diff --git a/src/Build.UnitTests/BackEnd/CommunicationsUtilities_Tests.cs b/src/Build.UnitTests/BackEnd/CommunicationsUtilities_Tests.cs
new file mode 100644
index 00000000000..ddfd878bd28
--- /dev/null
+++ b/src/Build.UnitTests/BackEnd/CommunicationsUtilities_Tests.cs
@@ -0,0 +1,95 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.Build.Framework;
+using Microsoft.Build.Internal;
+using Microsoft.Build.Shared;
+using Shouldly;
+using Xunit;
+
+namespace Microsoft.Build.Engine.UnitTests.BackEnd
+{
+    /// <summary>
+    /// Tests for <see cref="CommunicationsUtilities.GetHandshakeOptions"/> covering the parent
+    /// side of NET task host launches: the parent must suppress its own architecture bit on
+    /// the wire so already-shipped SDK children (whose <c>IsAllowedBitnessMismatch</c> tolerates
+    /// "parent sent no arch bit") accept the connection regardless of either process arch.
+    /// </summary>
+    public sealed class CommunicationsUtilities_Tests
+    {
+        [Theory]
+        [InlineData(XMakeAttributes.MSBuildArchitectureValues.x64)]
+        [InlineData(XMakeAttributes.MSBuildArchitectureValues.arm64)]
+        [InlineData(XMakeAttributes.MSBuildArchitectureValues.x86)]
+        public void GetHandshakeOptions_NetTaskHostParent_SuppressesArchBit(string parentArchitecture)
+        {
+            var parameters = new TaskHostParameters(
+                runtime: XMakeAttributes.MSBuildRuntimeValues.net,
+                architecture: parentArchitecture);
+
+            HandshakeOptions options = CommunicationsUtilities.GetHandshakeOptions(
+                taskHost: true,
+                taskHostParameters: parameters);
+
+            options.HasFlag(HandshakeOptions.NET).ShouldBeTrue();
+            options.HasFlag(HandshakeOptions.X64).ShouldBeFalse();
+            options.HasFlag(HandshakeOptions.Arm64).ShouldBeFalse();
+        }
+
+        [Fact]
+        public void GetHandshakeOptions_NonNetTaskHostParent_KeepsX64ArchBit()
+        {
+            var parameters = new TaskHostParameters(
+                runtime: XMakeAttributes.MSBuildRuntimeValues.clr4,
+                architecture: XMakeAttributes.MSBuildArchitectureValues.x64);
+
+            HandshakeOptions options = CommunicationsUtilities.GetHandshakeOptions(
+                taskHost: true,
+                taskHostParameters: parameters);
+
+            options.HasFlag(HandshakeOptions.X64).ShouldBeTrue();
+        }
+
+        [Fact]
+        public void GetHandshakeOptions_NonNetTaskHostParent_KeepsArm64ArchBit()
+        {
+            var parameters = new TaskHostParameters(
+                runtime: XMakeAttributes.MSBuildRuntimeValues.clr4,
+                architecture: XMakeAttributes.MSBuildArchitectureValues.arm64);
+
+            HandshakeOptions options = CommunicationsUtilities.GetHandshakeOptions(
+                taskHost: true,
+                taskHostParameters: parameters);
+
+            options.HasFlag(HandshakeOptions.Arm64).ShouldBeTrue();
+        }
+
+        [Fact]
+        public void GetHandshakeOptions_NetTaskHostChild_KeepsArchBit()
+        {
+            // The child invokes GetHandshakeOptions with TaskHostParameters.Empty; the helper
+            // then derives architectureFlagToSet from GetCurrentMSBuildArchitecture(). The
+            // suppression must not apply: the child needs to keep its own arch bit so
+            // already-deployed parents that still emit one continue to match.
+            HandshakeOptions options = CommunicationsUtilities.GetHandshakeOptions(
+                taskHost: true,
+                taskHostParameters: TaskHostParameters.Empty);
+
+            string currentArch = XMakeAttributes.GetCurrentMSBuildArchitecture();
+            if (currentArch.Equals(XMakeAttributes.MSBuildArchitectureValues.x64, System.StringComparison.OrdinalIgnoreCase))
+            {
+                options.HasFlag(HandshakeOptions.X64).ShouldBeTrue();
+            }
+            else if (currentArch.Equals(XMakeAttributes.MSBuildArchitectureValues.arm64, System.StringComparison.OrdinalIgnoreCase))
+            {
+                options.HasFlag(HandshakeOptions.Arm64).ShouldBeTrue();
+            }
+            else
+            {
+                // x86 or unknown: no arch bit is expected on the child side either.
+                options.HasFlag(HandshakeOptions.X64).ShouldBeFalse();
+                options.HasFlag(HandshakeOptions.Arm64).ShouldBeFalse();
+            }
+        }
+    }
+}
diff --git a/src/Build.UnitTests/BackEnd/NodeEndpointOutOfProcBase_Tests.cs b/src/Build.UnitTests/BackEnd/NodeEndpointOutOfProcBase_Tests.cs
new file mode 100644
index 00000000000..e0d3b465af1
--- /dev/null
+++ b/src/Build.UnitTests/BackEnd/NodeEndpointOutOfProcBase_Tests.cs
@@ -0,0 +1,77 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if NET
+
+using Microsoft.Build.BackEnd;
+using Microsoft.Build.Internal;
+using Shouldly;
+using Xunit;
+
+namespace Microsoft.Build.Engine.UnitTests.BackEnd
+{
+    /// <summary>
+    /// Tests for <see cref="NodeEndpointOutOfProcBase.IsAllowedBitnessMismatch"/>, the .NET task
+    /// host child-side tolerance that lets a parent which did not emit an architecture bit
+    /// (e.g. a .NET Framework MSBuild) connect to an SDK child running on x64 or arm64.
+    /// </summary>
+    public sealed class NodeEndpointOutOfProcBase_Tests
+    {
+        private const HandshakeOptions BaseNet = HandshakeOptions.TaskHost | HandshakeOptions.NET;
+
+        [Fact]
+        public void NoArchBitParent_X64Child_IsTolerated()
+        {
+            NodeEndpointOutOfProcBase.IsAllowedBitnessMismatch(
+                expectedOptions: (int)(BaseNet | HandshakeOptions.X64),
+                receivedOptions: (int)BaseNet).ShouldBeTrue();
+        }
+
+        [Fact]
+        public void NoArchBitParent_Arm64Child_IsTolerated()
+        {
+            NodeEndpointOutOfProcBase.IsAllowedBitnessMismatch(
+                expectedOptions: (int)(BaseNet | HandshakeOptions.Arm64),
+                receivedOptions: (int)BaseNet).ShouldBeTrue();
+        }
+
+        [Fact]
+        public void X64Parent_X64Child_NotConsideredMismatch()
+        {
+            // Equal handshakes never hit IsAllowedBitnessMismatch in production; verify it
+            // still returns false so the tolerance is scoped to the no-arch-bit parent only.
+            NodeEndpointOutOfProcBase.IsAllowedBitnessMismatch(
+                expectedOptions: (int)(BaseNet | HandshakeOptions.X64),
+                receivedOptions: (int)(BaseNet | HandshakeOptions.X64)).ShouldBeFalse();
+        }
+
+        [Fact]
+        public void X64Parent_Arm64Child_NotTolerated()
+        {
+            // True architecture mismatch (parent sent X64, child expects Arm64) is rejected.
+            NodeEndpointOutOfProcBase.IsAllowedBitnessMismatch(
+                expectedOptions: (int)(BaseNet | HandshakeOptions.Arm64),
+                receivedOptions: (int)(BaseNet | HandshakeOptions.X64)).ShouldBeFalse();
+        }
+
+        [Fact]
+        public void Arm64Parent_X64Child_NotTolerated()
+        {
+            NodeEndpointOutOfProcBase.IsAllowedBitnessMismatch(
+                expectedOptions: (int)(BaseNet | HandshakeOptions.X64),
+                receivedOptions: (int)(BaseNet | HandshakeOptions.Arm64)).ShouldBeFalse();
+        }
+
+        [Fact]
+        public void NoArchBitParent_NoArchBitChild_NotTolerated()
+        {
+            // The tolerance is scoped to x64/arm64 children; an x86-equivalent (no arch bit)
+            // child must not silently accept any handshake.
+            NodeEndpointOutOfProcBase.IsAllowedBitnessMismatch(
+                expectedOptions: (int)BaseNet,
+                receivedOptions: (int)BaseNet).ShouldBeFalse();
+        }
+    }
+}
+
+#endif
diff --git a/src/Framework/BackEnd/CommunicationsUtilities.cs b/src/Framework/BackEnd/CommunicationsUtilities.cs
index 195fbc08faf..d6c8dfba4f7 100644
--- a/src/Framework/BackEnd/CommunicationsUtilities.cs
+++ b/src/Framework/BackEnd/CommunicationsUtilities.cs
@@ -580,7 +580,25 @@ internal static HandshakeOptions GetHandshakeOptions(
             }
         }
 
