[BUG]New session fails with tool name validation error from a connected MCP connector

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

Starting a brand new chat session throws a validation error before I send any message:

tools.138.FrontendRemoteMcpToolDefinition.name: String should match pattern '^[a-zA-Z0-9_-]{1,64}$'

This appears to be a tool definition sourced from one of my connected MCP connectors whose combined
tool name (server-id prefix + tool name) exceeds the 64-character limit or otherwise fails the
required ^[a-zA-Z0-9_-]{1,64}$ naming pattern. The whole session's tool list build seems to fail
validation because of this one entry, blocking normal use of the home screen.

Steps to reproduce

1. Open a new chat session with multiple MCP connectors linked (I have a large number connected,

including a Facebook/Instagram management connector whose native tool names are quite long,
e.g. facebook_get_conversation_messages, instagram_create_carousel_container).

1. The error appears immediately: tools.138.FrontendRemoteMcpToolDefinition.name: String should

match pattern '^[a-zA-Z0-9_-]{1,64}$'

Expected behavior

A single connector's tool with an invalid/too-long name should either be dropped/skipped, or the
platform should truncate/hash long combined names, rather than failing validation for the entire
session's toolset.

Suspected cause

Combined tool names appear to be built as mcp__<36-char-server-id>__<tool-name>. For connectors
with long native tool names, this prefix pushes the combined string past 64 characters
(e.g. 5 + 36 + 2 + ~35 = ~78 chars), which fails the {1,64} length cap in the naming pattern.

Environment

• Client: Claude (chat/session home screen)
• Model: Claude Sonnet 5
• Date observed: 07-11-26

What Should Happen?

A single connector's tool with an invalid or too-long name should be dropped or skipped
(or the platform should truncate/hash long combined names), rather than failing
validation for the entire session's toolset and blocking the home screen.

Error Messages/Logs

tools.138.FrontendRemoteMcpToolDefinition.name: String should match pattern '^[a-zA-Z0-9_-]{1,64}$'

Steps to Reproduce

1. Have multiple MCP connectors linked to the account, including at least one whose

native tool names are long (e.g. facebook_get_conversation_messages,
instagram_create_carousel_container).

1. Open a brand new chat session.
2. The session fails before any message is sent, with:

tools.138.FrontendRemoteMcpToolDefinition.name: String should match pattern
'^[a-zA-Z0-9_-]{1,64}$'

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.149

Claude Code Version

v2.1.150

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_