protocol revision
Also written protocol version, protocolVersion
A protocol revision is the dated version of the MCP specification a connection runs on, written as a date such as 2025-11-25. The client proposes one during initialize and the server answers with the one it will actually use. MCP versions by date rather than by semantic version number.
What is a protocol revision?
A protocol revision is a dated snapshot of the specification, such as 2025-03-26, 2025-06-18, or 2025-11-25. It appears as protocolVersion in the initialize exchange.
Dates rather than version numbers is a deliberate choice. There is no implied promise about which changes are breaking, so a client cannot assume compatibility from the number's shape.
Which revision will a server actually use?
Whichever one it answers with, not the one you asked for. The client proposes; the server decides and states its choice in the response.
This is the part implementers most often get wrong, because a proposal that goes unchallenged looks like agreement. A client that assumes its own preference was accepted will send messages the server may not understand.
How common are older revisions in the wild?
Common enough that ignoring them breaks real connections. Of the 14 servers we connected to on 5 August 2026, 9 answered on 2025-11-25, 2 answered on 2025-06-18, and 3 answered on 2025-03-26.
So 5 of 14 were running a revision older than the current one on that date, including the largest server in the sample. Any figure we publish records the revision it was measured on, because a result stored today becomes uninterpretable once the handshake that produced it no longer exists.
Example
The 2026-07-28 revision is the sharpest case for why the recorded revision matters: it removes the initialize handshake and Mcp-Session-Id entirely, moves client info into _meta on every request, and adds server/discover. A tool count captured under 2025-11-25 was produced by a conversation that revision no longer has, so the number only means something alongside the revision it came from.