MCP connections not visible to model on first turn of Remote Trigger

Bug Description

MCP tools configured on a Remote Trigger via mcp_connections are not visible/accessible to the model on the first message turn. The model systematically reports "MCP servers Gmail and Zeplan are not connected to this session" and lists only base tools (Bash, Read, Write, Edit, Glob, Grep, WebFetch, ToolSearch).

However, when the user manually sends a follow-up message saying "you have access to these tools", the model immediately uses them successfully. This proves the MCP connections ARE properly configured and functional — the model just doesn't see them on the first turn.

Reproduction

1. Create a Remote Trigger with mcp_connections (e.g., Gmail + a custom MCP server)
2. Run the trigger
3. The model will report MCP tools are not available
4. Send a follow-up "you have access" → model uses MCP tools successfully

Diagnostic Results

• ToolSearch select:mcp__Gmail__gmail_get_profile → no results
• ToolSearch select:mcp__claude_ai_Gmail__gmail_get_profile → no results
• ToolSearch query:"+Gmail" → no results
• ToolSearch query:"mcp" → only returns WebFetch (not an MCP tool)

MCP tools are neither in the base tool list nor in deferred tools. Yet they ARE callable when the model is forced to try.

What was tried (all failed)

• Anti-ToolSearch instructions in the prompt
• Adding MCP tools to allowed_tools with both mcp__Gmail__* and mcp__claude_ai_Gmail__* prefixes
• Listing all 25 MCP tool names explicitly in allowed_tools
• Removing ToolSearch from allowed_tools
• Adding a CLAUDE.md with tool loading instructions in the git source
• Removing CLAUDE.md entirely
• Multi-turn events (2 user messages in the events array)
• Simplifying the prompt to not mention MCP/tools at all
• All combinations of the above

Nothing works. Only manual user intervention ("you have access") fixes it.

Environment

• Models affected: Claude Opus 4.6 AND Claude Sonnet 4.6 (both tested)
• Trigger ID: trig_01EfmhaMJ9rFdANLP9m2hP3B
• MCP connections: Gmail (connector via gmail.mcp.claude.com/mcp) + custom Zeplan MCP (Cloudflare Workers)
• Platform: macOS, Claude Code CLI
• Note: This trigger was working correctly ~1 week ago with the exact same configuration. No changes were made to the trigger config or MCP servers before it broke.

Expected Behavior

MCP tools defined in mcp_connections should be visible and callable by the model from the first turn, without requiring user intervention.

Possibly Related Issues

• #25894 (MCP tools not loaded as deferred tools when using mcp-remote proxy)
• #38245 (MCP server tools discovered but not accessible via ToolSearch)