diff --git a/app/src/main/kotlin/de/ids_mannheim/korapxmltools/KorapXmlTool.kt b/app/src/main/kotlin/de/ids_mannheim/korapxmltools/KorapXmlTool.kt
index 24fa849..3c80c69 100644
--- a/app/src/main/kotlin/de/ids_mannheim/korapxmltools/KorapXmlTool.kt
+++ b/app/src/main/kotlin/de/ids_mannheim/korapxmltools/KorapXmlTool.kt
@@ -424,6 +424,9 @@
     // Flag to track if TAR stream is still open
     @Volatile var tarStreamOpen = true
 
+    // Lock to synchronize TAR stream access between scanner and main thread
+    private val tarStreamLock = java.util.concurrent.locks.ReentrantLock()
+
     fun String.hasCorrespondingBaseZip(): Boolean {
         if (!this.matches(Regex(".*\\.([^/.]+)\\.zip$"))) return false
         val baseZip = this.replace(Regex("\\.([^/.]+)\\.zip$"), ".zip")
@@ -737,11 +740,17 @@
                     incrementalProgressBar?.step()
                 }
 
-                // Set flag before closing stream to prevent scanner from trying to write
-                tarStreamOpen = false
+                // Acquire lock before closing stream to prevent concurrent scanner access
+                tarStreamLock.lock()
+                try {
+                    // Set flag before closing stream to prevent scanner from trying to write
+                    tarStreamOpen = false
 
-                krillTarOutputStream!!.finish()
-                krillTarOutputStream!!.close()
+                    krillTarOutputStream!!.finish()
+                    krillTarOutputStream!!.close()
+                } finally {
+                    tarStreamLock.unlock()
+                }
 
                 // Close incremental progress bar if it was initialized
                 incrementalProgressBar?.close()
@@ -2979,19 +2988,25 @@
                     // We successfully claimed it - output now
                     val textData = krillData.remove(textId)
                     if (textData != null) {
-                        // Double-check stream is still open before writing
-                        if (tarStreamOpen) {
-                            try {
-                                outputKrillText(textId, textData)
-                                incrementalProgressBar?.step()
-                                outputCount++
-                                LOGGER.fine("Output text $textId (processed by ${relevantZips.size} ZIPs, ${krillData.size} still pending)")
-                            } catch (e: IOException) {
-                                // Stream may have been closed - stop trying to output
-                                LOGGER.warning("Cannot output text $textId: stream closed")
-                                tarStreamOpen = false
-                                break
+                        // Acquire lock to prevent concurrent access with stream closing
+                        tarStreamLock.lock()
+                        try {
+                            // Double-check stream is still open while holding the lock
+                            if (tarStreamOpen) {
+                                try {
+                                    outputKrillText(textId, textData)
+                                    incrementalProgressBar?.step()
+                                    outputCount++
+                                    LOGGER.fine("Output text $textId (processed by ${relevantZips.size} ZIPs, ${krillData.size} still pending)")
+                                } catch (e: IOException) {
+                                    // Stream may have been closed - stop trying to output
+                                    LOGGER.warning("Cannot output text $textId: stream closed")
+                                    tarStreamOpen = false
+                                    break
+                                }
                             }
+                        } finally {
+                            tarStreamLock.unlock()
                         }
                     }
 
@@ -3013,21 +3028,37 @@
     private fun stopIncrementalWriterThread() {
         if (incrementalOutputScheduler != null) {
             LOGGER.info("Stopping incremental writer scheduler")
+
+            // Set shutdown flag first to prevent scanner from starting new work
             shutdownIncrementalWriter = true
 
-            // Do one final scan before shutting down
-            scanAndOutputCompleteTexts()
+            // Use shutdownNow() to cancel any pending scheduled tasks immediately
+            val cancelledTasks = incrementalOutputScheduler?.shutdownNow()
+            LOGGER.fine("Cancelled ${cancelledTasks?.size ?: 0} pending scheduled tasks")
 
-            // Use shutdownNow() to cancel any scheduled tasks immediately
-            incrementalOutputScheduler?.shutdownNow()
+            // Wait for currently executing tasks to finish (increased timeout for safety)
             try {
-                if (!incrementalOutputScheduler!!.awaitTermination(5, java.util.concurrent.TimeUnit.SECONDS)) {
-                    LOGGER.warning("Writer scheduler did not terminate within timeout")
+                var waitTime = 0L
+                while (!incrementalOutputScheduler!!.awaitTermination(1, java.util.concurrent.TimeUnit.SECONDS)) {
+                    waitTime++
+                    if (waitTime >= 10) {
+                        LOGGER.warning("Writer scheduler did not terminate after ${waitTime}s, giving up")
+                        break
+                    }
+                    LOGGER.fine("Waiting for writer scheduler to terminate... (${waitTime}s)")
+                }
+                if (waitTime > 0) {
+                    LOGGER.info("Writer scheduler terminated after ${waitTime}s")
                 }
             } catch (e: InterruptedException) {
                 LOGGER.warning("Interrupted while stopping writer scheduler")
                 Thread.currentThread().interrupt()
             }
+
+            // Do one final scan after scheduler has stopped completely
+            LOGGER.info("Doing final scan for remaining texts...")
+            scanAndOutputCompleteTexts()
+
             incrementalOutputScheduler = null
         }
     }