Guide · Agent skill
How to use the Swift Protocol Dependency Injection Testing agent skill
Updated · written with AI from the project's own documentation and checked against our catalogue data
This guide explains how to install and use the swift-protocol-di-testing agent skill, a coding guide that tells your AI agent how to structure Swift code so file, network, and external-service access goes through small protocols. It is aimed at Swift and iOS/macOS developers who want unit tests that run without real I/O. By the end you will know what the skill changes in the code the agent writes, how to install it, and how to ask for it in plain language.
Swift Protocol Dependency Injection Testingaffaan-m/ECC · MIT Download the skill Quick start
- 1Download the ZIP. Download the skill ZIP from this page and unzip it to get the swift-protocol-di-testing folder containing SKILL.md.
- 2Place the skill folder. Move the folder into ~/.claude/skills/ for personal use or into your project's .claude/skills/ for a shared setup.
- 3Or upload it in the Claude apps. In the Claude apps, open Settings, go to Capabilities, and upload the skill under Skills.
- 4Ask for Swift code with dependencies. Describe the Swift type that touches a file, network or external API, and say you want it testable.
- 5Review the protocols and mocks. Check that protocols are small, mocks are configurable for errors, and only external boundaries are mocked.
- 6Run the Swift Testing tests. Run the generated tests with your Swift toolchain and confirm the error paths behave as expected.
What an agent skill is and what this one does
An agent skill is a folder with a SKILL.md file that gives your AI agent extra instructions for a specific kind of task. This skill covers Swift code that touches the file system, the network, iCloud, or other external APIs.
When you ask for Swift code or tests, the agent follows the pattern in the skill: it defines small protocols for each external concern, writes production implementations, and writes mock implementations that simulate success and failure. The tests are written with Apple's Swift Testing framework (import Testing). The skill is a guide the agent follows; it does not change your files on its own.
- Defines focused protocols such as FileSystemProviding, FileAccessorProviding and BookmarkStorageProviding.
- Provides default production implementations that call FileManager, Data and similar APIs.
- Provides mock classes with configurable readError and writeError properties and in-memory storage.
- Injects dependencies through initializer parameters with default values, so tests can pass mocks.
Install the skill (ZIP download or Claude apps upload)
Download the skill ZIP from this page and unzip it. The skill is delivered as a single file, SKILL.md. Put the unzipped skill folder in one of these locations:
- Personal skills folder: ~/.claude/skills/ (available across your projects).
- Project skills folder: .claude/skills/ inside your project (shared with anyone who works in that repository).
- Claude apps: open Settings, go to Capabilities, then upload the skill under Skills.
Trigger the skill with natural requests
You do not need a special command. Ask for Swift work that involves external dependencies, and mention testing or mocks if you want the protocol pattern. The skill's description covers protocol-based dependency injection, mocking the file system, network and external APIs, and Swift Testing.
Name the concern you want isolated (file access, network, iCloud container, bookmarks) and say whether you want tests. Concrete requests work best.
- Write a SyncManager actor that reads data.json from the iCloud container, and give it protocols so I can unit test it.
- Add tests with Swift Testing for what happens when the file read throws CocoaError.fileReadCorruptFile.
- Refactor my NetworkClient so I can mock its responses in tests without making real requests.
- Show me a Swift Testing case that checks a sync throws containerNotAvailable when there is no container.
Examples of the code it produces
The skill's core example defines a small protocol for file reads and writes. The agent writes the matching mock so a test can set readError and check the error path without touching disk.
The sync manager example injects its dependencies with default parameters. Production code gets the real implementations automatically; a test passes a mock in the initializer.
public protocol FileAccessorProviding: Sendable {
func read(from url: URL) throws -> Data
func write(_ data: Data, to url: URL) throws
func fileExists(at url: URL) -> Bool
}What you need before you start
The skill requires a Swift toolchain with the Swift Testing framework, which means code that uses import Testing. It does not run scripts and does not use the network. The patterns are Swift-only, so it is not useful for projects written in other languages.
- A Swift toolchain that includes the Swift Testing framework.
- A Swift project where you want unit tests for file, network, or iCloud code.
- Familiarity with Swift concurrency is helpful, since the skill covers actors and Sendable.
Safety notes: scripts and network
According to the skill's metadata, it runs no scripts and uses no network. It consists of a single SKILL.md instruction file. Because the agent still writes code into your project, review the generated protocols, mocks and tests as you would any code change before you commit them.
- No scripts are included or run.
- No network access is required.
- Check the agent's changes in your own version control before merging.
Tips for getting good results
The skill itself warns against over-engineering. Mock only boundaries such as the file system, network and external APIs, not internal types. Keep each protocol to one concern rather than a single large protocol. Avoid #if DEBUG switches as a substitute for injection.
- Keep protocols small: one concern each, not a god protocol.
- Mark protocols Sendable when they cross actor boundaries.
- Do not ask for a protocol for code that has no external dependencies; the skill says that adds unnecessary complexity.
- Give the agent your real external calls so it can mirror them in the protocol.
What people use it for
- Unit testing a sync manager that reads from an iCloud container without touching real storage.
- Testing how an app handles a corrupt-file or missing-file read by setting a mock error.
- Designing an actor-based service with Sendable protocols so it can be tested in app, test and SwiftUI preview contexts.
- Simulating network or API failures in Swift Testing to cover error handling that is hard to trigger for real.
Frequently asked questions
How do I install the swift-protocol-di-testing skill?
Download the skill ZIP from this page and unzip it. Place the folder in ~/.claude/skills/ for personal use or in a project's .claude/skills/ folder. You can also upload it in the Claude apps under Settings, then Capabilities, then Skills.
What does the swift-protocol-di-testing skill require?
It requires a Swift toolchain with the Swift Testing framework, used through import Testing. It does not run scripts and does not use the network.
Is the swift-protocol-di-testing skill free?
The skill is listed under the MIT licence. This page does not state pricing beyond that, so check the licence file in the source repository for the full terms.
Can I use this skill for a project that is not in Swift?
No. The patterns and examples are Swift-only, so the skill is only useful for Swift code. Projects in other languages should look for a skill written for their language.
Does the skill change my files automatically?
No. It is a coding guide the agent follows when it writes Swift code or tests. Any files the agent creates are still yours to review and commit.
When should I not use this skill?
Skip it for simple code with no external dependencies. The skill itself warns that a protocol adds unnecessary complexity there, and it advises against protocols for types without external access.
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Our verdict
This skill is a good fit for Swift and iOS/macOS developers who want unit tests for file, network, iCloud, or external API code, especially error paths that are hard to trigger in real environments. It is less useful for non-Swift projects, simple code with no external dependencies, or anyone looking for a finished app, because it only shapes the code the agent writes.
Download the skill