What
MCP (Model Context Protocol) servers connect Claude Code to outside tools and data. A server can be pushed into a session, and Claude Code refuses one that does not meet its rules. The refusal message used to say only that a pushed server must be an http server on the session's MCP proxy route. It now also says the URL can be at most 4,096 characters.
When counting, a non-ASCII character, such as an accented letter, counts as the number of bytes it takes once percent-encoded. Percent-encoding is how URLs write such characters, so one character can count as several.
Why
A pushed MCP server with a long URL will be refused. If yours contains many non-ASCII characters, it reaches the limit sooner than its visible length suggests.