fix: make Runtime Async opt-in and release 2.0.1

.editorconfig

@@ -1,4 +1,4 @@
-# Purview.BuildSdk 2.0.0 - managed file, do not edit
+# Purview.BuildSdk 2.0.1 - managed file, do not edit
 #
 # This file is written by Purview.BuildSdk and is replaced whenever the SDK version changes. The version
 # on the first line is how the SDK recognises a file it owns: a .editorconfig without that marker is

CHANGELOG.md

@@ -11,6 +11,18 @@ release summarise what the commits recorded rather than what was written at the
 
 ## [Unreleased]
 
+## [2.0.1] - 2026-11-12
+
+### Changed
+
+- **Runtime Async is off by default.** A `net11.0`+ target no longer compiles with
+  `<Features>runtime-async=on</Features>` unless asked: it changes how every `await` compiles, so it
+  is an opt-in rather than a baseline. Set `PurviewRuntimeAsync=true` — or `UseRuntimeAsync=true`,
+  the name the .NET team uses — to enable it; an explicit `false` under either name keeps it off.
+  This reverts the 2.0.0 default, so a repository that relied on it should opt in.
+
+## [2.0.0] - 2026-11-11
+
 ### Added
 
 - **`ServerShared` projects.** A `ServerShared*` project is referenced automatically by every

README.md

@@ -13,7 +13,7 @@ A reusable MSBuild SDK NuGet package that delivers standardised .NET project def
 | Feature | Detail |
 | -- | -- |
 | **Project type detection** | `IsCSharpProject`, `IsTestProject`, `IsSharedTestingProject`, `IsServerSharedProject`, `IsClientSideProject`, `IsContainerProject`, `IsWebSdkProject`, `IsAspireHostProject`, `IsBlazorWebAssemblyProject`, `IsBffProject`, … |
-| **.NET 11 opt-ins** | A `net11.0` target compiles with Runtime Async (`runtime-async=on`) and the C# 15 updated memory-safety rules (`MemorySafetyRules=2`), while `AllowUnsafeBlocks` stays at its compiler default (`false`); builds use MSBuild's multithreaded mode through a bootstrapped `Directory.Build.rsp` (see [.NET 11 features](#net-11-features)) |
+| **.NET 11 opt-ins** | A `net11.0` target compiles with the C# 15 updated memory-safety rules (`MemorySafetyRules=2`) and, when opted in with `PurviewRuntimeAsync=true`, Runtime Async (`runtime-async=on`), while `AllowUnsafeBlocks` stays at its compiler default (`false`); builds use MSBuild's multithreaded mode through a bootstrapped `Directory.Build.rsp` (see [.NET 11 features](#net-11-features)) |
 | **ServerShared** | A `ServerShared*` project is referenced automatically by every non-client-side project, and never by a WebAssembly client, browser-safe library, Razor class library or `*.Client`-style project; `PRSHR0001`/`PRSHR0002` report a reference that would still put it in a browser download (see [ServerShared](#servershared)) |
 | **Blazor and BFF** | Blazor WebAssembly clients, Blazor Web App hosts and BFF hosts are detected; `PurviewBrowserSafe` makes a library the client downloads fail the build on trim, AOT and browser-unsupported APIs, and the client checks every reference sets it (see [Blazor WebAssembly and BFF](#blazor-webassembly-and-bff)) |
 | **C# defaults** | `net10.0;net11.0` TFMs (overridable), `LangVersion=latest` (C# 15), `Nullable=enable`, `ImplicitUsings=enable`, deterministic builds |
@@ -44,7 +44,7 @@ A reusable MSBuild SDK NuGet package that delivers standardised .NET project def
     "runner": "Microsoft.Testing.Platform"
   },
   "msbuild-sdks": {
-    "Purview.BuildSdk": "2.0.0"
+    "Purview.BuildSdk": "2.0.1"
   }
 }
 ```
@@ -203,7 +203,7 @@ to a low-importance message) and only a copy that still fails after every retry
 The bootstrapped `.editorconfig` opens with a version marker:
 
