let ctx ← getContext
+  let concurrentCtx ← ctx.fork
+
+  let tasks : Array (AsyncTask α) ← xs.mapM fun ctxAsync => do
+    let childCtx ← concurrentCtx.fork
+    async (prio := prio)
+      (try
+        ctxAsync childCtx
+      catch err => do
+        concurrentCtx.cancel .cancel
+        throw err
+      finally
+        childCtx.cancel .cancel)
+
+  let result ← tasks.mapM await
+  return result
+
+/--
+Launches a `ContextAsync` computation in the background, discarding its result.
+
+The computation runs independently in the background in its own child context. The parent computation does not wait
+for background tasks to complete. This means that if the parent finishes its execution it will cause
+the cancellation of the background functions. See also `disown` for launching tasks that continue independently
+even after parent cancellation.
+-/
+@[inline, specialize]
+def background (action : ContextAsync α) (prio := Task.Priority.default) : ContextAsync Unit := do
+  let ctx ← getContext
+  let childCtx ← ctx.fork
+  Async.background (action childCtx *> childCtx.cancel .cancel) prio
+
+/--
+Launches a `ContextAsync` computation in the background, discarding its result. It's similar to `background`,
+but the child context is not automatically cancelled when the action completes. This allows the disowned
+computation to continue running independently, even if the parent context is cancelled. The child context
+will remain alive as long as the computation needs it. See also `background` for launching tasks that are
+cancelled when the parent finishes.
+-/
+@[inline, specialize]
+def disown (action : ContextAsync α) (prio := Task.Priority.default) : ContextAsync Unit := do
+  let childCtx ← CancellationContext.new
+  Async.background (action childCtx) prio
+
+/--
+Runs all computations concurrently and returns the first result. Each computation runs in its own child context;
+when the first completes successfully, all others are cancelled immediately.
+-/
+def raceAll [ForM ContextAsync c (ContextAsync α)] (xs : c)
+    (prio := Task.Priority.default) : ContextAsync α := do
+  let parent ← getContext
+  let promise ← IO.Promise.new
+
+  ForM.forM xs fun x => do
+    let ctx ← CancellationContext.fork parent
+    let task ← async (x ctx) prio
+
+    background do
+      try
+        let result ← await task
+        promise.resolve (.ok result)
+      catch e =>
+        discard $ promise.resolve (.error e)
+
+  let result ← await promise
+  parent.cancel .cancel
+  Async.ofExcept result
+
+/--
+Launches a `ContextAsync` computation as an asynchronous task with a forked child context.
+The child context is automatically cancelled when the task completes or fails.
+-/
+@[inline, specialize]
+def async (x : ContextAsync α) (prio := Task.Priority.default) : ContextAsync (AsyncTask α) :=
+  fun ctx => do
+    let childCtx ← ctx.fork
+    Async.async (try x childCtx finally childCtx.cancel .cancel) prio
+
+instance : MonadAsync AsyncTask ContextAsync where
+  async x prio := ContextAsync.async x prio
+
+instance : Functor ContextAsync where
+  map f x := fun ctx => f <$> x ctx
+
+instance : Monad ContextAsync where
+  pure a := fun _ => pure a
+  bind x f := fun ctx => x ctx >>= fun a => f a ctx
+
+instance : MonadLift IO ContextAsync where
+  monadLift x := fun _ => Async.ofIOTask (Task.pure <$> x)
+
+instance : MonadLift BaseIO ContextAsync where
+  monadLift x := fun _ => liftM (m := Async) x
+
+instance : MonadExcept IO.Error ContextAsync where
+  throw e := fun _ => throw e
+  tryCatch x h := fun ctx => tryCatch (x ctx) (fun e => h e ctx)
+
+instance : MonadFinally ContextAsync where
+  tryFinally' x f := fun ctx =>
+    tryFinally' (x ctx) (fun opt => f opt ctx)
+
+instance [Inhabited α] : Inhabited (ContextAsync α) where
+  default := fun _ => default
+
+instance : MonadAwait AsyncTask ContextAsync where
+  await t := fun _ => await t
+
+end ContextAsync
+
+/--
+Returns a selector that completes when the current context is cancelled.
+This is useful for selecting on cancellation alongside other asynchronous operations.
+-/
+def Selector.cancelled : ContextAsync (Selector Unit) := do
+  ContextAsync.doneSelector
+
+end Async
+end IO
+end Internal
+end Std
diff --git a/src/Std/Sync.lean b/src/Std/Sync.lean
index 65313a6ed7..c79ce06a5c 100644
--- a/src/Std/Sync.lean
+++ b/src/Std/Sync.lean
@@ -16,5 +16,6 @@ public import Std.Sync.Notify
 public import Std.Sync.Broadcast
 public import Std.Sync.StreamMap
 public import Std.Sync.CancellationToken
+public import Std.Sync.CancellationContext
 
 @[expose] public section
diff --git a/src/Std/Sync/CancellationContext.lean b/src/Std/Sync/CancellationContext.lean
new file mode 100644
index 0000000000..b62e931acb
--- /dev/null
+++ b/src/Std/Sync/CancellationContext.lean
@@ -0,0 +1,152 @@
+/-
+Copyright (c) 2025 Lean FRO, LLC. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Sofia Rodrigues
+-/
+module
+
+prelude
+public import Std.Data
+public import Init.System.Promise
+public import Init.Data.Queue
+public import Std.Sync.Mutex
+public import Std.Sync.CancellationToken
+public import Std.Internal.Async.Select
+
+public section
+
+/-!
+This module provides a tree-structured cancellation context called `CancellationToken` where cancelling a parent
+automatically cancels all child contexts.
+-/
+
+namespace Std
+open Std.Internal.IO.Async
+
+structure CancellationContext.State where
+  /--
+  Map of token IDs to optional tokens and their children.
+  -/
+  tokens : TreeMap UInt64 (CancellationToken × Array UInt64) := .empty
+
+  /--
+  Next available ID
+  -/
+  id : UInt64 := 1
+
+/--
+A cancellation context that allows multiple consumers to wait until cancellation is requested. Forms
+a tree structure where cancelling a parent cancels all children.
