diff --git a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
index c843ddb3df1..82b406ad2fd 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
@@ -1,5 +1,6 @@
 ### Fixed
 
+* Fix optimizer eliminating side-effectful receiver of unit-typed member access (e.g. inside `task { (effectful()).UnitProp }`). ([Issue #13099](https://github.com/dotnet/fsharp/issues/13099), [PR #19885](https://github.com/dotnet/fsharp/pull/19885))
 * Suppress hover/symbol resolution for wildcard `_` patterns inside `member _.…` bodies that incorrectly showed `val _: T` tooltip. ([PR #19760](https://github.com/dotnet/fsharp/pull/19760))
 * Deduplicate format specifier locations in computation expressions so editor tooling no longer reports duplicate entries for the same `%` specifier. ([Issue #16419](https://github.com/dotnet/fsharp/issues/16419), [PR #19791](https://github.com/dotnet/fsharp/pull/19791))
 * Reject non-function bindings for single-case and partial active pattern names with FS1209, matching the existing multi-case behavior. ([PR #19763](https://github.com/dotnet/fsharp/pull/19763))
diff --git a/src/Compiler/Optimize/Optimizer.fs b/src/Compiler/Optimize/Optimizer.fs
index 3cbb574598c..99328d04314 100644
--- a/src/Compiler/Optimize/Optimizer.fs
+++ b/src/Compiler/Optimize/Optimizer.fs
@@ -1715,6 +1715,24 @@ let TryEliminateBinding cenv _env bind e2 _m =
               | _ -> None
 
         let (DebugPoints(e2, recreate0)) = e2
+
+        // Protect side-effectful member receiver temporaries from being dropped by
+        // state-machine lowering.  After the optimizer inlines a unit-returning property
+        // access (e.g. task { (f()).End }), it produces:
+        //     let this = receiver in ()
+        // where 'this' has IsMemberThisVal = true.  LowerStateMachines.BindResumableCodeDefinitions
+        // treats any IsMemberThisVal binding as a resumable-code definition and removes it from
+        // the expression tree.  If the variable is unused the side-effectful receiver is silently
+        // lost.  Converting to Expr.Sequential here makes the expression invisible to that pass.
+        // See issue #13099.
+        let xUnusedInBody () =
+            let fvs = accFreeInExpr (CollectLocalsWithStackGuard()) e2 emptyFreeVars
+            not (Zset.contains vspec1 fvs.FreeLocals)
+
+        if vspec1.IsMemberThisVal && ExprHasEffect g e1 && xUnusedInBody () then
+            Some (mkSequential e1.Range e1 e2 |> recreate0)
+        else
+
         match e2 with 
 
          // Immediate consumption of value as itself 'let x = e in x'
diff --git a/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj b/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
index acfeca79f35..ac12744ef95 100644
--- a/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
+++ b/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
@@ -268,6 +268,7 @@
     <Compile Include="EmittedIL\ByRefTests.fs" />
     <Compile Include="EmittedIL\Misc\Misc.fs" />
     <Compile Include="Misc.fs" />
+    <Compile Include="Optimizations\TaskCEUnitPropertyAccess.fs" />
     <Compile Include="EmittedIL\operators\Operators.fs" />
     <Compile Include="EmittedIL\Platform\Platform.fs" />
     <Compile Include="EmittedIL\QueryExpressionStepping\QueryExpressionStepping.fs" />
diff --git a/tests/FSharp.Compiler.ComponentTests/Optimizations/TaskCEUnitPropertyAccess.fs b/tests/FSharp.Compiler.ComponentTests/Optimizations/TaskCEUnitPropertyAccess.fs
new file mode 100644
index 00000000000..9c2d3692afc
--- /dev/null
+++ b/tests/FSharp.Compiler.ComponentTests/Optimizations/TaskCEUnitPropertyAccess.fs
@@ -0,0 +1,186 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace Optimizations
+
+open Xunit
+open FSharp.Test.Compiler
+
+// Regression tests for https://github.com/dotnet/fsharp/issues/13099
+// The Release-mode optimizer was eliminating a side-effectful receiver expression
+// when its result flowed into a unit-typed member access (e.g. `.End`) inside `task { }`.
+//
+// In Debug, the call site is preserved and the side-effectful exception is observed.
+// In Release, the optimizer reduces the unit-typed property access to the unit constant
+// and drops the receiver, swallowing the exception.
+//
+// Each test compiles a snippet under both Debug (optimize=false) and Release
+// (optimize=true) and asserts the program completes without an unhandled exception.
+//
+// Note: snippets execute *in-process* via reflection. They MUST NOT call `exit`
+// because that would terminate the test runner. Use `failwith` (or simply let an
+// expected exception propagate) to signal test failure.
