SOTA Swift (2026)

Expert-level rules for producing and auditing production Swift, on any target. Swift is a language first and an Apple-platform toolkit second: a Vapor service on Linux, a CLI and an iOS app share every rule here. Platform concerns route elsewhere: app lifecycle, store review, Keychain, push and offline sync → sota-mobile; containers → sota-sandboxing; telemetry → sota-observability.

Version floor. swift.org publishes no end-of-life or support-window policy for older Swift releases (searched the swift.org site source, 2026-10-07); the nearest written bar is the Swift Server Workgroup's graduation criteria — "Support new GA versions of Swift within 30d" and a "documented support strategy for at least one previous major version". So pin the toolchain you build with, verify the latest release at swift.org, and treat the language mode as the load-bearing line: Swift 6 mode makes data-race safety a compile error. Feature notes below name the release that introduced each feature.

BUILD mode

1. Declare the toolchain and language mode in Package.swift (swift-tools-version: 6.x, swiftLanguageModes: — swiftLanguageVersions is deprecated since 6.1, SE-0441) and pin the toolchain for CI (rules/08).
2. Read the rules files that match the work (index below). Concurrency (rules/03) applies to any code with async, actors or shared state; security (rules/05) to anything touching input, files, processes, SQL or the network; server code also reads rules/06.
3. Write value types, exhaustive enums and non-optional straight-line code (rules/01); keep the public surface small and evolvable (rules/02).
4. Self-audit last: walk each loaded file's Audit checklist against the diff, then the router's principle 5 (abuse control, transport, tests, logs) before presenting.

AUDIT mode

1. Recon: Package.swift (tools version, language modes, unsafeFlags, plugins, macros, binary targets), Package.resolved, CI config, the server framework in use.
2. Run what is installed (never install unasked): the compiler in Swift 6 mode with -warnings-as-errors, SwiftLint, swift build --sbom-spec (6.4+), a dependency scanner (rules/09), CodeQL's Swift queries where CI has them.
3. Walk every applicable rules file's Audit checklist; each probe below is a grep you can run.
4. Report in the canonical format; severity per the table, chain closure per the sota router.

Severity conventions (refines the router's rules/03 §1 inside this skill)

Severity	Swift examples
Critical	SQL built with \(unsafeRaw:) from request input; certificateVerification: .none on a production client; /bin/sh -c with interpolated input; NSKeyedUnarchiver.unarchiveObject(with:) on untrusted bytes
High	data race behind nonisolated(unsafe) / @unchecked Sendable on reachable state; unauthenticated route group missing its guard middleware; session cookies left at Vapor's isSecure: false default; debug build shipped (raw error text to clients); plugin/macro dependency unreviewed on a Linux CI that runs them unsandboxed
Medium	try!/!/as! on input-derived values; Double for money; unbounded AsyncStream buffer; Task.detached without reason; ReDoS-prone regex on input
Low	try? on a write path; any P on a hot path; missing final; IUO outside two-phase init

Finding format

file:line | rule | severity | effort | fix — the router's canonical row; expand each surviving finding to the router's rules/03 §2 evidence block.

Rules index

File	Read this when…
rules/01-idioms-and-errors.md	Writing any Swift: value types vs classes, enums, optionals and force-unwraps, throws/typed throws/try?, integer overflow and money, dates and clocks
rules/02-api-design.md	Designing a module or package API: protocols vs generics (some/any), access control (public/package), library evolution and @frozen, deprecation, SemVer
rules/03-concurrency.md	Any async, actor, Task, Sendable or shared mutable state: Swift 6 mode, isolation escape hatches, reentrancy, structured concurrency, cancellation and timeouts, streams and backpressure
rules/04-memory-and-unsafe.md	Retain cycles, weak/unowned, resource ownership (defer, ~Copyable), unsafe pointers, C interop, strict memory safety
rules/05-security.md	Input, files, processes, SQL, HTTP, crypto, decoding, regex, logs, secrets — the language-level security surface
rules/06-server-side.md	Vapor/Hummingbird/SwiftNIO services: auth middleware order, cookie and session defaults, release vs debug, body limits, error responses, Linux Foundation
rules/07-performance.md	Profiling first, allocations, ARC traffic, existentials, copy-on-write, benchmarks in CI
rules/08-tooling-ci-testing.md	Toolchain pinning, warnings-as-errors, SwiftLint/swift-format and every suppression, Swift Testing/XCTest, sanitizers, flaky tests
rules/09-supply-chain.md	Package.resolved, version requirements, registries and package identity, plugins and macros (build-time code), binary targets, SBOM, vulnerability scanning, adopting a package

Top 10 non-negotiables

1. Swift 6 language mode for all first-party code; every nonisolated(unsafe), @unchecked Sendable, @preconcurrency and unowned(unsafe) carries a comment naming the invariant that makes it safe.
2. No !, try! or as! on values derived from input; guard let/throw instead.
3. Untyped throws on public APIs; typed throws only inside a closed boundary.
4. Structured concurrency first; every unstructured Task has an owner that cancels it; long loops check cancellation.
5. No retain cycles: escaping closures stored by self capture [weak self].
6. SQL binds, never interpolates: no \(unsafeRaw:)/unsafeSQL: with input; processes take an argument array, never /bin/sh -c.
7. TLS verification stays on; minimum TLS 1.2 set explicitly on NIOSSL clients.
8. Ship release builds (swift build -c release); production behaviour never depends on a debug build's #if DEBUG or Vapor's compile-time isRelease.
9. Commit Package.resolved and resolve with --force-resolved-versions in CI; review plugins and macros as build-time code — on Linux they run unsandboxed.
10. Warnings are errors in CI, and every SwiftLint/compiler suppression is visible and justified.

Operating notes

• Swift moves fast; Swift Evolution proposal status (swift.org/swift-evolution) is the primary source for whether a feature has shipped. "Accepted" is not "implemented".
• Facts here carry their verification date; re-check before pinning a version or quoting a default.