MCP: offer client roots so servers can write files

Sources/SwiftAgentKitMCP/MCPManager.swift

@@ -22,12 +22,35 @@ public actor MCPManager {
     /// readResource). Tool-call timeouts are configured on `MCPToolBridge`.
     public var requestTimeout: TimeInterval = 60
 
-    public init() {}
+    /// Folders this client offers servers as MCP roots. Servers that confine
+    /// file writes to the client's roots (chrome-devtools-mcp's screenshot
+    /// `filePath`, the filesystem server) refuse every path when a client
+    /// offers none. Empty = the `roots` capability is not declared.
+    public let roots: [URL]
+
+    public init(roots: [URL] = []) {
+        self.roots = roots
+    }
+
+    /// `roots` as MCP `Root`s: `file://` directory URIs, named by folder.
+    static func mcpRoots(from urls: [URL]) -> [Root] {
+        urls.map { url in
+            let dir = URL(fileURLWithPath: url.standardizedFileURL.path, isDirectory: true)
+            return Root(uri: dir.absoluteString, name: dir.lastPathComponent)
+        }
+    }
 
     /// Connect to an MCP server using the given configuration.
     @discardableResult
     public func connect(_ config: MCPClientConfig) async throws -> MCPServerInfo {
-        let client = Client(name: "SwiftAgentKit", version: "0.3.0-alpha.7")
+        let client = Client(
+            name: "SwiftAgentKit", version: "0.3.0-alpha.7",
+            capabilities: roots.isEmpty ? .init() : .init(roots: .init(listChanged: false))
+        )
+        if !roots.isEmpty {
+            let offered = Self.mcpRoots(from: roots)
+            await client.withRootsHandler { offered }
+        }
 
         let transport: Transport
         var process: Process?

Tests/SwiftAgentKitMCPTests/MCPManagerTests.swift

@@ -167,6 +167,58 @@ final class MCPManagerTests: XCTestCase {
         )
     }
 
+    func testRootsBecomeFileDirectoryURIs() {
+        let roots = MCPManager.mcpRoots(from: [
+            URL(fileURLWithPath: "/Users/someone"),
+            URL(fileURLWithPath: "/tmp/with space/../out"),
+        ])
+        XCTAssertEqual(roots.map(\.uri), ["file:///Users/someone/", "file:///tmp/out/"])
+        XCTAssertEqual(roots.map(\.name), ["someone", "out"])
+    }
+
+    func testManagerKeepsOfferedRoots() async {
+        let tmp = URL(fileURLWithPath: NSTemporaryDirectory())
+        let manager = MCPManager(roots: [tmp])
+        let roots = await manager.roots
+        XCTAssertEqual(roots, [tmp])
+        let none = await MCPManager().roots
+        XCTAssertTrue(none.isEmpty)
+    }
+
+    /// Live: chrome-devtools-mcp saves a screenshot inside an offered root.
+    /// Without roots it answers "is not within any of the configured workspace
+    /// roots" for every path. Needs npx + Chrome; pre-warm the package.
+    ///   SAK_LIVE_TESTS=1 swift test --filter testLiveChromeDevToolsWritesInsideRoot
+    func testLiveChromeDevToolsWritesInsideRoot() async throws {
+        try XCTSkipUnless(ProcessInfo.processInfo.environment["SAK_LIVE_TESTS"] == "1")
+
+        // Outside the temp directory: chrome-devtools-mcp always allows temp,
+        // so a temp folder would pass with no roots at all.
+        let dir = URL(fileURLWithPath: #filePath)
+            .deletingLastPathComponent().deletingLastPathComponent().deletingLastPathComponent()
+            .appendingPathComponent(".build/mcp-roots-\(UUID().uuidString)", isDirectory: true)
+        try FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: dir) }
+
+        let manager = MCPManager(roots: [dir])
+        defer { Task { await manager.disconnectAll() } }
+        _ = try await manager.connect(.stdio(
+            command: "npx",
+            args: ["-y", "chrome-devtools-mcp@latest", "--isolated", "--headless"]
+        ))
+
+        let tools = try await manager.bridgedTools()
+        let list = try XCTUnwrap(tools.first { $0.name == "list_pages" })
+        let pages = try await list.execute(parameters: [:]).result
+        // "1: about:blank [selected]" — the id of the open page.
+        let pageId = try XCTUnwrap(pages.split(separator: "\n")
+            .lazy.compactMap { Int($0.prefix { $0.isNumber }) }.first, pages)
+        let shot = try XCTUnwrap(tools.first { $0.name == "take_screenshot" })
+        let file = dir.appendingPathComponent("shot.png")
+        let result = try await shot.execute(parameters: ["pageId": pageId, "filePath": file.path])
+        XCTAssertTrue(FileManager.default.fileExists(atPath: file.path), "screenshot not written: \(result)")
+    }
+
     func testExecutableResolverReportsMissingCommand() {
         XCTAssertThrowsError(
             try MCPExecutableResolver.resolve(