claude.ai account MCP connectors intermittently fail to attach at session start (no connection attempt logged)

Environment

• Claude Code CLI v2.1.209 and v2.1.215, macOS (Apple Silicon, Darwin 25.5.0)
• Two user-added remote MCP connectors configured on the claude.ai account (both OAuth, streamable HTTP, attached via mcp-proxy.anthropic.com / "claude.ai proxy transport")

Symptom

Intermittently (~3 of the last 20 sessions on one machine), a new CLI session starts with none of the account's claude.ai connectors attached — their tools never appear in the session. Sessions started minutes before/after on the same machine and account attach both connectors normally.

Evidence

• Both connectors always vanish together, though they point at two unrelated servers with independent OAuth registrations — so the common failing layer appears to be the account-connector attach step at session start, not either server.
• Both MCP servers' logs show they were up and serving other clients (200 OK) at the exact minutes the failing sessions started.
• In ~/Library/Caches/claude-cli-nodejs/<project>/mcp-logs-claude-ai-*/, no log file is created at all for the failing sessions' start times — it looks like no connection attempt is made (or it fails before the transport logger initializes).
• From inside an affected session, claude mcp list succeeds: "Initializing claude.ai proxy transport… Successfully connected (transport: claudeai-proxy) in 843ms… hasTools:true". So config, OAuth tokens, and network are all fine.
• The connectors later attached mid-session (tools appeared in the deferred tool registry roughly 15–20 minutes in, possibly after the claude mcp list run) and worked normally once attached.
• Possible version correlation, small sample: 2 of 4 sessions on v2.1.215 (released ~2026-07-19) failed vs 1 of 16 on v2.1.209.

Expected

Account connectors attach at every session start, or a visible warning is emitted when the attach step fails, and the client retries.

Impact

Sessions that depend on account connectors for startup context silently run without them; nothing in the session UI indicates the connectors are missing.