kieronlanning · Nov 11, 2026 · Nov 10, 2026 · Nov 11, 2026
diff --git a/.config/dotnet-tools.json b/.config/dotnet-tools.json
@@ -3,7 +3,7 @@
   "isRoot": true,
   "tools": {
     "csharpier": {
-      "version": "1.3.0",
+      "version": "1.4.0",
       "commands": [
         "csharpier"
       ],
@@ -17,7 +17,7 @@
       "rollForward": false
     },
     "purview.build": {
-      "version": "0.8.2",
+      "version": "0.8.1",
       "commands": [
         "purview-build"
       ],

diff --git a/.config/purview-build.json b/.config/purview-build.json
@@ -23,6 +23,8 @@
 				"Sdk/Sdk.props",
 				"Sdk/Sdk.targets",
 				"Sdk/Targets/Blazor.targets",
+				"Sdk/Targets/Modern.targets",
+				"Sdk/Targets/ServerShared.targets",
 				"Sdk/Targets/VersionDetection.targets"
 			],
 			"purview.skills.testing": [

diff --git a/.editorconfig b/.editorconfig
@@ -1,4 +1,4 @@
-# Purview.BuildSdk 1.3.2 - managed file, do not edit
+# Purview.BuildSdk 2.0.0 - 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

diff --git a/.gitignore b/.gitignore
@@ -49,6 +49,7 @@ project.lock.json
 project.fragment.lock.json
 artifacts/
 
+*.rsp
 *_i.c
 *_p.c
 *_i.h
@@ -60,6 +61,9 @@ artifacts/
 *.pgc
 *.pgd
 *.rsp
+# The SDK bootstraps a managed response file at the repository root; it is tracked so the first build
+# in a fresh clone already runs MSBuild's multithreaded mode.
+!/Directory.Build.rsp
 *.sbr
 *.tlb
 *.tli

diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -11,6 +11,35 @@ release summarise what the commits recorded rather than what was written at the
 
 ## [Unreleased]
 
+### Added
+
+- **`ServerShared` projects.** A `ServerShared*` project is referenced automatically by every
+  non-client-side project, and never by a WebAssembly client, `PurviewBrowserSafe` library, Razor
+  class library or `*.Client`/`*.Browser`/`*.Wasm`/`*.WebAssembly`/`*.Components` project.
+  `IsClientSideProject` names the classification and can be set explicitly; `PRSHR0001` (client-side
+  reference) and `PRSHR0002` (Shared reference) catch a reference that would still reach a browser.
+  Opt the automatic reference out with `PurviewAutoServerSharedReference=false`.
+- **.NET 11 compiler opt-ins.** A `net11.0`+ target compiles with Runtime Async
+  (`<Features>runtime-async=on</Features>`) and the C# 15 updated memory-safety rules
+  (`MemorySafetyRules=2`), gated per-TFM so the `net10.0` leg is unchanged. `AllowUnsafeBlocks` is
+  left at the compiler default (`false`). Opt out with `PurviewRuntimeAsync=false`,
+  `UseRuntimeAsync=false`, or `MemorySafetyRules=0`; an unrecognised tier fails with `PRCFG0001`.
+- **Multithreaded builds.** A managed `Directory.Build.rsp` at the repository root enables MSBuild's
+  `-mt` on .NET 11, bootstrapped like `.editorconfig` and `global.json` and refreshed only when its
+  marker line is present. `PurviewMultiThreadedBuild=false` rewrites it to `-mt:false`.
+
+### Changed
+
+- **The default target frameworks are now `net10.0;net11.0`.** `Current` — the set that applies when a
+  project selects none — resolves to the current LTS plus the latest release, so a project that declares
+  neither `TargetFramework` nor `TargetFrameworks` now multi-targets. This is a breaking change: pin
+  `PurviewTargetFrameworksCurrent`, select another set, or declare the TFM explicitly to keep the previous
+  single-target output. `Supported` still names the same pair so a repository can pin the meaning rather
+  than depend on where `Current` moves next.
+- **`LangVersion` defaults to `latest` (C# 15)** instead of `preview`. C# 15 is the latest shipped
+  language version and includes union declarations; `preview` remains available per project when a
+  repository deliberately wants unshipped features. Roslyn components keep `LangVersion=latest`.
+
 ## [1.3.2] - 2026-11-09
 
