Implement client protocol version negotiation in server initialization (#107)

* Implement client version negotiation in server initialization

* Add test coverage for version negotiation

* Make Version.supported and Version.negotiate() APIs internal to not create new surface area

Author: mattt

Sources/MCP/Base/Versioning.swift

@@ -6,6 +6,23 @@ import Foundation
 ///
 /// - SeeAlso: https://spec.modelcontextprotocol.io/specification/2025-03-26/
 public enum Version {
-    /// The current protocol version.
-    public static let latest = "2025-03-26"
+    /// All protocol versions supported by this implementation, ordered from newest to oldest.
+    static let supported: Set<String> = [
+        "2025-03-26",
+        "2024-11-05",
+    ]
+
+    /// The latest protocol version supported by this implementation.
+    public static let latest = supported.max()!
+
+    /// Negotiates the protocol version based on the client's request and server's capabilities.
+    /// - Parameter clientRequestedVersion: The protocol version requested by the client.
+    /// - Returns: The negotiated protocol version. If the client's requested version is supported,
+    ///            that version is returned. Otherwise, the server's latest supported version is returned.
+    static func negotiate(clientRequestedVersion: String) -> String {
+        if supported.contains(clientRequestedVersion) {
+            return clientRequestedVersion
+        }
+        return latest
+    }
 }

Sources/MCP/Server/Server.swift

@@ -486,15 +486,20 @@ public actor Server {
                 try await hook(params.clientInfo, params.capabilities)
             }
 
-            // Set initial state
+            // Perform version negotiation
+            let clientRequestedVersion = params.protocolVersion
+            let negotiatedProtocolVersion = Version.negotiate(
+                clientRequestedVersion: clientRequestedVersion)
+
+            // Set initial state with the negotiated protocol version
             await self.setInitialState(
                 clientInfo: params.clientInfo,
                 clientCapabilities: params.capabilities,
-                protocolVersion: params.protocolVersion
+                protocolVersion: negotiatedProtocolVersion
             )
 
             return Initialize.Result(
-                protocolVersion: Version.latest,
+                protocolVersion: negotiatedProtocolVersion,
                 capabilities: await self.capabilities,
                 serverInfo: self.serverInfo,
                 instructions: nil

Tests/MCPTests/VersioningTests.swift

@@ -0,0 +1,53 @@
+import Testing
+
+@testable import MCP
+
+@Suite("Version Negotiation Tests")
+struct VersioningTests {
+    @Test("Client requests latest supported version")
+    func testClientRequestsLatestSupportedVersion() {
+        let clientVersion = Version.latest
+        let negotiatedVersion = Version.negotiate(clientRequestedVersion: clientVersion)
+        #expect(negotiatedVersion == Version.latest)
+    }
+
+    @Test("Client requests older supported version")
+    func testClientRequestsOlderSupportedVersion() {
+        let clientVersion = "2024-11-05"
+        let negotiatedVersion = Version.negotiate(clientRequestedVersion: clientVersion)
+        #expect(negotiatedVersion == "2024-11-05")
+    }
+
+    @Test("Client requests unsupported version")
+    func testClientRequestsUnsupportedVersion() {
+        let clientVersion = "2023-01-01"  // An unsupported version
+        let negotiatedVersion = Version.negotiate(clientRequestedVersion: clientVersion)
+        #expect(negotiatedVersion == Version.latest)
+    }
+
+    @Test("Client requests empty version string")
+    func testClientRequestsEmptyVersionString() {
+        let clientVersion = ""
+        let negotiatedVersion = Version.negotiate(clientRequestedVersion: clientVersion)
+        #expect(negotiatedVersion == Version.latest)
+    }
+
+    @Test("Client requests garbage version string")
+    func testClientRequestsGarbageVersionString() {
+        let clientVersion = "not-a-version"
+        let negotiatedVersion = Version.negotiate(clientRequestedVersion: clientVersion)
+        #expect(negotiatedVersion == Version.latest)
+    }
+
+    @Test("Server's supported versions correctly defined")
+    func testServerSupportedVersions() {
+        #expect(Version.supported.contains("2025-03-26"))
+        #expect(Version.supported.contains("2024-11-05"))
+        #expect(Version.supported.count == 2)
+    }
+
+    @Test("Server's latest version is correct")
+    func testServerLatestVersion() {
+        #expect(Version.latest == "2025-03-26")
+    }
+}