diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index e0fa46b..4275e8e 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -2,3 +2,8 @@
 **Vulnerability:** API endpoints in `backend/src/server.ts` taking user input (`projectId`, `jobId`) were directly joined with paths using `join` in `backend/src/store/localStore.ts` without proper sanitization. This allowed attackers to escape the project directory context and overwrite or read arbitrary files by sending payload containing `../` sequences.
 **Learning:** Even internal backend services handling project resources must securely sanitize all parameter values used for file operations to prevent path traversal outside expected boundaries.
 **Prevention:** Always use safe path sanitization utilities, like the implemented `safeJoin` and `toSafeRelativePath` in `backend/src/utils/path.ts`, to securely construct file paths and ensure the final path remains within the intended boundaries.
+
+## 2024-05-24 - [Prototype Pollution in Object Merging]
+**Vulnerability:** In `backend/src/services/aiRouter.ts`, the `mergeExtraBody` function copied properties from a user-provided JSON payload `extraBody` into a target object `payload` without explicitly filtering out `__proto__`, `constructor`, or `prototype` keys. This could allow an attacker to pollute the prototype chain of the target object.
+**Learning:** Functions that merge or copy properties from user-provided objects (especially JSON-parsed bodies) must proactively block or filter keys that can manipulate object prototypes (`__proto__`, `constructor`, `prototype`) to prevent Prototype Pollution vulnerabilities.
+**Prevention:** Always explicitly define a blocklist of dangerous keys, or safely construct objects using `Object.create(null)` when accepting arbitrary key-value pairs from external inputs.
diff --git a/backend/src/services/aiRouter.ts b/backend/src/services/aiRouter.ts
index cf9cb2e..d0b3cce 100644
--- a/backend/src/services/aiRouter.ts
+++ b/backend/src/services/aiRouter.ts
@@ -68,8 +68,8 @@ function isPlainObject(value: unknown): value is Record<string, unknown> {
 
 function mergeExtraBody(payload: JsonObject, extraBody: unknown): void {
   if (!isPlainObject(extraBody)) return;
-  // Never allow overriding required core fields.
-  const blocked = new Set(["model", "messages", "stream"]);
+  // Never allow overriding required core fields or prototype pollution.
+  const blocked = new Set(["model", "messages", "stream", "__proto__", "constructor", "prototype"]);
   for (const [key, value] of Object.entries(extraBody)) {
     if (blocked.has(key)) continue;
     payload[key] = value as any;