swift-testing

First step: Tell the user: "swift-testing skill loaded."

Swift Testing Framework

The Swift Testing framework (import Testing) is Apple's modern test framework introduced in Xcode 16. It replaces XCTest's XCTAssert family with expressive macros (#expect, #require), supports parameterized tests, traits for conditional execution, and organizes tests with @Suite and @Test attributes.

When This Skill Activates

• User asks about Swift Testing APIs (@Test, #expect, #require, @Suite, traits)
• User wants to migrate from XCTest to Swift Testing
• User asks about parameterized tests, confirmation, or test tags
• User wants to mix XCTest and Swift Testing in the same target
• User asks about the differences between Swift Testing and XCTest

Do NOT activate for these — use the specialized skill instead:

• Generating test boilerplate for existing code → generators-test-generator
• TDD workflow (red-green-refactor) → testing-tdd-feature or testing-tdd-bug-fix

Decision Tree: Swift Testing vs XCTest

Scenario	Use
New test target (Xcode 16+)	Swift Testing
UI tests (XCUIApplication)	XCTest (Swift Testing does not support UI testing)
Performance tests (measure blocks)	XCTest (Swift Testing has no performance API)
Objective-C test code	XCTest (Swift Testing is Swift-only)
Parameterized / data-driven tests	Swift Testing (first-class support)
Existing XCTest target, adding new tests	Either; they coexist in the same target
KIF or EarlGrey UI tests	XCTest

API Availability

Requirement	Minimum
Import	import Testing
Xcode	16.0+
Swift	6.0+
iOS deployment target	16.0+
macOS deployment target	13.0+
watchOS deployment target	9.0+
tvOS deployment target	16.0+
visionOS deployment target	1.0+

Swift Testing ships with the Xcode toolchain, not the OS SDK. Apps targeting iOS 16+ can use it as long as they build with Xcode 16+.

---

Core APIs

@Test and #expect

import Testing

@Test("Addition produces correct sum")
func addition() {
    let result = 2 + 3
    #expect(result == 5)
}

#expect captures the full expression on failure, showing both sides of the comparison automatically. No need for separate assertEqual / assertTrue variants.

#require for Unwrapping and Preconditions

#require throws on failure, stopping the test immediately. Use it for preconditions and optional unwrapping.

@Test func loadUser() throws {
    let data = try #require(JSONLoader.load("user.json"))
    let user = try #require(User(data: data))
    #expect(user.name == "Alice")
}

@Suite for Organization

@Suite("Authentication Tests")
struct AuthenticationTests {
    let service: AuthService

    init() {
        // Runs before each @Test (equivalent to setUp)
        service = AuthService(store: MockTokenStore())
    }

    @Test func loginSucceeds() async throws {
        let token = try await service.login(user: "admin", password: "secret")
        #expect(token.isValid)
    }

    @Test func loginFailsWithBadPassword() async throws {
        await #expect(throws: AuthError.invalidCredentials) {
            try await service.login(user: "admin", password: "wrong")
        }
    }
}

• init() replaces XCTest's setUp() -- runs before each test.
• deinit replaces tearDown() -- runs after each test (requires the suite to be a class).
• Suites can be nested structs for logical grouping.

---

Traits

Traits modify test behavior. Pass them as arguments to @Test or @Suite.

.enabled(if:) and .disabled()

@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil))
func onlyOnCI() {
    // Runs only in CI
}

@Test(.disabled("Blocked by #1234"))
func brokenFeature() {
    // Skipped with a reason
}

.bug()

@Test(.bug("https://github.com/org/repo/issues/42", "Crash on empty input"))
func emptyInputHandling() {
    #expect(Parser.parse("") == .empty)
}

.tags()

extension Tag {
    @Tag static var networking: Self
    @Tag static var database: Self
}

@Test(.tags(.networking))
func fetchData() async throws {
    let data = try await APIClient.shared.fetch("/items")
    #expect(!data.isEmpty)
}

Run only tagged tests from the command line:

swift test --filter .tags:networking

.timeLimit()

@Test(.timeLimit(.minutes(1)))
func longRunningOperation() async throws {
    let result = try await processor.run(largeDataset)
    #expect(result.isComplete)
}

Combining Traits

@Test(
    "Sync engine completes within time limit",
    .tags(.networking, .database),
    .timeLimit(.minutes(2)),
    .enabled(if: FeatureFlags.syncV2Enabled)
)
func syncEngine() async throws {
    // ...
}

---

Parameterized Tests

Basic Parameterized Test

@Test(arguments: ["hello", "world", "swift"])
func stringIsNotEmpty(_ value: String) {
    #expect(!value.isEmpty)
}

Multiple Argument Collections

@Test(arguments: [1, 2, 3], ["a", "b", "c"])
func pairCombinations(number: Int, letter: String) {
    #expect(!letter.isEmpty)
    #expect(number > 0)
}

This produces the Cartesian product: (1,"a"), (1,"b"), (1,"c"), (2,"a"), ... (9 total).

