Add docs for NETSDK1237 diagnostic

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Co-authored-by: jkoritzinsky <1571408+jkoritzinsky@users.noreply.github.com>

Author: Copilot

docs/core/tools/sdk-errors/index.md

@@ -374,3 +374,4 @@ This list is a complete list of the errors that you might get from the .NET SDK
 |NETSDK1211|EnableSingleFileAnalyzer is not supported for the target framework. Consider multi-targeting to a supported framework to enable single-file analysis, and set EnableSingleFileAnalyzer only for the supported frameworks. For example:<br>`<EnableSingleFileAnalyzer Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', '{0}'))">true</EnableSingleFileAnalyzer>`|
 |NETSDK1212|IsTrimmable and EnableTrimAnalyzer are not supported for the target framework. Consider multi-targeting to a supported framework to enable trimming, and set IsTrimmable only for the supported frameworks. For example:<br>`<IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>`|
 |NETSDK1213|Targeting .NET 8.0 or higher in Visual Studio 2022 17.7 is not supported.|
+|[NETSDK1237](netsdk1237.md)|Assembly '{0}' was listed in PublishReadyToRunPartialAssemblies but is being compiled into a composite image. Partial compilation is only supported for assemblies compiled separately. The assembly will be compiled fully into the composite image.|

docs/core/tools/sdk-errors/netsdk1237.md

@@ -0,0 +1,32 @@
+---
+title: "NETSDK1237: Assembly listed in PublishReadyToRunPartialAssemblies but compiled into a composite image"
+description: Learn about .NET SDK warning NETSDK1237, which occurs when an assembly is listed in PublishReadyToRunPartialAssemblies but is being compiled into a ReadyToRun composite image.
+ms.topic: error-reference
+ms.date: 04/13/2026
+f1_keywords:
+- NETSDK1237
+ai-usage: ai-generated
+---
+# NETSDK1237: Assembly listed in PublishReadyToRunPartialAssemblies but compiled into a composite image
+
+NETSDK1237 is a warning that an assembly listed in `PublishReadyToRunPartialAssemblies` is also being compiled into a [ReadyToRun composite image](../../deploying/ready-to-run.md). Partial compilation is only supported for assemblies compiled separately. The assembly is compiled fully into the composite image and the `PublishReadyToRunPartialAssemblies` setting for it is ignored.
+
+This situation occurs when you set both `PublishReadyToRunComposite` to `true` and include assemblies in `PublishReadyToRunPartialAssemblies` in your project file:
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <PublishReadyToRun>true</PublishReadyToRun>
+    <PublishReadyToRunComposite>true</PublishReadyToRunComposite>
+  </PropertyGroup>
+  <ItemGroup>
+    <!-- NETSDK1237 is emitted for each assembly listed here -->
+    <PublishReadyToRunPartialAssemblies Include="SomeAssembly" />
+  </ItemGroup>
+</Project>
+```
+
+To resolve this warning, choose one of the following options:
+
+- Remove the assemblies from `PublishReadyToRunPartialAssemblies` if you want them compiled fully into the composite image.
+- Set `PublishReadyToRunComposite` to `false` if you want to compile assemblies partially and separately instead of into a composite image.

docs/navigate/tools-diagnostics/toc.yml

@@ -91,6 +91,8 @@ items:
                 href: ../../core/tools/sdk-errors/netsdk1202.md
               - name: NETSDK1206
                 href: ../../core/tools/sdk-errors/netsdk1206.md
+              - name: NETSDK1237
+                href: ../../core/tools/sdk-errors/netsdk1237.md
           - name: BuildCheck rules
             items:
               - name: Index of rules