 ```editorconfig
-# Purview.BuildSdk 2.0.0 - managed file, do not edit
+# Purview.BuildSdk 2.0.1 - managed file, do not edit
 ```
 
 That line is how the SDK recognises a file **it** wrote. When the SDK version changes, a file carrying
@@ -378,7 +378,7 @@ Version detection logging is disabled by default. Set `VersionDetectionLogEnable
 | `AssemblyIdentifierClassName` | `<project name minus NamespacePrefix, dots removed>Assembly` | Overrides the name of the generated assembly identifier marker class. |
 | `AutoIncludeUsings` | `true` | Controls SDK-added global usings for `NamespacePrefix` and `RootNamespace`. |
 | `MemorySafetyRules` | `2` | C# 15 memory-safety tier for `net11.0`+ targets: `0` off, `1` preview pointer syntax, `2` updated rules (`updated-memory-safety-rules`, the default). `AllowUnsafeBlocks` is never changed. |
-| `PurviewRuntimeAsync` | `true` | Compiles `net11.0`+ targets with `<Features>runtime-async=on</Features>`. `UseRuntimeAsync=false` also opts out. |
+| `PurviewRuntimeAsync` | `false` | Set to `true` to compile `net11.0`+ targets with `<Features>runtime-async=on</Features>`. `UseRuntimeAsync=true` also opts in; an explicit `false` under either name keeps it off. |
 | `IsClientSideProject` | *(detected)* | `true` for a project whose assemblies are downloaded to a browser — a WebAssembly client, `PurviewBrowserSafe` library, Razor class library, or `*.Client`/`*.Browser`/`*.Wasm`/`*.WebAssembly`/`*.Components` name. Set explicitly to override; a `ServerShared` project is never added to a client-side project. |
 | `PurviewAutoServerSharedReference` | `true` | Adds `../ServerShared*/ServerShared*.csproj` to a non-client-side project. `false` removes the automatic reference. |
 | `DisablePurviewServerSharedReferenceCheck` | `false` | Suppresses `PRSHR0001`/`PRSHR0002`. |
@@ -583,7 +583,7 @@ later) target, so the `net10.0` leg of the default `net10.0;net11.0` set is unaf
 
 | Feature | Applied | Why |
 | -- | -- | -- |
-| Runtime Async V2 | `<Features>runtime-async=on</Features>` | .NET 11 no longer requires `EnablePreviewFeatures`, and the runtime libraries ship compiled with it. Cleaner live stack traces, lower await overhead, and continuations that skip `ExecutionContext` when nothing is ambient. Opt out with `PurviewRuntimeAsync=false` or `UseRuntimeAsync=false`. |
+| Runtime Async V2 (opt-in) | `<Features>runtime-async=on</Features>` when enabled | Enables Runtime Async for a target where the team has decided it is wanted: cleaner live stack traces, lower await overhead, and continuations that skip `ExecutionContext` when nothing is ambient. It is **off by default** because it changes how every `await` compiles; opt in with `PurviewRuntimeAsync=true` or `UseRuntimeAsync=true`. An explicit `false` under either name keeps it off. |
 | C# 15 updated memory-safety rules | `<Features>updated-memory-safety-rules</Features>` (`MemorySafetyRules=2`) | `unsafe` marks requires-unsafe APIs and the compiler records the choice in `MemorySafetyRulesAttribute`. Independent of `LangVersion`, so the default `latest` is enough. |
 | Unsafe code | **not** enabled | `AllowUnsafeBlocks` stays at the compiler default (`false`). The updated model is strictest with unsafe code disallowed; a project that needs pointers opts in itself. |
 | Multithreaded MSBuild | `Directory.Build.rsp` with `-mt` | Runs a build's projects concurrently in one process, which is 20–26% faster on large solutions per the MSBuild measurements. `-mt` is a host switch, so it cannot be set from a project; the response file is the only mechanism. `-nr:false` accompanies it because a node left by a non-`-mt` build can cache an `RoslynCodeTaskFactory` task with no assembly path and fail the next `-mt` build (`dotnet/msbuild#15213`); the short-lived server still gives Server GC. |

