bug: /mcp add on the first-ever MCP server fails to log in with "No MCP servers configured."

[mabaeyens]

Description

Running /mcp add <url> --scope ... --transport streamable-http to add the first MCP server in a running session persists the server correctly to config.toml, but the immediate OAuth login step fails:

Added OAuth MCP server my-server.
Starting OAuth login...
Error: No MCP servers configured.

config.toml is written correctly (verified with tomllib.load), and restarting Vibe then running /mcp login my-server works fine. The bug only affects the very first MCP server added to a session that started with zero configured servers.

Root cause

The interactive TUI constructs AgentLoop with defer_heavy_init=True (vibe/cli/cli.py:341). With that flag, mcp_registry stays None until _ensure_remote_registries() runs once during deferred init (_complete_init, vibe/core/agent_loop/_loop.py), and that method only creates the registry if self.mcp_registry is None and self.config.mcp_servers — i.e. only if servers already existed at startup.

/mcp add (vibe/cli/textual_ui/app.py::_mcp_add) persists the new server and calls AgentLoop.refresh_config(), but refresh_config() only does:

if self.mcp_registry is not None:
    self.mcp_registry.sync_active_servers(self.config.mcp_servers)

Since mcp_registry was None before this first server existed, it stays None forever in that session — refresh_config() never calls _ensure_remote_registries() to lazily create it. The subsequent _mcp_login call then sees self.agent_loop.mcp_registry is None and reports "No MCP servers configured.", even though the server is genuinely configured on disk.

Steps to reproduce

1. Start vibe in a project/user config with no [[mcp_servers]] entries.
2. Run /mcp add https://example.com/mcp --name my-server --scope read --transport streamable-http.
3. Observe: "Added OAuth MCP server my-server. Starting OAuth login..." followed by "Error: No MCP servers configured."
4. Run /mcp status — also reports "No MCP servers configured." in the same session.
5. Exit and restart vibe. Run /mcp login my-server — works correctly.

Expected behavior

/mcp add's automatic login (and any following /mcp login//mcp status) should work in the same session the server was added in, without requiring a restart.

Actual behavior

/mcp login and /mcp status report Error: No MCP servers configured. in the same session the first server was added, even though config.toml already contains the new [[mcp_servers]] entry on disk. The condition only clears after restarting Vibe.

Environment

• vibe --version: 2.19.0 (also present on main as of the v2.20.0 tag)
• Python version: 3.12
• OS: macOS

Error messages

Added OAuth MCP server my-server.
Starting OAuth login...
Error: No MCP servers configured.

Configuration

Reproducible with a minimal config.toml that has no [[mcp_servers]] entries at session start (the bug is specific to the empty-to-first-server transition, not to any particular server's settings). Example server entry added via /mcp add:

[[mcp_servers]]
name = "my-server"
url = "https://example.com/mcp"
transport = "streamable-http"
scope = "read"

I have a minimal fix + regression test ready — see linked PR #911.