+-/
+structure CancellationContext where
+  state : Std.Mutex CancellationContext.State
+  token : CancellationToken
+  id : UInt64
+
+namespace CancellationContext
+
+/--
+Creates a new root cancellation context.
+-/
+def new : BaseIO CancellationContext := do
+  let token ← Std.CancellationToken.new
+  return {
+    state := ← Std.Mutex.new { tokens := .empty |>.insert 0 (token, #[]) },
+    token,
+    id := 0
+  }
+
+/--
+Forks a child context from a parent. If the parent is already cancelled, returns the parent context.
+Otherwise, creates a new child that will be cancelled when the parent is cancelled.
+-/
+def fork (root : CancellationContext) : BaseIO CancellationContext := do
+  root.state.atomically do
+    if ← root.token.isCancelled then
+      return root
+
+    let token ← Std.CancellationToken.new
+
+    let st ← get
+    let newId := st.id
+
+    set { st with
+      id := newId + 1,
+      tokens := st.tokens.insert newId (token, #[])
+        |>.modify root.id (.map (·) (.push · newId))
+    }
+
+    return { state := root.state, token, id := newId }
+
+/--
+Recursively cancels a context and all its children with the given reason.
+-/
+private partial def cancelChildren (state : CancellationContext.State) (id : UInt64) (reason : CancellationReason) : BaseIO CancellationContext.State := do
+  let mut state := state
+
+  let some (token, children) := state.tokens.get? id
+    | return state
+
+  for tokenId in children do
+    state ← cancelChildren state tokenId reason
+
+  token.cancel reason
+
+  pure { state with tokens := state.tokens.erase id }
+
+/--
+Cancels this context and all child contexts with the given reason.
+-/
+def cancel (x : CancellationContext) (reason : CancellationReason) : BaseIO Unit := do
+  if ← x.token.isCancelled then
+    return
+
+  x.state.atomically do
+    let st ← get
+    let st ← cancelChildren st x.id reason
+    set st
+
+/--
+Checks if the context is cancelled.
+-/
+@[inline]
+def isCancelled (x : CancellationContext) : BaseIO Bool := do
+  x.token.isCancelled
+
+/--
+Returns the cancellation reason if the context is cancelled.
+-/
+@[inline]
+def getCancellationReason (x : CancellationContext) : BaseIO (Option CancellationReason) := do
+  x.token.getCancellationReason
+
+/--
+Waits for cancellation. Returns a task that completes when the context is cancelled.
+-/
+@[inline]
+def done (x : CancellationContext) : IO (AsyncTask Unit) :=
+  x.token.wait
+
+/--
+Creates a selector that waits for cancellation.
+-/
+@[inline]
+def doneSelector (x : CancellationContext) : Selector Unit :=
+  x.token.selector
+
+private partial def countAliveTokensRec (state : CancellationContext.State) (id : UInt64) : Nat :=
+  match state.tokens.get? id with
+  | none => 0
+  | some (_, children) => 1 + children.foldl (fun acc childId => acc + countAliveTokensRec state childId) 0
+
+/--
+Counts the number of alive (non-cancelled) tokens in the context tree, including
+this context and all its descendants.
+-/
+def countAliveTokens (x : CancellationContext) : BaseIO Nat := do
+  x.state.atomically do
+    let st ← get
+    return countAliveTokensRec st x.id
+
+end CancellationContext
+end Std
diff --git a/src/Std/Sync/CancellationToken.lean b/src/Std/Sync/CancellationToken.lean
index a674e4b23d..4f5c0debbf 100644
--- a/src/Std/Sync/CancellationToken.lean
+++ b/src/Std/Sync/CancellationToken.lean
@@ -23,6 +23,38 @@ that a cancellation has occurred.
 namespace Std
 open Std.Internal.IO.Async
 
+/--
+Reasons for cancellation.
+-/
+inductive CancellationReason where
+  /--
+  Cancelled due to a deadline or timeout
+  -/
+  | deadline
+
+  /--
+  Cancelled due to shutdown
+  -/
+  | shutdown
+
+  /--
+  Explicitly cancelled
+  -/
+  | cancel
+
+  /--
+  Custom cancellation reason
+  -/
+  | custom (msg : String)
+  deriving Repr, BEq
+
+instance : ToString CancellationReason where
+  toString
+    | .deadline => "deadline"
+    | .shutdown => "shutdown"
+    | .cancel => "cancel"
+    | .custom msg => s!"custom(\"{msg}\")"
+
 inductive CancellationToken.Consumer where
   | normal (promise : IO.Promise Unit)
   | select (finished : Waiter Unit)
@@ -44,9 +76,9 @@ The central state structure for a `CancellationToken`.
 -/
 structure CancellationToken.State where
   /--
-  Whether this token has been cancelled.
+  The cancellation reason if cancelled, none otherwise.
   -/
-  cancelled : Bool
+  reason : Option CancellationReason
 
   /--
   Consumers that are blocked waiting for cancellation.
@@ -63,24 +95,24 @@ structure CancellationToken where
 namespace CancellationToken
 
 /--
-Create a new cancellation token.
+Creates a new cancellation token.
 -/
 def new : BaseIO CancellationToken := do
-  return { state := ← Std.Mutex.new { cancelled := false, consumers := ∅ } }
+  return { state := ← Std.Mutex.new { reason := none, consumers := ∅ } }
 
 /--
-Cancel the token, notifying all currently waiting consumers with `true`.
+Cancels the token with the given reason, notifying all currently waiting consumers.
 Once cancelled, the token remains cancelled.
 -/
-def cancel (x : CancellationToken) : BaseIO Unit := do
+def cancel (x : CancellationToken) (reason : CancellationReason := .cancel) : BaseIO Unit := do
   x.state.atomically do
     let mut st ← get
 
-    if st.cancelled then
+    if st.reason.isSome then
       return
 
     let mut remainingConsumers := st.consumers
-    st := { cancelled := true, consumers := ∅ }
+    st := { reason := some reason, consumers := ∅ }
 
     while true do
       if let some (consumer, rest) := remainingConsumers.dequeue? then
@@ -92,21 +124,29 @@ def cancel (x : CancellationToken) : BaseIO Unit := do
     set st
 
 /--
-Check if the token is cancelled.