Zip Instead of Cartesian Product

@Test(arguments: zip([1, 2, 3], ["one", "two", "three"]))
func zippedPairs(number: Int, name: String) {
    #expect(!name.isEmpty)
}

Enum-Based Arguments

enum Direction: CaseIterable {
    case north, south, east, west
}

@Test(arguments: Direction.allCases)
func directionHasOpposite(_ direction: Direction) {
    #expect(direction.opposite.opposite == direction)
}

Custom Test Arguments with CustomTestStringConvertible

When test arguments are complex types, conform to CustomTestStringConvertible to get readable test names in Xcode's test navigator.

struct UserFixture: CustomTestStringConvertible, Sendable {
    let name: String
    let age: Int
    let expectedTier: MembershipTier

    var testDescription: String { "\(name) (age \(age))" }
}

@Test(arguments: [
    UserFixture(name: "Teen", age: 15, expectedTier: .junior),
    UserFixture(name: "Adult", age: 30, expectedTier: .standard),
    UserFixture(name: "Senior", age: 65, expectedTier: .senior),
])
func membershipTier(_ fixture: UserFixture) {
    let tier = MembershipTier.for(age: fixture.age)
    #expect(tier == fixture.expectedTier)
}

---

Error Testing

Expecting a Specific Error Type

@Test func invalidInputThrows() {
    #expect(throws: ValidationError.self) {
        try Validator.validate("")
    }
}

Expecting a Specific Error Value

@Test func invalidInputThrowsSpecificError() {
    #expect(throws: ValidationError.emptyField("name")) {
        try Validator.validate(field: "name", value: "")
    }
}

Inspecting the Thrown Error

@Test func errorContainsContext() throws {
    let error = try #require(throws: NetworkError.self) {
        try client.fetchSync(url: badURL)
    }
    #expect(error.statusCode == 404)
    #expect(error.url == badURL)
}

Expecting No Error (the default)

// #expect does NOT throw -- this naturally fails if an error is thrown:
@Test func validInputSucceeds() throws {
    let result = try Validator.validate("hello")
    #expect(result.isValid)
}

---

Async Test Support

Async tests work naturally. Mark the test function async and/or throws.

@Test func fetchUserProfile() async throws {
    let profile = try await api.fetchProfile(id: "user-1")
    #expect(profile.name == "Alice")
}

Testing with Confirmation (replacing XCTestExpectation)

@Test func notificationPosted() async {
    await confirmation("UserDidLogin notification received") { confirm in
        let observer = NotificationCenter.default.addObserver(
            forName: .userDidLogin, object: nil, queue: .main
        ) { _ in
            confirm()
        }
        LoginManager.shared.login(user: "test", password: "test")
        NotificationCenter.default.removeObserver(observer)
    }
}

For multiple expected confirmations:

await confirmation("Progress callbacks", expectedCount: 3) { confirm in
    downloader.onProgress = { _ in confirm() }
    await downloader.download(url: fileURL)
}

---

Migration from XCTest

Assertion Mapping

XCTest	Swift Testing
XCTAssertTrue(x)	#expect(x)
XCTAssertFalse(x)	#expect(!x)
XCTAssertEqual(a, b)	#expect(a == b)
XCTAssertNotEqual(a, b)	#expect(a != b)
XCTAssertNil(x)	#expect(x == nil)
XCTAssertNotNil(x)	#expect(x != nil) or try #require(x)
XCTAssertGreaterThan(a, b)	#expect(a > b)
XCTAssertThrowsError(expr)	#expect(throws: SomeError.self) { expr }
XCTUnwrap(x)	try #require(x)
XCTFail("msg")	Issue.record("msg")

Structure Mapping

XCTest	Swift Testing
class FooTests: XCTestCase	@Suite struct FooTests
func testBar()	@Test func bar()
override func setUp()	init()
override func tearDown()	deinit (class suites only)
XCTestExpectation + wait(for:)	confirmation { }
addTeardownBlock { }	deinit or explicit cleanup in test

