[BUG] MCP tool call stays "Running..." even though marionette_mcp connect succeeds

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When I use Claude Code with marionette_mcp, the MCP tool call stays in Running... forever and never returns in the UI.

This happens when calling the connect tool with a valid Flutter VM service URI.

My MCP config is:

```json
{
"mcpServers": {
"marionette": {
"type": "stdio",
"command": "marionette_mcp",
"args": []
}
}
}

The Claude Code UI shows the MCP tool call as still running, but the server log shows that the request actually succeeded:

[INFO][VmServiceContext] Connecting to app at ws://127.0.0.1:56608/dGVlzZ_Y7lQ=/ws
[INFO][VmServiceConnector] Connecting to VM service at ws://127.0.0.1:56608/dGVlzZ_Y7lQ=/ws
[INFO][VmServiceConnector] Connected to isolate: isolates/...
[FINE][VmServiceConnector] Calling extension: marionette.getVersion with args: {}

I also reproduced this by calling marionette_mcp manually over stdio, and it connects successfully there too.

So the MCP server appears to complete the request successfully, but Claude Code never resolves the tool call in the UI.

What Should Happen?

``md
Claude Code should return the connect tool result normally instead of leaving the MCP call in Running...`.

Error Messages/Logs

Steps to Reproduce

1. Configure an MCP server in .mcp.json:

```json
{
"mcpServers": {
"marionette": {
"type": "stdio",
"command": "marionette_mcp",
"args": []
}
}
}

1. Fully quit and restart Claude Code.
2. Open the project and ensure the marionette MCP server is shown as connected in /mcp.
3. Ask Claude Code to call only the connect MCP tool with a valid Flutter VM service URI, for example:

ws://127.0.0.1:56608/dGVlzZ_Y7lQ=/ws

1. Observe that the Claude Code UI stays at Running... forever.
2. Check /tmp/marionette_mcp.log and observe that the server-side connect actually succeeded.
3. Optionally, run marionette_mcp manually over stdio with the same URI and observe that it succeeds there too.

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

v2.1.79

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_