Add breaking change documentation for dotnet watch stderr logging

Co-authored-by: gewarren <[email protected]>

Author: Copilot

docs/core/compatibility/10.0.md

@@ -111,6 +111,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 | [`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 |
 | [`dotnet tool install --local` creates manifest by default](sdk/10.0/dotnet-tool-install-local-manifest.md) | Behavioral change | Preview 7 |
+| [`dotnet watch` logs internal-facing messages to stderr instead of stdout](sdk/10.0/dotnet-watch-stderr.md) | Behavioral change | RC 2 |
 | [project.json not supported in `dotnet restore`](sdk/10.0/dotnet-restore-project-json-unsupported.md) | Source incompatible | Preview 7 |
 | [SHA-1 fingerprint support deprecated in `dotnet nuget sign`](sdk/10.0/dotnet-nuget-sign-sha1-deprecated.md) | Behavioral change | Preview 1 |
 | [MSBUILDCUSTOMBUILDEVENTWARNING escape hatch removed](sdk/10.0/custom-build-event-warning.md) | Behavioral change | Preview 1 |

docs/core/compatibility/sdk/10.0/dotnet-watch-stderr.md

@@ -0,0 +1,42 @@
+---
+title: "Breaking change: 'dotnet watch' logs internal-facing messages to stderr instead of stdout"
+description: "Learn about the breaking change in .NET 10 RC 2 where 'dotnet watch' emits its log messages to stderr instead of stdout."
+ms.date: 01/16/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/45871
+---
+# 'dotnet watch' logs internal-facing messages to stderr instead of stdout
+
+Starting in .NET 10 RC 2, [`dotnet watch`](../../../tools/dotnet-watch.md) emits its own log messages to the `stderr` channel instead of `stdout`. This change is part of a general trend towards `dotnet` CLI commands not obscuring the `stdout` channel, which is often reserved for special semantics when running certain kinds of applications, like LSP or MCP servers.
+
+## Version introduced
+
+.NET 10 RC 2
+
+## Previous behavior
+
+Previously, [`dotnet watch`](../../../tools/dotnet-watch.md) emitted log messages to `stdout`.
+
+## New behavior
+
+Starting in .NET 10 RC 2, [`dotnet watch`](../../../tools/dotnet-watch.md) emits log messages to `stderr`.
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+This change is part of a general trend towards `dotnet` CLI commands not obscuring the `stdout` channel, which is often reserved or assumed to have special semantics when running certain kinds of applications, like LSP or MCP servers. This change was implemented in <https://github.com/dotnet/sdk/pull/50820>. In general, the CLI should get out of the way of your applications.
+
+## Recommended action
+
+Most users shouldn't need to take any action. If you need the `dotnet watch` messages on `stdout`, you can redirect the `stderr` stream to `stdout`. For example, use `2>&1` to redirect the `2` file descriptor for `stderr` to the `1` file descriptor for `stdout`:
+
+```bash
+dotnet watch 2>&1
+```
+
+## Affected APIs
+
+None.

docs/core/compatibility/toc.yml

@@ -128,6 +128,8 @@ items:
                 href: sdk/10.0/dotnet-package-list-restore.md
               - name: "`dotnet tool install --local` creates manifest by default"
                 href: sdk/10.0/dotnet-tool-install-local-manifest.md
+              - name: "`dotnet watch` logs internal-facing messages to stderr instead of stdout"
+                href: sdk/10.0/dotnet-watch-stderr.md
               - name: MSBUILDCUSTOMBUILDEVENTWARNING escape hatch removed
                 href: sdk/10.0/custom-build-event-warning.md
               - name: MSBuild custom culture resource handling