Ensure Request decoder handles missing/null params correctly (#22)

Author: mattt

Sources/MCP/Base/Messages.swift

@@ -101,11 +101,31 @@ extension Request {
         method = try container.decode(String.self, forKey: .method)
 
         if M.Parameters.self is NotRequired.Type {
+            // For NotRequired parameters, use decodeIfPresent or init()
             params =
                 (try container.decodeIfPresent(M.Parameters.self, forKey: .params)
                     ?? (M.Parameters.self as! NotRequired.Type).init() as! M.Parameters)
+        } else if let value = try? container.decode(M.Parameters.self, forKey: .params) {
+            // If params exists and can be decoded, use it
+            params = value
+        } else if !container.contains(.params)
+            || (try? container.decodeNil(forKey: .params)) == true
+        {
+            // If params is missing or explicitly null, use Empty for Empty parameters
+            // or throw for non-Empty parameters
+            if M.Parameters.self == Empty.self {
+                params = Empty() as! M.Parameters
+            } else {
+                throw DecodingError.dataCorrupted(
+                    DecodingError.Context(
+                        codingPath: container.codingPath,
+                        debugDescription: "Missing required params field"))
+            }
         } else {
-            params = try container.decode(M.Parameters.self, forKey: .params)
+            throw DecodingError.dataCorrupted(
+                DecodingError.Context(
+                    codingPath: container.codingPath,
+                    debugDescription: "Invalid params field"))
         }
     }
 }

Tests/MCPTests/RequestTests.swift

@@ -23,33 +23,33 @@ struct RequestTests {
 
     @Test("Request initialization with parameters")
     func testRequestInitialization() throws {
-        let id: ID = "test-id"
-        let params = TestMethod.Parameters(value: "test")
-        let request = Request<TestMethod>(id: id, method: TestMethod.name, params: params)
+        let id: ID = 1
+        let params = CallTool.Parameters(name: "test-tool")
+        let request = Request<CallTool>(id: id, method: CallTool.name, params: params)
 
         #expect(request.id == id)
-        #expect(request.method == TestMethod.name)
-        #expect(request.params.value == "test")
+        #expect(request.method == CallTool.name)
+        #expect(request.params.name == "test-tool")
     }
 
     @Test("Request encoding and decoding")
     func testRequestEncodingDecoding() throws {
-        let request = TestMethod.request(id: "test-id", TestMethod.Parameters(value: "test"))
+        let request = CallTool.request(id: 1, CallTool.Parameters(name: "test-tool"))
 
         let encoder = JSONEncoder()
         let decoder = JSONDecoder()
 
         let data = try encoder.encode(request)
-        let decoded = try decoder.decode(Request<TestMethod>.self, from: data)
+        let decoded = try decoder.decode(Request<CallTool>.self, from: data)
 
         #expect(decoded.id == request.id)
         #expect(decoded.method == request.method)
-        #expect(decoded.params.value == request.params.value)
+        #expect(decoded.params.name == request.params.name)
     }
 
     @Test("Empty parameters request encoding")
     func testEmptyParametersRequestEncoding() throws {
-        let request = EmptyMethod.request(id: "test-id")
+        let request = EmptyMethod.request(id: 1)
 
         let encoder = JSONEncoder()
         let decoder = JSONDecoder()
@@ -66,59 +66,198 @@ struct RequestTests {
     func testEmptyParametersRequestDecoding() throws {
         // Create a minimal JSON string
         let jsonString = """
-            {"jsonrpc":"2.0","id":"test-id","method":"empty.method"}
+            {"jsonrpc":"2.0","id":1,"method":"empty.method"}
             """
         let data = jsonString.data(using: .utf8)!
 
         let decoder = JSONDecoder()
         let decoded = try decoder.decode(Request<EmptyMethod>.self, from: data)
 
-        #expect(decoded.id == "test-id")
+        #expect(decoded.id == 1)
         #expect(decoded.method == EmptyMethod.name)
     }
 
     @Test("NotRequired parameters request decoding - with params")
     func testNotRequiredParametersRequestDecodingWithParams() throws {
         // Test decoding when params field is present
         let jsonString = """
-            {"jsonrpc":"2.0","id":"test-id","method":"ping","params":{}}
+            {"jsonrpc":"2.0","id":1,"method":"ping","params":{}}
             """
         let data = jsonString.data(using: .utf8)!
 
         let decoder = JSONDecoder()
         let decoded = try decoder.decode(Request<Ping>.self, from: data)
 
-        #expect(decoded.id == "test-id")
+        #expect(decoded.id == 1)
         #expect(decoded.method == Ping.name)
     }
 
     @Test("NotRequired parameters request decoding - without params")
     func testNotRequiredParametersRequestDecodingWithoutParams() throws {
         // Test decoding when params field is missing
         let jsonString = """
-            {"jsonrpc":"2.0","id":"test-id","method":"ping"}
+            {"jsonrpc":"2.0","id":1,"method":"ping"}
             """
         let data = jsonString.data(using: .utf8)!
 
         let decoder = JSONDecoder()
         let decoded = try decoder.decode(Request<Ping>.self, from: data)
 
-        #expect(decoded.id == "test-id")
+        #expect(decoded.id == 1)
         #expect(decoded.method == Ping.name)
     }
 
