diff --git a/quic/interop/plans/2026-05-06-interop-runner.md b/quic/interop/plans/2026-05-06-interop-runner.md
index c5d436837..4084614e7 100644
--- a/quic/interop/plans/2026-05-06-interop-runner.md
+++ b/quic/interop/plans/2026-05-06-interop-runner.md
@@ -26,16 +26,32 @@ reorder / migration scenarios that are awkward to reproduce in unit tests.
 
 ## Local iteration loop
 
-```
-# In our repo:
-make -C quic/interop build
+The fast path: `quic/interop/run-matrix.sh` clones the runner alongside
+this repo, sets up a venv, merges our `implementations.json` snippet,
+builds the endpoint image, and invokes `run.py`. All steps are
+idempotent so repeated invocations just iterate.
 
-# In a sibling clone of quic-interop-runner, merge our entry into
-# implementations.json (snippet checked in at quic/interop/quic-interop-runner-snippet.json):
+```
+# Single test against the most permissive peer:
+quic/interop/run-matrix.sh -s aioquic -t handshake
+
+# A focused triangulation:
+quic/interop/run-matrix.sh -s aioquic   -t handshake,chacha20
+quic/interop/run-matrix.sh -s quic-go   -t handshake,chacha20
+quic/interop/run-matrix.sh -s picoquic  -t handshake,chacha20
+
+# Tight inner loop — skip the image rebuild between test selections:
+SKIP_BUILD=1 quic/interop/run-matrix.sh -s aioquic -t transfer
+```
+
+Manual flow (if `run-matrix.sh` doesn't fit):
+
+```
+make -C quic/interop build
+# Then in a sibling clone of quic-interop-runner, merge our snippet:
 jq -s '.[0] * .[1]' implementations.json \
    ../amethyst/quic/interop/quic-interop-runner-snippet.json \
    > implementations.json.new && mv implementations.json.new implementations.json
-
 python run.py -d -i amethyst -s aioquic -t handshake,chacha20 --log-dir ./logs
 ```
 
diff --git a/quic/interop/run-matrix.sh b/quic/interop/run-matrix.sh
new file mode 100755
index 000000000..9ef7b4a2d
--- /dev/null
+++ b/quic/interop/run-matrix.sh
@@ -0,0 +1,98 @@
+#!/usr/bin/env bash
+# Drive the quic-interop-runner against the :quic-interop endpoint image.
+#
+# Idempotent: clones the runner alongside this repo if missing, sets up its
+# venv, registers `amethyst` in implementations.json, builds our endpoint
+# image, then invokes run.py with the user's args.
+#
+# Usage:
+#   quic/interop/run-matrix.sh                                      # full matrix vs aioquic
+#   quic/interop/run-matrix.sh -s aioquic -t handshake              # one test
+#   quic/interop/run-matrix.sh -s quic-go -t handshake,chacha20     # different peer
+#
+# Env overrides:
+#   RUNNER_DIR   — where to clone / find the runner (default: ../quic-interop-runner)
+#   LOG_DIR      — qlog / pcap output (default: $RUNNER_DIR/logs)
+#   SKIP_BUILD=1 — skip `make build` (image already current)
+set -euo pipefail
+
+SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+REPO_ROOT=$(cd "$SCRIPT_DIR/../.." && pwd)
+RUNNER_DIR="${RUNNER_DIR:-$REPO_ROOT/../quic-interop-runner}"
+LOG_DIR="${LOG_DIR:-$RUNNER_DIR/logs}"
+
+is_macos=false
+case "${OSTYPE:-}" in
+    darwin*) is_macos=true ;;
+esac
+
+install_hint() {
+    if $is_macos; then
+        echo "    brew install $1"
+    else
+        echo "    sudo apt-get install -y $1"
+    fi
+}
+
+need() {
+    if ! command -v "$1" >/dev/null 2>&1; then
+        echo "error: '$1' not found in PATH. Install with:" >&2
+        install_hint "$2" >&2
+        exit 1
+    fi
+}
+
+need docker docker.io
+need python3 python3
+need jq jq
+need git git
+need make make
+
+# Docker daemon must be reachable, not just installed. On macOS this is the
+# usual gotcha — Docker Desktop installed but not running.
+if ! docker info >/dev/null 2>&1; then
+    echo "error: docker daemon not reachable." >&2
+    if $is_macos; then
+        echo "  → launch Docker Desktop and retry." >&2
+    else
+        echo "  → 'sudo systemctl start docker' (and add yourself to the docker group)." >&2
+    fi
+    exit 1
+fi
+
+# 1. Clone the runner if missing.
+if [ ! -d "$RUNNER_DIR" ]; then
+    echo "==> cloning quic-interop-runner into $RUNNER_DIR"
+    git clone --depth 1 https://github.com/quic-interop/quic-interop-runner.git "$RUNNER_DIR"
+fi
+
+# 2. venv + Python deps.
+if [ ! -d "$RUNNER_DIR/.venv" ]; then
+    echo "==> creating venv at $RUNNER_DIR/.venv"
+    python3 -m venv "$RUNNER_DIR/.venv"
+fi
+"$RUNNER_DIR/.venv/bin/pip" install -q -r "$RUNNER_DIR/requirements.txt"
+
+# 3. Register our endpoint in implementations.json (idempotent).
+if ! jq -e '.amethyst' "$RUNNER_DIR/implementations.json" >/dev/null 2>&1; then
+    echo "==> registering 'amethyst' in implementations.json"
+    tmp=$(mktemp)
+    jq -s '.[0] * .[1]' \
+        "$RUNNER_DIR/implementations.json" \
+        "$SCRIPT_DIR/quic-interop-runner-snippet.json" \
+        > "$tmp"
+    mv "$tmp" "$RUNNER_DIR/implementations.json"
+fi
+
+# 4. Build the endpoint image (skippable for tight loops).
+if [ "${SKIP_BUILD:-0}" != "1" ]; then
+    echo "==> building amethyst-quic-interop image"
+    make -C "$SCRIPT_DIR" build
+fi
+
+# 5. Drive the runner.
+mkdir -p "$LOG_DIR"
+echo "==> running matrix (args: $* | logs: $LOG_DIR)"
+cd "$RUNNER_DIR"
+exec "$RUNNER_DIR/.venv/bin/python" run.py \
+    -d -i amethyst --log-dir "$LOG_DIR" "$@"