+Checks if the token is cancelled.
 -/
 def isCancelled (x : CancellationToken) : BaseIO Bool := do
   x.state.atomically do
     let st ← get
-    return st.cancelled
+    return st.reason.isSome
 
 /--
-Wait for cancellation. Returns a task that completes when cancelled,
+Gets the cancellation reason if the token is cancelled.
+-/
+def getCancellationReason (x : CancellationToken) : BaseIO (Option CancellationReason) := do
+  x.state.atomically do
+    let st ← get
+    return st.reason
+
+/--
+Waits for cancellation. Returns a task that completes when cancelled.
 -/
 def wait (x : CancellationToken) : IO (AsyncTask Unit) :=
   x.state.atomically do
     let st ← get
 
-    if st.cancelled then
+    if st.reason.isSome then
       return Task.pure (.ok ())
 
     let promise ← IO.Promise.new
@@ -118,7 +158,7 @@ def wait (x : CancellationToken) : IO (AsyncTask Unit) :=
       | none => throw (IO.userError "cancellation token dropped")
 
 /--
-Creates a selector that waits for cancellation
+Creates a selector that waits for cancellation.
 -/
 def selector (token : CancellationToken) : Selector Unit := {
   tryFn := do
@@ -131,7 +171,7 @@ def selector (token : CancellationToken) : Selector Unit := {
     token.state.atomically do
       let st ← get
 
-      if st.cancelled then
+      if st.reason.isSome then
         discard <| waiter.race (return false) (fun promise => do
           promise.resolve (.ok ())
           return true)
diff --git a/tests/lean/run/async_cancellation_reasons.lean b/tests/lean/run/async_cancellation_reasons.lean
new file mode 100644
index 0000000000..99ccad59fb
--- /dev/null
+++ b/tests/lean/run/async_cancellation_reasons.lean
@@ -0,0 +1,163 @@
+import Std.Internal.Async
+import Std.Sync
+
+open Std.Internal.IO Async
+
+-- Test basic cancellation with default reason
+def testBasicCancellationWithReason : Async Unit := do
+  let token ← Std.CancellationToken.new
+  assert! not (← token.isCancelled)
+
+  token.cancel
+  assert! (← token.isCancelled)
+
+  let reason ← token.getCancellationReason
+  assert! reason == some .cancel
+
+#eval testBasicCancellationWithReason.block
+
+-- Test cancellation with deadline reason
+def testDeadlineReason : Async Unit := do
+  let token ← Std.CancellationToken.new
+  assert! not (← token.isCancelled)
+
+  token.cancel .deadline
+  assert! (← token.isCancelled)
+
+  let reason ← token.getCancellationReason
+  assert! reason == some .deadline
+
+#eval testDeadlineReason.block
+
+-- Test cancellation with shutdown reason
+def testShutdownReason : Async Unit := do
+  let token ← Std.CancellationToken.new
+  token.cancel .shutdown
+
+  let reason ← token.getCancellationReason
+  assert! reason == some .shutdown
+
+#eval testShutdownReason.block
+
+-- Test cancellation with custom reason
+def testCustomReason : Async Unit := do
+  let token ← Std.CancellationToken.new
+  token.cancel (.custom "connection timeout")
+
+  let reason ← token.getCancellationReason
+  assert! reason == some (.custom "connection timeout")
+
+#eval testCustomReason.block
+
+-- Test that uncancelled token has no reason
+def testUncancelledNoReason : Async Unit := do
+  let token ← Std.CancellationToken.new
+
+  let reason ← token.getCancellationReason
+  assert! reason == none
+
+#eval testUncancelledNoReason.block
+
+-- Test context cancellation with reason
+def testContextCancellation : Async Unit := do
+  let ctx ← Std.CancellationContext.new
+  assert! not (← ctx.isCancelled)
+
+  ctx.cancel .shutdown
+  assert! (← ctx.isCancelled)
+
+  let reason ← ctx.token.getCancellationReason
+  assert! reason == some .shutdown
+
+#eval testContextCancellation.block
+
+-- Test context tree with different reasons
+def testContextTreeReasons : Async Unit := do
+  let root ← Std.CancellationContext.new
+  let child1 ← root.fork
+  let child2 ← root.fork
+  let grandchild ← child1.fork
+
+  -- Cancel root with shutdown reason
+  root.cancel .shutdown
+
+  -- All should be cancelled
+  assert! (← root.isCancelled)
+  assert! (← child1.isCancelled)
+  assert! (← child2.isCancelled)
+  assert! (← grandchild.isCancelled)
+
+  -- All should have the shutdown reason (propagated from root)
+  assert! (← root.token.getCancellationReason) == some .shutdown
+  assert! (← child1.token.getCancellationReason) == some .shutdown
+  assert! (← child2.token.getCancellationReason) == some .shutdown
+  assert! (← grandchild.token.getCancellationReason) == some .shutdown
+
+#eval testContextTreeReasons.block
+
+-- Test child cancellation doesn't affect parent
+def testChildCancellationIndependent : Async Unit := do
+  let root ← Std.CancellationContext.new
+  let child ← root.fork
+
+  -- Cancel child with deadline
+  child.cancel .deadline
+
+  -- Child should be cancelled with deadline reason
+  assert! (← child.isCancelled)
+  assert! (← child.token.getCancellationReason) == some .deadline
+
+  -- Parent should still be active
+  assert! not (← root.isCancelled)
+  assert! (← root.token.getCancellationReason) == none
+
+#eval testChildCancellationIndependent.block
+
+-- Test selector with reason
+def testSelectorWithReason : Async Unit := do
+  let token ← Std.CancellationToken.new
+  let completed ← Std.Mutex.new false
+  let reasonRef ← Std.Mutex.new none
+
+  let task ← async do
+    Selectable.one #[.case token.selector (fun _ => pure ())]
+    completed.atomically (set true)
+    reasonRef.atomically (set (← token.getCancellationReason))
+
+  assert! not (← completed.atomically get)
+
+  token.cancel .deadline
+  await task
