diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index de3b3676f..861d6fb8a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -103,8 +103,11 @@ pytest
 # Lint code
 ruff check nanobot/
 
-# Format code
-ruff format nanobot/
+# Format code — optional. The existing tree predates `ruff format`,
+# so running it across `nanobot/` produces a large unrelated diff
+# (E501 is ignored, so many existing lines exceed the 100-char setting).
+# Format only files you've actually touched, not the whole package.
+ruff format <files-you-changed>
 ```
 
 ## Contribution License