Model nested experimental values and extensions in Client.Capabilities

Sources/MCP/Client/Client.swift

@@ -125,19 +125,24 @@ public actor Client {
         /// Whether the client supports elicitation
         public var elicitation: Elicitation?
         /// Experimental features supported by the client
-        public var experimental: [String: String]?
+        public var experimental: [String: Value]?
+        /// Capability extensions declared by the client,
+        /// such as MCP Apps UI extensions
+        public var extensions: [String: Value]?
         /// Whether the client supports roots
         public var roots: Capabilities.Roots?
 
         public init(
             sampling: Sampling? = nil,
             elicitation: Elicitation? = nil,
-            experimental: [String: String]? = nil,
+            experimental: [String: Value]? = nil,
+            extensions: [String: Value]? = nil,
             roots: Capabilities.Roots? = nil
         ) {
             self.sampling = sampling
             self.elicitation = elicitation
             self.experimental = experimental
+            self.extensions = extensions
             self.roots = roots
         }
     }

Tests/MCPTests/RequestTests.swift

@@ -206,6 +206,61 @@ struct RequestTests {
         #expect(decoded.params.clientInfo.name == "unknown")
     }
 
+    @Test("Initialize parameters decoding - nested experimental and extensions")
+    func testInitializeParametersDecodingNestedCapabilities() throws {
+        let jsonString = """
+            {
+              "protocolVersion": "2025-11-25",
+              "capabilities": {
+                "experimental": {
+                  "openai/visibility": { "enabled": true }
+                },
+                "extensions": {
+                  "io.modelcontextprotocol/ui": {
+                    "mimeTypes": ["text/html;profile=mcp-app"]
+                  }
+                }
+              },
+              "clientInfo": { "name": "openai-mcp", "version": "1.0.0" }
+            }
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        let decoded = try decoder.decode(Initialize.Parameters.self, from: data)
+
+        #expect(decoded.protocolVersion == "2025-11-25")
+        #expect(decoded.clientInfo.name == "openai-mcp")
+        #expect(decoded.clientInfo.version == "1.0.0")
+
+        #expect(
+            decoded.capabilities.experimental?["openai/visibility"]
+                == .object(["enabled": .bool(true)]))
+        #expect(
+            decoded.capabilities.extensions?["io.modelcontextprotocol/ui"]
+                == .object(["mimeTypes": .array([.string("text/html;profile=mcp-app")])]))
+    }
+
+    @Test("Client capabilities round-trip with experimental and extensions")
+    func testClientCapabilitiesExperimentalExtensionsRoundTrip() throws {
+        let capabilities = Client.Capabilities(
+            experimental: ["openai/visibility": .object(["enabled": .bool(true)])],
+            extensions: [
+                "io.modelcontextprotocol/ui": .object([
+                    "mimeTypes": .array([.string("text/html;profile=mcp-app")])
+                ])
+            ]
+        )
+
+        let encoder = JSONEncoder()
+        let decoder = JSONDecoder()
+
+        let data = try encoder.encode(capabilities)
+        let decoded = try decoder.decode(Client.Capabilities.self, from: data)
+
+        #expect(decoded == capabilities)
+    }
+
     @Test("Invalid parameters request decoding")
     func testInvalidParametersRequestDecoding() throws {
         let jsonString = """