[BUG] ToolSearch Creates tool_reference Objects Exceeding 64-Character API Limit makes claude code unresponsive

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?

Environment

Claude Code Version: 2.1.29 (latest as of 2026-02-01)
OS: macOS tahoe
MCP Server: IBM FlashSystem MCP (stdio-based)
Comparison Client: Codex (handles same MCP server without issues)

Description

Claude Code's ToolSearch tool creates tool_reference objects that exceed the Claude API's 64-character tool name limit, causing complete conversation failure and making certain MCP servers unusable.

Critical Impact

Severity: Critical - Completely breaks Claude Code functionality:

• Entire conversation becomes unresponsive after ToolSearch executes
• Claude Code cannot display any responses (stuck/frozen)
• Users must disable affected MCP servers to recover
• Prevents use of legitimate MCP servers with reasonable tool names

Error Details

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "messages.154.content.0.tool_result.content.0.tool_reference.tool_name: String should have at most 64 characters"
  },
  "request_id": "req_011CXgofvhjjsvvX622ot7ob"
}

Error Location: tool_result.content.0.tool_reference.tool_name

This indicates the issue is in how Claude Code constructs tool_reference objects, not in the actual tool names themselves.

What Should Happen?

The 64-character limit is reasonable, but Claude Code shouldn't be adding metadata that pushes valid tool names over the limit.

MCP servers with tool names under 64 characters should work
Tool discovery should succeed without API errors
tool_reference objects should respect API limits
Claude Code should handle tools the same way Codex does

Error Messages/Logs

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "messages.154.content.0.tool_result.content.0.tool_reference.tool_name: String should have at most 64 characters"
  },
  "request_id": "req_011CXgofvhjjsvvX622ot7ob"
}

Steps to Reproduce

1. Configure an MCP server with tools that have names longer than 64 characters (e.g., IBM FlashSystem MCP)

(https://github.com/IBM/ibm-flashsystems-mcpserver)

1. Load the MCP server in Claude Code
2. Use ToolSearch or similar tool discovery mechanism
3. Claude Code creates tool_reference objects with names exceeding 64 characters
4. When sending the tool results back to the Claude API, the conversation fails with "String should have at most 64 characters" error
5. Claude Code becomes stuck/unresponsive

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.29

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

The exact same MCP server configuration works flawlessly in Codex

When Claude Code's ToolSearch returns MCP tools, it creates tool_reference objects that apparently include:

Additional metadata
Prefixes or namespacing
MCP server identifiers
Or other data that pushes the total over 64 characters
This transformation happens inside Claude Code, not in the MCP server.