Bug: Custom remote MCP server tools not accessible in claude.ai chat UI (Tool not found)

Environment

• claude.ai standard chat UI (web)
• Custom remote MCP server hosted via NGROK
• Server registered in Settings → Connectors, refresh works, connection is healthy
• Server-side confirmed working normally
• Anthropic cloud access whitelisted on server side

Problem

Claude cannot invoke any tools from the custom remote MCP server in chat, despite the server appearing connected and tools being visible in the Connectors settings page.

Symptoms

• Connectors page shows the server and its tool list correctly
• Claude cannot call any tools from that server in chat — all attempts return Tool not found
• The same server works correctly from other AI clients (GPT, Codex)

Attempted tool name patterns (all failed with Tool not found)

• mcp__<server-name>__<tool>
• mcp__<url-segments>__<tool>
• Various capitalizations and separator combinations

Related issues

• #23251 — Custom MCP server tools not accessible in claude.ai standard chat
• #53489 — claude.ai MCP connectors not passed into session
• #11146 — MCP server access missing in web sessions

Root cause hypothesis

Anthropic's cloud infrastructure is not passing custom remote connector tool lists into Claude's context. The connection itself succeeds (OAuth completes, tools visible in Connectors UI), but the tools are invisible to Claude during the conversation.