diff --git a/docs/plans/2026-08-04-authz-permissions-implementation.md b/docs/plans/2026-08-04-authz-permissions-implementation.md
index 10c01502..f8b6ee49 100644
--- a/docs/plans/2026-08-04-authz-permissions-implementation.md
+++ b/docs/plans/2026-08-04-authz-permissions-implementation.md
@@ -2155,6 +2155,20 @@ function wantsJson(ctx: Context): boolean {
   return accept.includes("application/json") && !accept.includes("text/html");
 }
 
+/**
+ * Header values must be Latin-1, so a localized path would otherwise throw
+ * inside `new Response` and 500 on a denial path. Encode ONLY the codepoints
+ * that cannot be sent: encodeURI would also escape "%", corrupting a target
+ * that already carries a percent-encoded return path.
+ */
+function headerSafePath(value: string): string {
+  let out = "";
+  for (const character of value) {
+    out += character.codePointAt(0)! <= 0x7f ? character : encodeURIComponent(character);
+  }
+  return out;
+}
+
 /** Reject anything that could navigate off-site or inject a header. */
 function isLocalPath(value: string): boolean {
   if (!value.startsWith("/") || value.startsWith("//") || value.includes("\\")) return false;
@@ -2196,9 +2210,7 @@ export function guardPermission(permission: string, options: GuardOptions = {}):
         return new Response(null, {
           status: 303,
           headers: {
-            // Header values must be Latin-1; a localized path like /accounts
-            // in non-ASCII would otherwise throw and 500 on a denial path.
-            location: encodeURI(options.redirectTo),
+            location: headerSafePath(options.redirectTo),
             "cache-control": "private, no-store",
           },
         });