apply_patch: one file per call, folders named as folders

Sources/SwiftAgentKitTools/FileSystemTools.swift

@@ -188,6 +188,7 @@ public struct PatchFileTool: AgentTool {
     public let name = "apply_patch"
     public let description = """
     Edit an existing file by applying a unified diff (git / `diff -u` format). \
+    One file per call: `path` is that file; to change two files, call it twice. \
     Provide the smallest diff that makes the change — one or more `@@` hunks with \
     a few lines of surrounding context; `-` lines are removed, `+` lines added. \
     Line numbers in `@@` headers may be approximate (matched by context, tolerant \
@@ -238,6 +239,23 @@ public struct PatchFileTool: AgentTool {
         if let policy, let reason = policy.blockReason(for: path) {
             return .error(toolCallId: "", toolName: name, message: "Refusing to patch \(raw): \(reason).")
         }
+        // One file per call. A diff spanning two files had all its hunks tried
+        // against `path`, and the second file's hunk "didn't match" (Naseem,
+        // 2026-10-09: PomoApp.swift + project.pbxproj in one patch).
+        let targets = UnifiedDiff.targets(in: patch)
+        if targets.count > 1 {
+            return .error(toolCallId: "", toolName: name, message: """
+            This patch changes \(targets.count) files (\(targets.joined(separator: ", "))); apply_patch takes \
+            one file per call. Nothing was changed. Send one apply_patch per file, each with that file's `path`.
+            """)
+        }
+        var isDir: ObjCBool = false
+        if FileManager.default.fileExists(atPath: path, isDirectory: &isDir), isDir.boolValue {
+            return .error(toolCallId: "", toolName: name, message: """
+            \(raw) is a folder. `path` must be the file the diff changes\
+            \(targets.first.map { " (this diff names \($0))" } ?? ""). Nothing was changed.
+            """)
+        }
         guard let data = FileManager.default.contents(atPath: path) else {
             return .error(toolCallId: "", toolName: name,
                 message: "Cannot read file to patch: \(raw). Use write_file to create a new file.")

Sources/SwiftAgentKitTools/UnifiedDiff.swift

@@ -109,6 +109,22 @@ enum UnifiedDiff {
         return hunks.isEmpty ? nil : hunks
     }
 
+    /// The files a diff changes, from its `+++` headers (a leading `a/`/`b/`
+    /// dropped, /dev/null skipped), in order and distinct. Empty when the diff
+    /// has no headers — the tool's `path` names its one file.
+    static func targets(in patch: String) -> [String] {
+        var out: [String] = []
+        for raw in patch.components(separatedBy: "\n") where raw.hasPrefix("+++ ") {
+            var name = String(raw.dropFirst(4))
+            if let tab = name.firstIndex(of: "\t") { name = String(name[..<tab]) }
+            name = name.trimmingCharacters(in: .whitespaces)
+            if name == "/dev/null" || name.isEmpty { continue }
+            if name.hasPrefix("a/") || name.hasPrefix("b/") { name.removeFirst(2) }
+            if !out.contains(name) { out.append(name) }
+        }
+        return out
+    }
+
     /// Extract the old-file start line from an `@@ -a,b +c,d @@` header.
     private static func parseOldStart(_ header: String) -> Int? {
         guard let dash = header.firstIndex(of: "-") else { return nil }

Tests/SwiftAgentKitToolsTests/UnifiedDiffTests.swift

@@ -149,4 +149,50 @@ struct UnifiedDiffTests {
         #expect(nearby.contains("line10"))
         #expect(nearby.contains("10 |"))   // numbered, so the model can re-anchor
     }
+
+    // MARK: - One file per call (Naseem, 2026-10-09)
+
+    /// A model put PomoApp.swift's and project.pbxproj's hunks in one patch
+    /// with path=PomoApp.swift; the pbxproj hunk "didn't match" the Swift file.
+    @Test func targetsListsEachFileTheDiffChanges() {
+        let patch = """
+        --- a/Pomo/PomoApp.swift
+        +++ b/Pomo/PomoApp.swift
+        @@ -1,1 +1,1 @@
+        -a
+        +b
+        --- a/Pomo.xcodeproj/project.pbxproj
+        +++ b/Pomo.xcodeproj/project.pbxproj
+        @@ -1,1 +1,1 @@
+        -c
+        +d
+        """
+        #expect(UnifiedDiff.targets(in: patch) == ["Pomo/PomoApp.swift", "Pomo.xcodeproj/project.pbxproj"])
+        #expect(UnifiedDiff.targets(in: "--- a/f.txt\n+++ b/f.txt\n@@ -1 +1 @@\n-a\n+b") == ["f.txt"])
+        #expect(UnifiedDiff.targets(in: "@@ -1 +1 @@\n-a\n+b").isEmpty)
+    }
+
+    @Test func aMultiFilePatchIsRefusedWithBothNames() async throws {
+        let dir = FileManager.default.temporaryDirectory.appendingPathComponent("sak-patch-\(UUID().uuidString)")
+        try FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: dir) }
+        let file = dir.appendingPathComponent("a.txt")
+        try "a\n".write(to: file, atomically: true, encoding: .utf8)
+        let patch = "--- a/a.txt\n+++ b/a.txt\n@@ -1 +1 @@\n-a\n+A\n--- a/b.txt\n+++ b/b.txt\n@@ -1 +1 @@\n-b\n+B"
+        let r = try await PatchFileTool().execute(parameters: ["path": file.path, "patch": patch])
+        #expect(r.isError)
+        #expect(r.result.contains("a.txt") && r.result.contains("b.txt"))
+        #expect(r.result.contains("one file per call"))
+        #expect(try String(contentsOf: file, encoding: .utf8) == "a\n")
+    }
+
+    @Test func aFolderPathIsNamedAsAFolder() async throws {
+        let dir = FileManager.default.temporaryDirectory.appendingPathComponent("sak-patch-\(UUID().uuidString)")
+        try FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: dir) }
+        let r = try await PatchFileTool().execute(parameters: ["path": dir.path, "patch": "@@ -1 +1 @@\n-a\n+b"])
+        #expect(r.isError)
+        #expect(r.result.contains("is a folder"))
+        #expect(!r.result.contains("Use write_file"))
+    }
 }