diff --git a/tests/OpenCode.Sdk.Tests/Hosting/BackgroundService/Discovery/LoopbackTransportTests.cs b/tests/OpenCode.Sdk.Tests/Hosting/BackgroundService/Discovery/LoopbackTransportTests.cs
index ebd7da87b..4594a8448 100644
--- a/tests/OpenCode.Sdk.Tests/Hosting/BackgroundService/Discovery/LoopbackTransportTests.cs
+++ b/tests/OpenCode.Sdk.Tests/Hosting/BackgroundService/Discovery/LoopbackTransportTests.cs
@@ -1,9 +1,10 @@
+using System.Globalization;
 using System.Net;
 using System.Net.Sockets;
 using OpenCode.Sdk.Internal;
 using OpenCode.Sdk.Tests.Hosting.BackgroundService.Support;
-using OpenCode.Sdk.Tests.Support;
 #if !NET
+using OpenCode.Sdk.Tests.Support;
 using TUnit.Assertions.Enums;
 #endif
 
@@ -22,6 +23,13 @@ namespace OpenCode.Sdk.Tests.Hosting.BackgroundService.Discovery;
 /// </summary>
 public sealed class LoopbackTransportTests
 {
+    /// <summary>The first port the closed endpoint tries; below every platform's ephemeral range.</summary>
+    private const int FirstClosedPortCandidate = 20000;
+
+    private const int ClosedPortCandidates = 1000;
+
+    private static int s_lastClosedPortCandidate = FirstClosedPortCandidate - 1;
+
     [Test]
     [Arguments("http://127.0.0.1:4096/")]
     [Arguments("http://[::1]:4096/")]
@@ -71,7 +79,7 @@ public async Task CreateHandler_Should_Keep_The_Default_Connect_For_Another_Endp
     [Test]
     public async Task CreateHandler_Should_Connect_A_Windows_Loopback_Exchange_On_A_Socket_That_Took_The_Option()
     {
-        var endpoint = await ClosedEndpointAsync();
+        var endpoint = ClosedEndpoint();
         var connector = new RecordingLoopbackConnector();
         using var handler = new LoopbackTransport(connector).CreateHandler(endpoint);
         using var invoker = new HttpMessageInvoker(handler, disposeHandler: false);
@@ -100,7 +108,7 @@ public async Task CreateHandler_Should_Connect_A_Windows_Loopback_Exchange_On_A_
     [Test]
     public async Task IsListeningAsync_Should_Report_A_Refused_Loopback_Port_As_Not_Listening()
     {
-        var endpoint = await ClosedEndpointAsync();
+        var endpoint = ClosedEndpoint();
         var connector = new RecordingLoopbackConnector();
 
         var listening = await new LoopbackTransport(connector).IsListeningAsync(endpoint, CancellationToken.None);
@@ -142,7 +150,7 @@ public async Task IsListeningAsync_Should_Connect_Without_The_Token()
     [Test]
     public async Task IsListeningAsync_Should_Report_A_Refusal_That_Completes_While_The_Token_Is_Cancelled_As_Not_Listening()
     {
-        var endpoint = await ClosedEndpointAsync();
+        var endpoint = ClosedEndpoint();
         using var bound = new CancellationTokenSource();
         var connector = new ScriptedLoopbackConnector(bound, new SocketException((int)SocketError.ConnectionRefused));
 
@@ -159,7 +167,7 @@ public async Task IsListeningAsync_Should_Report_A_Refusal_That_Completes_While_
     [MethodDataSource(nameof(ClosedSocketFailures))]
     public async Task IsListeningAsync_Should_Throw_The_Token_Cancellation_For_A_Connect_The_Token_Cut_Short(Exception failure)
     {
-        var endpoint = await ClosedEndpointAsync();
+        var endpoint = ClosedEndpoint();
         using var bound = new CancellationTokenSource();
         var connector = new ScriptedLoopbackConnector(bound, failure);
 
@@ -251,11 +259,42 @@ public async Task DisableSynRetransmission_Should_Be_Accepted_On_Windows_And_Not
 
     private static LoopbackTransport PlatformTransport() => new(new LoopbackSocketConnector());
 
-    /// <summary>A loopback port that was listening a moment ago and is closed now.</summary>
-    private static async Task<Uri> ClosedEndpointAsync()
+    /// <summary>
+    /// A loopback port nothing listens on, found free by a bind and released. It lies below every
+    /// platform's ephemeral range (Windows and macOS from 49152, Linux from 32768), so no parallel
+    /// test binding port 0 can be handed it and turn the refusal into a connect. A port a disposed
+    /// server had just released from that range could be (#421). Every call takes its own
+    /// candidates, so a parallel caller is never binding the port this one connects to: macOS times
+    /// a bound, non-listening port out instead of refusing it.
+    /// </summary>
+    private static Uri ClosedEndpoint()
     {
-        await using var server = LoopbackHttpServer.Start(static _ => new LoopbackHttpResponse { StatusCode = HttpStatusCode.OK });
-        return server.Endpoint;
+        for (var attempt = 0; attempt < ClosedPortCandidates; attempt++)
+        {
+            var port = Interlocked.Increment(ref s_lastClosedPortCandidate);
+            if (IsFree(port))
+            {
+                return new Uri($"http://127.0.0.1:{port.ToString(CultureInfo.InvariantCulture)}");
+            }
+        }
+
+        throw new InvalidOperationException("No free loopback port below the ephemeral ranges was found for the closed endpoint.");
+    }
+
+    /// <summary>Whether a bind to the loopback port succeeds; the probe socket is released at once.</summary>
+    private static bool IsFree(int port)
+    {
+        using var probe = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        try
+        {
+            probe.Bind(new IPEndPoint(IPAddress.Loopback, port));
+            return true;
+        }
+        catch (SocketException exception) when (exception.SocketErrorCode is SocketError.AddressAlreadyInUse or SocketError.AccessDenied)
+        {
+            // Taken, or inside a range Windows has excluded (WinNAT, Hyper-V); any other fault propagates.
+            return false;
+        }
     }
 
     private static bool UseProxy(HttpMessageHandler handler) => handler switch