diff --git a/AGENTS.md b/AGENTS.md
index 69b0df68f..45eed9ec2 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -118,6 +118,7 @@
 - Keep files concise; extract helpers instead of “V2” copies. Use existing patterns for CLI options and dependency injection via `createDefaultDeps`.
 - Aim to keep files under ~700 LOC; guideline only (not a hard guardrail). Split/refactor when it improves clarity or testability.
 - Naming: use **OpenClaw** for product/app/docs headings; use `openclaw` for CLI command, package/binary, paths, and config keys.
+- Written English: use American spelling and grammar in code, comments, docs, and UI strings (e.g. "color" not "colour", "behavior" not "behaviour", "analyze" not "analyse").
 
 ## Release Channels (Naming)
 
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index c7808db9c..a4bb0e173 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -92,6 +92,7 @@ Welcome to the lobster tank! 🦞
 - Describe what & why
 - Reply to or resolve bot review conversations you addressed before asking for review again
 - **Include screenshots** — one showing the problem/before, one showing the fix/after (for UI or visual changes)
+- Use American English spelling and grammar in code, comments, docs, and UI strings
 
 ## Review Conversations Are Author-Owned