Swift Project Standard

Swift Project Standard

This skill is the guiding standard for any Swift work. Apply it by default; don't wait to be asked.

Its spine: trust is placed not in the model, but in the machine-checkable code that constrains it. In Swift, like Rust, most of that scaffold is the compiler — types, optionality, exhaustiveness, and (in Swift 6) data-race safety are enforced at compile time with no type erasure, so there's no runtime type-checker to bolt on (unlike the Python sibling's beartype). The job here is to (a) keep the few escape hatches shut and turn on Swift 6 complete strict concurrency, (b) push every external dependency behind a protocol so the model is hot-swappable, (c) organize deep and name-navigable, and (d) mechanize the implicit knowledge a human would otherwise hold — via a zero-warning gate, a per-target contract, and drift guards. The agent's output ceiling equals the tightness of that loop.

Baseline: Swift 6.x (// swift-tools-version: 6.0, swiftLanguageModes: [.v6]), complete strict concurrency, warnings-as-errors at the gate, swift-format (toolchain-bundled), SwiftLint (community, --strict), Codable + newtypes, os.Logger, a Codable AppConfig, and XcodeGen for a thin app shell when there's a UI.

This standard is the project-level engineering charter (the gate + architecture conventions). For how to write the implementation inside a target, it delegates to the existing implementation skills — swift-concurrency, swift-error-handling, swift-protocols, swift-data-flow, swift-testing, swift-networking, swift-persistence, and the rest — rather than restating them. Cite them where relevant; don't duplicate their content.

When starting a new project

python scripts/scaffold.py <package_name> --target <dir> --domains ingestion retrieval generation agents

This mirrors assets/templates/ into an SPM package (Package.swift + Kernel/Domain/Adapters targets + an App executable + a domain target per --domains + a CLAUDE.md per target + ci.sh + ADR), substitutes __PACKAGE__, and registers each domain target in Package.swift's targets/products. Module names (import Kernel, import Domain, …) are project-independent — only the package name, the executable name, and CLAUDE.md headers carry the literal name, so import paths never depend on it. When adapting an existing repo, copy from assets/templates/ by hand.

Then: swift build, and ./ci.sh (or make check) to verify. swift format ships with the toolchain; swiftlint needs brew install swiftlint once. Pass --app to run xcodegen and emit a thin .xcodeproj app shell that links the AppCore library product.

When working on existing code

Apply the rules below, keep new code strict, and verify structural invariants:

python scripts/check_conformance.py <project_root>

It parses the manifest via swift package dump-package (JSON, cleaner than regex) and checks the mechanically enforceable invariants (SPM present, tools ≥ 6 + v6 language mode, target-per-domain, the Domain target has zero vendor-SDK dependencies, warnings-as-errors wired in ci.sh or the manifest, .swift-format + .swiftlint.yml present, ci.sh chaining swift-format/swiftlint/build/test, a CLAUDE.md in every Sources/<Target>). Everything else is enforced by the gate and by applying the standard.

The non-negotiables

Full rationale in references/standard.md.

1. The compiler is the static guarantee; turn on Swift 6 and keep the escape hatches shut. // swift-tools-version: 6.0 + swiftLanguageModes: [.v6] gives complete strict concurrency — compile-time data-race safety, the Swift analog of Rust's Send/Sync. No bolted-on runtime type check — Swift doesn't need one. The escape hatches are banned: no ! force-unwrap, no try!, no as!, no implicitly-unwrapped optionals, no @unchecked Sendable. SwiftLint enforces force_unwrapping/force_try/force_cast. A site that genuinely needs one takes the controlled exit: // swiftlint:disable:next force_unwrapping — reason (explicit, auditable, greppable — the Swift analog of a justified # type: ignore), and nonisolated(unsafe) / @unchecked Sendable only with a // SAFETY: note and a CLAUDE.md entry.

2. No silent failures: throws + typed throws(E) + a concrete Error, not !/try! sprinkled around. Recoverable failures are throws; Swift 6 typed throws (throws(ProviderError)) makes the error set part of the signature, the analog of Rust's Result<T, E>. Vendor/network/timeout errors normalize at the adapter boundary into ProviderError; program bugs (precondition violations) trap or propagate — never swallowed into a fallback. See swift-error-handling.

3. Boundaries: parse, don't validate — Codable + newtypes. Everything crossing a boundary (config, LLM output, tool results, files) decodes into a strongly-typed value via Codable; encode constraints in newtypes with a validating init (init(_:) throws or failable init?) so invalid states are unrepresentable rather than runtime-checked. This is the Swift mirror of Python's pydantic and Rust's serde + newtype.

4. Model-agnostic: every external AI dependency behind a protocol in Domain; SDKs only in Adapters. The Domain target has zero SDK dependencies (the checker parses the resolved manifest to enforce this); protocols stay usable as any LLM existentials. Real SDKs live in a separate target gated by an SPM package trait (.product(..., condition: .when(traits: ...))) — the precise analog of Cargo features — and normalize to ProviderError. The composition root (App) is the one place that selects an impl by config and injects it (any LLM, the analog of Box<dyn>). A deterministic MockProvider is the default (not a test stub) so the executable, tests, and CI run offline with no SDK or key. See swift-protocols for the any/some/protocol-witness tradeoff.

5. Completion = one zero-warning gate, the agent's only correctness judge. ./ci.sh runs swift format lint --strict → swiftlint --strict → swift build -Xswiftc -warnings-as-errors → swift test (offline, Mock default, smoke + conformance), under set -euo pipefail; an app scenario appends xcodebuild ... SWIFT_TREAT_WARNINGS_AS_ERRORS=YES SWIFT_STRICT_CONCURRENCY=complete. Run it after every change; fix until green; never "looks fine, commit". Pin it with a pre-push hook; CI mirrors it.

