diff --git a/Sources/CodexBarCore/TelemetryFolderMigration.swift b/Sources/CodexBarCore/TelemetryFolderMigration.swift
index b5cca44..362fc2e 100644
--- a/Sources/CodexBarCore/TelemetryFolderMigration.swift
+++ b/Sources/CodexBarCore/TelemetryFolderMigration.swift
@@ -27,8 +27,12 @@ public struct TelemetryFolderMigration {
             guard let stage = Self.stage(of: entry) else { continue }
             do {
                 let values = try entry.resourceValues(forKeys: [.isRegularFileKey, .isSymbolicLinkKey])
-                guard values.isRegularFile == true, values.isSymbolicLink != true else { continue }
-                if stage == .migrated {
+                guard values.isRegularFile == true, values.isSymbolicLink != true else {
+                    // removeItem would follow a directory into its contents, so only regular files are ever touched.
+                    if stage != .live { warnings.append("Left \(entry.lastPathComponent) alone in the old telemetry folder: it isn't a regular file") }
+                    continue
+                }
+                if stage == .migrated || stage == .mergingDone {
                     try fileManager.removeItem(at: entry)
                     continue
                 }
@@ -62,7 +66,7 @@ public struct TelemetryFolderMigration {
             return true
         }
         return entries.contains { entry in
-            guard let stage = Self.stage(of: entry), stage != .migrated,
+            guard let stage = Self.stage(of: entry), stage == .live || stage == .merging,
                   let values = try? entry.resourceValues(forKeys: [.isRegularFileKey, .isSymbolicLinkKey]) else { return false }
             return values.isRegularFile == true && values.isSymbolicLink != true
         }
@@ -116,8 +120,20 @@ public struct TelemetryFolderMigration {
             try? handle.truncate(atOffset: size)
             throw error
         }
-        let migrated = merging.deletingPathExtension().appendingPathExtension(Self.migratedExtension)
-        try fileManager.moveItem(at: merging, to: migrated)
+        // The lines are in the target now, so from here the source must never go back to a name that gets appended again.
+        let base = merging.deletingPathExtension()
+        let migrated = base.appendingPathExtension(Self.migratedExtension)
+        do {
+            try fileManager.moveItem(at: merging, to: migrated)
+        } catch {
+            do {
+                try fileManager.removeItem(at: merging)
+            } catch {
+                try fileManager.moveItem(at: merging, to: base.appendingPathExtension(Self.mergingDoneExtension))
+                throw error
+            }
+            return
+        }
         try fileManager.removeItem(at: migrated)
     }
 
@@ -134,13 +150,17 @@ public struct TelemetryFolderMigration {
     private static let mergingExtension = "merging"
     private static let migratedExtension = "migrated"
 
-    private enum Stage { case live, merging, migrated }
+    private static let mergingDoneExtension = "merging-done"
+
+    /// `merging` and `mergingDone` are both leftovers, but only `merging` still needs its lines appended.
+    private enum Stage { case live, merging, mergingDone, migrated }
 
     private static func stage(of url: URL) -> Stage? {
         if isTelemetryFile(url) { return .live }
         guard isTelemetryFile(url.deletingPathExtension()) else { return nil }
         switch url.pathExtension {
         case mergingExtension: return .merging
+        case mergingDoneExtension: return .mergingDone
         case migratedExtension: return .migrated
         default: return nil
         }
diff --git a/Tests/CodexBarCoreTests/TelemetryFolderMigrationTests.swift b/Tests/CodexBarCoreTests/TelemetryFolderMigrationTests.swift
index fc3f88c..b2f209f 100644
--- a/Tests/CodexBarCoreTests/TelemetryFolderMigrationTests.swift
+++ b/Tests/CodexBarCoreTests/TelemetryFolderMigrationTests.swift
@@ -15,6 +15,25 @@ private final class UnreadableFileManager: FileManager, @unchecked Sendable {
     }
 }
 
