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 fe8a68137..12ed601d6 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
@@ -198,7 +198,21 @@ fun main() {
                     initialVersion = initialVersion,
                     keyLogPath = keyLogPath,
                     qlogDir = qlogDir,
-                    parallel = (testcase == "multiplexing"),
+                    // The runner reuses TESTCASE_CLIENT=transfer for the
+                    // multiplexing testcase — discrimination is by URL
+                    // count, not testcase name. We were checking
+                    // testcase == "multiplexing" which is NEVER true
+                    // (we'd see TESTCASE=multiplexing only on a
+                    // hypothetical client where the runner explicitly
+                    // sets it). Symptom: 60s timeout with 1421/2000
+                    // files at 1 stream / RTT — exactly the serial
+                    // client.get(...) path. Confirmed via the boot log:
+                    //   [boot] TESTCASE=transfer; transfer mode:
+                    //   parallel=false urls=1999
+                    //
+                    // Cheap whitespace tokenization here just counts;
+                    // runTransferTest re-parses into URI[] inside.
+                    parallel = requests.split(Regex("\\s+")).count { it.isNotBlank() } > 1,
                 )
             }
 
diff --git a/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/WriterDebug.kt b/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/WriterDebug.kt
index 0fbd586f2..7b5d0facf 100644
--- a/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/WriterDebug.kt
+++ b/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/WriterDebug.kt
@@ -39,4 +39,4 @@ var writerDebugEnabled: Boolean = false
  * docker layer cache didn't serve a stale jar). Bump when adding new
  * trace lines to make them traceable from the wire run.
  */
-const val WRITER_DEBUG_BUILD_ID: String = "2026-05-07-parallel-branch-v1"
+const val WRITER_DEBUG_BUILD_ID: String = "2026-05-07-fix-parallel-detection-v1"