+
+  assert! (← completed.atomically get)
+  assert! (← reasonRef.atomically get) == some Std.CancellationReason.deadline
+
+#eval testSelectorWithReason.block
+
+-- Test wait with reason
+def testWaitWithReason : Async Unit := do
+  let token ← Std.CancellationToken.new
+
+  let task ← async do
+    let _ ← await (← token.wait)
+    token.getCancellationReason
+
+  Async.sleep 10
+  token.cancel (.custom "test reason")
+
+  let reason ← await task
+  assert! reason == some (.custom "test reason")
+
+#eval testWaitWithReason.block
+
+-- Test multiple cancellations (first one wins)
+def testMultipleCancellations : Async Unit := do
+  let token ← Std.CancellationToken.new
+
+  token.cancel .deadline
+  token.cancel .shutdown -- This should be ignored
+
+  let reason ← token.getCancellationReason
+  assert! reason == some .deadline -- First reason should persist
+
+#eval testMultipleCancellations.block
diff --git a/tests/lean/run/cancellation_context.lean b/tests/lean/run/cancellation_context.lean
new file mode 100644
index 0000000000..73562d42d8
--- /dev/null
+++ b/tests/lean/run/cancellation_context.lean
@@ -0,0 +1,251 @@
+import Std.Internal.Async
+import Std.Sync
+
+open Std.Internal.IO Async
+
+/-- Test basic tree cancellation -/
+partial def testCancelTree : IO Unit := do
+  let mutex ← Std.Mutex.new 0
+  let context ← Std.CancellationContext.new
+
+  Async.block do
+
+    let rec loop (x : Nat) (parent : Std.CancellationContext) : Async Unit := do
+      match x with
+      | 0 => do
+          await (← parent.done)
+          mutex.atomically (modify (· + 1))
+
+      | n + 1 => do
+          background (loop n (← parent.fork))
+          background (loop n (← parent.fork))
+          await (← parent.done)
+          mutex.atomically (modify (· + 1))
+
+    background (loop 3 context)
+    Async.sleep 500
+    context.cancel .cancel
+    Async.sleep 1000
+
+  assert! (← context.countAliveTokens) == 0
+  let size ← mutex.atomically get
+  IO.println s!"cancelled {size}"
+
+/--
+info: cancelled 15
+-/
+#guard_msgs in
+#eval testCancelTree
+
+/-- Test cancellation with different reasons -/
+def testCancellationReasons : IO Unit := do
+  let ctx ← Std.CancellationContext.new
+  let (reason1, reason2, reason3, reason4) ← Async.block do
+    -- Test with .cancel reason
+    let ctx1 ← ctx.fork
+    ctx1.cancel .cancel
+    let some reason1 ← ctx1.getCancellationReason | return (none, none, none, none)
+
+    -- Test with .deadline reason
+    let ctx2 ← ctx.fork
+    ctx2.cancel .deadline
+    let some reason2 ← ctx2.getCancellationReason | return (none, none, none, none)
+
+    -- Test with .shutdown reason
+    let ctx3 ← ctx.fork
+    ctx3.cancel .shutdown
+    let some reason3 ← ctx3.getCancellationReason | return (none, none, none, none)
+
+    -- Test with custom reason
+    let ctx4 ← ctx.fork
+    ctx4.cancel (.custom "test error")
+    let some reason4 ← ctx4.getCancellationReason | return (none, none, none, none)
+
+    return (some reason1, some reason2, some reason3, some reason4)
+
+  if let some r1 := reason1 then IO.println s!"Reason 1: {r1}"
+  if let some r2 := reason2 then IO.println s!"Reason 2: {r2}"
+  if let some r3 := reason3 then IO.println s!"Reason 3: {r3}"
+  if let some r4 := reason4 then IO.println s!"Reason 4: {r4}"
+
+  assert! (← ctx.countAliveTokens) == 1
+
+/--
+info: Reason 1: cancel
+Reason 2: deadline
+Reason 3: shutdown
+Reason 4: custom("test error")
+-/
+#guard_msgs in
+#eval testCancellationReasons
+
+/-- Test cancellation propagates reason to children -/
+def testReasonPropagation : IO Unit := do
+  let (parentReason, child1Reason, child2Reason, grandchildReason) ← Async.block do
+    let parent ← Std.CancellationContext.new
+    let child1 ← parent.fork
+    let child2 ← parent.fork
+    let grandchild ← child1.fork
+
+    parent.cancel (.custom "parent cancelled")
+    Async.sleep 100
+
+    let some parentReason ← parent.getCancellationReason | return (none, none, none, none)
+    let some child1Reason ← child1.getCancellationReason | return (none, none, none, none)
+    let some child2Reason ← child2.getCancellationReason | return (none, none, none, none)
+    let some grandchildReason ← grandchild.getCancellationReason | return (none, none, none, none)
+
+    return (some parentReason, some child1Reason, some child2Reason, some grandchildReason)
+
+  if let some r := parentReason then IO.println s!"Parent: {r}"
+  if let some r := child1Reason then IO.println s!"Child1: {r}"
+  if let some r := child2Reason then IO.println s!"Child2: {r}"
+  if let some r := grandchildReason then IO.println s!"Grandchild: {r}"
+
+/--
+info: Parent: custom("parent cancelled")
+Child1: custom("parent cancelled")
+Child2: custom("parent cancelled")
+Grandchild: custom("parent cancelled")
+-/
+#guard_msgs in
+#eval testReasonPropagation
+
+/-- Test cancellation in the middle of work -/
+def testCancelInMiddle : IO Unit := do
+  let counter ← Std.Mutex.new 0
+  let cancelledCounter ← Std.Mutex.new 0
+
+  let (finalCount, cancelledCount) ← Async.block do
+    let context ← Std.CancellationContext.new
+
+    -- Worker that does work until cancelled
+    let worker (ctx : Std.CancellationContext) : Async Unit := do
+      for _ in [0:100] do
+        if ← ctx.isCancelled then
+          cancelledCounter.atomically (modify (· + 1))
+          break
+        counter.atomically (modify (· + 1))
+        Async.sleep 10
+
+    -- Start 5 workers
+    for _ in [0:5] do
+      background (worker context)
+
