dotnet · radekdoulik · Jun 15, 2026 · Jun 16, 2026 · Jun 16, 2026 · Jun 16, 2026
diff --git a/eng/pipelines/libraries/outerloop.yml b/eng/pipelines/libraries/outerloop.yml
@@ -107,3 +107,47 @@ extends:
                       testScope: outerloop
                       creator: dotnet-bot
                       extraHelixArguments: /p:BuildTargetFramework=net481
+
+        # WebAssembly CoreCLR - run the library test suites on Chrome with the slow
+        # [OuterLoop] tests enabled. Uses -testscope all (not outerloop) on purpose:
+        # the outerloop scope adds a '-trait category=OuterLoop' include filter, and the
+        # wasm test host (xharness) treats a matching include as "always run", short-
+        # circuiting the '-notrait category=failing' exclude. That leaks [OuterLoop]
+        # tests from classes disabled via [ConditionalClass] (e.g. on
+        # IsMultithreadingSupported) onto single-threaded wasm, where they block the
+        # one thread and hang the job. -testscope all carries no OuterLoop include, so
+        # the failing-tagged classes are skipped as intended; it also covers the slow
+        # suites' innerloop tests.
+        # Gated on includeLinuxOuterloop (browser_wasm builds on a Linux host) so the
+        # wasm suites run in the umbrella and -linux outerloop definitions only, not in
+        # every platform-split (-windows/-osx) outerloop definition.
+        - ${{ if eq(variables['includeLinuxOuterloop'], true) }}:
+          - template: /eng/pipelines/common/platform-matrix.yml
+            parameters:
+              jobTemplate: /eng/pipelines/common/global-build-job.yml
+              helixQueuesTemplate: /eng/pipelines/libraries/helix-queues-setup.yml
+              buildConfig: Release
+              runtimeFlavor: coreclr
+              platforms:
+              - browser_wasm
+              jobParameters:
+                testScope: all
+                nameSuffix: LibraryTestsCoreCLR
+                buildArgs: -s clr+libs+host+packs+libs.tests -c $(_BuildConfig) -testscope all /p:ArchiveTests=true /p:BrowserHost=$(_hostedOs) /p:InstallChromeForTests=true /p:AotHostArchitecture=x64 /p:AotHostOS=$(_hostedOS) /maxcpucount:1
+                # This lane runs the slow suites with a 04:00:00 per-suite xharness
+                # timeout and a per-batch Helix cap of 300 min (see
+                # eng/testing/tests.wasm.targets and src/libraries/sendtohelix-browser.targets).
+                # The job waits synchronously for the slowest batch, so it must outlast that
+                # batch plus build/queue overhead.
+                timeoutInMinutes: 600
+                includeAllPlatforms: ${{ variables['isRollingBuild'] }}
+                # extra steps, run tests
+                postBuildSteps:
+                  - template: /eng/pipelines/libraries/helix.yml
+                    parameters:
+                      scenarios:
+                      - WasmTestOnChrome
+                      testScope: all
+                      creator: dotnet-bot
+                      testRunNamePrefixSuffix: CoreCLR_$(_BuildConfig)
+                      extraHelixArguments: /p:BrowserHost=$(_hostedOs)
diff --git a/eng/testing/tests.wasm.targets b/eng/testing/tests.wasm.targets
@@ -35,6 +35,12 @@
     <_ShellCommandSeparator Condition="'$(OS)' == 'Windows_NT'">&amp;&amp;</_ShellCommandSeparator>
     <_ShellCommandSeparator Condition="'$(OS)' != 'Windows_NT'">&amp;&amp;</_ShellCommandSeparator>
     <XUnitUseRandomizedTestOrderer Condition="'$(XUnitUseRandomizedTestOrderer)' == '' and '$(IsTestProject)' == 'true'">true</XUnitUseRandomizedTestOrderer>
+    <!-- Outerloop wasm/CoreCLR runs the slow [OuterLoop] suites; give each suite a much larger
+         per-suite xharness timeout so a long suite is not killed mid-run (xharness exit code 71).
+         Innerloop keeps the existing 01:30:00 (CoreCLR) / 00:30:00 budgets. Keep this in sync with
+         the per-batch and AzDO job timeouts in src/libraries/sendtohelix-browser.targets and
+         eng/pipelines/libraries/outerloop.yml (suite <= batch <= job). -->
+    <WasmXHarnessTestsTimeout Condition="'$(WasmXHarnessTestsTimeout)' == '' and '$(RuntimeFlavor)' == 'CoreCLR' and ('$(TestScope)' == 'outerloop' or '$(TestScope)' == 'all')">04:00:00</WasmXHarnessTestsTimeout>
     <WasmXHarnessTestsTimeout Condition="'$(WasmXHarnessTestsTimeout)' == '' and '$(RuntimeFlavor)' == 'CoreCLR'">01:30:00</WasmXHarnessTestsTimeout>
     <WasmXHarnessTestsTimeout Condition="'$(WasmXHarnessTestsTimeout)' == ''">00:30:00</WasmXHarnessTestsTimeout>
     <RunWorkingDirectory>$(BundleDir)</RunWorkingDirectory>

