Make sure to properly deal with non-required parameters

Author: odrobnik

Sources/SwiftMCP/Extensions/MCPToolMetadata+Arguments.swift

@@ -0,0 +1,44 @@
+//
+//  MCPToolMetadata+Arguments.swift
+//  SwiftMCP
+//
+//  Created by Oliver Drobnik on 28.03.25.
+//
+
+import Foundation
+
+/**
+ Extension to provide utility methods for working with MCP tools.
+ */
+extension MCPToolMetadata {
+	/**
+	 Enriches a dictionary of arguments with default values for any missing parameters.
+	 
+	 - Parameters:
+	   - arguments: The original arguments dictionary
+	 
+	 - Returns: A new dictionary with default values added for missing parameters
+	 - Throws: MCPToolError if required parameters are missing or if parameter conversion fails
+	 */
+	public func enrichArguments(_ arguments: [String: Sendable]) throws -> [String: Sendable] {
+		// Create a copy of the arguments dictionary
+		var enrichedArguments = arguments
+		
+		// Check for missing required parameters and add default values
+		for param in parameters {
+			// If we don't have an argument for this parameter
+			if enrichedArguments[param.name] == nil {
+				// If it has a default value, use it
+				if let defaultValue = param.defaultValue {
+					enrichedArguments[param.name] = defaultValue
+				}
+				// If it's required and has no default value, throw an error
+				else if param.isRequired {
+					throw MCPToolError.missingRequiredParameter(parameterName: param.name)
+				}
+			}
+		}
+		
+		return enrichedArguments
+	}
+}

Sources/SwiftMCP/Models/Tools/MCPTool.swift

@@ -62,54 +62,6 @@ extension MCPTool: Codable {
 	}
 }
 
-/**
- Extension to provide utility methods for working with MCP tools.
- */
-extension MCPTool {
-	/**
-	 Enriches a dictionary of arguments with default values for any missing parameters.
-	 
-	 - Parameters:
-	   - arguments: The original arguments dictionary
-	   - object: The object containing the function metadata
-	 
-	 - Returns: A new dictionary with default values added for missing parameters
-	 - Throws: MCPToolError if required parameters are missing or if parameter conversion fails
-	 */
-	public func enrichArguments(_ arguments: [String: Sendable], forObject object: Any) throws -> [String: Sendable] {
-		// Use the provided function name or fall back to the tool's name
-		let metadataKey = "__mcpMetadata_\(name)"
-		
-		// Create a copy of the arguments dictionary
-		var enrichedArguments = arguments
-		
-		// Find the metadata for the function using reflection
-		let mirror = Mirror(reflecting: object)
-		guard let child = mirror.children.first(where: { $0.label == metadataKey }),
-			  let metadata = child.value as? MCPToolMetadata else {
-			// If no metadata is found, return the original arguments
-			return arguments
-		}
-		
-		// Check for missing required parameters
-		for param in metadata.parameters {
-			if enrichedArguments[param.name] == nil && param.defaultValue == nil {
-				throw MCPToolError.missingRequiredParameter(parameterName: param.name)
-			}
-		}
-		
-		// Add default values for parameters that are missing from the arguments dictionary
-		for param in metadata.parameters {
-			if enrichedArguments[param.name] == nil, let defaultValue = param.defaultValue {
-				
-				enrichedArguments[param.name] = defaultValue
-			}
-		}
-		
-		return enrichedArguments
-	}
-}
-
 /**
  Extension to make JSONSchema conform to Codable
  */

Sources/SwiftMCP/Protocols/MCPServer.swift

@@ -465,4 +465,29 @@ public extension MCPServer {
     var mcpTools: [MCPTool] {
         return mcpToolMetadata.convertedToTools()
     }
+	
+	// MARK: - Internal Helpers
+	
+	/**
+	 Provides metadata for a function annotated with `@MCPTool`.
+	 
+	 - Parameter toolName: The name of the tool
+	 - Returns: The metadata for the tool
+	 
+	 This property uses runtime reflection to gather tool metadata from properties
+	 generated by the `@MCPTool` macro.
+	 */
+	func mcpToolMetadata(for toolName: String) -> MCPToolMetadata?
+	{
+		let metadataKey = "__mcpMetadata_\(toolName)"
+		
+		// Find the metadata for the function using reflection
+		let mirror = Mirror(reflecting: self)
+		guard let child = mirror.children.first(where: { $0.label == metadataKey }),
+			  let metadata = child.value as? MCPToolMetadata else {
+			return nil
+		}
+		
+		return metadata
+	}
 }

