diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index 7c362ab53..013305399 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -25,6 +25,7 @@
   - Python: `cd python && uv pip install -e ".[dev]"` → `uv run pytest` (E2E tests use the test harness)
   - Go: `cd go && go test ./...`
   - .NET: `cd dotnet && dotnet test test/GitHub.Copilot.SDK.Test.csproj`
+  - **.NET testing note:** Never add `InternalsVisibleTo` to any project file when writing tests. Tests must only access public APIs.
 
 ## Testing & E2E tips ⚙️