Basic Tests
@Test Traits
#expect and #require
@Suite and Test Organization
Execution Model
XCTest Migration Boundaries
Known Issues
Additional Patterns
Common Mistakes
Test Attachments
Exit Testing
Version-Gated APIs
Review Checklist
References

Basic Tests

import Testing

@Test("User can update their display name")
func updateDisplayName() {
    var user = User(name: "Alice")
    user.name = "Bob"
    #expect(user.name == "Bob")
}

`@Test` Traits

@Test("Validates email format")                                    // display name
@Test(.tags(.validation, .email))                                  // tags
@Test(.disabled("Server migration in progress"))                   // disabled
@Test(.enabled(if: ProcessInfo.processInfo.environment["CI"] != nil)) // conditional
@Test(.bug("https://github.com/org/repo/issues/42"))               // bug reference
@Test(.timeLimit(.minutes(1)))                                     // time limit
@Test("Timeout handling", .tags(.networking), .timeLimit(.seconds(30))) // combined

#expect and #require

// #expect records failure but continues execution
#expect(result == 42)
#expect(name.isEmpty == false)
#expect(items.count > 0, "Items should not be empty")

// #expect with error type checking
#expect(throws: ValidationError.self) {
    try validate(email: "not-an-email")
}

// #expect with specific error value
#expect {
    try validate(email: "")
} throws: { error in
    guard let err = error as? ValidationError else { return false }
    return err == .empty
}

// #require records failure AND stops test (like XCTUnwrap)
let user = try #require(await fetchUser(id: 1))
#expect(user.name == "Alice")

// #require for optionals -- unwraps or fails
let first = try #require(items.first)
#expect(first.isValid)

Rule: Use #require when subsequent assertions depend on the value. Use #expect for independent checks.

`@Suite` and Test Organization

See references/testing-patterns.md for suite organization, confirmation patterns, known-issue handling, and execution-model details.

Execution Model

Swift Testing runs tests in parallel by default. Do not assume test order, shared suite instances, or exclusive access to mutable state unless you explicitly design for it.

@Suite(.serialized)
struct KeychainTests {
    @Test func storesToken() throws { /* ... */ }
    @Test func deletesToken() throws { /* ... */ }
}

Use .serialized when a test or suite must run one-at-a-time because it touches shared external state. It does not make unrelated tests outside that scope run serially.

Rules:

Each test must set up its own state.
Shared mutable globals are a bug unless protected or intentionally serialized.
@Suite(.serialized) is for exclusive execution, not for expressing logical ordering between tests.
If tests depend on sequence, combine them into one test or move the sequence into shared helper code.

XCTest Migration Boundaries

Swift Testing unit tests do not inherit from XCTestCase. Declare @Test on free functions or methods on suite types such as struct, class, or actor; use static or class methods when instance fixtures are unnecessary.

XCTest and Swift Testing can coexist during migration. Migrate one file or suite at a time, compare discovery/pass/fail/skip counts, and keep UI automation, performance benchmarks, and common snapshot flows on XCTest/XCUITest or snapshot tooling. Separate files or targets when that makes runner expectations clearer.

For Xcode 27-era mixed helpers, check the configured interoperability mode rather than claiming cross-framework APIs are forbidden. Older test plans inherit limited; new projects use complete; strict and none are also available. Prefer complete or strict during migration and use SWIFT_TESTING_XCTEST_INTEROP_MODE for SwiftPM when needed. See references/testing-advanced.md for the mode matrix and toolchain gates.

Do not mechanically replace every XCTest assertion with #expect; preserve required unwraps and unconditional failures with these migration defaults:

XCTAssert* -> #expect(...)
XCTUnwrap or any value required by later checks -> try #require(...)
XCTFail("...") or manual unconditional issues -> Issue.record("...")
UI tests, performance benchmarks, and common snapshot-test flows stay on XCTest/XCUITest or snapshot tooling.
Put @available on individual @Test functions, not on suite types or their containing types.

See references/testing-patterns.md for migration examples and references/testing-advanced.md for Swift/Xcode version gates.

