diff --git a/docs/Diagnostics.md b/docs/Diagnostics.md
new file mode 100644
index 0000000..55ba66a
--- /dev/null
+++ b/docs/Diagnostics.md
@@ -0,0 +1,51 @@
+# Diagnostics (message IDs)
+
+Messages emitted by `MSBuild.SDK.SystemWeb` use the `SYSWEB` prefix followed by a number.
+This page is the central registry of those messages.
+
+| ID | Severity | Meaning |
+|----|----------|---------|
+| `SYSWEB000` | Error | Invalid value for the `MissingWebApplicationsTargetPathAction` property (expected `Ignore`, `Skip`, `Warn` or `Error`). |
+| `SYSWEB001` | Error / Warning / message | `Microsoft.WebApplication.targets` could not be resolved at `$(WebApplicationsTargetPath)`. Configurable via the `MissingWebApplicationsTargetPathAction` property — see below. |
+
+## `MissingWebApplicationsTargetPathAction`
+
+`Microsoft.WebApplication.targets` ships with Visual Studio / the Visual Studio Build Tools, not with
+the .NET SDK. When building with the `dotnet` CLI (Core MSBuild), `$(MSBuildExtensionsPath32)` points at
+the .NET SDK rather than a Visual Studio install, so the default `$(WebApplicationsTargetPath)` may not
+exist and the build would otherwise fail with a cryptic `MSB4019`.
+
+The `MissingWebApplicationsTargetPathAction` property controls what happens when that file cannot be found:
+
+| Value | Behaviour |
+|-------|-----------|
+| `Ignore` | Do nothing. |
+| `Skip` | Emit a high-importance informational message (`SYSWEB001`). |
+| `Warn` | Emit a warning (`SYSWEB001`). |
+| `Error` | Emit an error and fail the build (`SYSWEB001`). **Default.** |
+
+> With `Ignore`, `Skip` or `Warn` the build continues **without** importing `Microsoft.WebApplication.targets`.
+> For a real web project, later targets that depend on it may still fail. These options are intended for
+> scenarios where the web targets are genuinely not needed.
+
+### How to resolve a missing `Microsoft.WebApplication.targets`
+
+- Install Visual Studio, or the Visual Studio Build Tools, with the **web development build tools** workload.
+- Build from a **Visual Studio Developer command prompt** (which sets the correct environment).
+- Set `VSToolsPath` (or `WebApplicationsTargetPath`) explicitly, e.g. computed via
+  [vswhere](https://github.com/microsoft/vswhere) in a CI pre-build step:
+
+```xml
+<!-- e.g. in Directory.Build.props, or pass via -p: on the command line -->
+<PropertyGroup>
+  <VSToolsPath>C:\Program Files\Microsoft Visual Studio\2022\Enterprise\MSBuild\Microsoft\VisualStudio\v17.0</VSToolsPath>
+</PropertyGroup>
+```
+
+To opt out of the failure instead:
+
+```xml
+<PropertyGroup>
+  <MissingWebApplicationsTargetPathAction>Warn</MissingWebApplicationsTargetPathAction>
+</PropertyGroup>
+```
diff --git a/docs/README.md b/docs/README.md
index 6916138..a259950 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -83,6 +83,9 @@ At evaluation time, MSBuild adds implicit imports at the top and bottom of the p
 
 # Useful Information
 
+## Diagnostics
+- [Message IDs and the `MissingWebApplicationsTargetPathAction` property](Diagnostics.md)
+
 ## Binding Redirects
 - [How to show Suggested Binding Redirects](Binding_Redirects/How-to-show-Suggested-Binding-Redirects.md)
 - [Autogenerating Binding Redirects](Binding_Redirects/Autogenerating-Binding-Redirects.md)
diff --git a/src/MSBuild.SDK.SystemWeb/Sdk/Sdk.targets b/src/MSBuild.SDK.SystemWeb/Sdk/Sdk.targets
index d7b0dbd..30eaf79 100644
--- a/src/MSBuild.SDK.SystemWeb/Sdk/Sdk.targets
+++ b/src/MSBuild.SDK.SystemWeb/Sdk/Sdk.targets
@@ -15,6 +15,7 @@
     <VisualStudioVersion Condition="'$(VisualStudioVersion)' == ''">15.0</VisualStudioVersion>
     <VSToolsPath Condition="'$(VSToolsPath)' == ''">$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)</VSToolsPath>
     <WebApplicationsTargetPath Condition=" '$(WebApplicationsTargetPath)' == '' ">$(VSToolsPath)\WebApplications\Microsoft.WebApplication.targets</WebApplicationsTargetPath>
+    <MissingWebApplicationsTargetPathAction Condition="'$(MissingWebApplicationsTargetPathAction)' == ''">Error</MissingWebApplicationsTargetPathAction>
   </PropertyGroup>
 
   <!-- Default item excludes -->
@@ -25,5 +26,26 @@
   <!-- Inject BindingRedirects targets after Microsoft.NET.Sdk is injected -->
   <Import Project="MSBuild.SDK.SystemWeb.BindingRedirects.targets" />
 
-  <Import Project="$(WebApplicationsTargetPath)" />
+  <Import Project="$(WebApplicationsTargetPath)" Condition="Exists('$(WebApplicationsTargetPath)')" />
+
+  <!-- When Microsoft.WebApplication.targets is missing, react per $(MissingWebApplicationsTargetPathAction)
+       instead of failing with a cryptic MSB4019. See docs/Diagnostics.md (SYSWEB000/SYSWEB001). -->
+  <Target Name="_SystemWebCheckWebApplicationsTargetPath"
+          BeforeTargets="BeforeBuild"
+          Condition="!Exists('$(WebApplicationsTargetPath)')">
+
+    <PropertyGroup>
+      <_SystemWebMissingWebTargetsText>SYSWEB001: 'Microsoft.WebApplication.targets' was not found at '$(WebApplicationsTargetPath)'. Install Visual Studio or the Visual Studio Build Tools with the "web development build tools" workload, build from a Visual Studio Developer command prompt, or set the 'VSToolsPath' (or 'WebApplicationsTargetPath') property to a valid Visual Studio tools path. Use the 'MissingWebApplicationsTargetPathAction' property (Ignore | Skip | Warn | Error) to control this behaviour.</_SystemWebMissingWebTargetsText>
+    </PropertyGroup>
+
+    <!-- Reject unknown values up front. -->
+    <Error Code="SYSWEB000"
+           Condition="'$(MissingWebApplicationsTargetPathAction)' != 'Ignore' and '$(MissingWebApplicationsTargetPathAction)' != 'Skip' and '$(MissingWebApplicationsTargetPathAction)' != 'Warn' and '$(MissingWebApplicationsTargetPathAction)' != 'Error'"
+           Text="SYSWEB000: Unknown MissingWebApplicationsTargetPathAction '$(MissingWebApplicationsTargetPathAction)'. Valid values are: Ignore, Skip, Warn, Error." />
+
+    <Error   Code="SYSWEB001" Text="$(_SystemWebMissingWebTargetsText)" Condition="'$(MissingWebApplicationsTargetPathAction)' == 'Error'" />
+    <Warning Code="SYSWEB001" Text="$(_SystemWebMissingWebTargetsText)" Condition="'$(MissingWebApplicationsTargetPathAction)' == 'Warn'" />
+    <Message Importance="high" Text="$(_SystemWebMissingWebTargetsText)" Condition="'$(MissingWebApplicationsTargetPathAction)' == 'Skip'" />
+    <!-- 'Ignore' => no output -->
+  </Target>
 </Project>