diff --git a/src/libraries/Common/tests/System/Net/Http/ResponseStreamTest.cs b/src/libraries/Common/tests/System/Net/Http/ResponseStreamTest.cs
@@ -565,6 +565,10 @@ public async Task BrowserHttpHandler_StreamingRequest_CancelRequest(bool cancelA
 
         [OuterLoop]
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser))]
+        // Browser+CoreCLR currently surfaces a TaskCanceledException (100s HttpClient.Timeout)
+        // instead of the expected HttpRequestException ("TypeError: Failed to fetch"); the
+        // CoreCLR-wasm streaming-request-over-HTTP/1 path differs from Mono-wasm (which passes).
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129758", typeof(PlatformDetection), nameof(PlatformDetection.IsCoreCLR))]
         public async Task BrowserHttpHandler_StreamingRequest_Http1Fails()
         {
             var WebAssemblyEnableStreamingRequestKey = new HttpRequestOptionsKey<bool>("WebAssemblyEnableStreamingRequest");

diff --git a/src/libraries/System.Collections.Immutable/tests/Frozen/FrozenDictionaryTests.cs b/src/libraries/System.Collections.Immutable/tests/Frozen/FrozenDictionaryTests.cs
@@ -422,6 +422,7 @@ public void ContainsKey_WithNonAscii(int percentageWithNonAscii)
 
         [Fact]
         [OuterLoop]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129973", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsCoreCLR))]
         public void ToFrozenDictionary_WithExtremelyLargeStrings()
         {
             // Test case with extremely large strings that exceed length bucket boundaries.

diff --git a/src/libraries/System.Collections.Immutable/tests/Frozen/FrozenSetTests.cs b/src/libraries/System.Collections.Immutable/tests/Frozen/FrozenSetTests.cs
@@ -379,6 +379,7 @@ public void Contains_WithNonAscii(int percentageWithNonAscii)
 
         [Fact]
         [OuterLoop]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129973", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsCoreCLR))]
         public void ToFrozenSet_WithExtremelyLargeStrings()
         {
             // Test case with extremely large strings that exceed length bucket boundaries.

diff --git a/src/libraries/System.Console/tests/WindowAndCursorProps.cs b/src/libraries/System.Console/tests/WindowAndCursorProps.cs
@@ -312,6 +312,7 @@ public void BeepWithFrequency_Unix_ThrowsPlatformNotSupportedException()
 