+    -- Let them run for a bit, then cancel
+    Async.sleep 200
+    context.cancel .deadline
+
+    -- Wait for them to finish
+    Async.sleep 500
+
+    let finalCount ← counter.atomically get
+    let cancelledCount ← cancelledCounter.atomically get
+    return (finalCount, cancelledCount)
+
+  IO.println s!"Completed {finalCount} iterations before cancellation"
+  IO.println s!"{cancelledCount} workers detected cancellation"
+
+/-- Test cancellation before forking -/
+def testCancelBeforeFork : IO Unit := do
+  let (isSame, isChildCancelled) ← Async.block do
+    let ctx ← Std.CancellationContext.new
+    ctx.cancel .cancel
+
+    -- Fork after cancellation should return same context
+    let child ← ctx.fork
+    let isSame := ctx.id == child.id
+    let isChildCancelled ← child.isCancelled
+
+    return (isSame, isChildCancelled)
+
+  IO.println s!"Same context: {isSame}, Child cancelled: {isChildCancelled}"
+
+/--
+info: Same context: true, Child cancelled: true
+-/
+#guard_msgs in
+#eval testCancelBeforeFork
+
+/-- Test deep tree cancellation with reason -/
+partial def testDeepTreeCancellation : IO Unit := do
+  let depths ← Std.Mutex.new ([] : List (Nat × Std.CancellationReason))
+
+  let (count, allSameReason) ← Async.block do
+    let root ← Std.CancellationContext.new
+
+    let rec makeTree (depth : Nat) (ctx : Std.CancellationContext) : Async Unit := do
+      if depth == 0 then
+        await (← ctx.done)
+        if let some reason ← ctx.getCancellationReason then
+          depths.atomically (modify (·.cons (depth, reason)))
+      else
+        let child1 ← ctx.fork
+        let child2 ← ctx.fork
+        background (makeTree (depth - 1) child1)
+        background (makeTree (depth - 1) child2)
+        await (← ctx.done)
+        if let some reason ← ctx.getCancellationReason then
+          depths.atomically (modify (·.cons (depth, reason)))
+
+    background (makeTree 4 root)
+    Async.sleep 200
+    root.cancel (.custom "deep tree cancel")
+    Async.sleep 500
+
+    let results ← depths.atomically get
+    let count := results.length
+    let allSameReason := results.all fun (_, r) => r == .custom "deep tree cancel"
+    return (count, allSameReason)
+
+  IO.println s!"Cancelled {count} nodes, all with same reason: {allSameReason}"
+
+/--
+info: Cancelled 31 nodes, all with same reason: true
+-/
+#guard_msgs in
+#eval testDeepTreeCancellation
+
+/-- Test counting alive tokens -/
+def testCountAliveTokens : IO Unit := do
+  let (count0, count1, count2, count3, count4) ← Async.block do
+    let root ← Std.CancellationContext.new
+    let count0 ← root.countAliveTokens -- Root only
+
+    -- Fork 3 children
+    let child1 ← root.fork
+    let child2 ← root.fork
+    let _child3 ← root.fork
+    let count1 ← root.countAliveTokens -- Root + 3 children = 4
+
+    -- Cancel one child (and its subtree)
+    child1.cancel .cancel
+    Async.sleep 100
+    let count2 ← root.countAliveTokens -- Root + 2 children = 3
+
+    -- Fork a grandchild from child2
+    let _grandchild ← child2.fork
+    let count3 ← root.countAliveTokens -- Root + 2 children + 1 grandchild = 4
+
+    -- Cancel root (should cancel everything)
+    root.cancel .cancel
+    Async.sleep 100
+    let count4 ← root.countAliveTokens -- All cancelled = 0
+
+    return (count0, count1, count2, count3, count4)
+
+  IO.println s!"Initial (root only): {count0}"
+  IO.println s!"After forking 3 children: {count1}"
+  IO.println s!"After cancelling 1 child: {count2}"
+  IO.println s!"After forking grandchild: {count3}"
+  IO.println s!"After cancelling root: {count4}"
+
+/--
+info: Initial (root only): 1
+After forking 3 children: 4
+After cancelling 1 child: 3
+After forking grandchild: 4
+After cancelling root: 0
+-/
+#guard_msgs in
+#eval testCountAliveTokens
diff --git a/tests/lean/run/context_async.lean b/tests/lean/run/context_async.lean
new file mode 100644
index 0000000000..ffabcae20f
--- /dev/null
+++ b/tests/lean/run/context_async.lean
@@ -0,0 +1,698 @@
+import Std.Internal.Async
+import Std.Sync
+
+open Std.Internal.IO Async
+
+/-- Test ContextAsync cancellation check -/
+def testIsCancelled : IO Unit := do
+  let (before, after) ← Async.block do
+    ContextAsync.run do
+      let before ← ContextAsync.isCancelled
+      ContextAsync.cancel .cancel
+      Async.sleep 50
+      let after ← ContextAsync.isCancelled
+      return (before, after)
+
+  IO.println s!"Before: {before}, After: {after}"
+
+/--
+info: Before: false, After: true
+-/
+#guard_msgs in
+#eval testIsCancelled
+
+/-- Test ContextAsync cancellation reason -/
+def testGetCancellationReason : IO Unit := do
+  let res ← Async.block do
+    ContextAsync.run do
+      ContextAsync.cancel (.custom "test reason")
+      Async.sleep 50
+      let some reason ← ContextAsync.getCancellationReason
+        | return "ERROR: No reason found"
+      return s!"Reason: {reason}"
+
+  IO.println res
+
+/--
+info: Reason: custom("test reason")
+-/
+#guard_msgs in
+#eval testGetCancellationReason
+
+/-- Test awaitCancellation -/
+def testAwaitCancellation : IO Unit := do
+  let received ← Std.Mutex.new false
+
+  Async.block do
+    let started ← Std.Mutex.new false
+
+    ContextAsync.run do
+      discard <| ContextAsync.concurrently
+        (do
+          started.atomically (set true)
+          ContextAsync.awaitCancellation
+          received.atomically (set true))
+        (do
+          -- Wait for task to start
+          while !(← started.atomically get) do
+            Async.sleep 10
