diff --git a/Sources/MCP/Client/Client.swift b/Sources/MCP/Client/Client.swift
index 453a8fdc..be32df76 100644
--- a/Sources/MCP/Client/Client.swift
+++ b/Sources/MCP/Client/Client.swift
@@ -344,7 +344,7 @@ public actor Client {
     }
 
     public func callTool(name: String, arguments: [String: Value]? = nil) async throws -> (
-        content: [Tool.Content], isError: Bool
+        content: [Tool.Content], isError: Bool?
     ) {
         _ = try checkCapability(\.tools, "Tools")
         let request = CallTool.request(.init(name: name, arguments: arguments))
diff --git a/Sources/MCP/Server/Tools.swift b/Sources/MCP/Server/Tools.swift
index aacb282b..aaa07492 100644
--- a/Sources/MCP/Server/Tools.swift
+++ b/Sources/MCP/Server/Tools.swift
@@ -154,9 +154,9 @@ public enum CallTool: Method {
 
     public struct Result: Hashable, Codable, Sendable {
         public let content: [Tool.Content]
-        public let isError: Bool
+        public let isError: Bool?
 
-        public init(content: [Tool.Content], isError: Bool) {
+        public init(content: [Tool.Content], isError: Bool? = nil) {
             self.content = content
             self.isError = isError
         }
diff --git a/Tests/MCPTests/RoundtripTests.swift b/Tests/MCPTests/RoundtripTests.swift
index ef314de4..014ca85b 100644
--- a/Tests/MCPTests/RoundtripTests.swift
+++ b/Tests/MCPTests/RoundtripTests.swift
@@ -52,13 +52,13 @@ struct RoundtripTests {
             }
 
             guard let a = request.arguments?["a"]?.intValue,
-                  let b = request.arguments?["b"]?.intValue
+                let b = request.arguments?["b"]?.intValue
             else {
                 return CallTool.Result(
                     content: [.text("Did not receive valid arguments")], isError: true)
             }
 
-            return CallTool.Result(content: [.text("\(a + b)")], isError: false)
+            return CallTool.Result(content: [.text("\(a + b)")])
         }
 
         let client = Client(name: "TestClient", version: "1.0")
@@ -96,7 +96,7 @@ struct RoundtripTests {
 
         let callToolTask = Task {
             let result = try await client.callTool(name: "add", arguments: ["a": 1, "b": 2])
-            #expect(result.isError == false)
+            #expect(result.isError == nil)
             #expect(result.content == [.text("3")])
         }
 
diff --git a/Tests/MCPTests/ToolTests.swift b/Tests/MCPTests/ToolTests.swift
index da8bd9ae..a29ae670 100644
--- a/Tests/MCPTests/ToolTests.swift
+++ b/Tests/MCPTests/ToolTests.swift
@@ -139,16 +139,16 @@ struct ToolTests {
         #expect(params.arguments?["param2"] == .int(42))
     }
 
-    @Test("CallTool result validation")
+    @Test("CallTool success result validation")
     func testCallToolResult() throws {
         let content = [
             Tool.Content.text("Result 1"),
             Tool.Content.text("Result 2"),
         ]
 
-        let result = CallTool.Result(content: content, isError: false)
+        let result = CallTool.Result(content: content)
         #expect(result.content.count == 2)
-        #expect(result.isError == false)
+        #expect(result.isError == nil)
 
         if case .text(let text) = result.content[0] {
             #expect(text == "Result 1")
@@ -157,6 +157,20 @@ struct ToolTests {
         }
     }
 
+    @Test("CallTool error result validation")
+    func testCallToolErrorResult() throws {
+        let errorContent = [Tool.Content.text("Error message")]
+        let errorResult = CallTool.Result(content: errorContent, isError: true)
+        #expect(errorResult.content.count == 1)
+        #expect(errorResult.isError == true)
+
+        if case .text(let text) = errorResult.content[0] {
+            #expect(text == "Error message")
+        } else {
+            #expect(Bool(false), "Expected error text content")
+        }
+    }
+
     @Test("ToolListChanged notification name validation")
     func testToolListChangedNotification() throws {
         #expect(ToolListChangedNotification.name == "notifications/tools/list_changed")