Add breaking change documentation for dotnet new sln defaulting to SLNX format (#48195)

Author: Copilot

docs/core/compatibility/10.0.md

@@ -105,6 +105,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 | [.NET CLI `--interactive` defaults to `true` in user scenarios](sdk/10.0/dotnet-cli-interactive.md) | Behavioral change | Preview 3 |
 | [.NET tool packaging creates RuntimeIdentifier-specific tool packages](sdk/10.0/dotnet-tool-pack-publish.md) | Behavioral change | Preview 6 |
 | [Default workload configuration from 'loose manifests' to 'workload sets' mode](sdk/10.0/default-workload-config.md) | Behavioral change | Preview 2 |
+| [`dotnet new sln` defaults to SLNX file format](sdk/10.0/dotnet-new-sln-slnx-default.md) | Behavioral change | RC 1 |
 | [`dotnet package list` performs restore](sdk/10.0/dotnet-package-list-restore.md) | Behavioral change | Preview 4 |
 | [`dotnet restore` audits transitive packages](sdk/10.0/nugetaudit-transitive-packages.md) | Behavioral change | Preview 3 |
 | [project.json not supported in `dotnet restore`](sdk/10.0/dotnet-restore-project-json-unsupported.md) | Source incompatible | Preview 7 |

docs/core/compatibility/sdk/10.0/dotnet-new-sln-slnx-default.md

@@ -0,0 +1,65 @@
+---
+title: "Breaking change - `dotnet new sln` defaults to SLNX file format"
+description: "Learn about the breaking change in .NET 10 where `dotnet new sln` creates SLNX-format solution files instead of SLN-format files."
+ms.date: 08/30/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/48192
+---
+
+# `dotnet new sln` defaults to SLNX file format
+
+In .NET 10, `dotnet new sln` generates an [SLNX-format](https://devblogs.microsoft.com/visualstudio/new-simpler-solution-file-format/) solution file instead of an SLN-formatted solution file.
+
+## Version introduced
+
+.NET 10 RC 1
+
+## Previous behavior
+
+Previously, `dotnet new sln` created a SLN-format solution file similar to:
+
+```sln
+# Visual Studio Version 17
+VisualStudioVersion = 17.0.31903.59
+MinimumVisualStudioVersion = 10.0.40219.1
+Global
+        GlobalSection(SolutionConfigurationPlatforms) = preSolution
+                Debug|Any CPU = Debug|Any CPU
+                Release|Any CPU = Release|Any CPU
+        EndGlobalSection
+        GlobalSection(SolutionProperties) = preSolution
+                HideSolutionNode = FALSE
+        EndGlobalSection
+EndGlobal
+```
+
+## New behavior
+
+Starting in .NET 10, `dotnet new sln` creates a SLNX-format solution file similar to:
+
+```xml
+<Solution>
+</Solution>
+```
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+The .NET SDK [added support for SLNX files](https://devblogs.microsoft.com/dotnet/introducing-slnx-support-dotnet-cli/) in version 9.0.200, and it's proven to be a stable, understandable format for developers. It's well-supported by all major .NET tooling and is much easier for developers to maintain. This breaking change aims to encourage the use of the SLNX format.
+
+## Recommended action
+
+If you desire an SLN-formatted solution file, pass the `--format sln` option to the command:
+
+`dotnet new sln --format sln`
+
+## Affected APIs
+
+None.
+
+## See also
+
+- [.NET default templates for dotnet new](../../../tools/dotnet-new-sdk-templates.md)

docs/core/compatibility/toc.yml

@@ -118,6 +118,8 @@ items:
                 href: sdk/10.0/dotnet-restore-project-json-unsupported.md
               - name: Default workload configuration from 'loose manifests' to 'workload sets' mode
                 href: sdk/10.0/default-workload-config.md
+              - name: "`dotnet new sln` defaults to SLNX file format"
+                href: sdk/10.0/dotnet-new-sln-slnx-default.md
               - name: "`dotnet package list` performs restore"
                 href: sdk/10.0/dotnet-package-list-restore.md
               - name: MSBUILDCUSTOMBUILDEVENTWARNING escape hatch removed

docs/core/tools/dotnet-sln.md

@@ -1,7 +1,7 @@
 ---
 title: dotnet sln command
 description: The dotnet-sln command provides a convenient option to add, remove, and list projects in a solution file.
-ms.date: 03/26/2025
+ms.date: 08/29/2025
 ---
 # dotnet sln
 
@@ -27,24 +27,27 @@ The `dotnet sln` command provides a convenient way to list and modify projects i
 
 To use the `dotnet sln` command, the solution file must already exist. If you need to create one, use the [dotnet new](dotnet-new.md) command with the `sln` template name.
 
-The following example creates a *.sln* file in the current folder, with the same name as the folder:
+The following example creates an *.slnx* file in the current folder, with the same name as the folder:
 
 ```dotnetcli
 dotnet new sln
 ```
 
-The following example creates a *.sln* file in the current folder, with the specified file name:
+The following example creates an *.slnx* file in the current folder, with the specified file name:
 
 ```dotnetcli
 dotnet new sln --name MySolution
 ```
 
-The following example creates a *.sln* file in the specified folder, with the same name as the folder:
+The following example creates an *.slnx* file in the specified folder, with the same name as the folder:
 
 ```dotnetcli
 dotnet new sln --output MySolution
 ```
 
+> [!NOTE]
+> In .NET 9 and earlier versions, `dotnet new sln` creates an *.sln* file instead of an *.slnx* file.
+
 ## Arguments
 
 - **`SOLUTION_FILE`**