Fix/decode tool call (#36)

* Adding unit test

* Decode entire request to strongly typed Method call. This allows NotRequired parameters to properly decode from null, which wouldn't be possible when decoding from the 'null' JSON encoding directly.

Sources/MCP/Base/Messages.swift

@@ -161,12 +161,11 @@ final class TypedRequestHandler<M: Method>: RequestHandlerBox, @unchecked Sendab
         let decoder = JSONDecoder()
 
         // Create a concrete request from the type-erased one
-        let data = try encoder.encode(request.params)
-        let params = try decoder.decode(M.Parameters.self, from: data)
-        let typedRequest = Request<M>(id: request.id, method: M.name, params: params)
+        let data = try encoder.encode(request)
+        let request = try decoder.decode(Request<M>.self, from: data)
 
         // Handle with concrete type
-        let response = try await _handle(typedRequest)
+        let response = try await _handle(request)
 
         // Convert result to AnyMethod response
         switch response.result {

Tests/MCPTests/ToolTests.swift

@@ -111,7 +111,7 @@ struct ToolTests {
         let emptyParams = ListTools.Parameters()
         #expect(emptyParams.cursor == nil)
     }
-    
+
     @Test("ListTools request decoding with omitted params")
     func testListToolsRequestDecodingWithOmittedParams() throws {
         // Test decoding when params field is omitted
@@ -126,7 +126,7 @@ struct ToolTests {
         #expect(decoded.id == "test-id")
         #expect(decoded.method == ListTools.name)
     }
-    
+
     @Test("ListTools request decoding with null params")
     func testListToolsRequestDecodingWithNullParams() throws {
         // Test decoding when params field is null
@@ -205,4 +205,37 @@ struct ToolTests {
     func testToolListChangedNotification() throws {
         #expect(ToolListChangedNotification.name == "notifications/tools/list_changed")
     }
+
+    @Test("ListTools handler invocation without params")
+    func testListToolsHandlerWithoutParams() async throws {
+        let jsonString = """
+            {"jsonrpc":"2.0","id":1,"method":"tools/list"}
+            """
+        let jsonData = jsonString.data(using: .utf8)!
+
+        let anyRequest = try JSONDecoder().decode(AnyRequest.self, from: jsonData)
+
+        let handler = TypedRequestHandler<ListTools> { request in
+            #expect(request.method == ListTools.name)
+            #expect(request.id == 1)
+            #expect(request.params.cursor == nil)
+
+            let testTool = Tool(name: "test_tool", description: "Test tool for verification")
+            return ListTools.response(id: request.id, result: ListTools.Result(tools: [testTool]))
+        }
+
+        let response = try await handler(anyRequest)
+
+        if case .success(let value) = response.result {
+            let encoder = JSONEncoder()
+            let decoder = JSONDecoder()
+            let data = try encoder.encode(value)
+            let result = try decoder.decode(ListTools.Result.self, from: data)
+
+            #expect(result.tools.count == 1)
+            #expect(result.tools[0].name == "test_tool")
+        } else {
+            #expect(Bool(false), "Expected success result")
+        }
+    }
 }