Allow params field to be omitted in requests for methods that don't require them (#17)

* Allow `params` field to be omitted in requests for methods with empty parameters

* Add ping request to roundtrip test

* Generalize decoding of non-required parameters

Sources/MCP/Base/Messages.swift

@@ -2,7 +2,13 @@ import Foundation
 
 private let jsonrpc = "2.0"
 
-public struct Empty: Hashable, Codable, Sendable {}
+public protocol NotRequired {
+    init()
+}
+
+public struct Empty: NotRequired, Hashable, Codable, Sendable {
+    public init() {}
+}
 
 // MARK: -
 
@@ -78,14 +84,11 @@ public struct Request<M: Method>: Hashable, Identifiable, Codable, Sendable {
         try container.encode(jsonrpc, forKey: .jsonrpc)
         try container.encode(id, forKey: .id)
         try container.encode(method, forKey: .method)
-        if M.Parameters.self != Empty.self {
-            try container.encode(params, forKey: .params)
-        } else {
-            // Encode empty object for Empty parameters
-            try container.encode(Empty(), forKey: .params)
-        }
+        try container.encode(params, forKey: .params)
     }
+}
 
+extension Request {
     public init(from decoder: Decoder) throws {
         let container = try decoder.container(keyedBy: CodingKeys.self)
         let version = try container.decode(String.self, forKey: .jsonrpc)
@@ -95,15 +98,11 @@ public struct Request<M: Method>: Hashable, Identifiable, Codable, Sendable {
         }
         id = try container.decode(ID.self, forKey: .id)
         method = try container.decode(String.self, forKey: .method)
-        if M.Parameters.self == Empty.self {
-            if (try? container.decodeNil(forKey: .params)) != nil {
-                params = Empty() as! M.Parameters
-            } else if (try? container.decode(Empty.self, forKey: .params)) != nil {
-                params = Empty() as! M.Parameters
-            } else {
-                // If params field is missing, use Empty
-                params = Empty() as! M.Parameters
-            }
+
+        if M.Parameters.self is NotRequired.Type {
+            params =
+                (try container.decodeIfPresent(M.Parameters.self, forKey: .params)
+                    ?? (M.Parameters.self as! NotRequired.Type).init() as! M.Parameters)
         } else {
             params = try container.decode(M.Parameters.self, forKey: .params)
         }

Sources/MCP/Client/Client.swift

@@ -305,7 +305,12 @@ public actor Client {
         -> (prompts: [Prompt], nextCursor: String?)
     {
         _ = try checkCapability(\.prompts, "Prompts")
-        let request = ListPrompts.request(.init(cursor: cursor))
+        let request: Request<ListPrompts>
+        if let cursor = cursor {
+            request = ListPrompts.request(.init(cursor: cursor))
+        } else {
+            request = ListPrompts.request(.init())
+        }
         let result = try await send(request)
         return (prompts: result.prompts, nextCursor: result.nextCursor)
     }
@@ -323,7 +328,12 @@ public actor Client {
         resources: [Resource], nextCursor: String?
     ) {
         _ = try checkCapability(\.resources, "Resources")
-        let request = ListResources.request(.init(cursor: cursor))
+        let request: Request<ListResources>
+        if let cursor = cursor {
+            request = ListResources.request(.init(cursor: cursor))
+        } else {
+            request = ListResources.request(.init())
+        }
         let result = try await send(request)
         return (resources: result.resources, nextCursor: result.nextCursor)
     }
@@ -338,7 +348,12 @@ public actor Client {
 
     public func listTools(cursor: String? = nil) async throws -> [Tool] {
         _ = try checkCapability(\.tools, "Tools")
-        let request = ListTools.request(.init(cursor: cursor))
+        let request: Request<ListTools>
+        if let cursor = cursor {
+            request = ListTools.request(.init(cursor: cursor))
+        } else {
+            request = ListTools.request(.init())
+        }
         let result = try await send(request)
         return result.tools
     }

Sources/MCP/Server/Prompts.swift

@@ -153,10 +153,14 @@ public struct Prompt: Hashable, Codable, Sendable {
 public enum ListPrompts: Method {
     public static let name: String = "prompts/list"
 
-    public struct Parameters: Hashable, Codable, Sendable {
+    public struct Parameters: NotRequired, Hashable, Codable, Sendable {
         public let cursor: String?
+        
+        public init() {
+            self.cursor = nil
+        }
 
-        public init(cursor: String? = nil) {
+        public init(cursor: String) {
             self.cursor = cursor
         }
     }

Sources/MCP/Server/Resources.swift

@@ -99,10 +99,14 @@ public struct Resource: Hashable, Codable, Sendable {
 public enum ListResources: Method {
     public static let name: String = "resources/list"
 
-    public struct Parameters: Hashable, Codable, Sendable {
+    public struct Parameters: NotRequired, Hashable, Codable, Sendable {
         public let cursor: String?
 
-        public init(cursor: String? = nil) {
+        public init() {
+            self.cursor = nil
+        }
+        
+        public init(cursor: String) {
             self.cursor = cursor
         }
     }

Sources/MCP/Server/Tools.swift

@@ -118,10 +118,14 @@ public struct Tool: Hashable, Codable, Sendable {
 public enum ListTools: Method {
     public static let name = "tools/list"
 
-    public struct Parameters: Hashable, Codable, Sendable {
+    public struct Parameters: NotRequired, Hashable, Codable, Sendable {
         public let cursor: String?
+        
+        public init() {
+            self.cursor = nil
+        }
 
-        public init(cursor: String? = nil) {
+        public init(cursor: String) {
             self.cursor = cursor
         }
     }