diff --git a/quic/interop/Dockerfile b/quic/interop/Dockerfile
new file mode 100644
index 000000000..21b225c86
--- /dev/null
+++ b/quic/interop/Dockerfile
@@ -0,0 +1,16 @@
+# quic-interop-runner endpoint image for :quic.
+#
+# Build (from repo root):
+#   ./gradlew :quic-interop:installDist
+#   docker build -t amethyst-quic-interop -f quic/interop/Dockerfile .
+#
+# Or run `make build` inside quic/interop/.
+FROM martenseemann/quic-network-simulator-endpoint:latest
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends openjdk-21-jre-headless \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY quic/interop/build/install/quic-interop /opt/quic-interop
+COPY quic/interop/run_endpoint.sh /run_endpoint.sh
+RUN chmod +x /run_endpoint.sh /opt/quic-interop/bin/quic-interop
diff --git a/quic/interop/Makefile b/quic/interop/Makefile
new file mode 100644
index 000000000..b905c0f84
--- /dev/null
+++ b/quic/interop/Makefile
@@ -0,0 +1,39 @@
+# Local iteration loop for the quic-interop-runner endpoint image.
+#
+#   make build              # compile JVM dist + build Docker image
+#   make smoke              # quick handshake test against a local picoquic
+#   make image-name         # print the image tag (for use in implementations.json)
+#
+# These targets are deliberately thin wrappers — the real harness is the
+# `quic-interop-runner` repo (clone separately and register `amethyst` with
+# image=amethyst-quic-interop:latest, role=client).
+IMAGE          ?= amethyst-quic-interop:latest
+REPO_ROOT      := $(shell git rev-parse --show-toplevel)
+DIST_DIR       := build/install/quic-interop
+PICOQUIC_PORT  ?= 4433
+
+.PHONY: build dist docker smoke image-name clean
+
+build: docker
+
+dist:
+	cd $(REPO_ROOT) && ./gradlew :quic-interop:installDist
+
+docker: dist
+	cd $(REPO_ROOT) && docker build -t $(IMAGE) -f quic/interop/Dockerfile .
+
+smoke: docker
+	@docker rm -f amethyst-quic-interop-smoke >/dev/null 2>&1 || true
+	docker run --rm --name amethyst-quic-interop-smoke \
+	    --network host \
+	    -e ROLE=client \
+	    -e TESTCASE=handshake \
+	    -e REQUESTS="https://127.0.0.1:$(PICOQUIC_PORT)/" \
+	    $(IMAGE)
+
+image-name:
+	@echo $(IMAGE)
+
+clean:
+	cd $(REPO_ROOT) && ./gradlew :quic-interop:clean
+	docker image rm -f $(IMAGE) 2>/dev/null || true
diff --git a/quic/interop/build.gradle.kts b/quic/interop/build.gradle.kts
new file mode 100644
index 000000000..839f2f5d6
--- /dev/null
+++ b/quic/interop/build.gradle.kts
@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+
+plugins {
+    alias(libs.plugins.jetbrainsKotlinJvm)
+    application
+}
+
+kotlin {
+    jvmToolchain(21)
+    compilerOptions {
+        jvmTarget.set(JvmTarget.JVM_21)
+    }
+}
+
+sourceSets {
+    main {
+        kotlin.srcDir("src/main/kotlin")
+    }
+}
+
+dependencies {
+    implementation(project(":quic"))
+    implementation(libs.kotlinx.coroutines.core)
+}
+
+application {
+    mainClass.set("com.vitorpamplona.quic.interop.runner.InteropClientKt")
+    applicationName = "quic-interop"
+}
diff --git a/quic/interop/plans/2026-05-06-interop-runner.md b/quic/interop/plans/2026-05-06-interop-runner.md
new file mode 100644
index 000000000..0dd0ac2f9
--- /dev/null
+++ b/quic/interop/plans/2026-05-06-interop-runner.md
@@ -0,0 +1,63 @@
+# quic-interop-runner endpoint — Phase 0 scaffolding
+
+Date: 2026-05-06
+
+## Why
+
+We want the [`quic-interop-runner`](https://github.com/quic-interop/quic-interop-runner)
+matrix as a **bug-finding harness**, not a vanity scoreboard. Each peer impl
+(quiche, aioquic, picoquic, ngtcp2, msquic, mvfst, lsquic, neqo, kwik) enforces
+different parts of RFC 9000/9001/9002/9114 strictly, so failures triangulate
+to specific bugs in `:quic`. The runner's ns-3 sim also exposes loss /
+reorder / migration scenarios that are awkward to reproduce in unit tests.
+
+## What's in Phase 0
+
+- `:quic-interop` Gradle module (JVM-only application, registered at
+  `quic/interop/` via `settings.gradle`).
+- `InteropClient.kt` reads the runner's env-var contract (`ROLE`, `TESTCASE`,
+  `REQUESTS`, …) and dispatches by testcase. Phase 0 implements only
+  `handshake`; everything else returns `127` (runner-skip).