docs/wiki/Configuration-Reference.md

@@ -37,7 +37,7 @@ See [Version Detection](Version-Detection.md) for the full resolution rules.
 | `AssemblyIdentifierClassName` | `<project name minus NamespacePrefix, dots removed>Assembly` | Overrides the name of the generated assembly identifier marker class. |
 | `AutoIncludeUsings` | `true` | Controls SDK-added global usings for `NamespacePrefix` and `RootNamespace`. |
 | `MemorySafetyRules` | `2` | C# 15 memory-safety tier on `net11.0`+ targets: `0` off, `1` preview pointer syntax (sets `LangVersion=preview` when the project has not chosen one), `2` updated rules (`updated-memory-safety-rules`). An unrecognised value fails with `PRCFG0001`. `AllowUnsafeBlocks` is never changed. |
-| `PurviewRuntimeAsync` | `true` | Adds `<Features>runtime-async=on</Features>` on `net11.0`+ targets. `PurviewRuntimeAsync=false` or `UseRuntimeAsync=false` opts out. |
+| `PurviewRuntimeAsync` | `false` | Set to `true` to add `<Features>runtime-async=on</Features>` on `net11.0`+ targets. `UseRuntimeAsync=true` also opts in; an explicit `false` under either name keeps it off. |
 | `IsClientSideProject` | *(detected)* | `true` when the project is downloaded to a browser: a WebAssembly client, a `PurviewBrowserSafe` library, a Razor class library, or a `*.Client`/`*.Browser`/`*.Wasm`/`*.WebAssembly`/`*.Components` name. Set explicitly to override the detection either way. |
 | `PurviewAutoServerSharedReference` | `true` | Adds `../ServerShared*/ServerShared*.csproj` to every non-client-side project. `false` disables it. |
 | `DisablePurviewServerSharedReferenceCheck` | `false` | Turns off `PRSHR0001` and `PRSHR0002`. |
@@ -186,7 +186,7 @@ unchanged.
 
 | Property | Applied | Notes |
 | -- | -- | -- |
-| `PurviewRuntimeAsync` | `true` | `<Features>runtime-async=on</Features>`. `UseRuntimeAsync=false` also opts out. |
+| `PurviewRuntimeAsync` | `false` | When set to `true`, adds `<Features>runtime-async=on</Features>`. `UseRuntimeAsync=true` also opts in; an explicit `false` under either name keeps it off. |
 | `MemorySafetyRules` | `2` | `updated-memory-safety-rules`; `1` sets `LangVersion=preview` for the pointer-syntax tier; `0` disables. `PRCFG0001` on an unknown value. |
 | `AllowUnsafeBlocks` | *(compiler default: false)* | Never set by the SDK. |
 
@@ -301,7 +301,7 @@ read them through `build_property.<PropertyName>`:
 | `DisablePurviewServerSharedReferenceCheck` | When `true`, suppresses `PRSHR0001`/`PRSHR0002`. |
 | `PurviewServerSharedReferenceCheckAsError` | When `true`, reports `PRSHR0001`/`PRSHR0002` as errors. |
 | `MemorySafetyRules` | C# 15 memory-safety tier applied to `net11.0`+ targets. |
-| `PurviewRuntimeAsync` | When `true` (default), `net11.0`+ targets compile with `runtime-async=on`. |
+| `PurviewRuntimeAsync` | When `true`, `net11.0`+ targets compile with `runtime-async=on`. Defaults to `false`; `UseRuntimeAsync=true` also opts in. |
 | `PurviewMultiThreadedBuild` | When `true` (default), a managed `Directory.Build.rsp` enables MSBuild `-mt`. |
 | `EditorConfigFilePath` | Path to the SDK-provided `.editorconfig` injected into `@(EditorConfigFiles)`. |
 | `RepositoryEditorConfigFilePath` | Destination path for bootstrapping a physical repo-level `.editorconfig`. |

docs/wiki/Getting-Started.md

@@ -14,7 +14,7 @@ Add `Purview.BuildSdk` to the `msbuild-sdks` section so MSBuild can resolve the
     "runner": "Microsoft.Testing.Platform"
   },
   "msbuild-sdks": {
-    "Purview.BuildSdk": "2.0.0"
+    "Purview.BuildSdk": "2.0.1"
   }
 }
 ```