+module TaskCEUnitPropertyAccess =
+
+    let private run (optimize: bool) (source: string) =
+        Fsx source
+        |> asExe
+        |> withOptimization optimize
+        |> compileExeAndRun
+        |> shouldSucceed
+        |> ignore
+
+    [<InlineData(false)>]   // Debug
+    [<InlineData(true)>]    // Release
+    [<Theory>]
+    let ``TaskCE_UnitPropertyAccess_PreservesReceiverSideEffects`` (optimize: bool) =
+        run optimize """
+type SomeOutputType() =
+    member x.End = ()
+
+let someFunctionWithReturnType () =
+    failwith "This should be raised"
+    SomeOutputType()
+
+let theTaskAtHand () =
+    task { (someFunctionWithReturnType ()).End }
+
+try
+    theTaskAtHand().Wait()
+    failwith "Expected exception was not raised; the optimizer dropped the side-effectful receiver."
+with
+| :? System.AggregateException as ex when ex.InnerException.Message = "This should be raised" -> ()
+"""
+
+    [<InlineData(false)>]
+    [<InlineData(true)>]
+    [<Theory>]
+    let ``UnitPropertyAccess_OnSideEffectfulReceiver_OutsideTaskCE_Raises`` (optimize: bool) =
+        run optimize """
+type SomeOutputType() =
+    member x.End = ()
+
+let someFunctionWithReturnType () =
+    failwith "boom"
+    SomeOutputType()
+
+let test () = (someFunctionWithReturnType ()).End
+
+try
+    test ()
+    failwith "Expected exception was not raised; the optimizer dropped the side-effectful receiver."
+with
+| ex when ex.Message = "boom" -> ()
+"""
+
+    [<InlineData(false)>]
+    [<InlineData(true)>]
+    [<Theory>]
+    let ``TaskCE_UnitMethodCall_PreservesReceiverSideEffects`` (optimize: bool) =
+        run optimize """
+type SomeOutputType() =
+    member x.Finish() = ()
+
+let someFunctionWithReturnType () =
+    failwith "boom"
+    SomeOutputType()
+
+let theTaskAtHand () =
+    task { (someFunctionWithReturnType ()).Finish() }
+
+try
+    theTaskAtHand().Wait()
+    failwith "Expected exception was not raised; the optimizer dropped the side-effectful receiver."
+with
+| :? System.AggregateException as ex when ex.InnerException.Message = "boom" -> ()
+"""
+
+    [<InlineData(false)>]
+    [<InlineData(true)>]
+    [<Theory>]
+    let ``TaskCE_UnitPropertyAccess_RunsBothReceiverAndGetterEffects`` (optimize: bool) =
+        run optimize """
+let mutable counter = 0
+
+type Tracker() =
+    member x.Done = counter <- counter + 1
+
+let makeTracker () =
+    counter <- counter + 1
+    Tracker()
+
+let theTaskAtHand () =
+    task { (makeTracker ()).Done }
+
+theTaskAtHand().Wait()
+if counter <> 2 then
+    failwithf "Expected counter=2 (receiver + getter side effects) but got %d" counter
+"""
+
+    [<InlineData(false)>]
+    [<InlineData(true)>]
+    [<Theory>]
+    let ``TaskCE_NonUnitPropertyAccess_OnSideEffectfulReceiver_Raises`` (optimize: bool) =
+        run optimize """
+type HasValue() =
+    member x.Value = 42
+
+let makeHasValue () =
+    failwith "boom"
+    HasValue()
+
+let theTaskAtHand () =
+    task { let _ = (makeHasValue ()).Value in () }
+
+try
+    theTaskAtHand().Wait()
+    failwith "Expected exception was not raised."
+with
+| :? System.AggregateException as ex when ex.InnerException.Message = "boom" -> ()
+"""
+
+    [<InlineData(false)>]
+    [<InlineData(true)>]
+    [<Theory>]
+    let ``AsyncCE_UnitPropertyAccess_PreservesReceiverSideEffects`` (optimize: bool) =
+        run optimize """
+type SomeOutputType() =
+    member x.End = ()
+
+let someFunctionWithReturnType () =
+    failwith "boom"
+    SomeOutputType()
+
+let theAsyncAtHand () =
+    async { (someFunctionWithReturnType ()).End }
+
+try
+    theAsyncAtHand () |> Async.RunSynchronously
+    failwith "Expected exception was not raised."
+with
+| ex when ex.Message = "boom" -> ()
+"""
+
+    [<InlineData(false)>]
+    [<InlineData(true)>]
+    [<Theory>]
+    let ``TaskCE_NestedUnitPropertyAccess_PreservesReceiverSideEffects`` (optimize: bool) =
+        run optimize """
+type Inner() =
+    member x.End = ()
+
+type Outer() =
+    member x.Inner = Inner()
+
+let makeOuter () =
+    failwith "boom"
+    Outer()
+
+let theTaskAtHand () =
+    task { (makeOuter ()).Inner.End }
+
+try
+    theTaskAtHand().Wait()
+    failwith "Expected exception was not raised; the optimizer dropped the side-effectful receiver."
