You add a local MCP server whose start-up command takes its own flags, and you type the whole thing after the server name without a double-dash separator. The command fails complaining about an option it does not recognise. Why?

The principle — A command line has two audiences here: the options Claude Code reads, and the command it will run to start your server. Something has to mark the boundary between them.

Why the key is correct — The separator is that mark. Everything after it is passed to the server untouched, so the server's own flags are only safe once they sit on the far side of it. Leave it out and those flags are still inside the region Claude Code parses for itself, which is why one of them comes back as unrecognised.

Why the others are wrong — Pass-through does not begin at the server name; it begins at the separator. The separator is not decoration, it is the boundary. And quoting solves a shell problem rather than an argument-parsing one, so no amount of it moves the boundary.

Remember this — The separator is where your options stop and the server's begin.

Sources — Anthropic's Claude Code MCP reference.