What you see	What it means
`401 Authorization header is required`	The client hasn't signed in yet. Authenticate it (`/mcp` in Claude Code).
`401 Access token has expired`	It should have refreshed. Authenticate again.
`401 This connection has been disconnected`	It was disconnected under Dev Tools → MCP, or a refresh token was reused. Authenticate again.
`401 Access token has been revoked`	The API client was revoked on the Dev Tools page.
`401 This connection does not cover any company…`	You've left every company it was connected to. Edit it, or connect again.
`403 This connection was not granted the scope(s)…`	You didn't give it that permission at that company. The tool's own answer carries an approval link; or edit the connection.
`permission_required`	The connection lacks a permission the call needs. Show the `approvalUrl` to the person, then retry once they approve — see Asking for more access.
`Input validation error`	An argument the tool's schema refuses — often an id with a `/` in it.
`403 ... missing required scope(s)`	An API token without that scope. Create a client that has it.
`403 Your account does not have the scope(s)…`	You granted it, but no longer hold it yourself.
`401 Company has outstanding charges`	Billing is locked; the whole API is closed until it's settled.
`429 Too Many Requests`	The company's API rate limit. Wait for the window named in the response.
A tool you expected isn't listed	No company granted it the scope — `whoami` lists each company's tools.

Anything else comes straight from the underlying API endpoint, with its status code, so it means what it means in the API reference.