MCP Apps SDK callServerTool() fails - "Invalid JSON-RPC message received" in Claude Desktop

[yaniv-golan]

Describe the bug
When an MCP App UI calls app.callServerTool() from the @modelcontextprotocol/ext-apps SDK (v1.0.1), Claude Desktop rejects the message with a Zod validation error. The MCP Apps specification explicitly supports tools/call from UIs during the Interactive Phase, but Claude Desktop's App Bridge does not accept the message format.

To Reproduce

1. Create an MCP server with a tool that has an associated UI
2. In the UI's JavaScript, load the SDK and call callServerTool() after connecting:

import { App } from 'https://cdn.jsdelivr.net/npm/@modelcontextprotocol/ext-apps@1.0.1/+esm';

const app = new App({ name: "test", version: "1.0.0" });

app.connect().then(async () => {
  const result = await app.callServerTool('my-tool', { someArg: 'value' });
});

3. Run the tool in Claude Desktop so the UI loads
4. Observe the error toast in Claude Desktop

Expected behavior
Per the MCP Apps specification (2026-01-26), UIs can send tools/call requests to the host during the Interactive Phase. The host should proxy these to the MCP server and return results to the UI.

Logs

Invalid JSON-RPC message received: [ { "code": "invalid_union", "unionErrors": [ { "issues": [ { "code": "invalid_type", "expected": "object", "received": "string", "path": [ "params" ], "message": "Expected object, received string" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_type", "expected": "object", "received": "string", "path": [ "params" ], "message": "Expected object, received string" }, { "code": "unrecognized_keys", "keys": [ "id" ], "path": [], "message": "Unrecognized key(s) in object: 'id'" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_type", "expected": "object", "received": "undefined", "path": [ "result" ], "message": "Required" }, { "code": "unrecognized_keys", "keys": [ "method", "params" ], "path": [], "message": "Unrecognized key(s) in object: 'method', 'params'" } ], "name": "ZodError" }, { "issues": [ { "code": "invalid_type", "expected": "object", "received": "undefined", "path": [ "error" ], "message": "Required" }, { "code": "unrecognized_keys", "keys": [ "method", "params" ], "path": [], "message": "Unrecognized key(s) in object: 'method', 'params'" } ], "name": "ZodError" } ], "path": [], "message": "Invalid input" } ]

Additionally, the MCP server connection drops after the error ("Unable to reach [server-name]").

Additional context

• Claude Desktop: Latest version (Jan 28, 2026)
• SDK: @modelcontextprotocol/ext-apps@1.0.1
• OS: macOS

The Zod errors suggest a mismatch between what the SDK sends via postMessage and what Claude Desktop's App Bridge schema expects. This blocks UI-initiated tool calls, which are needed for use cases like polling for progress updates during long-running operations.

[yaniv-golan]

resources/read is also affected.

[ochafik]

Hey @yaniv-golan , thanks for the report!

The callServerTool syntax in your example needs fixing:

From:

const result = await app.callServerTool('my-tool', { someArg: 'value' });

To:

const result = await app.callServerTool({name: 'my-tool', arguments: { someArg: 'value' }});

Let us know if you still face issues, happy to reopen this.