diff --git a/packages/client-runtime/src/mcpApps/host.test.ts b/packages/client-runtime/src/mcpApps/host.test.ts
index 898588737c31f..76f6574f5012f 100644
--- a/packages/client-runtime/src/mcpApps/host.test.ts
+++ b/packages/client-runtime/src/mcpApps/host.test.ts
@@ -203,8 +203,10 @@ describe("makeMcpAppHost", () => {
   });
 
   it("proxies tools/call and reports a refusal as a JSON-RPC error", async () => {
+    const calls: Array<unknown> = [];
     const { host, sent } = setup({
       callTool: async (input) => {
+        calls.push(input.arguments);
         if (input.name === "delete_everything")
           throw new McpAppHostRefusal({ message: "Declined by the user." });
         return { content: [], structuredContent: { ok: true } };
@@ -217,7 +219,14 @@ describe("makeMcpAppHost", () => {
       method: "tools/call",
       params: { name: "delete_everything", arguments: {} },
     });
+    host.receive({
+      jsonrpc: "2.0",
+      id: 9,
+      method: "tools/call",
+      params: { name: "refresh", arguments: { since: undefined, limit: 5 } },
+    });
     await flush();
+    expect(calls.at(-1)).toStrictEqual({ limit: 5 });
     expect(sent).toContainEqual({
       jsonrpc: "2.0",
       id: 7,
diff --git a/packages/client-runtime/src/mcpApps/host.ts b/packages/client-runtime/src/mcpApps/host.ts
index 847771fb09dbd..42938a6d7f005 100644
--- a/packages/client-runtime/src/mcpApps/host.ts
+++ b/packages/client-runtime/src/mcpApps/host.ts
@@ -247,10 +247,16 @@ export function makeMcpAppHost(options: McpAppHostOptions): McpAppHost {
           fail(id, -32602, "tools/call needs a tool name and object arguments.");
           return;
         }
+        let jsonArgs: Record<string, unknown>;
+        try {
+          // postMessage keeps undefined and other non-JSON values the RPC encoding rejects.
+          jsonArgs = JSON.parse(JSON.stringify(args));
+        } catch {
+          fail(id, -32602, "tools/call arguments must be JSON.");
+          return;
+        }
         answer(id, () =>
-          options
-            .callTool({ name, arguments: args as Record<string, unknown> })
-            .then(normalizeMcpAppToolResult),
+          options.callTool({ name, arguments: jsonArgs }).then(normalizeMcpAppToolResult),
         );
         return;
       }