"handshaking with MCP server failed: connection closed: initialize response" in Codex. Server exits or prints to stdout first

Error (Codex CLI or app, at start-up, for a local stdio MCP server):

MCP client for `<name>` failed to start: MCP startup failed: handshaking with MCP server
failed: connection closed: initialize response
MCP startup incomplete (failed: <name>)

What it means: Codex started the server's command, sent initialize, and the connection closed before a valid answer came back. The message is the same whatever the reason, so find the reason first.

Causes and fixes, most common first:

1. The server itself fails at start (missing environment variable, wrong Node version, broken npx cache). Run the exact command and args from ~/.codex/config.toml in a terminal, and read the server's errors in the Codex log (~/.codex/log/codex-tui.log, lines MCP server stderr); a Codex maintainer found a missing API key this way in issue 6020.
2. The server prints logs to stdout. The MCP spec says a stdio server "MUST NOT write anything to its stdout that is not a valid MCP message"; Codex then sees garbage instead of the handshake. Send the server's logging to stderr (one reporter fixed AvaloniaUI.MCP by moving its console logger to stderr).
3. Windows, shared background server: the Codex daemon keeps the PATH it started with, so tools installed later (npx via nvm, uvx) are not found and the server exits. If the same server starts with codex --no-daemon (which bypasses the shared background server, CLI 0.156.0 and later), that is the cause. Use --no-daemon, or restart the background server from a shell that has the new PATH, as the reporter of issue 55888 did.

A remote server on the old SSE transport fails at the same step with a different message (HTTP status client error (404 Not Found)): Codex supports Streamable HTTP only, per a Codex maintainer in issue 6020.

Versions: reported from Codex CLI 0.53.0 (Oct 2025) to 0.163.0 (Oct 2026), on macOS, Linux and Windows.

Sources:

• https://github.com/openai/codex/issues/6020 (Codex maintainer's comments, 4 and 5 Nov 2025; stdout cause, 24 Nov 2025; Windows daemon cause, 21 Oct 2026)
• https://github.com/openai/codex/issues/55888
• MCP spec, stdio transport: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports
• Codex changelog, CLI 0.156.0 (--no-daemon, /daemon): https://learn.chatgpt.com/docs/changelog

Verified: the stdout rule from the MCP spec, the flags from OpenAI's changelog, and the log location and SSE answer from a Codex maintainer (OpenAI's gpeal). Not verified: the Windows PATH cause comes from one reporter, not OpenAI; nothing here was run.