Lazy/on-demand startup for MCP servers

What variant of Codex are you using?

CLI

What feature would you like to see?

Problem

Codex initializes and starts every enabled MCP server when a local session
is opened, even if none of that server's tools are used.

With multiple MCP servers configured, this can:

• Increase session startup time
• Start unnecessary local processes
• Consume CPU and memory unnecessarily
• Produce startup errors for servers unrelated to the current task

Environment:

• Component: Codex local client / MCP
• Codex CLI version: 0.152.0
• MCP transport: STDIO

Proposed solution

Add an optional lazy startup mode for MCP servers:

[mcp_servers.example]
startup_mode = "lazy"

Expected behavior:

1. Do not start the MCP process when a session opens.
2. Load cached or declarative tool metadata so the tools remain discoverable.
3. Start and initialize the server on the first tool invocation.
4. Reuse the initialized server for the remainder of the session.
5. Keep "eager" as the default mode for backward compatibility.

If automatic lazy startup is difficult because tool discovery requires MCP
initialization, an in-session command would also help:

/mcp start <server-name>
/mcp stop <server-name>

This would avoid requiring users to edit config.toml and restart Codex just
to use an MCP server temporarily.

### Additional information

_No response_

Start by tracing MCP server initialization in the Codex CLI and how config.toml handles the proposed startup_mode setting. Check how STDIO tool discovery and first invocation currently work. Done means supporting lazy startup without launching unused servers, reusing the initialized server, preserving eager as the default, and covering the behavior with tests.