diff --git a/quic/interop/inspect-testcase.sh b/quic/interop/inspect-testcase.sh
new file mode 100755
index 000000000..f3266ba95
--- /dev/null
+++ b/quic/interop/inspect-testcase.sh
@@ -0,0 +1,107 @@
+#!/usr/bin/env bash
+# Per-testcase deep-dive diagnostic. Use after running any single
+# testcase that failed:
+#   ./quic/interop/inspect-testcase.sh longrtt
+#
+# Auto-finds the most recent run dir that has the named testcase and
+# reports the runner status line + qlog summary + frame histograms
+# (sent/received) + connection-close events. Designed to fit in one
+# screen of output.
+set -o pipefail
+
+TC="${1:-}"
+if [[ -z "$TC" ]]; then
+    echo "usage: $0 <testcase>   (e.g. longrtt, retry, http3)" >&2
+    exit 2
+fi
+
+REPO_ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+RUNNER_LOGS="${REPO_ROOT}/../quic-interop-runner/logs"
+
+# Most recent run dir that has the named testcase.
+RUN_DIR=""
+for d in $(ls -1dt "$RUNNER_LOGS"/run-* 2>/dev/null); do
+    if [[ -d "$d"/*amethyst*/"$TC" ]] 2>/dev/null; then
+        RUN_DIR="$d"
+        break
+    fi
+    # Fallback for shells that don't expand the glob in the test:
+    if ls -d "$d"/*amethyst*/"$TC" >/dev/null 2>&1; then
+        RUN_DIR="$d"
+        break
+    fi
+done
+if [[ -z "$RUN_DIR" ]]; then
+    echo "no run dir with testcase '$TC' under $RUNNER_LOGS" >&2
+    exit 1
+fi
+echo "==> run dir: $RUN_DIR"
+TC_DIR="$(ls -1d "$RUN_DIR"/*amethyst*/"$TC" 2>/dev/null | head -n 1)"
+echo "==> testcase dir: $TC_DIR"
+
+echo
+echo "=============== runner status ==============="
+if [[ -f "${RUN_DIR}.stdout.log" ]]; then
+    grep -E "Test: $TC took" "${RUN_DIR}.stdout.log" | tail -n 5
+else
+    echo "(no .stdout.log — run-matrix.sh predates the tee, status not saved)"
+fi
+
+echo
+echo "=============== file sizes generated for this testcase ==============="
+if [[ -f "${RUN_DIR}.stdout.log" ]]; then
+    # 'Generated random file: NAME of size: N' lines printed before
+    # each test run. Extract the ones that came RIGHT BEFORE this
+    # testcase's "Running test case" line.
+    awk -v tc="$TC" '
+        /^Running test case: / { current = $0 }
+        /^Generated random file:/ { last_files = last_files "\n" $0 }
+        $0 ~ ("Running test case: " tc) {
+            print last_files
+            last_files = ""
+            exit
+        }
+    ' "${RUN_DIR}.stdout.log" | tail -n 10
+else
+    echo "(no .stdout.log)"
+fi
+
+echo
+echo "=============== qlog event-type histogram ==============="
+QLOG="$(ls -1 "$TC_DIR"/client/qlog/*.sqlog "$TC_DIR"/client/qlog/*.qlog 2>/dev/null | head -n 1 || true)"
+if [[ -z "$QLOG" ]]; then
+    echo "(no qlog — connection probably never made it past TLS)"
+else
+    echo "(qlog: $QLOG, $(wc -l <"$QLOG" | tr -d ' ') lines)"
+    grep -oE '"name":"[^"]+"' "$QLOG" | sort | uniq -c | sort -rn | head
+fi
+
+if [[ -n "$QLOG" ]]; then
+    echo
+    echo "=============== transport_parameters (peer's flow-control budget) ==============="
+    grep '"name":"transport:parameters_set"' "$QLOG"
+
+    echo
+    echo "=============== connection_closed (smoking gun for spec violations) ==============="
+    grep '"name":"transport:connection_closed"' "$QLOG" | head -n 5 \
+        || echo "(none — connection didn't formally close; ran out of time?)"
+
+    echo
+    echo "=============== last 10 sent / received packets (steady-state shape) ==============="
+    echo "-- received --"
+    grep '"name":"transport:packet_received"' "$QLOG" | tail -n 10
+    echo "-- sent --"
+    grep '"name":"transport:packet_sent"' "$QLOG" | tail -n 10
+
+    echo
+    echo "=============== first/last received packet timestamps (transfer rate hint) ==============="
+    FIRST=$(grep '"name":"transport:packet_received"' "$QLOG" | head -n 1 | grep -oE '"time":[0-9]+' | head -n 1)
+    LAST=$(grep '"name":"transport:packet_received"' "$QLOG" | tail -n 1 | grep -oE '"time":[0-9]+' | head -n 1)
+    PKTS=$(grep -c '"name":"transport:packet_received"' "$QLOG")
+    echo "first=$FIRST last=$LAST pkts=$PKTS"
+fi
+
+echo
+echo "=============== server stderr (last 30 lines) ==============="
+tail -n 30 "$TC_DIR/server/stderr.log" 2>/dev/null \
+    || echo "(no server/stderr.log)"