diff --git a/quic/interop/inspect-multiplexing.sh b/quic/interop/inspect-multiplexing.sh
index 50c27ecbf..02481cadb 100755
--- a/quic/interop/inspect-multiplexing.sh
+++ b/quic/interop/inspect-multiplexing.sh
@@ -38,7 +38,12 @@ echo "=============== writer-side debug traces (DEBUG=1 build only) ============
 # `grep -c` exits 1 on no matches AND prints "0", so a naive
 # `grep -c ... || echo 0` doubles up. Suppress the exit code
 # instead.
-WRITER_LINES=$(grep -cE '\[(writer|batch|interop)' "$CASE_DIR/output.txt" 2>/dev/null) || WRITER_LINES=0
+echo
+echo "=============== boot line (verifies image has latest debug build) ==============="
+grep '\[boot\]' "$CASE_DIR/output.txt" 2>/dev/null | head -n 5 \
+    || echo "(no [boot] lines — image is older than the boot log; rebuild with: DEBUG=1 ./quic/interop/run-matrix.sh ...)"
+
+WRITER_LINES=$(grep -cE '\[(writer|batch|interop|boot)' "$CASE_DIR/output.txt" 2>/dev/null) || WRITER_LINES=0
 if [[ "$WRITER_LINES" -gt 0 ]]; then
     echo "($WRITER_LINES diagnostic lines; [batch] / [interop] entries first then first 30 [writer.app]:)"
     # Use `|| true` because grep returns 1 on no matches and the
diff --git a/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt b/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt
index d9ce893f0..ccda033a7 100644
--- a/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt
+++ b/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt
@@ -85,8 +85,15 @@ private const val MULTIPLEX_PARALLELISM = 64
 fun main() {
     // Single env-var check, propagated to library code that opts into
     // verbose tracing only when this is set.
-    if (System.getenv("QUIC_INTEROP_DEBUG") == "1") {
+    val debugEnv = System.getenv("QUIC_INTEROP_DEBUG")
+    if (debugEnv == "1") {
         com.vitorpamplona.quic.connection.writerDebugEnabled = true
+        System.err.println(
+            "[boot] DEBUG=1; writerDebugEnabled=true; build_id=" +
+                "${com.vitorpamplona.quic.connection.WRITER_DEBUG_BUILD_ID}",
+        )
+    } else {
+        System.err.println("[boot] DEBUG=${debugEnv ?: "(unset)"} writerDebugEnabled=false")
     }
 
     val role = System.getenv("ROLE") ?: "client"
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 6955974aa..ff3aebd7b 100644
--- a/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/WriterDebug.kt
+++ b/quic/src/commonMain/kotlin/com/vitorpamplona/quic/connection/WriterDebug.kt
@@ -32,3 +32,11 @@ package com.vitorpamplona.quic.connection
  */
 @Volatile
 var writerDebugEnabled: Boolean = false
+
+/**
+ * Build identifier injected into the boot log so we can verify the
+ * deployed image actually has the latest debug code (i.e. that the
+ * 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-batch-log-v1"