Sources/SwiftMCPMacros/MCPServerMacro.swift

@@ -141,13 +141,13 @@ public struct MCPServerMacro: MemberMacro, ExtensionMacro {
 /// - Returns: The result of the tool call
 /// - Throws: MCPToolError if the tool doesn't exist or cannot be called
 public func callTool(_ name: String, arguments: [String: Sendable]) async throws -> (Codable & Sendable) {
-   // Find the tool by name
-   guard let tool = mcpTools.first(where: { $0.name == name }) else {
+   // Find the tool metadata by name
+   guard let metadata = mcpToolMetadata(for: name) else {
       throw MCPToolError.unknownTool(name: name)
    }
    
    // Enrich arguments with default values
-   let enrichedArguments = try tool.enrichArguments(arguments, forObject: self)
+   let enrichedArguments = try metadata.enrichArguments(arguments)
    
    // Call the appropriate wrapper method based on the tool name
    switch name {

Tests/SwiftMCPTests/MCPToolArgumentEnrichingTests.swift

@@ -14,14 +14,12 @@ import SwiftMCP
 func testEnrichArguments() throws {
     let calculator = Calculator()
 
-    // Get the add tool from the calculator
-    guard let tool = calculator.mcpTools.first(where: { $0.name == "add" }) else {
-        throw TestError("Could not find add tool")
-    }
+    // Get the add tool metadata from the calculator
+    let metadata = unwrap(calculator.mcpToolMetadata(for: "add"))
 
     // Test enriching arguments
     let arguments: [String: Sendable] = ["a": 2, "b": 3]
-    let enrichedArguments = try tool.enrichArguments(arguments, forObject: calculator as Any)
+    let enrichedArguments = try metadata.enrichArguments(arguments)
 
     // Check that the arguments were not changed
     #expect(enrichedArguments.count == 2)
@@ -33,14 +31,12 @@ func testEnrichArguments() throws {
 func testEnrichArgumentsWithExplicitFunctionName() throws {
     let calculator = Calculator()
 
-    // Get the add tool from the calculator
-    guard let tool = calculator.mcpTools.first(where: { $0.name == "add" }) else {
-        throw TestError("Could not find add tool")
-    }
+    // Get the add tool metadata from the calculator
+    let metadata = unwrap(calculator.mcpToolMetadata(for: "add"))
 
     // Test enriching arguments with explicit function name
     let arguments: [String: Sendable] = ["a": 2, "b": 3]
-    let enrichedArguments = try tool.enrichArguments(arguments, forObject: calculator as Any)
+    let enrichedArguments = try metadata.enrichArguments(arguments)
 
     // Check that the arguments were not changed
     #expect(enrichedArguments.count == 2)
@@ -52,14 +48,12 @@ func testEnrichArgumentsWithExplicitFunctionName() throws {
 func testEnrichArgumentsWithNoDefaults() throws {
     let calculator = Calculator()
 
-    // Get a tool from the calculator
-    guard let tool = calculator.mcpTools.first(where: { $0.name == "add" }) else {
-        throw TestError("Could not find add tool")
-    }
+    // Get the add tool metadata from the calculator
+    let metadata = unwrap(calculator.mcpToolMetadata(for: "add"))
 
     // Test enriching arguments with no default values
     let arguments: [String: Sendable] = ["a": 2, "b": 3]
-    let enrichedArguments = try tool.enrichArguments(arguments, forObject: calculator as Any)
+    let enrichedArguments = try metadata.enrichArguments(arguments)
 
     // Check that the arguments were not changed
     #expect(enrichedArguments.count == 2)
@@ -71,29 +65,25 @@ func testEnrichArgumentsWithNoDefaults() throws {
 func testEnrichArgumentsWithMissingRequiredArgument() throws {
     let calculator = Calculator()
 
