fix: preserve falsy structuredContent values (0, false, '', null)

- Replace truthy checks with 'structuredContent' in result to detect property existence
- Add comprehensive tests for falsy values: 0, false, empty string, null
- Ensures valid structured outputs are not dropped due to falsy evaluation
- Addresses P1 feedback from @chatgpt-codex-connector about data loss
- Maintains parity with Python implementation behavior

Author: boice-sealint

.gitignore

@@ -148,5 +148,4 @@ openai-agents-python
 bundled/
 
 .wrangler/
-.dev.vars
-package-lock.json
+.dev.vars

packages/agents-core/src/mcp.ts

@@ -477,19 +477,19 @@ export function mcpToFunctionTool(
 
     // JS ergonomics: return objects/arrays; include structuredContent when enabled
     if (result.content && result.content.length === 1) {
-      if (server.useStructuredContent && (result as any).structuredContent) {
+      if (server.useStructuredContent && 'structuredContent' in result) {
         return [result.content[0], (result as any).structuredContent];
       }
       return result.content[0];
     } else if (result.content && result.content.length > 1) {
       const outputs: any[] = result.content.map((c) => c);
-      if (server.useStructuredContent && (result as any).structuredContent) {
+      if (server.useStructuredContent && 'structuredContent' in result) {
         outputs.push((result as any).structuredContent);
       }
       return outputs;
     } else if (
       server.useStructuredContent &&
-      (result as any).structuredContent
+      'structuredContent' in result
     ) {
       return (result as any).structuredContent;
     }

packages/agents-core/test/mcpStructuredContent.test.ts

@@ -198,4 +198,78 @@ describe('MCP structuredContent handling', () => {
       { foo: 1 },
     ]);
   });
+
+  it('preserves falsy structuredContent values when enabled', async () => {
+    const server = new StubServer(
+      's',
+      [
+        {
+          name: 't',
+          description: '',
+          inputSchema: {
+            type: 'object',
+            properties: {},
+            required: [],
+            additionalProperties: false,
+          },
+        },
+      ],
+      true,
+    );
+
+    // Test different falsy values
+    const falsyValues = [0, false, '', null];
+    
+    for (const falsyValue of falsyValues) {
+      // Test with single content + falsy structuredContent
+      (server as any).callTool = async () => ({
+        content: [{ type: 'text', text: 'hello' }],
+        structuredContent: falsyValue,
+      });
+
+      const tool = mcpToFunctionTool(
+        {
+          name: 't',
+          description: '',
+          inputSchema: {
+            type: 'object',
+            properties: {},
+            required: [],
+            additionalProperties: false,
+          },
+        },
+        server,
+        false,
+      );
+
+      const out = await tool.invoke({} as any, '{}');
+      expect(out).toEqual([{ type: 'text', text: 'hello' }, falsyValue]);
+    }
+
+    // Test with no content + falsy structuredContent
+    for (const falsyValue of falsyValues) {
+      (server as any).callTool = async () => ({
+        content: [],
+        structuredContent: falsyValue,
+      });
+
+      const tool = mcpToFunctionTool(
+        {
+          name: 't',
+          description: '',
+          inputSchema: {
+            type: 'object',
+            properties: {},
+            required: [],
+            additionalProperties: false,
+          },
+        },
+        server,
+        false,
+      );
+
+      const out = await tool.invoke({} as any, '{}');
+      expect(out).toEqual(falsyValue);
+    }
+  });
 });