+with
+| :? System.AggregateException as ex when ex.InnerException.Message = "boom" -> ()
+"""
diff --git a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_net10.0.bsl b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_net10.0.bsl
index 51b95b60067..424ee9bee24 100644
--- a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_net10.0.bsl
+++ b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_net10.0.bsl
@@ -6,6 +6,6 @@
 [IL]: Error [ReturnPtrToStack]: : Internal.Utilities.Text.Lexing.LexBuffer`1::get_LexemeView()][offset 0x00000019] Return type is ByRef, TypedReference, ArgHandle, or ArgIterator.
 [IL]: Error [ReturnPtrToStack]: : FSharp.Compiler.CodeAnalysis.ItemKeyStore::ReadKeyString([System.Reflection.Metadata]System.Reflection.Metadata.BlobReader&)][offset 0x00000026] Return type is ByRef, TypedReference, ArgHandle, or ArgIterator.
 [IL]: Error [ReturnPtrToStack]: : FSharp.Compiler.CodeAnalysis.ItemKeyStore::ReadFirstKeyString()][offset 0x00000070] Return type is ByRef, TypedReference, ArgHandle, or ArgIterator.
-[IL]: Error [StackUnexpected]: : FSharp.Compiler.CodeAnalysis.ItemKeyStoreBuilder::writeRange([FSharp.Compiler.Service]FSharp.Compiler.Text.Range)][offset 0x00000011][found address of '[FSharp.Compiler.Service]FSharp.Compiler.Text.Range'][expected Native Int] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : FSharp.Compiler.CodeAnalysis.ItemKeyStoreBuilder::writeRange([FSharp.Compiler.Service]FSharp.Compiler.Text.Range)][offset 0x0000000F][found address of '[FSharp.Compiler.Service]FSharp.Compiler.Text.Range'][expected Native Int] Unexpected type on the stack.
 [IL]: Error [ExpectedNumericType]: : FSharp.Compiler.EditorServices.SemanticClassificationKeyStoreBuilder::WriteAll([FSharp.Compiler.Service]FSharp.Compiler.EditorServices.SemanticClassificationItem[])][offset 0x0000001D][found address of '[FSharp.Compiler.Service]FSharp.Compiler.EditorServices.SemanticClassificationItem'] Expected numeric type on the stack.
 [IL]: Error [UnmanagedPointer]: : FSharp.Compiler.Interactive.Shell+Utilities+pointerToNativeInt::Invoke(object)][offset 0x00000007] Unmanaged pointers are not a verifiable type.
diff --git a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
index 51b95b60067..424ee9bee24 100644
--- a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
+++ b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
@@ -6,6 +6,6 @@
 [IL]: Error [ReturnPtrToStack]: : Internal.Utilities.Text.Lexing.LexBuffer`1::get_LexemeView()][offset 0x00000019] Return type is ByRef, TypedReference, ArgHandle, or ArgIterator.
 [IL]: Error [ReturnPtrToStack]: : FSharp.Compiler.CodeAnalysis.ItemKeyStore::ReadKeyString([System.Reflection.Metadata]System.Reflection.Metadata.BlobReader&)][offset 0x00000026] Return type is ByRef, TypedReference, ArgHandle, or ArgIterator.
 [IL]: Error [ReturnPtrToStack]: : FSharp.Compiler.CodeAnalysis.ItemKeyStore::ReadFirstKeyString()][offset 0x00000070] Return type is ByRef, TypedReference, ArgHandle, or ArgIterator.
-[IL]: Error [StackUnexpected]: : FSharp.Compiler.CodeAnalysis.ItemKeyStoreBuilder::writeRange([FSharp.Compiler.Service]FSharp.Compiler.Text.Range)][offset 0x00000011][found address of '[FSharp.Compiler.Service]FSharp.Compiler.Text.Range'][expected Native Int] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : FSharp.Compiler.CodeAnalysis.ItemKeyStoreBuilder::writeRange([FSharp.Compiler.Service]FSharp.Compiler.Text.Range)][offset 0x0000000F][found address of '[FSharp.Compiler.Service]FSharp.Compiler.Text.Range'][expected Native Int] Unexpected type on the stack.
 [IL]: Error [ExpectedNumericType]: : FSharp.Compiler.EditorServices.SemanticClassificationKeyStoreBuilder::WriteAll([FSharp.Compiler.Service]FSharp.Compiler.EditorServices.SemanticClassificationItem[])][offset 0x0000001D][found address of '[FSharp.Compiler.Service]FSharp.Compiler.EditorServices.SemanticClassificationItem'] Expected numeric type on the stack.
 [IL]: Error [UnmanagedPointer]: : FSharp.Compiler.Interactive.Shell+Utilities+pointerToNativeInt::Invoke(object)][offset 0x00000007] Unmanaged pointers are not a verifiable type.