 ### Changed

diff --git a/Directory.Build.rsp b/Directory.Build.rsp
@@ -0,0 +1,4 @@
+# Purview.BuildSdk - managed file
+# Managed by Purview.BuildSdk. Delete the marker line to take ownership of this file.
+-mt
+-nr:false
diff --git a/Directory.Packages.props b/Directory.Packages.props
@@ -1,6 +1,6 @@
 <Project>
 	<PropertyGroup>
-		<TUnitVersion>1.76.0</TUnitVersion>
+		<TUnitVersion>1.83.0</TUnitVersion>
 		<MicrosoftRoslynVersion>5.9.0</MicrosoftRoslynVersion>
 	</PropertyGroup>
 	<ItemGroup>

diff --git a/README.md b/README.md
diff --git a/docs/wiki/Configuration-Reference.md b/docs/wiki/Configuration-Reference.md
diff --git a/docs/wiki/Getting-Started.md b/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": "1.3.2"
+    "Purview.BuildSdk": "2.0.0"
   }
 }
 ```

diff --git a/docs/wiki/Home.md b/docs/wiki/Home.md
@@ -34,8 +34,8 @@ repo root inherits everything automatically.
   `.csproj` filename, the `Sdk` attribute, and on-disk markers.
 - **Engineering conventions** — repository layout, project naming, namespace identity, test readability,
   and the default boundaries between source, test, and shared test-support projects.
-- **C# defaults** — `net10.0` TFM (overridable, `netstandard2.0` for Roslyn components),
-  `LangVersion=preview` (`latest` for Roslyn components), `Nullable=enable`, `ImplicitUsings=enable`,
+- **C# defaults** — `net10.0;net11.0` TFMs (overridable, `netstandard2.0` for Roslyn components),
+  `LangVersion=latest` (C# 15, including unions), `Nullable=enable`, `ImplicitUsings=enable`,
   deterministic builds, `ManagePackageVersionsCentrally=true`.
 - **Code style** — an `.editorconfig` baked into the package, applied via `EditorConfigFilePath`, and
   auto-bootstrapped to the repo root if missing; `EnforceCodeStyleInBuild=true`,
@@ -65,7 +65,7 @@ repo root inherits everything automatically.
 
 ## Requirements
 
-- .NET SDK 10.0 or later (the default TFM for source projects is `net10.0`).
+- .NET SDK 11.0 or later (the default TFMs for source projects are `net10.0` and `net11.0`).
 - A `NamespacePrefix` set before the SDK import.
 - A `Directory.Packages.props` at the repo root when Central Package Management is used (the SDK sets
   `ManagePackageVersionsCentrally=true`).

diff --git a/docs/wiki/Project-Type-Detection.md b/docs/wiki/Project-Type-Detection.md
@@ -15,6 +15,8 @@ wiki.
 | `IsTestProject` | Project name matches `^.*\.([\w|_]\w*)Tests?$` — a target name, a dot, then any word before `Test`/`Tests`. The captured word becomes `TestingType`. |
 | `IsSharedTestingProject` | Project name is one of `SharedTestingFramework`, `SharedTestingInfrastructure`, `SharedTestingInfra`, `SharedTestingUtilities`, `SharedTestingUtils`, `SharedTestingLibrary`, `SharedTestingLib`, `SharedTestingHelpers`. |
 | `IsSharedProject` | Project name is one of `Shared`, `SharedFramework`, `SharedInfrastructure`, `SharedInfra`, `SharedUtilities`, `SharedUtils`, `SharedLibrary`, `SharedLib`, `SharedHelpers`. |
+| `IsServerSharedProject` | Project name is one of `ServerShared`, `ServerSharedFramework`, `ServerSharedInfrastructure`, `ServerSharedInfra`, `ServerSharedUtilities`, `ServerSharedUtils`, `ServerSharedLibrary`, `ServerSharedLib`, `ServerSharedHelpers`. |
+| `IsClientSideProject` | A WebAssembly client, a `PurviewBrowserSafe` library, a Razor class library, or a project named `*.Client`, `*.ClientShared`, `*.Browser`, `*.Wasm`, `*.WebAssembly`, `*.Components`, `Components`, or `ClientShared`. An explicit value overrides the detection. |
 | `IsContainerProject` | A `Dockerfile`, `dockerfile`, or `Dockerfile.dev` exists in the project directory. |
 | `IsSdkProject` | An `Sdk` value is parsed from the `<Project>`/`<Import>` element. |
 | `IsWebSdkProject` | `SdkProjectName` is `Microsoft.NET.Sdk.Web`. |