6. Two lint gates, both --strict. swift-format owns formatting (the cargo fmt analog); SwiftLint owns lint and the escape-hatch bans (the clippy analog). They are complementary, not redundant — keep both, both strict, both in the gate.

Structure: SPM package, target-per-domain

The depth comes from the package (multiple targets), not from one target with deep folders. An SPM target ≈ a Cargo crate: target-level dependency isolation plus a resolvable Package.swift. "Fix the reranker" should resolve to Sources/Retrieval/... with no search.

• A target per bounded context (Sources/<Domain>/), plus the fixed infra targets and a thin executable:
  • Kernel — cross-cutting infra: Config.swift (Codable AppConfig: Sendable; defaults < configs/settings.json < env APP_* with __ nesting), Logging.swift (os.Logger; an enum Log factory; logProvenance(...); privacy discipline), Prompts.swift (Bundle.module resources + strict rendering). Named Kernel, not Core. Zero external deps.
  • Domain — ports (protocols) + models + boundary errors; zero SDK deps.
  • Adapters — protocol impls; MockProvider is the default; real SDKs in a trait-gated sibling target.
  • domain targets (Retrieval/Generation/…) depend on Domain + Kernel, never on Adapters or SDKs.
  • App — the executable and composition root; wires concrete adapters by config. The Xcode app target plays this role in a UI project and links the AppCore library product.
• Dependency direction: domain target → Domain + Kernel; Adapters → Domain + Kernel; App → everything. Domain is zero-SDK.
• A CLAUDE.md in every target (the layered-context rule applied per directory): the root one is a routing table (hard constraints + where to look); each target's states its responsibility, dependency direction, and local contract (Domain's "zero SDK", Adapters' "errors normalize", Kernel's "payloads never logged"). The checker requires one per target.
• Go deep: split a target into submodules by sub-capability; names map to paths.

Navigability

Naming-as-path is to navigation what types are to interface contracts — and SPM lifts it to the target level. Group by capability, not by Models//Utils/. Nest until leaf files have a single clear responsibility. A name should resolve to a path with no search.

Principles for AI-touching code (advisory)

Beyond the type system — for any code where a model produces output. Upper-level discipline; not all mechanically checkable.

• Constrain, don't ask. Push non-negotiable properties (no fabrication, must-cite, no privilege escalation) into deterministic control flow so the model physically cannot violate them — don't rely on the prompt. Synthesize answers from typed values in code; discard model prose on the critical path.
• Narrow the emission surface. Make the model pick from a typed enum of options or call tools returning a tri-state result; take final values from tool results, not free-form model text. A Swift enum (with associated values, exhaustively matched) is the natural controlled surface.
• Guardrails are deterministic, independent, never pluggable. Intent parsing can be swapped; safety decisions are deterministic code re-evaluated from raw input, not trusting a pluggable component's output.

Driving AI on big work (advisory)

Treat AI as supervisable labor, not unsupervised autopilot.

• Decision-first: a numbered, immutable ADR (docs/adr/) before coding — context + chosen option + rejected alternatives and why. Rejected-reasons stop it re-walking excluded paths.
• TDD red-light first; tests are the immutable spec. Write the failing test (Swift Testing @Test or XCTest), then implement to green; never weaken a test to pass. See swift-testing.
• Numbered steps, each independently green; one commit per step through the full gate. No giant diffs.
• Adversarial independent review. After writing, run a separate, hostile, multi-perspective review prioritizing what tests can't cover (diagrams, docs, tradeoffs).

Scale to project size

The target split, per-target CLAUDE.md, ADRs, and provider seam are real overhead — overkill for a 200-line CLI. Present them as triggered, scalable patterns ("the moment you call an LLM/embedding/vector store, put it behind a protocol in Domain"; "split a target out the moment it takes a second responsibility"), not blanket mandates. A single-target package with files + the two lint gates + the zero-warning gate is a perfectly good small-scale instantiation.

Scale along two axes, not one — size and domain. The standard has a universal spine that holds for any Swift project whether or not it touches AI: Swift 6 + complete strict concurrency + warnings-as-errors, the zero-warning gate, throws/typed throws with the escape hatches shut (!/try!/as!), Codable + newtypes at boundaries, SPM target-per-domain + deep naming-as-path, os.Logger, a strongly-typed AppConfig, swift-format + SwiftLint. The rest is an AI-triggered layer that only switches on once the project actually calls an LLM / embedding / vector store: the Domain protocol seam + zero-SDK domain target, MockProvider-as-default, bundled prompts + strict rendering, logProvenance, and the constrain-don't-ask discipline. A pure library / CLI / app that never calls a model should take the spine in full and skip the AI layer outright — bolting MockProvider or prompt-embedding onto such a project is cargo-culting, not conformance.

Resources

• references/standard.md — the full standard with rationale and complete code for every target (read for the why, edge cases, or exact module contents).
• assets/templates/ — exact SPM boilerplate; __PACKAGE__ is the only placeholder. Includes Kernel/Domain/Adapters/App, .swift-format, .swiftlint.yml, ci.sh, project.yml (XcodeGen), a per-target CLAUDE.md, configs/settings.json, and smoke/conformance tests.
• scripts/scaffold.py — generate a conforming package.
• scripts/check_conformance.py — verify structural invariants (incl. Domain-zero-SDK and per-target CLAUDE.md).