-    // Get a tool from the calculator
-    guard let tool = calculator.mcpTools.first(where: { $0.name == "add" }) else {
-        throw TestError("Could not find add tool")
-    }
+    // Get the add tool metadata from the calculator
+    let metadata = unwrap(calculator.mcpToolMetadata(for: "add"))
 
     // Test enriching arguments with a missing required argument
     #expect(throws: MCPToolError.self, "Should notice missing parameter") {
-        try tool.enrichArguments(["a": 2], forObject: calculator)
+        try metadata.enrichArguments(["a": 2])
     }
 }
 
 @Test
 func testEnrichArgumentsWithTypeConversion() throws {
     let calculator = Calculator()
 
-    // Get a tool from the calculator
-    guard let tool = calculator.mcpTools.first(where: { $0.name == "add" }) else {
-        throw TestError("Could not find add tool")
-    }
+    // Get the add tool metadata from the calculator
+    let metadata = unwrap(calculator.mcpToolMetadata(for: "add"))
 
     // Test enriching arguments with string values that need to be converted
     let arguments: [String: Sendable] = ["a": "2", "b": "3"]
-    let enrichedArguments = try tool.enrichArguments(arguments, forObject: calculator as Any)
+    let enrichedArguments = try metadata.enrichArguments(arguments)
 
     // Check that the arguments were not changed (enrichArguments doesn't do type conversion)
     #expect(enrichedArguments.count == 2)
@@ -105,23 +95,21 @@ func testEnrichArgumentsWithTypeConversion() throws {
 func testSubtractArguments() throws {
     let calculator = Calculator()
 
-    // Get a tool from the calculator
-    guard let tool = calculator.mcpTools.first(where: { $0.name == "subtract" }) else {
-        throw TestError("Could not find subtract tool")
-    }
+    // Get the subtract tool metadata from the calculator
+    let metadata = unwrap(calculator.mcpToolMetadata(for: "subtract"))
 
     // Test with no arguments - should throw missing required parameter
     #expect(throws: MCPToolError.self, "Should notice missing parameter") {
-        try tool.enrichArguments([:], forObject: calculator)
+        try metadata.enrichArguments([:])
     }
 
     // Test with partial arguments - should throw missing required parameter
     #expect(throws: MCPToolError.self, "Should notice missing parameter") {
-        try tool.enrichArguments(["b": 5], forObject: calculator)
+        try metadata.enrichArguments(["b": 5])
     }
 
     // Test with all arguments - no defaults should be added
-    let allArgs = try tool.enrichArguments(["a": 20, "b": 5], forObject: calculator)
+    let allArgs = try metadata.enrichArguments(["a": 20, "b": 5])
     #expect(allArgs.count == 2)
     #expect(allArgs["a"] as? Int == 20)
     #expect(allArgs["b"] as? Int == 5)
@@ -131,23 +119,21 @@ func testSubtractArguments() throws {
 func testMultiplyArguments() throws {
     let calculator = Calculator()
 
-    // Get a tool from the calculator
-    guard let tool = calculator.mcpTools.first(where: { $0.name == "multiply" }) else {
-        throw TestError("Could not find multiply tool")
-    }
+    // Get the multiply tool metadata from the calculator
+    let metadata = unwrap(calculator.mcpToolMetadata(for: "multiply"))
 
     // Test with no arguments - should throw missing required parameter
     #expect(throws: MCPToolError.self, "Should notice missing parameter") {
-        try tool.enrichArguments([:], forObject: calculator)
+        try metadata.enrichArguments([:])
     }
 
     // Test with partial arguments - should throw missing required parameter
     #expect(throws: MCPToolError.self, "Should notice missing parameter") {
-        try tool.enrichArguments(["b": 5], forObject: calculator)
+        try metadata.enrichArguments(["b": 5])
     }
 
     // Test with all arguments - no defaults should be added
-    let allArgs = try tool.enrichArguments(["a": 20, "b": 5], forObject: calculator)
+    let allArgs = try metadata.enrichArguments(["a": 20, "b": 5])
     #expect(allArgs.count == 2)
     #expect(allArgs["a"] as? Int == 20)
     #expect(allArgs["b"] as? Int == 5)