@@ -87,6 +89,16 @@ The SDK ships the following full `TestType` set, in addition to the standard `Un
   (`../Shared*/Shared*.csproj`); test projects automatically reference
   `../SharedTesting*/SharedTesting*.csproj`.
 
+### ServerShared projects (`IsServerSharedProject=true`)
+
+- Referenced automatically by every non-client-side project via wildcard
+  (`../ServerShared*/ServerShared*.csproj`).
+- Never referenced automatically by a WebAssembly client, a `PurviewBrowserSafe` library, a Razor class
+  library, a project named with a client-side segment, or another `Shared`/`SharedTesting` project.
+  `IsClientSideProject` overrides the classification either way.
+- `PRSHR0001` reports a client-side project that references one; `PRSHR0002` reports a `Shared`
+  project that does. `PurviewBrowserSafe="true"` on the reference records an accepted decision.
+
 ### Container projects
 
 - `InvariantGlobalization=true`, `PublishAot=true`, `DockerDefaultTargetOS=Linux`,

diff --git a/docs/wiki/Repository-Bootstrap.md b/docs/wiki/Repository-Bootstrap.md
@@ -66,9 +66,41 @@ file, and honours `PurviewRepoBootstrapMode`:
 | `DisableAutoCopySdkFiles` | `false` | Master switch that disables repo-level SDK file bootstrapping. |
 | `PurviewRepoBootstrapMode` | `IfMissing` | `IfMissing`, `Always`, `WarnOnDrift` or `Never`. |
 
+## `Directory.Build.rsp` bootstrapping
+
+`-mt` is an MSBuild **host** switch, so no project property can enable it: the host reads its switches
+before evaluation. A `Directory.Build.rsp` in any ancestor of a project is the one file the CLI reads
+that can, so the SDK writes one at the repository root:
+
+```text
+# Purview.BuildSdk - managed file
+# Managed by Purview.BuildSdk. Delete the marker line to take ownership of this file.
+-mt
+-nr:false
+```
+
+`-nr:false` is deliberate. A node left by a non-`-mt` build can hold a cached `RoslynCodeTaskFactory`
+task with no assembly path, and the first `-mt` build that reuses it fails with `MSB0001`
+([dotnet/msbuild#15213](https://github.com/dotnet/msbuild/issues/15213)) — which is exactly the
+transition a repository makes when this file first appears. A short-lived server still provides the
+Server GC `-mt` expects. `-mt` requires .NET 11; the bootstrap is skipped on an older SDK.
+
+The file is SDK-managed: a destination carrying the marker line is refreshed, one without it belongs
+to the developer and is never touched. `PurviewMultiThreadedBuild=false` rewrites it to `-mt:false`
+rather than deleting it, so opting out cannot leave a stale copy enabling the mode.
+
+Like the other bootstraps it writes atomically, retries quietly, and honours `PurviewRepoBootstrapMode`:
+
+| Property | Default | Description |
+| -- | -- | -- |
+| `BootstrapMultiThreadedBuildRspToRepoRoot` | `true` | Creates the `Directory.Build.rsp` when missing. |
+| `RepositoryDirectoryBuildRspFilePath` | *(auto-detected)* | Override the destination path. |
+| `PurviewMultiThreadedBuild` | `true` | `false` writes `-mt:false` instead of `-mt`. |
+| `DisableAutoCopySdkFiles` | `false` | Master switch that disables repo-level SDK file bootstrapping. |
+
 ## Retry and failure behaviour
 
-Both bootstraps share the same retry contract:
+All bootstraps share the same retry contract:
 
 | Property | Default | Description |
 | -- | -- | -- |
@@ -82,11 +114,11 @@ error by default, including the destination path and the underlying OS error.
 
 ## Repository root discovery
 
-Both bootstrap targets locate the repository root by:
+All bootstrap targets locate the repository root by:
 
 1. Running `git rev-parse --show-toplevel` from the `Directory.Build.props` directory.
 2. Falling back to probing upward for a `.git` marker.
