API Error: tool_reference.tool_name exceeds 64 characters in long conversations with MCP tools

Description

After approximately 116 messages in a conversation using MCP tools, all subsequent MCP tool calls fail with an API validation error related to tool name length.

Error Message

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.116.content.0.tool_result.content.0.tool_reference.tool_name: String should have at most 64 characters"},"request_id":"req_011CXbVTreDNnLEycry4qgrM"}

Reproduction Steps

1. Install the worktree-manager plugin from the marketplace:

``
/plugin marketplace add danielraffel/generous-corp-marketplace
/plugin install worktree-manager@generous-corp-marketplace
``

1. Use Claude Code in a long conversation (~100+ messages)
2. Invoke any MCP tool from the plugin (e.g., /worktree-manager:list)
3. Observe the API error

Observations

• The MCP tool names themselves are within the 64-character limit (longest is mcp__plugin_worktree-manager_worktree__worktree_delete_branch at 61 characters)
• The tool actually executes successfully (returns "success": true) before the error occurs
• The error appears to be in how tool results are formatted/referenced in the conversation history
• Starting a new session resolves the issue
• The error persists across all MCP tool calls once it starts occurring

Environment

• Plugin: worktree-manager from generous-corp-marketplace
• Marketplace repo: https://github.com/danielraffel/generous-corp-marketplace
• The issue appears to affect MCP tool result formatting, not the plugin itself

Expected Behavior

MCP tools should continue to work regardless of conversation length.

Actual Behavior

After ~116 messages, all MCP tool calls fail with tool_reference.tool_name: String should have at most 64 characters error.