diff --git a/open-sse/handlers/chatCore/sanitization.ts b/open-sse/handlers/chatCore/sanitization.ts
index 62b43615ed..b93f3ac2c3 100644
--- a/open-sse/handlers/chatCore/sanitization.ts
+++ b/open-sse/handlers/chatCore/sanitization.ts
@@ -46,7 +46,7 @@ export function sanitizeChatRequestBody(
   }
 
   if (Array.isArray(body.tools)) {
-    body.tools = body.tools.filter((tool: Record<string, unknown>) => {
+    const tools = body.tools.filter((tool: Record<string, unknown>) => {
       const toolType = typeof tool.type === "string" ? tool.type : "";
       if (toolType && toolType !== "function" && !tool.function && tool.name === undefined) {
         return true;
@@ -56,7 +56,7 @@ export function sanitizeChatRequestBody(
       return name && String(name).trim().length > 0;
     });
 
-    body.tools = body.tools.map((tool) => sanitizeOpenAITool(tool) as (typeof body.tools)[number]);
+    body.tools = tools.map((tool) => sanitizeOpenAITool(tool));
   }
 
   return body;
diff --git a/tests/unit/chatcore-extracted-modules-3821.test.ts b/tests/unit/chatcore-extracted-modules-3821.test.ts
index 009412f98e..e8a689f6ca 100644
--- a/tests/unit/chatcore-extracted-modules-3821.test.ts
+++ b/tests/unit/chatcore-extracted-modules-3821.test.ts
@@ -49,6 +49,7 @@ test("sanitizeChatRequestBody: strips empty message name and filters nameless to
       ],
       tools: [
         { type: "function", function: { name: "real_tool", parameters: {} } },
+        { type: "web_search_preview" },
         { type: "function", function: { name: "" } }, // dropped — empty name
         { type: "function", function: {} }, // dropped — no name
       ],
@@ -62,8 +63,9 @@ test("sanitizeChatRequestBody: strips empty message name and filters nameless to
   assert.equal(messages[1].name, "keepme", "non-empty name kept");
 
   const tools = out.tools as Array<Record<string, unknown>>;
-  assert.equal(tools.length, 1, "only the named tool survives");
+  assert.equal(tools.length, 2, "the named function and built-in tool survive");
   assert.equal((tools[0].function as Record<string, unknown>).name, "real_tool");
+  assert.deepEqual(tools[1], { type: "web_search_preview" });
 });
 
 test("checkIdempotencyCache returns { hit:null, idempotencyKey } on a miss", async () => {