 3. Finally falling back to the `Directory.Build.props` directory itself.
 
-Set the `RepositoryEditorConfigFilePath` / `RepositoryGlobalJsonFilePath` properties explicitly to
-override discovery.
+Set the `RepositoryEditorConfigFilePath`, `RepositoryGlobalJsonFilePath` or
+`RepositoryDirectoryBuildRspFilePath` property explicitly to override discovery.
diff --git a/global.json b/global.json
@@ -1,5 +1,7 @@
 {
-	"test": {
-		"runner": "Microsoft.Testing.Platform"
-	}
-}
+  "sdk": { "version": "11.0.100" },
+
+  "test": {
+	"runner": "Microsoft.Testing.Platform"
+  }
+}
diff --git a/package.json b/package.json
@@ -1,6 +1,6 @@
 {
 	"name": "build-sdk",
-	"version": "1.3.2",
+	"version": "2.0.0",
 	"homepage": "https://purview.dev/projects/build-sdk/",
 	"bugs": {
 		"url": "https://github.com/purview-dev/build-sdk/issues"
@@ -9,4 +9,4 @@
 		"type": "git",
 		"url": "git+https://github.com/purview-dev/build-sdk.git"
 	}
-}
+}
diff --git a/skills/purview-buildsdk-configuration/SKILL.md b/skills/purview-buildsdk-configuration/SKILL.md
@@ -41,7 +41,7 @@ in `Directory.Build.props` unless the setting is deliberately project-specific.
 | `TestingFramework` | `TUnit` | `TUnit`, `Xunit` or `None` |
 | `SubstituteFramework` | `TUnitMocks` | `TUnitMocks`, `NSubstitute` or `None` |
 | `TestDataFramework` | `Bogus` | `Bogus` or `None` |
-| `TargetFramework` | `net10.0` | `netstandard2.0` when `IsRoslynComponent=true` |
+| `TargetFramework` | `net10.0;net11.0` | `netstandard2.0` when `IsRoslynComponent=true` |
 | `IsPackable` | `false` | Opts a project into the packable defaults |
 | `UsePackageJsonVersion` | `true` | `true`, `false` or `Strict` — version from `package.json` |
 | `ExcludePurviewTelemetry` | `false` | Drops the telemetry source generator |
@@ -50,6 +50,11 @@ in `Directory.Build.props` unless the setting is deliberately project-specific.
 | `PurviewBrowserSafe` | *(unset)* | On a class library a WebAssembly client downloads: trim, AOT and browser-unsupported APIs fail the build |
 | `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`+ |
+| `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` |
 
 The full set, grouped by area and with the behaviour each one changes, is in
 [references/properties.md](references/properties.md). Read it rather than guessing a property name —
@@ -89,11 +94,15 @@ is the SDK's answer, and it stays correct when the rules change. The exported se
 
 ## Repository bootstrap
 
-The SDK bootstraps two repository-wide files from the package: `.editorconfig` and `global.json`.
+The SDK bootstraps three repository-wide files from the package: `.editorconfig`, `global.json` and
+`Directory.Build.rsp`.
 
 - `PurviewRepoBootstrapMode` — `IfMissing` (default), `Always`, `WarnOnDrift`, `Never`
 - `DisableAutoCopySdkFiles` — the master off switch
 - `PurviewRepoBootstrapCopyRetries` / `...RetryDelayMilliseconds` / `...FailureAsError`
+- `BootstrapMultiThreadedBuildRspToRepoRoot` / `PurviewMultiThreadedBuild` — the response file that
+  enables MSBuild `-mt` (and `-nr:false`, which avoids a `RoslynCodeTaskFactory` cache left by a
+  non-`-mt` build)
 
 Every project in a solution writes those same destinations, so the write is race-tolerant by
 design: an existing file is never overwritten unless asked, each write is staged into a temporary

diff --git a/skills/purview-buildsdk-configuration/references/properties.md b/skills/purview-buildsdk-configuration/references/properties.md
@@ -21,7 +21,7 @@ consuming project supplies nothing.
 
 | Property | Default | Behaviour |
 | -- | -- | -- |
-| `TargetFramework` | `net10.0` | Applied when neither `TargetFramework` nor `TargetFrameworks` is set. A project that explicitly declares `IsRoslynComponent=true` defaults to `netstandard2.0` |
