MCP server failures and reserved names get clearer, shared error messages

What

MCP servers are outside programs that give Claude Code extra tools. When one cannot be loaded, Claude Code records an error for it. Two related things changed:

• Shared error builder: failed MCP servers, including ones blocked by managed policy (settings an organization sets for all its users), now get their error from one shared builder (E_) that is given the server's name and configuration. This replaces fixed messages and the hard-coded POLICY_BLOCKED error code. The DISABLED path is kept.
• Reserved names: Claude Code used to reject only the name "__proto__", with the message '"proto" is a reserved MCP server name and was not loaded'. It now checks every MCP server name against a list of reserved names. A server with a reserved name is skipped with a message saying the name cannot be used as a server name and a "Rename this server in your MCP config" hint. That message stays visible instead of being replaced by "connection failed".

Why

Error messages for MCP servers blocked under managed policy can now be more specific, and the wording or codes of MCP failure messages may differ from before. If a server uses a reserved name, you are told to rename it rather than seeing a vague connection failure.