-        if (!string.IsNullOrEmpty(architectureFlagToSet))
+        // For the NET task host, the launched child process's architecture is determined by
+        // what the .NET SDK shipped (x64 today, arm64 in the future), not by the parent's
+        // process architecture. Emitting the parent's arch bit here creates a wire-level
+        // mismatch with already-shipped SDK children whose arch differs (e.g. arm64 VS
+        // launching an x64 SDK MSBuild.dll). Suppress the arch bit so the child's existing
+        // IsAllowedBitnessMismatch tolerance ("parent sent no arch bit") accepts the
+        // connection regardless of either side's process architecture.
+        //
+        // Only the parent suppresses — i.e. when GetHandshakeOptions is invoked with concrete
+        // taskHostParameters describing a specific runtime/architecture to launch. The child
+        // (which calls GetHandshakeOptions with TaskHostParameters.Empty) keeps its own arch
+        // bit so that already-deployed parents that still emit an arch bit continue to match
+        // (or fall through to IsAllowedBitnessMismatch's tolerance).
+        bool isNetTaskHostParent =
+            taskHost &&
+            !taskHostParameters.IsEmpty &&
+            XMakeAttributes.MSBuildRuntimeValues.net.Equals(taskHostParameters.Runtime, StringComparison.OrdinalIgnoreCase);
+
+        if (!isNetTaskHostParent && !string.IsNullOrEmpty(architectureFlagToSet))
         {
             if (architectureFlagToSet!.Equals(XMakeAttributes.MSBuildArchitectureValues.x64, StringComparison.OrdinalIgnoreCase))
             {
diff --git a/src/Shared/NodeEndpointOutOfProcBase.cs b/src/Shared/NodeEndpointOutOfProcBase.cs
index 16828229734..032c6f1c64b 100644
--- a/src/Shared/NodeEndpointOutOfProcBase.cs
+++ b/src/Shared/NodeEndpointOutOfProcBase.cs
@@ -584,21 +584,31 @@ private bool IsHandshakePartValid(KeyValuePair<string, int> component, int hands
 