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "build-sdk",
-	"version": "2.0.0",
+	"version": "2.0.1",
 	"homepage": "https://purview.dev/projects/build-sdk/",
 	"bugs": {
 		"url": "https://github.com/purview-dev/build-sdk/issues"

skills/purview-buildsdk-configuration/SKILL.md

@@ -51,7 +51,7 @@ in `Directory.Build.props` unless the setting is deliberately project-specific.
 | `IsBffProject` | name ends `Bff` | Classifies a BFF host; `false` opts out |
 | `PurviewBrowserSafeReferenceCheckAsError` | `false` | Makes `PRBLZ0001` (a client reference that is not browser-safe) an error |
 | `MemorySafetyRules` | `2` | C# 15 memory-safety tier on `net11.0`+: `0` off, `1` preview syntax, `2` updated rules |
-| `PurviewRuntimeAsync` | `true` | `runtime-async=on` on `net11.0`+ |
+| `PurviewRuntimeAsync` | `false` | Opts a `net11.0`+ target into `runtime-async=on`; `UseRuntimeAsync=true` also opts in |
 | `IsClientSideProject` | *(computed)* | Whether the project's assemblies can reach a browser; overrides the `ServerShared` classification |
 | `PurviewAutoServerSharedReference` | `true` | Adds a sibling `ServerShared` reference to a non-client-side project |
 | `PurviewMultiThreadedBuild` | `true` | Bootstraps a `Directory.Build.rsp` enabling MSBuild `-mt` |

skills/purview-buildsdk-configuration/references/properties.md

@@ -41,7 +41,7 @@ consuming project supplies nothing.
 | Property | Default | Behaviour |
 | -- | -- | -- |
 | `MemorySafetyRules` | `2` | C# 15 memory-safety tier on `net11.0`+ targets: `0` off, `1` preview pointer syntax (sets `LangVersion=preview` when none was chosen), `2` `updated-memory-safety-rules`. Unknown values fail with `PRCFG0001`. Never touches `AllowUnsafeBlocks` |
-| `PurviewRuntimeAsync` | `true` | `runtime-async=on` on `net11.0`+ targets. `UseRuntimeAsync=false` also opts out |
+| `PurviewRuntimeAsync` | `false` | Set to `true` for `runtime-async=on` on `net11.0`+ targets. `UseRuntimeAsync=true` also opts in; an explicit `false` under either name keeps it off |
 | `PurviewMultiThreadedBuild` | `true` | Bootstraps a managed `Directory.Build.rsp` with `-mt`/`-nr:false`; `false` writes `-mt:false` |
 | `BootstrapMultiThreadedBuildRspToRepoRoot` | `true` | Whether that bootstrapped response file is written at all |
 | `IsClientSideProject` | *(computed)* | `true` for a WebAssembly client, `PurviewBrowserSafe` library, Razor class library, or `*.Client`/`*.Browser`/`*.Wasm`/`*.WebAssembly`/`*.Components` name. Explicit value wins |

src/src/BuildSdk/Sdk/Props/Defaults.props

@@ -196,7 +196,9 @@
 		     and PRSHR0002). Warning by default; set this to true to fail the build instead. -->
 		<PurviewServerSharedReferenceCheckAsError Condition="'$(PurviewServerSharedReferenceCheckAsError)' == ''"
 			>false</PurviewServerSharedReferenceCheckAsError>
-		<PurviewRuntimeAsync Condition="'$(PurviewRuntimeAsync)' == ''">true</PurviewRuntimeAsync>
+		<!-- Runtime Async is resolved per-TFM in Targets/Modern.targets, after the project body, so
+		     PurviewRuntimeAsync=true or UseRuntimeAsync=true works from the project file as well as
+		     from Directory.Build.props. It defaults to off: it changes how every await compiles. -->
 		<!-- MSBuild's single-process multithreaded build (-mt). Applied through the bootstrapped
 		     Directory.Build.rsp, because -mt is a build-host switch and cannot be set from a project. -->
 		<PurviewMultiThreadedBuild Condition="'$(PurviewMultiThreadedBuild)' == ''">true</PurviewMultiThreadedBuild>