+/// Fails the rename that marks an append as done, and optionally the delete that would follow it.
+private final class UnmarkableFileManager: FileManager, @unchecked Sendable {
+    let failsDeletes: Bool
+    init(failsDeletes: Bool) {
+        self.failsDeletes = failsDeletes
+        super.init()
+    }
+    override func moveItem(at srcURL: URL, to dstURL: URL) throws {
+        if dstURL.pathExtension == "migrated" {
+            throw CocoaError(.fileWriteFileExists, userInfo: [NSFilePathErrorKey: dstURL.path])
+        }
+        try super.moveItem(at: srcURL, to: dstURL)
+    }
+    override func removeItem(at URL: URL) throws {
+        if failsDeletes { throw CocoaError(.fileWriteNoPermission, userInfo: [NSFilePathErrorKey: URL.path]) }
+        try super.removeItem(at: URL)
+    }
+}
+
 final class TelemetryFolderMigrationTests: XCTestCase {
     private var home: URL!
     private var legacy: URL!
@@ -179,6 +198,51 @@ final class TelemetryFolderMigrationTests: XCTestCase {
         XCTAssertFalse(FileManager.default.fileExists(atPath: legacy.path))
     }
 
+    func testDirectoriesAndLinksNamedLikeLeftoversAreReportedAndNeverRemoved() throws {
+        let name = "codexbar-metrics-2026-10-11.jsonl"
+        try write("inside.txt", "keep", in: legacy.appending(path: "\(name).migrated", directoryHint: .isDirectory))
+        let outside = home.appending(path: "outside.jsonl")
+        try Data("secret\n".utf8).write(to: outside)
+        try FileManager.default.createSymbolicLink(at: legacy.appending(path: "\(name).merging"), withDestinationURL: outside)
+        let migration = TelemetryFolderMigration(home: home)
+
+        XCTAssertFalse(migration.hasPendingFiles())
+        XCTAssertEqual(migration.run().count, 2)
+
+        XCTAssertEqual(try read("inside.txt", in: legacy.appending(path: "\(name).migrated", directoryHint: .isDirectory)), "keep")
+        XCTAssertNotNil(try? FileManager.default.destinationOfSymbolicLink(atPath: legacy.appending(path: "\(name).merging").path))
+        XCTAssertEqual(try String(contentsOf: outside, encoding: .utf8), "secret\n")
+        XCTAssertFalse(FileManager.default.fileExists(atPath: destination.path))
+    }
+
+    func testFailedMigratedRenameAfterAppendFallsBackToDeletingTheSource() throws {
+        let name = "codexbar-metrics-2026-10-11.jsonl"
+        try write(name, "new\n", in: destination)
+        try write(name, "old\n", in: legacy)
+
+        XCTAssertEqual(TelemetryFolderMigration(home: home, fileManager: UnmarkableFileManager(failsDeletes: false)).run(), [])
+        _ = TelemetryFolderMigration(home: home).run()
+
+        XCTAssertEqual(try read(name, in: destination), "new\nold\n")
+        XCTAssertFalse(FileManager.default.fileExists(atPath: legacy.path))
+    }
+
+    func testSourceThatCantBeMarkedOrDeletedAfterAppendIsNeverAppendedAgain() throws {
+        let name = "codexbar-metrics-2026-10-11.jsonl"
+        try write(name, "new\n", in: destination)
+        try write(name, "old\n", in: legacy)
+
+        let stuck = TelemetryFolderMigration(home: home, fileManager: UnmarkableFileManager(failsDeletes: true))
+        XCTAssertEqual(stuck.run().count, 1)
+        XCTAssertFalse(stuck.hasPendingFiles())
+        XCTAssertTrue(FileManager.default.fileExists(atPath: legacy.appending(path: "\(name).merging-done").path))
+
+        XCTAssertEqual(TelemetryFolderMigration(home: home).run(), [])
+
+        XCTAssertEqual(try read(name, in: destination), "new\nold\n")
+        XCTAssertFalse(FileManager.default.fileExists(atPath: legacy.path))
+    }
+
     func testLeftoverMigratedFileIsOnlyDeleted() throws {
         let name = "codexbar-metrics-2026-10-11.jsonl"
         try write(name, "new\n", in: destination)