diff --git a/shared/constants.go b/shared/constants.go
index a3d6a41..937d84e 100644
--- a/shared/constants.go
+++ b/shared/constants.go
@@ -8,5 +8,6 @@ const (
 	// where multiple releases in a day are required
 	Version = "0.0.1 pre-alpha"
 	// Username for the server actor
-	ServerActorName = "server.actor"
+	ServerActorName    = "server.actor"
+	FeedUsernameSuffix = "-feed"
 )
diff --git a/shared/validation.go b/shared/validation.go
new file mode 100644
index 0000000..eb72dbc
--- /dev/null
+++ b/shared/validation.go
@@ -0,0 +1,25 @@
+package shared
+
+import (
+	"strings"
+
+	"git.mstar.dev/mstar/goutils/sliceutils"
+)
+
+var forbiddenUsernames = []string{
+	"server.actor",
+	"feed",
+}
+
+// Reports whether a given user name is valid (for non-internal systems)
+//
+// TODO: Include compat check for Mastodon?
+func ValidateUsername(username string) bool {
+	if strings.HasSuffix(username, FeedUsernameSuffix) {
+		return false
+	}
+	if sliceutils.Contains(forbiddenUsernames, username) {
+		return false
+	}
+	return true
+}