     [Fact]
     [OuterLoop] // clears the screen, not very inner-loop friendly
+    [SkipOnPlatform(TestPlatforms.Browser, "Console.Clear is not supported on Browser")]
     public static void Clear_Invoke_Success()
     {
         if (!OperatingSystem.IsWindows() || (!Console.IsInputRedirected && !Console.IsOutputRedirected))

diff --git a/src/libraries/System.Linq.Expressions/tests/CompilerTests.cs b/src/libraries/System.Linq.Expressions/tests/CompilerTests.cs
@@ -14,6 +14,9 @@ public static class CompilerTests
         [Theory]
         [ClassData(typeof(CompilationTypes))]
         [OuterLoop("Takes over a minute to complete")]
+        // Compiling a 10,000-deep expression tree takes hours on the single-threaded wasm
+        // interpreter, exceeding the test timeout. Still covered on Mono and other platforms.
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129973", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsCoreCLR))]
         public static void CompileDeepTree_NoStackOverflow(bool useInterpreter)
         {
             var e = (Expression)Expression.Constant(0);

diff --git a/...ies/System.Linq.Parallel/tests/Combinatorial/CancellationParallelQueryCombinationTests.cs b/...ies/System.Linq.Parallel/tests/Combinatorial/CancellationParallelQueryCombinationTests.cs
@@ -603,7 +603,7 @@ public static void Min_OperationCanceledException_PreCanceled(Labeled<Func<Paral
             AssertThrows.AlreadyCanceled(source => operation.Item(source, () => { }).Min());
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))] // Observing "eventual" cancellation requires concurrent workers; unsupported on single-threaded platforms.
         [ActiveIssue("https://github.com/dotnet/runtime/issues/22626", TestPlatforms.Linux)]
         [MemberData(nameof(UnaryCancelingOperators))]
         [MemberData(nameof(BinaryCancelingOperators))]

diff --git a/src/libraries/System.Linq.Parallel/tests/DegreeOfParallelismTests.cs b/src/libraries/System.Linq.Parallel/tests/DegreeOfParallelismTests.cs
@@ -47,7 +47,7 @@ public static void DegreeOfParallelism(Labeled<ParallelQuery<int>> labeled, int
             Assert.Equal(Functions.SumRange(0, count), labeled.Item.WithDegreeOfParallelism(degree).Sum());
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))] // Coordinates work across threads via a Barrier; deadlocks/fails on single-threaded platforms.
         [MemberData(nameof(DegreeData), new[] { 1, 4, 32 }, new int[] { /* same as count */ })]
         [OuterLoop]
         public static void DegreeOfParallelism_Barrier(Labeled<ParallelQuery<int>> labeled, int count, int degree)
@@ -59,7 +59,7 @@ public static void DegreeOfParallelism_Barrier(Labeled<ParallelQuery<int>> label
             }
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))] // Pipelined enumeration relies on concurrent producer threads; unsupported on single-threaded platforms.
         [MemberData(nameof(DegreeData), new int[] { /* Sources.OuterLoopCount */ }, new[] { 1, 4, 64, 128 })]
         [OuterLoop]
         public static void DegreeOfParallelism_Pipelining(Labeled<ParallelQuery<int>> labeled, int count, int degree)
@@ -74,7 +74,7 @@ public static void DegreeOfParallelism_Pipelining(Labeled<ParallelQuery<int>> la
             }
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))] // Pipelined enumeration relies on concurrent producer threads; unsupported on single-threaded platforms.
         [MemberData(nameof(DegreeData), new[] { 1, 4 }, new int[] { /* same as count */ })]
         [MemberData(nameof(DegreeData), new[] { 32 }, new[] { 4 })]
         [OuterLoop]

diff --git a/src/libraries/System.Linq.Parallel/tests/ExchangeTests.cs b/src/libraries/System.Linq.Parallel/tests/ExchangeTests.cs
@@ -105,7 +105,7 @@ public static void Partitioning_Default(Labeled<ParallelQuery<int>> labeled, int
             }
         }
 
