Protocol Versioning

Protocol Versioning ensures that MCP clients and servers can understand each other even as the standard evolves. During the initialization phase, both parties negotiate which version of the protocol to use.

Negotiation Process

1. Client Proposes: The client sends a single protocol version (the latest it supports) in the initialize request.
2. Server Responds: If the server supports that version, it responds with the same version. If not, the server responds with another protocol version it supports (typically its latest).
3. Two-Way Agreement: If the client doesn't support the server's response version, the client should disconnect. This is a two-way negotiation where both parties must find common ground.
4. Fallback: If no common version is found, the connection is gracefully closed with a version mismatch error.

Versioning prevents breaking changes from disrupting existing MCP integrations.

Questions & Answers

Why is "Protocol Versioning" essential for MCP ecosystem stability?

Versioning ensures that clients (hosts) and servers can continue to communicate effectively as the protocol evolves. It prevents breaking changes from disrupting existing integrations by ensuring both parties are using a compatible set of rules.

Describe the version negotiation process during the initialization handshake.

The client sends a single protocol version (the latest it supports) in the initialize request. If the server supports that version, it responds with the same version. If not, the server proposes another version it supports. This is a two-way negotiation where both parties must agree on a common version or disconnect.

What happens if a client and server cannot agree on a common protocol version?

If the negotiation fails and no common version is found, the connection process is halted and the interaction is gracefully closed with a version mismatch error to prevent unpredictable behavior.