Constant PROTOCOL_VERSION 

pub const PROTOCOL_VERSION: &str = "1";

Protocol version, distinct from the daemon version.

It tracks the /v1/ path namespace rather than the crate version: a patch to the daemon does not change the wire format, and a client that pinned a daemon version would refuse an upgrade it is compatible with.

What a client should do on a mismatch, stated here because it is the question a version number exists to answer:

• Same protocol_version, different daemon_version — proceed. The path namespace and every shape in $defs are unchanged; only the implementation moved. This is the normal case and must not be treated as an error.
• Different protocol_version — do not proceed against /v1/. A new protocol version means a shape or a status code changed meaning, and a client built for 1 cannot tell a changed meaning from a bug. Fetch /v1/schema, which stays unauthenticated precisely so this is diagnosable, and fail with the two versions named.
• The microvms-agentd-version response header disagrees with the schema document’s daemon_version — you are talking to two daemons through one endpoint, or to a proxy that rewrote a response. Treat it as a transport fault, not a version negotiation.

There is deliberately no negotiation request: the daemon serves exactly one protocol version, and a client that has read this document already knows which.