-        [Theory]
+        [ConditionalTheory] // Delegates to Partitioning_Default, which skips (SkipTestException) when multithreading is unavailable.
         [OuterLoop]
         [MemberData(nameof(PartitioningData), new int[] { /* Sources.OuterLoopCount */ })]
         public static void Partitioning_Default_Longrunning(Labeled<ParallelQuery<int>> labeled, int count, int partitions)
@@ -129,7 +129,7 @@ public static void Partitioning_Striped(Labeled<ParallelQuery<int>> labeled, int
             }
         }
 
-        [Theory]
+        [ConditionalTheory] // Delegates to Partitioning_Striped, which skips (SkipTestException) when multithreading is unavailable.
         [OuterLoop]
         [MemberData(nameof(PartitioningData), new int[] { /* Sources.OuterLoopCount */ })]
         public static void Partitioning_Striped_Longrunning(Labeled<ParallelQuery<int>> labeled, int count, int partitions)

diff --git a/src/libraries/System.Linq.Parallel/tests/System.Linq.Parallel.Tests.csproj b/src/libraries/System.Linq.Parallel/tests/System.Linq.Parallel.Tests.csproj
@@ -5,6 +5,15 @@
     <!-- https://github.com/dotnet/runtime/issues/126862 -->
     <EnableTrimAnalyzer>false</EnableTrimAnalyzer>
   </PropertyGroup>
+  <!-- The full [OuterLoop] combinatorial matrix expands to ~195k cases on a single-threaded
+       wasm browser session: it produces a ~72 MB test-results file that exceeds xharness's
+       30 MB upload limit and grows the wasm heap to ~1.3 GB, crashing the browser tab at
+       teardown. PLINQ has no real parallelism on a single thread, so this OuterLoop coverage
+       adds little value here. Skip OuterLoop for this suite on the wasm/CoreCLR outerloop lane
+       only; its innerloop tests still run and all other lanes are unaffected. -->
+  <PropertyGroup Condition="'$(TargetOS)' == 'browser' and '$(RuntimeFlavor)' == 'CoreCLR' and ('$(TestScope)' == 'all' or '$(TestScope)' == 'outerloop')">
+    <WithoutCategories>$(WithoutCategories);OuterLoop</WithoutCategories>
+  </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonTestPath)System\Diagnostics\Tracing\TestEventListener.cs"
              Link="Common\System\Diagnostics\Tracing\TestEventListener.cs" />

diff --git a/src/libraries/System.Linq.Parallel/tests/WithCancellationTests.cs b/src/libraries/System.Linq.Parallel/tests/WithCancellationTests.cs
@@ -82,7 +82,7 @@ public static IEnumerable<object[]> ProducerBlocked_Data()
         //
         // To specifically verify this test, it was checked that the Async channels were blocked in TryEnqueChunk before Dispose() was called
         //  -> this was verified manually, but is not simple to automate
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))] // Exercises blocked producer/consumer threads on the async channel; unsupported on single-threaded platforms.
         [OuterLoop] // explicit timeouts / delays
         [MemberData(nameof(ProducerBlocked_Data))]
         public static void WithCancellation_DisposedEnumerator_ChannelCancellation_ProducerBlocked(Labeled<ParallelQuery<int>> labeled, int count)

diff --git a/src/libraries/System.Memory/tests/EncodingExtensions/EncodingExtensionsTests.cs b/src/libraries/System.Memory/tests/EncodingExtensions/EncodingExtensionsTests.cs
@@ -354,6 +354,7 @@ public static void GetBytes_Encoding_ReadOnlySequence_IBufferWriter_SingleSegmen
         [Fact]
         [OuterLoop] // this test takes ~10 seconds on modern hardware since it operates over GBs of data
         [ActiveIssue("https://github.com/dotnet/runtime/issues/79883", TestRuntimes.Mono)]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129973", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsCoreCLR))]
         public static void GetBytes_Encoding_ReadOnlySequence_IBufferWriter_LargeMultiSegment()
         {
             ReadOnlySequence<char> sequence = GetLargeRepeatingReadOnlySequence<char>(AllScalarsAsUtf16, 1500); // ~ 3.2bn chars of UTF-16 input
@@ -509,6 +510,7 @@ public static void GetChars_Encoding_ReadOnlySequence_IBufferWriter_SingleSegmen
 
