Fix null toolOutput being sent as text 'null'

Check for both null and undefined before delivering tool-result
notification. Previously null passed through and was stringified.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ochafik

src/openai/transport.test.ts

@@ -431,7 +431,9 @@ describe("OpenAITransport", () => {
 
     test("converts null _meta to undefined in tool result", async () => {
       // Simulate null being set (e.g., from JSON parsing where null is valid)
-      (mockOpenAI as unknown as { toolResponseMetadata: null }).toolResponseMetadata = null;
+      (
+        mockOpenAI as unknown as { toolResponseMetadata: null }
+      ).toolResponseMetadata = null;
 
       const transport = new OpenAITransport();
       const messages: unknown[] = [];
@@ -452,6 +454,28 @@ describe("OpenAITransport", () => {
       expect(toolResultNotification?.params?._meta).toBeUndefined();
     });
 
+    test("does not deliver tool-result when toolOutput is null", async () => {
+      // Simulate null being set (e.g., from JSON parsing)
+      (mockOpenAI as unknown as { toolOutput: null }).toolOutput = null;
+
+      const transport = new OpenAITransport();
+      const messages: unknown[] = [];
+      transport.onmessage = (msg) => {
+        messages.push(msg);
+      };
+
+      transport.deliverInitialState();
+
+      await new Promise((resolve) => setTimeout(resolve, 0));
+
+      const toolResultNotification = messages.find(
+        (m: unknown) =>
+          (m as { method?: string }).method === "ui/notifications/tool-result",
+      );
+      // Should NOT deliver tool-result when toolOutput is null
+      expect(toolResultNotification).toBeUndefined();
+    });
+
     test("does not deliver notifications when data is missing", async () => {
       delete mockOpenAI.toolInput;
       delete mockOpenAI.toolOutput;

src/openai/transport.ts

@@ -495,8 +495,8 @@ export class OpenAITransport implements Transport {
       });
     }
 
-    // Deliver tool output if available
-    if (this.openai.toolOutput !== undefined) {
+    // Deliver tool output if available (check for both null and undefined)
+    if (this.openai.toolOutput != null) {
       queueMicrotask(() => {
         this.onmessage?.({
           jsonrpc: "2.0",