diff --git a/sigil/errors.py b/sigil/errors.py
index ca80da5..8cbe2e3 100644
--- a/sigil/errors.py
+++ b/sigil/errors.py
@@ -63,4 +63,8 @@ class CodeExpired(SigilError):
 
 
 class CodeUsed(SigilError):
-    """One-time code already redeemed. Codes are single use."""
\ No newline at end of file
+    """One-time code already redeemed. Codes are single use."""
+
+class RateLimited(SigilError):
+    """A rate limit was exhausted. Distinct so callers can back off rather
+    than treat it as a credential failure."""