         [Fact]
         [OuterLoop] // this test takes ~10 seconds on modern hardware since it operates over GBs of data
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129973", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsCoreCLR))]
         public static void GetChars_Encoding_ReadOnlySequence_IBufferWriter_LargeMultiSegment()
         {
             ReadOnlySequence<byte> sequence = GetLargeRepeatingReadOnlySequence<byte>(AllScalarsAsUtf8, 1500); // ~ 6.5bn bytes of UTF-8 input

diff --git a/src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientTest.cs b/src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientTest.cs
@@ -1123,7 +1123,7 @@ await IgnoreExceptions(async () =>
 
         [Fact]
         [OuterLoop]
-        [SkipOnPlatform(TestPlatforms.Android, "Synchronous Send is not supported on Android")]
+        [SkipOnPlatform(TestPlatforms.Android | TestPlatforms.Browser, "Synchronous Send is not supported on Android or Browser")]
         public async Task Send_TimeoutRequestContent_Throws()
         {
             var semaphore = new SemaphoreSlim(0);

diff --git a/src/libraries/System.Net.Http/tests/FunctionalTests/MetricsTest.cs b/src/libraries/System.Net.Http/tests/FunctionalTests/MetricsTest.cs
@@ -372,6 +372,7 @@ public Task RequestDuration_Success_Recorded(string method, HttpStatusCode statu
 
         [OuterLoop("Uses external server.")]
         [ConditionalFact]
+        [SkipOnPlatform(TestPlatforms.Browser, "NameResolution (System.Net.Dns) is not supported on Browser")]
         public async Task ExternalServer_DurationMetrics_Recorded()
         {
             if (UseVersion == HttpVersion.Version30)

diff --git a/src/libraries/System.Net.WebSockets.Client/tests/ClientWebSocketOptionsTests.cs b/src/libraries/System.Net.WebSockets.Client/tests/ClientWebSocketOptionsTests.cs
@@ -45,6 +45,7 @@ public static void Proxy_Roundtrips()
         }
 
         [OuterLoop]
+        [SkipOnPlatform(TestPlatforms.Browser, "Proxy not supported on browser")]
         [ConditionalTheory(typeof(ClientWebSocketOptionsTests), nameof(WebSocketsSupported)), MemberData(nameof(EchoServers))]
         public async Task Proxy_SetNull_ConnectsSuccessfully(Uri server)
         {

diff --git a/src/libraries/System.Private.Xml.Linq/tests/xNodeBuilder/FunctionalTests.cs b/src/libraries/System.Private.Xml.Linq/tests/xNodeBuilder/FunctionalTests.cs
@@ -18,6 +18,11 @@ public partial class XNodeBuilderFunctionalTests : TestModule
         // Test Module
         [Fact]
         [OuterLoop]
+        // This ModuleCore entry point runs the full XNodeBuilder writer suite. On wasm the runtime
+        // fails to shut down cleanly after it completes, so the test harness hangs until it times
+        // out. It is [OuterLoop], so it never ran on single-threaded wasm before; gate it off
+        // CoreCLR Browser (Mono keeps running) to keep the lane green while preserving coverage elsewhere.
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129973", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsCoreCLR))]
         public static void RunTests()
         {
             TestInput.CommandLine = "";

diff --git a/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/GCTests.cs b/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/GCTests.cs
@@ -297,7 +297,7 @@ private class TestObject
         }
 
         [OuterLoop]
-        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsPreciseGcSupported))]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsPreciseGcSupported), nameof(PlatformDetection.IsMultithreadingSupported))] // Races finalization across threads; meaningless on single-threaded platforms.
         public static void WaitForPendingFinalizersRaces()
         {
             Task.Run(Test);

diff --git a/src/libraries/System.Runtime/tests/System.Threading.Tasks.Tests/MethodCoverage.cs b/src/libraries/System.Runtime/tests/System.Threading.Tasks.Tests/MethodCoverage.cs
@@ -22,7 +22,7 @@ public static void TaskWait_MaxInt32()
         }
 