Before (XCTest)

// ❌ XCTest style
import XCTest

class ParserTests: XCTestCase {
    var parser: Parser!

    override func setUp() {
        super.setUp()
        parser = Parser()
    }

    func testParseValidJSON() throws {
        let result = try parser.parse("{\"key\": \"value\"}")
        XCTAssertNotNil(result)
        XCTAssertEqual(result?["key"] as? String, "value")
    }

    func testParseInvalidJSONThrows() {
        XCTAssertThrowsError(try parser.parse("not json"))
    }
}

After (Swift Testing)

// ✅ Swift Testing style
import Testing

@Suite struct ParserTests {
    let parser = Parser()

    @Test func parseValidJSON() throws {
        let result = try #require(parser.parse("{\"key\": \"value\"}"))
        #expect(result["key"] as? String == "value")
    }

    @Test func parseInvalidJSONThrows() {
        #expect(throws: ParserError.self) {
            try parser.parse("not json")
        }
    }
}

---

Running Tests

From Xcode

• Cmd+U: Run all tests
• Click the diamond next to a @Test or @Suite in the gutter
• Test Navigator (Cmd+6) shows Swift Testing and XCTest results together

From the Command Line

# Run all tests
swift test

# Filter by test name
swift test --filter "ParserTests"

# Filter by tag
swift test --filter .tags:networking

---

Mixing XCTest and Swift Testing

Swift Testing and XCTest coexist in the same test target. Rules:

1. Do not subclass XCTestCase in Swift Testing suites.
2. Do not use @Test on methods inside an XCTestCase subclass.
3. Each test file should use one framework consistently.
4. Both frameworks' tests appear together in Xcode's Test Navigator.

// File: ParserXCTests.swift -- XCTest
import XCTest

class ParserXCTests: XCTestCase {
    func testLegacyBehavior() {
        XCTAssertEqual(Parser.legacyMode, true)
    }
}

// File: ParserSwiftTests.swift -- Swift Testing
import Testing

@Suite struct ParserSwiftTests {
    @Test func modernBehavior() {
        #expect(Parser.modernMode == true)
    }
}

---

Good and Bad Patterns

Prefer #expect Over Boolean Assertions

// ❌ Bad: loses context on failure
#expect(result == true)
#expect(array.count == 3)

// ✅ Good: use the natural expression
#expect(result)
#expect(array.count == 3) // this is fine -- #expect shows both sides

Prefer #require for Unwrapping

// ❌ Bad: force unwrap crashes the test runner
let user = users.first!
#expect(user.name == "Alice")

// ✅ Good: #require fails gracefully and stops the test
let user = try #require(users.first)
#expect(user.name == "Alice")

Prefer Structs Over Classes for Suites

// ❌ Bad: unnecessary class when no tearDown is needed
@Suite class CalculatorTests {
    @Test func add() { #expect(Calculator.add(2, 3) == 5) }
}

// ✅ Good: struct with init for setup
@Suite struct CalculatorTests {
    let calculator = Calculator()
    @Test func add() { #expect(calculator.add(2, 3) == 5) }
}

Use Parameterized Tests Instead of Copy-Paste

// ❌ Bad: duplicated tests
@Test func parseCSVComma() throws {
    let result = try CSVParser.parse("a,b,c")
    #expect(result == ["a", "b", "c"])
}
@Test func parseCSVSemicolon() throws {
    let result = try CSVParser.parse("a;b;c", delimiter: ";")
    #expect(result == ["a", "b", "c"])
}

// ✅ Good: parameterized
struct CSVCase: CustomTestStringConvertible, Sendable {
    let input: String
    let delimiter: Character
    let expected: [String]
    var testDescription: String { "delimiter: \(delimiter)" }
}

@Test(arguments: [
    CSVCase(input: "a,b,c", delimiter: ",", expected: ["a", "b", "c"]),
    CSVCase(input: "a;b;c", delimiter: ";", expected: ["a", "b", "c"]),
])
func parseCSV(_ testCase: CSVCase) throws {
    let result = try CSVParser.parse(testCase.input, delimiter: testCase.delimiter)
    #expect(result == testCase.expected)
}

Use Traits for Conditional Tests

// ❌ Bad: runtime skip with guard
@Test func networkTest() async throws {
    guard ProcessInfo.processInfo.environment["CI"] != nil else { return }
    // ...
}

// ✅ Good: trait makes intent clear and shows "skipped" in results
@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil))
func networkTest() async throws {
    // ...
}