+
+          Async.sleep 100
+          ContextAsync.cancel .shutdown)
+
+    Async.sleep 200
+
+  let _ ← received.atomically get
+  IO.println "Cancellation received"
+
+
+def testSelectorCancellationFail : IO Unit := do
+  let received ← Std.Mutex.new false
+
+  let result ← Async.block do
+    let ctx ← Std.CancellationContext.new
+    let started ← Std.Mutex.new false
+
+
+    let result ← do
+      try
+        ContextAsync.runIn ctx do
+        discard <| ContextAsync.concurrently
+          (do
+            started.atomically (set true)
+            let res ← Selectable.one #[
+              .case (← ContextAsync.doneSelector) (fun _ => pure true),
+              .case (← Selector.sleep 2000) (fun _ => pure false)
+            ]
+            received.atomically (set res))
+          (do
+            throw (.userError "failed")
+            return ())
+        return Except.ok ()
+      catch err =>
+        return Except.error err
+
+    Async.sleep 500
+
+    return result
+
+  let _ ← received.atomically get
+  IO.println "Cancellation received"
+
+  if let Except.error err := result then
+    throw err
+
+/--
+info: Cancellation received
+---
+error: failed
+-/
+#guard_msgs in
+#eval testSelectorCancellationFail
+
+/-- Test concurrently with both tasks succeeding -/
+def testConcurrently : IO Unit := do
+  let (a, b) ← Async.block do
+    ContextAsync.run do
+      ContextAsync.concurrently
+        (do
+          Async.sleep 100
+          return 42)
+        (do
+          Async.sleep 150
+          return "hello")
+
+  IO.println s!"Results: {a}, {b}"
+
+/--
+info: Results: 42, hello
+-/
+#guard_msgs in
+#eval testConcurrently
+
+/-- Test race with first task winning -/
+def testRace : IO Unit := do
+  let result ← Async.block do
+    ContextAsync.run do
+      ContextAsync.race
+        (do
+          Async.sleep 50
+          return "fast")
+        (do
+          Async.sleep 200
+          return "slow")
+
+  IO.println s!"Winner: {result}"
+
+/--
+info: Winner: fast
+-/
+#guard_msgs in
+#eval testRace
+
+/-- Test concurrentlyAll -/
+def testConcurrentlyAll : IO Unit := do
+  let results ← Async.block do
+    ContextAsync.run do
+      let tasks := #[
+        (do Async.sleep 50; return 1),
+        (do Async.sleep 100; return 2),
+        (do Async.sleep 75; return 3)
+      ]
+      ContextAsync.concurrentlyAll tasks
+
+  IO.println s!"All results: {results}"
+
+/--
+info: All results: #[1, 2, 3]
+-/
+#guard_msgs in
+#eval testConcurrentlyAll
+
+/-- Test background task with cancellation -/
+def testBackground : IO Unit := do
+  let counter ← Std.Mutex.new 0
+
+  Async.block do
+    ContextAsync.run do
+      discard <| ContextAsync.concurrently
+        (do
+          for _ in [0:10] do
+            if ← ContextAsync.isCancelled then
+              break
+            counter.atomically (modify (· + 1))
+            Async.sleep 50)
+        (do
+          -- Let it run for a bit
+          Async.sleep 150
+          ContextAsync.cancel .cancel)
+
+    Async.sleep 200
+
+  let final ← counter.atomically get
+  IO.println s!"Counter reached: {final}"
+
+/-- Test fork cancellation isolation -/
+def testForkCancellation : IO Unit := do
+  let parent ← Std.CancellationContext.new
+  let childCancelled ← Std.Mutex.new false
+  let parentCancelled ← Std.Mutex.new false
+
+  Async.block do
+    ContextAsync.runIn parent do
+      discard <| ContextAsync.concurrentlyAll #[
+        (do
+          let child ← ContextAsync.getContext
+          Async.sleep 100
+          child.cancel .cancel
+          childCancelled.atomically (set true)),
+        (do
+        Async.sleep 200
+        if ← parent.isCancelled then
+          parentCancelled.atomically (set true))
+      ]
+
+  let childWasCancelled ← childCancelled.atomically get
+  let parentWasCancelled ← parentCancelled.atomically get
+
+  IO.println s!"Child cancelled: {childWasCancelled}, Parent cancelled: {parentWasCancelled}"
+
+/--
+info: Child cancelled: true, Parent cancelled: false
+-/
+#guard_msgs in
+#eval testForkCancellation
+
+/-- Test doneSelector -/
+partial def testNestedFork : IO Unit := do
+  let res ← Async.block do
+    ContextAsync.run do
+      let ctx ← ContextAsync.getContext
+      let sel ← ContextAsync.doneSelector
+
+      let (_, result) ← ContextAsync.concurrently
+        (do
+          Async.sleep 100
+          ctx.cancel .deadline)
+        (Selectable.one #[.case sel (fun _ => pure true)])
+
+      return result
+
+  IO.println s!"Done selector triggered: {res}"
+
+/--
+info: Done selector triggered: true
+-/
+#guard_msgs in
+#eval testNestedFork
+
+/-- Test Selector.cancelled -/
+def testSelectorCancelled : IO Unit := do
+  let res ← Async.block do
+    ContextAsync.run do
+      let ctx ← ContextAsync.getContext
+      let sel ← Selector.cancelled
+
+      let (_, result) ← ContextAsync.concurrently
+        (do
+          Async.sleep 150
+          ctx.cancel .shutdown)
+        (Selectable.one #[.case sel (fun _ => pure true)])
+
+      return result
+
+  IO.println s!"Selector.cancelled triggered: {res}"
+
+/--
+info: Selector.cancelled triggered: true
+-/
+#guard_msgs in
+#eval testSelectorCancelled
+
+/-- Test MonadLift instances -/
+def testMonadLift : IO Unit := do
+  let (msg1, msg2) ← Async.block do
+    ContextAsync.run do
+      -- Lift from IO
+      let msg1 : String := "From IO"
+
+      -- Lift from BaseIO
+      let msg2 : String := "From BaseIO"
+
+      -- Lift from Async
+      let _ ← (Async.sleep 50 : Async Unit)