         //EH
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
         [OuterLoop]
         public static void TaskContinuation()
         {

diff --git a/...me/tests/System.Threading.Tasks.Tests/System.Runtime.CompilerServices/TaskAwaiterTests.cs b/...me/tests/System.Threading.Tasks.Tests/System.Runtime.CompilerServices/TaskAwaiterTests.cs
@@ -262,7 +262,7 @@ public static void GetResult_Completed_Success()
         }
 
         [OuterLoop]
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
         public static void GetResult_NotCompleted_BlocksUntilCompletion()
         {
             var tcs = new TaskCompletionSource<bool>();

diff --git a/src/libraries/System.Runtime/tests/System.Threading.Tasks.Tests/Task/TaskAPMTest.cs b/src/libraries/System.Runtime/tests/System.Threading.Tasks.Tests/Task/TaskAPMTest.cs
@@ -64,7 +64,7 @@ public void WaitUntilCompleteTechnique(bool hasReturnType)
             Assert.False(asyncResult.CompletedSynchronously, "Should not have completed synchronously.");
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
         [OuterLoop]
         [InlineData(true)]
         [InlineData(false)]
@@ -89,7 +89,7 @@ public void PollUntilCompleteTechnique(bool hasReturnType)
             Assert.False(asyncResult.CompletedSynchronously, "Should not have completed synchronously.");
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
         [OuterLoop]
         [InlineData(true)]
         [InlineData(false)]

diff --git a/...ies/System.Runtime/tests/System.Threading.Tasks.Tests/TaskScheduler/TaskSchedulerTests.cs b/...ies/System.Runtime/tests/System.Threading.Tasks.Tests/TaskScheduler/TaskSchedulerTests.cs
@@ -18,7 +18,7 @@ public static class TaskSchedulerTests
     {
         // Just ensure we eventually complete when many blocked tasks are created.
         [OuterLoop]
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
         public static void RunBlockedInjectionTest()
         {
             Debug.WriteLine("* RunBlockedInjectionTest() -- if it deadlocks, it failed");
@@ -209,7 +209,7 @@ public static void RunBuggySchedulerTests()
             }
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
         [OuterLoop]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/89921", typeof(PlatformDetection), nameof(PlatformDetection.IsAlpine), nameof(PlatformDetection.IsMonoRuntime))]
         public static void RunSynchronizationContextTaskSchedulerTests()

diff --git a/src/libraries/System.Runtime/tests/System.Threading.Timer.Tests/TimerFiringTests.cs b/src/libraries/System.Runtime/tests/System.Threading.Timer.Tests/TimerFiringTests.cs
@@ -106,7 +106,7 @@ public void Timer_FiresOnlyOnce_OnDueTime_With_InfinitePeriod()
         }
 
         [OuterLoop("Waits seconds")]
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))] // blocking waits and timer firing require threads, unavailable on single-threaded platforms
         public void Timer_ChangeToDelete_DoesntFire()
         {
             RetryHelper.Execute(() =>

diff --git a/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/BitStackTests.cs b/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/BitStackTests.cs
@@ -86,6 +86,7 @@ public static void BitStackPushPop(int bitLength)
         [OuterLoop]
         [InlineData(3_200_000)]
         [InlineData(int.MaxValue / 32 + 1)]    // 67_108_864
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/129973", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsCoreCLR))]
         public static void BitStackPushPopLarge(int bitLength)
         {
             BitStack bitStack = default;
-Original file line number
+Diff line change
@@ Expand Up / @@ -22,7 +22,7 @@ public static void TaskWait_MaxInt32() @@
             }
             //EH
-            [Fact]
+            [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
             [OuterLoop]
             public static void TaskContinuation()
             {
@@ Expand Down @@