diff --git a/SPEC.md b/SPEC.md
index 32deae2..98cecfb 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -391,4 +391,4 @@ Abuse control is quotas, size caps and rate limits: per-IP connection and `SEND`
 2 bad version   5 auth failed     8 rate limited
 ```
 
-A status is the first byte of every response body (§6.1), which MAY be followed by a UTF-8 reason string. Clients MUST NOT parse the reason.
+A status is the first byte of every response body (§6.1), which MAY be followed by a UTF-8 reason string. Clients MUST NOT parse the reason. Codes not listed are unassigned: a client MUST treat one it does not know as a failure of the operation, MUST NOT retry automatically, and SHOULD surface the number.