+
+      return (msg1, msg2)
+
+  IO.println msg1
+  IO.println msg2
+  IO.println "All lifts work"
+
+/--
+info: From IO
+From BaseIO
+All lifts work
+-/
+#guard_msgs in
+#eval testMonadLift
+
+/-- Test exception handling in ContextAsync -/
+def testExceptionHandling : IO Unit := do
+  let res ← Async.block do
+    ContextAsync.run do
+      try
+        throw (IO.userError "test error")
+        return "Should not reach here"
+      catch e =>
+        return s!"Caught: {e}"
+
+  IO.println res
+
+/--
+info: Caught: test error
+-/
+#guard_msgs in
+#eval testExceptionHandling
+
+/-- Test tryFinally in ContextAsync -/
+def testTryFinally : IO Unit := do
+  let cleaned ← Std.Mutex.new false
+
+  Async.block do
+    ContextAsync.run do
+      try
+        ContextAsync.cancel .cancel
+        ContextAsync.awaitCancellation
+      finally
+        cleaned.atomically (set true)
+
+  let wasCleanedUp ← cleaned.atomically get
+  IO.println s!"Cleanup ran: {wasCleanedUp}"
+
+/--
+info: Cleanup ran: true
+-/
+#guard_msgs in
+#eval testTryFinally
+
+/-- Test race with cancellation -/
+def testRaceWithCancellation : IO Unit := do
+  let ctx ← Std.CancellationContext.new
+  let leftCancelled ← Std.Mutex.new false
+  let rightCancelled ← Std.Mutex.new false
+
+  Async.block do
+    ContextAsync.runIn ctx do
+      let _ ← ContextAsync.race
+        (do
+          try
+            Async.sleep 500
+            return "left"
+          finally
+            if ← ContextAsync.isCancelled then
+              leftCancelled.atomically (set true))
+        (do
+          try
+            Async.sleep 50
+            return "right"
+          finally
+            if ← ContextAsync.isCancelled then
+              rightCancelled.atomically (set true))
+
+      Async.sleep 1000
+
+  let left ← leftCancelled.atomically get
+  let right ← rightCancelled.atomically get
+  IO.println s!"Left cancelled: {left}, Right cancelled: {right}"
+
+/--
+info: Left cancelled: true, Right cancelled: false
+-/
+#guard_msgs in
+#eval testRaceWithCancellation
+
+/-- Test complex concurrent workflow -/
+def testComplexWorkflow : IO Unit := do
+  let results ← Std.Mutex.new ([] : List String)
+
+  Async.block do
+    ContextAsync.run do
+      -- Run multiple concurrent operations
+      let (a, b) ← ContextAsync.concurrently
+        (do
+          Async.sleep 50
+          results.atomically (modify ("A"::·))
+          return 1)
+        (do
+          Async.sleep 75
+          results.atomically (modify ("B"::·))
+          return 2)
+
+      -- Additional concurrent task
+      discard <| ContextAsync.concurrently
+        (do
+          Async.sleep 100
+          results.atomically (modify ("BG"::·)))
+        (do
+          Async.sleep 200
+          results.atomically (modify (s!"Sum:{a+b}"::·)))
+
+  let final ← results.atomically get
+  IO.println s!"Results: {final.reverse}"
+
+/--
+info: Results: [A, B, BG, Sum:3]
+-/
+#guard_msgs in
+#eval testComplexWorkflow
+
+def testConcurrentlyAllException : IO Unit := do
+  let ref ← IO.mkRef ""
+
+  try
+    Async.block do
+      ContextAsync.run do
+        let tasks := #[
+          (do
+            Async.sleep 1000
+            if ← ContextAsync.isCancelled then
+              ref.set "cancelled"
+              return
+            else
+              ref.set "not cancelled"
+            Async.sleep 500
+            if ← ContextAsync.isCancelled then
+              ref.modify (· ++ ", cancelled")
+            else
+              ref.modify (· ++ ", not cancelled")),
+          (do
+            Async.sleep 250
+            throw (IO.userError "Error: Hello"))
+        ]
+        discard <| ContextAsync.concurrentlyAll tasks
+  finally
+    IO.println (← ref.get)
+
+/--
+info: cancelled
+---
+error: Error: Hello
+-/
+#guard_msgs in
+#eval testConcurrentlyAllException
+
+/-- Test that tasks in ContextAsync.run are not cancelled when run completes -/
+def test0 : IO Unit := do
+  let ref ← IO.mkRef false
+
+  Async.block do
+    ContextAsync.run do
+      Async.sleep 100
+      if ← ContextAsync.isCancelled then
+        ref.set true
+
+  IO.sleep 200
+  IO.println s!"{← ref.get}"
+
+/--
+info: false
+-/
+#guard_msgs in
+#eval test0
+
+/-- Test that background tasks are cancelled when ContextAsync.run completes -/
+def test1 : IO Unit := do
+  let ref ← IO.mkRef false
+
+  Async.block do
+    ContextAsync.run do
+      ContextAsync.background do
+        Async.sleep 100
+        if ← ContextAsync.isCancelled then
+          ref.set true
+
+  IO.sleep 200
+  IO.println s!"{← ref.get}"
+
+/--
+info: true
+-/
+#guard_msgs in
+#eval test1
+
+/-- Test that nested background tasks (ContextAsync.background in ContextAsync.background) are cancelled -/
+def test2 : IO Unit := do
+  let ref ← IO.mkRef false
+
+  Async.block do
+    ContextAsync.run do
+      ContextAsync.background do
+        ContextAsync.background do
+          Async.sleep 100
+          if ← ContextAsync.isCancelled then
+            ref.set true
+
+  IO.sleep 200
+  IO.println s!"{← ref.get}"
+
+/--
+info: true
+-/
+#guard_msgs in
+#eval test2
+
+/-- Test that ContextAsync.background in Async.background is cancelled -/
+def test2' : IO Unit := do
+  let ref ← IO.mkRef false
+
+  Async.block do
+    ContextAsync.run do
+      Async.background do
+        ContextAsync.background do
+          Async.sleep 100
+          if ← ContextAsync.isCancelled then
+            ref.set true
+
+  IO.sleep 200
+  IO.println s!"{← ref.get}"
+
+/--
+info: true
+-/
