diff --git a/src/tools/mcp/mcp.test.mjs b/src/tools/mcp/mcp.test.mjs
index ff94659f0..0df9bb955 100644
--- a/src/tools/mcp/mcp.test.mjs
+++ b/src/tools/mcp/mcp.test.mjs
@@ -233,6 +233,7 @@ test('resources are listed and read, and tools name their UI resource', async ()
   });
   assert.deepEqual((await call('tools/list')).tools[0]._meta, {
     ui: { resourceUri: 'ui://fixture/view' },
+    'ui/resourceUri': 'ui://fixture/view',
   });
   assert.deepEqual((await call('resources/list')).resources, [
     {
diff --git a/src/tools/mcp/protocol.js b/src/tools/mcp/protocol.js
index 67858833e..446d01869 100644
--- a/src/tools/mcp/protocol.js
+++ b/src/tools/mcp/protocol.js
@@ -40,7 +40,14 @@ export function toMcpTools(catalog, { descriptions = {}, decorate } = {}) {
       inputSchema: tool.inputSchema,
       annotations: { title, ...tool.annotations },
       ...(tool.ui
-        ? { _meta: { ui: { resourceUri: tool.ui.resourceUri } } }
+        ? {
+            // The flat key is the earlier form of the same field, which some
+            // hosts still read; the MCP Apps SDK sends both.
+            _meta: {
+              ui: { resourceUri: tool.ui.resourceUri },
+              'ui/resourceUri': tool.ui.resourceUri,
+            },
+          }
         : {}),
     };
     return decorate