src/src/BuildSdk/Sdk/Sdk.props

@@ -564,7 +564,7 @@
 			<Description>The C# 15 memory-safety tier: 0 off, 1 preview syntax, 2 updated rules (default). AllowUnsafeBlocks is left unset.</Description>
 		</CompilerVisibleProperty>
 		<CompilerVisibleProperty Include="PurviewRuntimeAsync">
-			<Description>When true (default), a net11.0+ project compiles with the runtime-async compiler feature. UseRuntimeAsync=false opts out.</Description>
+			<Description>When true, a net11.0+ project compiles with the runtime-async compiler feature. Defaults to false; UseRuntimeAsync=true also opts in.</Description>
 		</CompilerVisibleProperty>
 		<CompilerVisibleProperty Include="PurviewMultiThreadedBuild">
 			<Description>When true (default), boots a Directory.Build.rsp that enables MSBuild's multithreaded build (-mt).</Description>

src/src/BuildSdk/Sdk/Targets/Modern.targets

@@ -7,15 +7,15 @@
 	     property. That matters: TargetFrameworks defaults to net10.0;net11.0, so
 	     a switch that improves net11.0 must not be applied to the net10.0 leg.
 
-	     Two compiler features are enabled for a net11.0+ target:
+	     One compiler feature is enabled by default for a net11.0+ target, and
+	     a second is available as an opt-in:
 
-	       runtime-async=on          Runtime Async V2. .NET 11 no longer needs
-	                                 EnablePreviewFeatures for it, and the
-	                                 runtime libraries themselves ship built with
-	                                 it, so an application built without it runs
-	                                 against libraries that did opt in.
+	       runtime-async=on          Runtime Async V2. Off by default: it
+	                                 changes how every await compiles, so it is
+	                                 opted into with PurviewRuntimeAsync=true
+	                                 (or UseRuntimeAsync=true), never assumed.
 
-	       updated-memory-safety-rules
+	       updated-memory-safety-rules (default)
 	                                 The C# 15 memory-safety model: 'unsafe'
 	                                 marks requires-unsafe APIs and the caller
 	                                 must acknowledge the access. Independent of
@@ -43,7 +43,11 @@
 	</PropertyGroup>
 
 	<PropertyGroup Label=".NET 11 feature selection" Condition="'$(_PurviewTargetsNet11OrLater)' == 'true'">
-		<PurviewRuntimeAsync Condition="'$(PurviewRuntimeAsync)' == ''">true</PurviewRuntimeAsync>
+		<!-- Runtime Async is opt-in. The .NET team's property name opts in too, and an explicit false
+		     under either name wins, so a repository can turn it off regardless of how it was enabled. -->
+		<PurviewRuntimeAsync Condition="'$(PurviewRuntimeAsync)' == '' AND '$(UseRuntimeAsync)' != ''"
+			>$(UseRuntimeAsync)</PurviewRuntimeAsync>
+		<PurviewRuntimeAsync Condition="'$(PurviewRuntimeAsync)' == ''">false</PurviewRuntimeAsync>
 		<MemorySafetyRules Condition="'$(MemorySafetyRules)' == ''">2</MemorySafetyRules>
 		<_PurviewMemorySafetyRulesNormalized>$([System.String]::Copy('$(MemorySafetyRules)').Trim().ToLowerInvariant())</_PurviewMemorySafetyRulesNormalized>
 		<!-- The planned stable property's tiers: 0/off, 1 (preview syntax), 2 (updated rules). -->