Update AnyNotification to include notification params (#86)

* Update AnyNotification to not wipe out params that come with a notification

* Add test coverage for notification with params

* Allow params to be omitted when empty

---------

Co-authored-by: Stephen Tallent <stallent@mercury.io>
Co-authored-by: Mattt Zmuda <mattt@loopwork.com>

Sources/MCP/Base/Messages.swift

@@ -278,7 +278,7 @@ public protocol Notification: Hashable, Codable, Sendable {
 /// A type-erased notification for message handling
 struct AnyNotification: Notification, Sendable {
     static var name: String { "" }
-    typealias Parameters = Empty
+    typealias Parameters = Value
 }
 
 extension AnyNotification {
@@ -325,10 +325,11 @@ public struct Message<N: Notification>: Hashable, Codable, Sendable {
         }
         method = try container.decode(String.self, forKey: .method)
 
-        // Handle params field being optional
+        if N.Parameters.self is NotRequired.Type {
-        if N.Parameters.self == Empty.self {
+            // For NotRequired parameters, use decodeIfPresent or init()
-            // For Empty parameters, use Empty() if params is missing or null
+            params =
-            params = Empty() as! N.Parameters
+                (try container.decodeIfPresent(N.Parameters.self, forKey: .params)
+                    ?? (N.Parameters.self as! NotRequired.Type).init() as! N.Parameters)
         } else if let value = try? container.decode(N.Parameters.self, forKey: .params) {
             // If params exists and can be decoded, use it
             params = value

Tests/MCPTests/NotificationTests.swift

@@ -86,4 +86,87 @@ struct NotificationTests {
 
         #expect(decoded.method == InitializedNotification.name)
     }
+
+    @Test("Resource updated notification with parameters")
+    func testResourceUpdatedNotification() throws {
+        let params = ResourceUpdatedNotification.Parameters(uri: "test://resource")
+        let notification = ResourceUpdatedNotification.message(params)
+
+        #expect(notification.method == ResourceUpdatedNotification.name)
+        #expect(notification.params.uri == "test://resource")
+
+        let encoder = JSONEncoder()
+        let decoder = JSONDecoder()
+
+        let data = try encoder.encode(notification)
+
+        // Verify the exact JSON structure
+        let json = try JSONDecoder().decode([String: Value].self, from: data)
+        #expect(json["jsonrpc"] == "2.0")
+        #expect(json["method"] == "notifications/resources/updated")
+        #expect(json["params"] != nil)
+        #expect(json.count == 3, "Should contain jsonrpc, method, and params fields")
+
+        // Verify we can decode it back
+        let decoded = try decoder.decode(Message<ResourceUpdatedNotification>.self, from: data)
+        #expect(decoded.method == ResourceUpdatedNotification.name)
+        #expect(decoded.params.uri == "test://resource")
+    }
+
+    @Test("AnyNotification decoding - without params")
+    func testAnyNotificationDecodingWithoutParams() throws {
+        // Test decoding when params field is missing
+        let jsonString = """
+            {"jsonrpc":"2.0","method":"notifications/initialized"}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        let decoded = try decoder.decode(AnyMessage.self, from: data)
+
+        #expect(decoded.method == InitializedNotification.name)
+    }
+
+    @Test("AnyNotification decoding - with null params")
+    func testAnyNotificationDecodingWithNullParams() throws {
+        // Test decoding when params field is null
+        let jsonString = """
+            {"jsonrpc":"2.0","method":"notifications/initialized","params":null}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        let decoded = try decoder.decode(AnyMessage.self, from: data)
+
+        #expect(decoded.method == InitializedNotification.name)
+    }
+
+    @Test("AnyNotification decoding - with empty params")
+    func testAnyNotificationDecodingWithEmptyParams() throws {
+        // Test decoding when params field is empty
+        let jsonString = """
+            {"jsonrpc":"2.0","method":"notifications/initialized","params":{}}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        let decoded = try decoder.decode(AnyMessage.self, from: data)
+
+        #expect(decoded.method == InitializedNotification.name)
+    }
+
+    @Test("AnyNotification decoding - with non-empty params")
+    func testAnyNotificationDecodingWithNonEmptyParams() throws {
+        // Test decoding when params field has values
+        let jsonString = """
+            {"jsonrpc":"2.0","method":"notifications/resources/updated","params":{"uri":"test://resource"}}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        let decoded = try decoder.decode(AnyMessage.self, from: data)
+
+        #expect(decoded.method == ResourceUpdatedNotification.name)
+        #expect(decoded.params.objectValue?["uri"]?.stringValue == "test://resource")
+    }
 }