[BUG] MCP client rejects valid tools/list response due to strict validation of ttlMs/cacheScope fields (Roblox Studio MCP server)

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?

Description

A user-configured stdio MCP server connects successfully (status: connected) but always reports tool_count: 0, across brand-new sessions, app restarts, and even a full OS reboot. The problem persists indefinitely — no retry ever recovers it.

Environment

• Claude Code desktop app (Code tab), Windows 11
• MCP server: Roblox Studio's own official MCP bridge (StudioMCP.exe), launched via a project .mcp.json:

```json
{
"mcpServers": {
"Roblox_Studio": {
"command": "cmd.exe",
"args": ["/c", "cd /d %LOCALAPPDATA%\\Roblox && .\\mcp.bat"]
}
}
}
Root cause (found via local logs)
%LOCALAPPDATA%\claude-cli-nodejs\Cache\<project>\mcp-logs-Roblox-Studio\*.jsonl shows the server connects fine, then tools/list fails client-side schema validation and is retried/abandoned:

Connection established with capabilities: {"hasTools":true, ... "negotiatedProtocolVersion":"2026-07-28"}
tools/list failed (Invalid result for tools/list: [
{ "path": ["ttlMs"], "code": "invalid_type", "message": "Invalid input" },
{ "code": "invalid_value", "values": ["public","private"], "path": ["cacheScope"], "message": "Invalid input" }
]); retrying in 250ms
...
Failed to fetch tools: Invalid result for tools/list: [...]
The server's tools/list response apparently includes extra per-tool/result metadata fields (ttlMs, cacheScope) tied to a newer MCP protocol revision. The Claude Code MCP client's schema validator rejects the whole response instead of tolerating/ignoring the unrecognized fields, permanently leaving the server at 0 tools for the session's lifetime.

Verification that the server itself is fine
I bypassed Claude Code and manually spoke MCP JSON-RPC to the exact same spawn command (cmd.exe /c cd /d %LOCALAPPDATA%\Roblox && .\mcp.bat) via a small Node script emulating the client. The server responded correctly and returned a full, valid list of 28 tools (upload_image, search_game_tree, etc.) in under 100ms, for multiple negotiated protocol versions. So the Roblox Studio-side bridge is working correctly; the failure is specific to Claude Code's response validation.

Expected behavior
tools/list results with additional/unrecognized fields (like ttlMs, cacheScope) should not cause the entire tool list to be discarded — either the schema should tolerate/pass through extra fields, or unknown-field validation errors should not zero out an otherwise valid tool list.

Actual behavior
The MCP server is marked connected but exposes tool_count: 0 permanently, with no way to recover short of the underlying bug being fixed (confirmed via session_connectors_status across several fresh sessions, app restarts, and a full reboot).

What Should Happen?

The tools/list response from the Roblox Studio MCP server should be accepted and its tools exposed normally, even though it includes extra metadata fields (ttlMs, cacheScope) from a newer MCP protocol revision. Unrecognized/extra fields in a tool's result should be tolerated (or ignored) rather than causing the entire tools/list response to be rejected and the server permanently stuck at 0 tools.

Error Messages/Logs

{"debug":"Starting connection with timeout of 30000ms", ...}
{"debug":"Successfully connected (transport: stdio) in 169ms", ...}
{"debug":"Connection established with capabilities: {\"hasTools\":true,\"hasPrompts\":false,\"hasResources\":false,\"hasResourceSubscribe\":false,\"serverVersion\":{\"name\":\"RobloxStudio\",\"version\":\"1.0.0\"},\"protocolEra\":\"modern\",\"negotiatedProtocolVersion\":\"2026-07-28\"}", ...}
{"debug":"tools/list failed (Invalid result for tools/list: [\n  {\n    \"expected\": \"number\",\n    \"code\": \"invalid_type\",\n    \"path\": [\n      \"ttlMs\"\n    ],\n    \"message\": \"Invalid input\"\n  },\n  {\n    \"code\": \"invalid_value\",\n    \"values\": [\n      \"public\",\n      \"private\"\n    ],\n    \"path\": [\n      \"cacheScope\"\n    ],\n    \"message\": \"Invalid input\"\n  }\n]); retrying in 250ms", ...}
{"debug":"tools/list failed (Invalid result for tools/list: [...]); retrying in 500ms", ...}
{"debug":"tools/list failed (Invalid result for tools/list: [...]); retrying in 1000ms", ...}
{"error":"Failed to fetch tools: Invalid result for tools/list: [\n  {\n    \"expected\": \"number\",\n    \"code\": \"invalid_type\",\n    \"path\": [\n      \"ttlMs\"\n    ],\n    \"message\": \"Invalid input\"\n  },\n  {\n    \"code\": \"invalid_value\",\n    \"values\": [\n      \"public\",\n      \"private\"\n    ],\n    \"path\": [\n      \"cacheScope\"\n    ],\n    \"message\": \"Invalid input\"\n  }\n]", ...}
(Log source: %LOCALAPPDATA%\claude-cli-nodejs\Cache\C--github-project1\mcp-logs-Roblox-Studio\*.jsonl)

Resultado final observado via session_connectors_status:

{"name":"Roblox_Studio","kind":"user","transport":"stdio","status":"connected","tool_count":0}

Steps to Reproduce

Para o campo "Steps to Reproduce":

1. Install Roblox Studio (recent version that ships the official StudioMCP.exe MCP bridge).
2. Add a project .mcp.json pointing at it:
   {
     "mcpServers": {
       "Roblox_Studio": {
         "command": "cmd.exe",
         "args": ["/c", "cd /d %LOCALAPPDATA%\\Roblox && .\\mcp.bat"]
       }
     }
   }
3. Open Roblox Studio (single instance) and open a Claude Code session in that project.
4. Check the server's status, e.g. via the "session_connectors_status" tool/equivalent MCP inspector — it shows status "connected" but tool_count: 0.
5. Try asking Claude to use any Roblox Studio tool — none are available.
6. Open a brand-new Claude Code session (or restart the app, or reboot the OS) and repeat step 4 — tool_count is still 0 every time.
7. Inspect %LOCALAPPDATA%\claude-cli-nodejs\Cache\<project>\mcp-logs-Roblox-Studio\*.jsonl — it shows the connection succeeding but every tools/list call failing schema validation on the "ttlMs" and "cacheScope" fields (see Error Messages/Logs), so the client gives up and reports 0 tools.
8. As independent verification, bypass Claude Code and speak raw MCP JSON-RPC (initialize + notifications/initialized + tools/list) directly to the same "cmd.exe /c cd /d %LOCALAPPDATA%\Roblox && .\mcp.bat" command via a small script — the server responds correctly with a full list of 28 tools, proving the Roblox Studio side is not the problem.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Versão 2.9939.2 (d3e504)

Platform

Anthropic API

Operating System

Windows 11

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_