     @Test("NotRequired parameters request decoding - with null params")
     func testNotRequiredParametersRequestDecodingWithNullParams() throws {
         // Test decoding when params field is null
         let jsonString = """
-            {"jsonrpc":"2.0","id":"test-id","method":"ping","params":null}
+            {"jsonrpc":"2.0","id":1,"method":"ping","params":null}
             """
         let data = jsonString.data(using: .utf8)!
 
         let decoder = JSONDecoder()
         let decoded = try decoder.decode(Request<Ping>.self, from: data)
 
-        #expect(decoded.id == "test-id")
+        #expect(decoded.id == 1)
         #expect(decoded.method == Ping.name)
     }
+
+    @Test("Required parameters request decoding - missing params")
+    func testRequiredParametersRequestDecodingMissingParams() throws {
+        let jsonString = """
+            {"jsonrpc":"2.0","id":1,"method":"tools/call"}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        #expect(throws: DecodingError.self) {
+            _ = try decoder.decode(Request<CallTool>.self, from: data)
+        }
+    }
+
+    @Test("Required parameters request decoding - null params")
+    func testRequiredParametersRequestDecodingNullParams() throws {
+        let jsonString = """
+            {"jsonrpc":"2.0","id":1,"method":"tools/call","params":null}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        #expect(throws: DecodingError.self) {
+            _ = try decoder.decode(Request<CallTool>.self, from: data)
+        }
+    }
+
+    @Test("Empty parameters request decoding - with null params")
+    func testEmptyParametersRequestDecodingNullParams() throws {
+        let jsonString = """
+            {"jsonrpc":"2.0","id":1,"method":"empty.method","params":null}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        let decoded = try decoder.decode(Request<EmptyMethod>.self, from: data)
+
+        #expect(decoded.id == 1)
+        #expect(decoded.method == EmptyMethod.name)
+    }
+
+    @Test("Empty parameters request decoding - with empty object params")
+    func testEmptyParametersRequestDecodingEmptyParams() throws {
+        let jsonString = """
+            {"jsonrpc":"2.0","id":1,"method":"empty.method","params":{}}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        let decoded = try decoder.decode(Request<EmptyMethod>.self, from: data)
+
+        #expect(decoded.id == 1)
+        #expect(decoded.method == EmptyMethod.name)
+    }
+
+    @Test("Initialize request decoding - requires params")
+    func testInitializeRequestDecodingRequiresParams() throws {
+        // Test missing params field
+        let missingParams = """
+            {"jsonrpc":"2.0","id":"test-id","method":"initialize"}
+            """
+        let decoder = JSONDecoder()
+        #expect(throws: DecodingError.self) {
+            _ = try decoder.decode(
+                Request<Initialize>.self, from: missingParams.data(using: .utf8)!)
+        }
+
+        // Test null params
+        let nullParams = """
+            {"jsonrpc":"2.0","id":"test-id","method":"initialize","params":null}
+            """
+        #expect(throws: DecodingError.self) {
+            _ = try decoder.decode(Request<Initialize>.self, from: nullParams.data(using: .utf8)!)
+        }
+
+        // Verify that empty object params works (since fields have defaults)
+        let emptyParams = """
+            {"jsonrpc":"2.0","id":"test-id","method":"initialize","params":{}}
+            """
+        let decoded = try decoder.decode(
+            Request<Initialize>.self, from: emptyParams.data(using: .utf8)!)
+        #expect(decoded.params.protocolVersion == Version.latest)
+        #expect(decoded.params.clientInfo.name == "unknown")
+    }
+
+    @Test("Invalid parameters request decoding")
+    func testInvalidParametersRequestDecoding() throws {
+        let jsonString = """
+            {"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"invalid":"value"}}
+            """
+        let data = jsonString.data(using: .utf8)!
+
+        let decoder = JSONDecoder()
+        #expect(throws: DecodingError.self) {
+            _ = try decoder.decode(Request<CallTool>.self, from: data)
+        }
+    }
+
+    @Test("NotRequired parameters request decoding")
+    func testNotRequiredParametersRequestDecoding() throws {
+        // Test with missing params
+        let missingParams = """
+            {"jsonrpc":"2.0","id":1,"method":"tools/list"}
+            """
+        let decoder = JSONDecoder()
+        let decodedMissing = try decoder.decode(
+            Request<ListTools>.self,
+            from: missingParams.data(using: .utf8)!)
+        #expect(decodedMissing.id == 1)
+        #expect(decodedMissing.method == ListTools.name)
+        #expect(decodedMissing.params.cursor == nil)
+
+        // Test with null params
+        let nullParams = """
+            {"jsonrpc":"2.0","id":1,"method":"tools/list","params":null}
+            """
+        let decodedNull = try decoder.decode(
+            Request<ListTools>.self,
+            from: nullParams.data(using: .utf8)!)
+        #expect(decodedNull.params.cursor == nil)
+
+        // Test with empty object params
+        let emptyParams = """
+            {"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}
+            """
+        let decodedEmpty = try decoder.decode(
+            Request<ListTools>.self,
+            from: emptyParams.data(using: .utf8)!)
+        #expect(decodedEmpty.params.cursor == nil)
+
+        // Test with provided cursor
+        let withCursor = """
+            {"jsonrpc":"2.0","id":1,"method":"tools/list","params":{"cursor":"next-page"}}
+            """
+        let decodedWithCursor = try decoder.decode(
+            Request<ListTools>.self,
+            from: withCursor.data(using: .utf8)!)
+        #expect(decodedWithCursor.params.cursor == "next-page")
+    }
 }