diff --git a/AGENTS.md b/AGENTS.md
index f214bcc..7b8cbcc 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -17,6 +17,11 @@ Two roles are covered: **writing code** and **reviewing pull requests**.
   (`Transience`, `ErrorOrigin`) are the only value-type exception.
 - Keep changes small and focused. Treat renamed error codes, diagnostic IDs and
   public types as breaking changes.
+- Adding a new project? Also add its GUID to `FirstClassErrors.sln`'s
+  `GlobalSection(NestedProjects)`, nested under the `src` or `tests` solution
+  folder like its siblings — a project left out of that section sits loose at
+  the solution root instead of grouped with the rest. This has recurred
+  several times; check it whenever a `.csproj` is added.
 
 ## Architecture decisions (code changes)
 
diff --git a/CLAUDE.md b/CLAUDE.md
index f1e3d72..1803198 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -36,6 +36,13 @@ errors should stay structured, documented, and close to the code.
 * `FirstClassErrors.Usage`         — usage examples
 * `doc/`                           — documentation: `handwritten/` (`for-users`, `for-maintainers`) and `generated/` (CI/CD living docs)
 
+When adding a new project to the solution, also add its GUID to
+`FirstClassErrors.sln`'s `GlobalSection(NestedProjects)`, nested under the
+`src` or `tests` solution folder like its siblings — a project missing from
+that section shows up loose at the solution root in Visual Studio/Rider
+instead of grouped with the rest. This has been missed and fixed after the
+fact several times; check it every time a `.csproj` is added.
+
 ## Change guidelines
 
 * Keep changes small, focused, and aligned with the requested task.
diff --git a/FirstClassErrors.sln b/FirstClassErrors.sln
index 3131b88..ee394bd 100644
--- a/FirstClassErrors.sln
+++ b/FirstClassErrors.sln
@@ -381,6 +381,8 @@ Global
 		{75EA57DD-0128-4EB9-ADBE-567BFF602A93} = {B7C3D08D-EFC5-4F5D-8DE4-5B7938354DBB}
 		{F2C739E7-6F2E-4256-9C87-9D6F1168DF4A} = {02EA681E-C7D8-13C7-8484-4AC65E1B71E8}
 		{BC4F2BE8-55C9-4AA4-90FA-699EC83A8985} = {B7C3D08D-EFC5-4F5D-8DE4-5B7938354DBB}
+		{BF67DBAE-EBE1-45CE-86E5-B2E9E3D4EF6B} = {02EA681E-C7D8-13C7-8484-4AC65E1B71E8}
+		{75FD5E2C-A871-4659-8A01-13461FED84EB} = {B7C3D08D-EFC5-4F5D-8DE4-5B7938354DBB}
 		{1FC2C501-8842-4ABE-845E-000ED6575DD6} = {B7C3D08D-EFC5-4F5D-8DE4-5B7938354DBB}
 		{D12EE45B-78F8-4CD4-8E67-E4A9DDA01AE6} = {B7C3D08D-EFC5-4F5D-8DE4-5B7938354DBB}
 		{04707ACA-FB2E-43BF-A12C-28E01BF5F60D} = {B7C3D08D-EFC5-4F5D-8DE4-5B7938354DBB}