diff --git a/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt b/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt
index 449269e84..d9ce893f0 100644
--- a/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt
+++ b/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt
@@ -364,8 +364,20 @@ private fun runTransferTest(
                             // (responses dribble in over a long stretch).
                             val debug = System.getenv("QUIC_INTEROP_DEBUG") == "1"
                             val transferStartMs = nowMs()
+                            if (debug) {
+                                System.err.println(
+                                    "[interop] multiplex start: total_urls=${urls.size} " +
+                                        "MULTIPLEX_PARALLELISM=$MULTIPLEX_PARALLELISM " +
+                                        "expected_chunks=${(urls.size + MULTIPLEX_PARALLELISM - 1) / MULTIPLEX_PARALLELISM}",
+                                )
+                            }
                             urls.chunked(MULTIPLEX_PARALLELISM).forEachIndexed { chunkIdx, chunk ->
                                 val chunkStartMs = nowMs()
+                                if (debug && chunkIdx < 3) {
+                                    System.err.println(
+                                        "[interop] chunk=$chunkIdx size=${chunk.size} starting prepareRequests",
+                                    )
+                                }
                                 // Single lock-held batch open + enqueue.
                                 // Without this, openBidiStream's per-call
                                 // lock acquire / release lets the send loop
diff --git a/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/QuicConnection.kt b/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/QuicConnection.kt
index aeaafd667..e23cabdaf 100644
--- a/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/QuicConnection.kt
+++ b/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/QuicConnection.kt
@@ -800,9 +800,18 @@ class QuicConnection(
         init: (QuicStream, I) -> R,
     ): List<R> {
         if (items.isEmpty()) return emptyList()
-        return streamsLock.withLock {
-            items.map { init(openBidiStreamLocked(), it) }
+        val streamsBefore = if (writerDebugEnabled) streams.size else 0
+        val result =
+            streamsLock.withLock {
+                items.map { init(openBidiStreamLocked(), it) }
+            }
+        if (writerDebugEnabled) {
+            System.err.println(
+                "[batch] openBidiStreamsBatch items=${items.size} returned=${result.size} " +
+                    "streamsList_before=$streamsBefore streamsList_after=${streams.size}",
+            )
         }
+        return result
     }
 
     /**