MCP protocol versions and capability negotiation

MCP evolves, and it does so in a way that’s easy to reason about: the protocol is date-versioned. Every revision is identified by its release date in YYYY-MM-DD form — no semantic-version guessing about what “2.3” means, just the date the spec shipped.

Negotiation at initialize

The first thing a client and server do when they connect is initialize. In that handshake two things get settled:

1. The protocol version. The client proposes a version; the server responds with the highest version both support, and echoes that agreed version back. From then on, both speak that revision.
2. Capabilities. Each side advertises what it can do. The server declares tools, prompts, and/or resources (with sub-flags like listChanged or subscribe). The client declares sampling, elicitation, and/or roots. Each side then uses only what the other advertised — see capability negotiation for what those client-side capabilities unlock.

The elegance is that a newer client and an older server (or vice versa) can still talk: they simply meet at the highest revision they share and use the intersection of their capabilities.

Notable revisions

Version	What it brought
2024-11-05	The first widely-adopted revision.
2025-03-26	Streamable HTTP transport; authorization guidance.
2025-06-18	Refinements and a broader capability set.

A live example of the vantage gap

Here’s a subtle point that connects back to what a server can see: the negotiated version is agreed at initialize, but whether a server can report it back to you depends on the host surfacing it. When mcp-glimpse’s introspect_mcp_context runs with no snapshot, it reports the negotiated protocol version when the host surfaces it — and when the host doesn’t, the field honestly reads “not surfaced.”

That “not surfaced” is not a bug; it’s the vantage limit made visible. A server knows the version it negotiated, but it can only tell you what the host chose to pass along. It’s a small, concrete reminder that a server’s view of your environment is exactly as wide as the host lets it be — and no wider.

To see which version your own client actually negotiated, connect mcp-glimpse and run introspect_mcp_context; the “This Connection” view reports the wire-level facts a server can observe first-hand.