diff --git a/.claude/CLAUDE.md b/.claude/CLAUDE.md
index cbce208e..b7d292ba 100644
--- a/.claude/CLAUDE.md
+++ b/.claude/CLAUDE.md
@@ -36,3 +36,12 @@ Don't hand-edit a file that has the generated header — the next
 `sdk:generate` run silently overwrites it. Instead, fix the emitter or the
 spec upstream and regenerate. If a requested fix lands in a generated file,
 say so and point at the upstream repo rather than patching it here.
+
+## Spelling
+
+Write American English in hand-maintained code, comments, docs and test names
+(`behavior`, `canceled`, `honor`), per the
+[WorkOS writing style guide](https://app.notion.com/p/workos/Writing-In-Our-Product-6975915fa9a74150916c669d562afe16#5d75728e33a040efa879e598340b8523).
+Never respell a name or value that already ships: public types, methods,
+parameters, API fields and enum values, generated files, and third-party
+commands or identifiers (e.g. `phpstan analyse`) keep their spelling.
diff --git a/tests/HttpClientTest.php b/tests/HttpClientTest.php
index 4cb5309f..79b3a4f7 100644
--- a/tests/HttpClientTest.php
+++ b/tests/HttpClientTest.php
@@ -309,7 +309,7 @@ public function testResolveUrlPreservesEncodedIdAsSingleSegment(): void
 
         // And confirm the documented caveat: a raw `/` in an unencoded ID is
         // NOT contained — it splits into a new segment. This pins the current
-        // behaviour so a future refactor that silently changes it is caught.
+        // behavior so a future refactor that silently changes it is caught.
         $client3 = new HttpClient(
             apiKey: 'test_key',
             clientId: null,