Package.swift

// swift-tools-version: 6.1
import PackageDescription

let package = Package(
    name: "SwiftFormatPlugin",
    products: [
        .plugin(name: "SwiftFormatPlugin", targets: ["SwiftFormatPlugin"])
    ],
    targets: [
        .plugin(name: "SwiftFormatPlugin", capability: .buildTool)
    ]
)

SwiftFormatPlugin.swift

import Foundation
import PackagePlugin

@main
struct SwiftFormatPlugin: BuildToolPlugin {
    func createBuildCommands(context: PluginContext, target: Target) async throws -> [Command] {
        guard let sourceFiles = target.sourceModule?.sourceFiles else { return [] }

        let tool = try context.tool(named: "swift-format")
        let paths = sourceFiles.map(\.url)
            .filter { $0.pathExtension == "swift" }
            .map{ $0.path(percentEncoded: false) }

        return [
            .prebuildCommand(
                displayName: "Checking the format of Swift files in \(target.name)",
                executable: tool.url,
                arguments: ["lint", "--parallel", "--no-color-diagnostics"] + paths,
                outputFilesDirectory: context.pluginWorkDirectoryURL
            )
        ]
    }
}

#if canImport(XcodeProjectPlugin)
import XcodeProjectPlugin

extension SwiftFormatPlugin: XcodeBuildToolPlugin {
    func createBuildCommands(context: XcodePluginContext, target: XcodeTarget) throws -> [Command] {
        let tool = try context.tool(named: "swift-format")

        let paths = target.inputFiles.map(\.url)
            .filter { $0.pathExtension == "swift" }
            .map{ $0.path(percentEncoded: false) }

        return [
            .prebuildCommand(
                displayName: "Checking the format of Swift files in \(target.displayName)",
                executable: tool.url,
                arguments: ["lint", "--parallel", "--no-color-diagnostics"] + paths,
                outputFilesDirectory: context.pluginWorkDirectoryURL
            )
        ]
    }
}

#endif