Known Issues

Mark expected failures so they do not cause test failure:

withKnownIssue("Propane tank is empty") {
    #expect(truck.grill.isHeating)
}

// Intermittent / flaky failures
withKnownIssue(isIntermittent: true) {
    #expect(service.isReachable)
}

// Conditional known issue
withKnownIssue {
    #expect(foodTruck.grill.isHeating)
} when: {
    !hasPropane
}

If no known issues are recorded, Swift Testing records a distinct issue notifying you the problem may be resolved.

Additional Patterns

See references/testing-patterns.md for parameterized tests, tags and suites, async testing, traits, and execution-model details.

Test Attachments

Attach diagnostic data to test results for debugging failures. See references/testing-patterns.md for full examples.

@Test func generateReport() async throws {
    let report = try generateReport()
    Attachment.record(report.data, named: "report.json")
    #expect(report.isValid)
}

For image attachments and their toolchain gate, use the canonical table in Version-Gated APIs.

Exit Testing

Test code that calls exit(), fatalError(), or preconditionFailure() on a supported runtime. State the exact gate from Version-Gated APIs when correcting exit-test code.

@Test func invalidInputCausesExit() async {
    await #expect(processExitsWith: .failure) {
        processInvalidInput()  // calls fatalError()
    }
}

Version-Gated APIs

For advanced APIs, state the exact toolchain and runtime gate beside the correction. This is the canonical summary; references/testing-advanced.md contains the detailed matrix and examples.

@Test func exitsWithCapturedCode() async {
    let expectedCode: Int32 = 42
    await #expect(processExitsWith: .failure) { [expectedCode] in
        exit(expectedCode)
    }
}

User code to correct	Current guidance
`#expect(exitsWith:)`	Use `await #expect(processExitsWith: .failure) { ... }`. Exit testing requires Swift 6.2 / Xcode 26.0 or newer and is supported on macOS, Linux, FreeBSD, OpenBSD, and Windows runtime targets, not iOS, tvOS, or watchOS. For an iOS app target, test fatal-path logic through a smaller non-exiting API or a supported host/tool target.
Exit-test closure reads outer values	Add an explicit capture list, for example `{ [expectedCode] in ... }`. Exit-test capture lists require the Swift 6.3 compiler; captured values must be `Sendable` and `Codable`.
`Test.cancel()` in a test that awaits work	Make the test `async throws` and call `try Test.cancel("reason")`. `Test.cancel(_:)` requires Swift 6.3 / Xcode 26.4-era support.
`Issue.record(..., severity: .warning)`	Use `Issue.record("message", severity: .warning)`. Warning severity is reported but does not fail the test, and requires Swift 6.3 / Xcode 26.4-era support.
`Attachment(image, named:).record()`	Use `Attachment.record(image, named: "name", as: .png)`. Import `Testing` plus the relevant image framework; Apple-platform image values include `UIImage`, `CGImage`, `CIImage`, and `NSImage`. Image attachment recording requires Swift 6.3 / Xcode 26.4-era support.

Common Mistakes

Testing implementation, not behavior. Test what the code does, not how.
No error path tests. If a function can throw, test the throw path.
Flaky async tests or sleeps. Use confirmation, clock injection, or concurrency primitives instead of sleeping.
Shared mutable state between tests. Each test sets up its own state via init() in @Suite.
Missing accessibility identifiers in UI tests. XCUITest queries rely on them.
Not testing cancellation. If code supports Task cancellation, verify it cancels cleanly.
Unclear migration boundaries. Follow XCTest Migration Boundaries instead of treating either framework as an all-or-nothing choice.
Non-Sendable helpers shared across tests. Make shared helpers Sendable; annotate MainActor-dependent test code with @MainActor.
Treating serialization as ordering. Tests run in parallel by default; .serialized protects exclusive state but does not make one test feed another.

Review Checklist

References

Testing patterns: references/testing-patterns.md
Advanced testing (warnings, cancellation, image attachments): references/testing-advanced.md

swift-testing

Swift Testing

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