+#guard_msgs in
+#eval test2'
+
+/-- Test that Async.background in ContextAsync.background is cancelled -/
+def test2'' : IO Unit := do
+  let ref ← IO.mkRef false
+
+  Async.block do
+    ContextAsync.run do
+      ContextAsync.background do
+        Async.background do
+          Async.sleep 100
+          if ← ContextAsync.isCancelled then
+            ref.set true
+
+  IO.sleep 200
+  IO.println s!"{← ref.get}"
+
+/--
+info: true
+-/
+#guard_msgs in
+#eval test2''
+
+/-- Test concurrently with first task succeeding immediately, others checking cancellation -/
+def testConcurrentlySuccessWithCancellation : IO Unit := do
+  let task2Cancelled ← Std.Mutex.new false
+  let task3Cancelled ← Std.Mutex.new false
+
+  let results ← Async.block do
+    ContextAsync.run do
+      ContextAsync.concurrentlyAll #[
+        (do
+          return "first"),
+        (do
+          -- Second task waits and checks for cancellation
+          let res ← Selectable.one #[
+          .case (← ContextAsync.doneSelector) (fun _ => pure true),
+          .case (← Selector.sleep 500) (fun _ => pure false)
+          ]
+
+          task2Cancelled.atomically (set (res))
+          return "second"),
+        (do
+          let res ← Selectable.one #[
+          .case (← ContextAsync.doneSelector) (fun _ => pure true),
+          .case (← Selector.sleep 500) (fun _ => pure false)
+          ]
+
+          task3Cancelled.atomically (set (res))
+          return "third")
+      ]
+
+  let t2 ← task2Cancelled.atomically get
+  let t3 ← task3Cancelled.atomically get
+  IO.println s!"Results: {results}"
+  IO.println s!"Task2 cancelled: {t2}, Task3 cancelled: {t3}"
+
+/--
+info: Results: #[first, second, third]
+Task2 cancelled: false, Task3 cancelled: false
+-/
+#guard_msgs in
+#eval testConcurrentlySuccessWithCancellation
+
+/-- Test concurrently with first task failing, others checking for cancellation -/
+def testConcurrentlyFailWithCancellation : IO Unit := do
+  let task2Cancelled ← Std.Mutex.new false
+  let task3Cancelled ← Std.Mutex.new false
+
+  let results ← Async.block do
+      ContextAsync.run do
+        try
+          let result ← ContextAsync.concurrentlyAll #[
+            (do
+              -- First task fails immediately
+              throw (IO.userError "first task failed")),
+            (do
+              -- Second task waits and checks for cancellation
+              let res ← Selectable.one #[
+              .case (← ContextAsync.doneSelector) (fun _ => pure true),
+              .case (← Selector.sleep 2000) (fun _ => pure false)
+              ]
+
+              task2Cancelled.atomically (set (res))
+              return "second"),
+            (do
+              let res ← Selectable.one #[
+              .case (← ContextAsync.doneSelector) (fun _ => pure true),
+              .case (← Selector.sleep 2000) (fun _ => pure false)
+              ]
+
+              task3Cancelled.atomically (set (res))
+              return "third")
+          ]
+          return Except.ok result
+        catch e =>
+          Async.sleep 500
+          return Except.error e
+
+  let t2 ← task2Cancelled.atomically get
+  let t3 ← task3Cancelled.atomically get
+
+  match results with
+  | .ok results => IO.println s!"Results: {results}"
+  | .error e => IO.println s!"Error: {e}"
+
+  IO.println s!"Task2 cancelled: {t2}, Task3 cancelled: {t3}"
+
+/--
+info: Error: first task failed
+Task2 cancelled: true, Task3 cancelled: true
+-/
+#guard_msgs in
+#eval testConcurrentlyFailWithCancellation
+
+/-- Test concurrently with both tasks succeeding, checking cancellation status -/
+def testConcurrentlySuccessWithCancellation2Tasks : IO Unit := do
+  let task2Cancelled ← Std.Mutex.new false
+
+  let (r1, r2) ← Async.block do
+    ContextAsync.run do
+      ContextAsync.concurrently
+        (do return "first")
+        (do
+          -- Second task waits and checks for cancellation
+          let res ← Selectable.one #[
+            .case (← ContextAsync.doneSelector) (fun _ => pure true),
+            .case (← Selector.sleep 500) (fun _ => pure false)
+          ]
+
+          task2Cancelled.atomically (set res)
+          return "second")
+
+  let t2 ← task2Cancelled.atomically get
+  IO.println s!"Results: {r1}, {r2}"
+  IO.println s!"Task2 cancelled: {t2}"
+
+/--
+info: Results: first, second
+Task2 cancelled: false
+-/
+#guard_msgs in
+#eval testConcurrentlySuccessWithCancellation2Tasks
+
+/-- Test concurrently with first task failing, second task checking for cancellation -/
+def testConcurrentlyFailWithCancellation2Tasks : IO Unit := do
+  let task2Cancelled ← Std.Mutex.new false
+
+  try
+    Async.block do
+      ContextAsync.run do
+        let (_ : (String × String)) ← ContextAsync.concurrently
+          (do
+            -- First task fails immediately
+            throw (IO.userError "first task failed") : ContextAsync String)
+          (do
+            -- Second task waits and checks for cancellation
+            let res ← Selectable.one #[
+              .case (← ContextAsync.doneSelector) (fun _ => pure true),
+              .case (← Selector.sleep 2000) (fun _ => pure false)
+            ]
+
+            task2Cancelled.atomically (set res)
+            return "second")
+  catch e =>
+    IO.sleep 500
+    let t2 ← task2Cancelled.atomically get
+    IO.println s!"Error: {e}"
+    IO.println s!"Task2 cancelled: {t2}"
+
+/--
+info: Error: first task failed
+Task2 cancelled: true
+-/
+#guard_msgs in
+#eval testConcurrentlyFailWithCancellation2Tasks