 #if NET
         /// <summary>
-        /// NET Task host allows MSBuild.exe to connect to it even if they have bitness mismatch.
-        /// 0x00FFFFFF is the handshake version included in component, the rest is the node type.
-        /// </summary>
-        private bool IsAllowedBitnessMismatch(int expectedOptions, int receivedOptions)
+        /// The .NET task host child tolerates a parent that did not emit an architecture bit
+        /// on the wire (which is indistinguishable from x86) when the child itself is x64 or
+        /// arm64. This is the .NET Framework parent → .NET SDK child scenario: the parent
+        /// typically cannot describe the SDK child's architecture, but the launched task host
+        /// process is whatever the SDK ships (x64 on Windows-x64, arm64 on Windows-arm64).
+        ///
+        /// True cross-arch mismatches (e.g. parent sent X64 but child expects Arm64, or vice
+        /// versa) remain rejected.
+        ///
+        /// The lower 24 bits (mask 0x00FFFFFF) carry the HandshakeOptions flags; the upper
+        /// byte carries the handshake version and is ignored here.
+        /// </summary>
+        internal static bool IsAllowedBitnessMismatch(int expectedOptions, int receivedOptions)
         {
             var expectedNodeType = (HandshakeOptions)(expectedOptions & 0x00FFFFFF);
             var receivedNodeType = (HandshakeOptions)(receivedOptions & 0x00FFFFFF);
 
-            // not X64 or Arm64 means we are running on x86
+            // not X64 or Arm64 means the wire-form architecture is x86 (or no arch bit).
             bool receivedIsX86 = !Handshake.IsHandshakeOptionEnabled(receivedNodeType, HandshakeOptions.X64) &&
                                  !Handshake.IsHandshakeOptionEnabled(receivedNodeType, HandshakeOptions.Arm64);
 
             bool expectedIsX64 = Handshake.IsHandshakeOptionEnabled(expectedNodeType, HandshakeOptions.X64);
+            bool expectedIsArm64 = Handshake.IsHandshakeOptionEnabled(expectedNodeType, HandshakeOptions.Arm64);
 
-            return receivedIsX86 && expectedIsX64;
+            return receivedIsX86 && (expectedIsX64 || expectedIsArm64);
         }
 #endif