diff --git a/ui/src/ui/chat/tool-helpers.test.ts b/ui/src/ui/chat/tool-helpers.test.ts
index 7965a7df93e..18fdc0d72a4 100644
--- a/ui/src/ui/chat/tool-helpers.test.ts
+++ b/ui/src/ui/chat/tool-helpers.test.ts
@@ -159,6 +159,10 @@ describe("tool-helpers", () => {
     it("returns null for undefined arguments", () => {
       expect(formatToolPayloadForSidebar(undefined)).toBeNull();
     });
+
+    it("returns null for null arguments", () => {
+      expect(formatToolPayloadForSidebar(null)).toBeNull();
+    });
   });
 
   describe("buildToolSidebarContent", () => {
diff --git a/ui/src/ui/chat/tool-helpers.ts b/ui/src/ui/chat/tool-helpers.ts
index d95a695b2a9..0ceb6351c6b 100644
--- a/ui/src/ui/chat/tool-helpers.ts
+++ b/ui/src/ui/chat/tool-helpers.ts
@@ -27,7 +27,7 @@ export function formatToolOutputForSidebar(text: string): string {
  * Uses JSON code blocks for structured values and preserves plain text input.
  */
 export function formatToolPayloadForSidebar(value: unknown): string | null {
-  if (value === undefined) {
+  if (value === undefined || value === null) {
     return null;
   }
   if (typeof value === "string") {