diff --git a/docs/plans/2026-08-05-inter-app-comms-implementation.md b/docs/plans/2026-08-05-inter-app-comms-implementation.md
index 46245b9a..f62afbac 100644
--- a/docs/plans/2026-08-05-inter-app-comms-implementation.md
+++ b/docs/plans/2026-08-05-inter-app-comms-implementation.md
@@ -1460,8 +1460,14 @@ export function implement<Procedures extends AnyProcedures>(
     contract,
 
     async invoke(procedureName, payload, identity) {
-      const def = contract.procedures[procedureName as keyof Procedures];
-      const handler = handlers[procedureName as keyof Procedures];
+      // Object.hasOwn, not plain indexing: "constructor", "toString" and every
+      // other Object.prototype member otherwise resolve as truthy, and a
+      // prototype member carries no `permission`, so the gate below is skipped
+      // entirely and an unintended function runs with attacker-controlled input.
+      const known =
+        Object.hasOwn(contract.procedures, procedureName) && Object.hasOwn(handlers, procedureName);
+      const def = known ? contract.procedures[procedureName as keyof Procedures] : undefined;
+      const handler = known ? handlers[procedureName as keyof Procedures] : undefined;
       if (!def || !handler) {
         return failure(RPC_ERROR_CODES.unknown, `No procedure '${contract.name}/${procedureName}'`);
       }
@@ -2313,8 +2319,12 @@ export async function handleRpcRequest(
   }
 
   const [, , , serviceName, procedureName] = url.pathname.split("/");
-  const service = serviceName ? services.get(serviceName) : undefined;
-  if (!service || !procedureName) {
+  // Constrain the segment before it is used as a lookup key, so a prototype
+  // member can never be reached even if a future implement() regresses.
+  const SAFE_SEGMENT = /^[A-Za-z0-9_-]+$/;
+  const service =
+    serviceName && SAFE_SEGMENT.test(serviceName) ? services.get(serviceName) : undefined;
+  if (!service || !procedureName || !SAFE_SEGMENT.test(procedureName)) {
     return json({ ok: false, code: "RPC_UNKNOWN", message: "Unknown procedure", retryable: false });
   }