+| `TargetFramework` | `net10.0;net11.0` | Applied when neither `TargetFramework` nor `TargetFrameworks` is set. A project that explicitly declares `IsRoslynComponent=true` defaults to `netstandard2.0` |
 | `IsRoslynComponent` | *(unset)* | When explicitly `true`: a single `netstandard2.0` target, `LangVersion=latest`, `Nullable=enable`, `TreatWarningsAsErrors=true`, `Deterministic=true`, extended analyzer rules, SourceLink with `EmbedUntrackedSources=true`, no dependency file, telemetry exclusion, and `PrivateAssets=all` on `Microsoft.CodeAnalysis.*` references |
 | `IsRoslynComponentOnly` | `true` for Roslyn components | Analyzer-only package: `IncludeBuildOutput=false`, `IncludeSymbols=false`, portable PDB packed beside the analyzer under `analyzers/dotnet/cs/`. Set `false` for a dual-role component using normal library symbol packaging |
 | `DisableRoslynCompilerDefaultsValidation` | `false` | Opts out of `ValidateRoslynComponentCompilerSettings`, which fails the pack of a packable Roslyn component missing `LangVersion`, `Nullable`, `TreatWarningsAsErrors` or `EnforceExtendedAnalyzerRules` |
@@ -36,6 +36,19 @@ consuming project supplies nothing.
 | `DisablePurviewTestContextRuleSet` | `false` | Makes test and shared-testing projects enforce the production API-surface rules too |
 | `PurviewPolicyExemptNoWarn` | *(derived)* | The SDK-injected `NoWarn` entries `ValidatePurviewStylePolicy` accepts |
 
+## .NET 11 features and ServerShared
+
+| 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 |
+| `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 |
+| `PurviewAutoServerSharedReference` | `true` | Adds `../ServerShared*/ServerShared*.csproj` to a non-client-side project |
+| `PurviewServerSharedReferenceCheckAsError` | `false` | Makes `PRSHR0001`/`PRSHR0002` errors |
+| `DisablePurviewServerSharedReferenceCheck` | `false` | Turns `PRSHR0001`/`PRSHR0002` off |
+
 ## Testing
 
 | Property | Default | Behaviour |
@@ -119,6 +132,9 @@ reference was judged browser-safe when it cannot say so itself.
 | `RepositoryEditorConfigFilePath` | *(derived)* | Overrides the destination `.editorconfig` |
 | `BootstrapGlobalJsonToRepoRoot` | `true` | |
 | `RepositoryGlobalJsonFilePath` | *(derived)* | Overrides the destination `global.json` |
+| `BootstrapMultiThreadedBuildRspToRepoRoot` | `true` | Creates a managed `Directory.Build.rsp` enabling `-mt` |
+| `RepositoryDirectoryBuildRspFilePath` | *(derived)* | Overrides the destination `Directory.Build.rsp` |
+| `PurviewMultiThreadedBuild` | `true` | `false` rewrites a managed rsp to `-mt:false` |
 | `PurviewBuildSdkVersionForGlobalJson` | restored package version, else the version it was packed as | |
 | `PurviewRepoBootstrapCopyRetries` | `3` | |
 | `PurviewRepoBootstrapCopyRetryDelayMilliseconds` | `500` | |
@@ -148,7 +164,11 @@ Exported to analyzers and generators as `build_property.<PropertyName>`:
 - `ExcludePurviewTelemetry`, `ExcludeMSTelemetryExtension`
 - `EnableAssemblyNameGeneration`, `DisableAutoInternalsVisibleTo`, `DisableGenerateAssemblyInfoClass`,
   `DisableGenerateAssemblyIdentifierClass`, `AssemblyIdentifierClassName`
-- `IsCSharpProject`, `IsTestProject`, `IsSharedTestingProject`, `IsSharedProject`
+- `IsCSharpProject`, `IsTestProject`, `IsSharedTestingProject`, `IsSharedProject`,
+  `IsServerSharedProject`, `IsClientSideProject`
+- `PurviewAutoServerSharedReference`, `DisablePurviewServerSharedReferenceCheck`,
+  `PurviewServerSharedReferenceCheckAsError`
+- `MemorySafetyRules`, `PurviewRuntimeAsync`, `PurviewMultiThreadedBuild`
 - `TestingType`, `TargetProjectName`
 - `IsContainerProject`, `IsSdkProject`, `SdkProjectName`, `IsWebProject`, `IsWebSdkProject`,
   `IsWorkerSdkProject`, `IsAspireHostProject`, `IsCLIProject`