+- `Dockerfile` based on `martenseemann/quic-network-simulator-endpoint` +
+  OpenJDK 21 runtime, copies the `installDist` output.
+- `run_endpoint.sh` sources the base image's `/setup.sh` then execs our JVM
+  binary.
+- `Makefile` wrappers: `make build`, `make smoke`, `make clean`.
+
+## Local iteration loop
+
+```
+# In our repo:
+make -C quic/interop build
+
+# In a sibling clone of quic-interop-runner, add to implementations.json:
+"amethyst": {
+    "image": "amethyst-quic-interop:latest",
+    "url":   "https://github.com/vitorpamplona/amethyst",
+    "role":  "client"
+}
+
+python run.py -d -i amethyst -s aioquic -t handshake --log-dir ./logs
+```
+
+Inspect `./logs/<run>/client_qlog/*.qlog` in qvis when something breaks.
+
+## Phase ladder (excerpt — full plan in conversation)
+
+| Phase | Goal | Tests | Exit criterion |
+|---|---|---|---|
+| 0 | Minimum harness | `handshake` | one test reproducible end-to-end ✅ |
+| 1 | Triangulate handshake bugs | + `versionnegotiation`, `chacha20` | green vs aioquic + quiche + picoquic |
+| 2 | Streams + loss + multiplexing | + `transfer`, `multiplexing`, `*loss`, `http3` | green vs aioquic + quiche; soak 500/500 |
+| 3 | Edge cases | `retry`, `resumption`, `zerortt`, `keyupdate`, `rebinding-*`, `blackhole`, `amplificationlimit` | every test green or unsupported-127 with a written reason |
+| 4 | CI gate | nightly Phases 1–2; PR-blocking subset on every push | qlogs uploaded as artifacts on red |
+
+## Known follow-ups (NOT in Phase 0)
+
+- `SSLKEYLOGFILE` and `QLOGDIR` are not yet wired through to `:quic` — the
+  runner will set them but our endpoint ignores them. Phase 1 must surface
+  TLS keys (so Wireshark can decrypt the sim's pcap captures) and qlog
+  output (so qvis is useful).
+- Server role: we are client-first. Reassess after Phase 3.
+- WebTransport is **not** part of the standard interop matrix; it needs a
+  separate harness against `moq-rs` / chrome-headless.
diff --git a/quic/interop/run_endpoint.sh b/quic/interop/run_endpoint.sh
new file mode 100755
index 000000000..fdbcf2146
--- /dev/null
+++ b/quic/interop/run_endpoint.sh
@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# quic-interop-runner endpoint entry point.
+#
+# The base image (martenseemann/quic-network-simulator-endpoint) sets up
+# routing in /setup.sh; we source it then dispatch to our JVM client based
+# on the ROLE env var pushed in by the runner.
+set -euo pipefail
+
+# shellcheck disable=SC1091
+source /setup.sh
+
+case "${ROLE:-client}" in
+    client)
+        exec /opt/quic-interop/bin/quic-interop
+        ;;
+    server)
+        # Phase 0: server role unimplemented. Exit 127 so the runner skips.
+        echo "server role not implemented" >&2
+        exit 127
+        ;;
+    *)
+        echo "unknown ROLE=${ROLE}" >&2
+        exit 127
+        ;;
+esac
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
new file mode 100644
index 000000000..b9b1a0504
--- /dev/null
+++ b/quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt
@@ -0,0 +1,139 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quic.interop.runner
+
+import com.vitorpamplona.quic.connection.QuicConnection
+import com.vitorpamplona.quic.connection.QuicConnectionConfig
+import com.vitorpamplona.quic.connection.QuicConnectionDriver
+import com.vitorpamplona.quic.tls.PermissiveCertificateValidator
+import com.vitorpamplona.quic.transport.UdpSocket
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.SupervisorJob
+import kotlinx.coroutines.cancel
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.runBlocking
+import kotlinx.coroutines.withTimeoutOrNull
+import java.net.URI
+import kotlin.system.exitProcess
+
+/**
+ * Endpoint that implements the [quic-interop-runner](https://github.com/quic-interop/quic-interop-runner)
+ * Docker contract. Reads the env-var protocol the runner pushes into each
+ * client container and dispatches by [TESTCASE].
+ *
+ * Exit codes:
+ *  - `0`   — testcase passed
+ *  - `1`   — testcase failed (bug in our impl, OR partner)
+ *  - `127` — testcase not implemented (runner skips, doesn't fail)
+ */
+private const val EXIT_OK = 0
+private const val EXIT_FAIL = 1
+private const val EXIT_UNSUPPORTED = 127
+
+private const val HANDSHAKE_TIMEOUT_SEC = 10L
+
+fun main() {
+    val role = System.getenv("ROLE") ?: "client"
+    if (role != "client") {
+        System.err.println("server role not implemented")
+        exitProcess(EXIT_UNSUPPORTED)
+    }
+
+    val testcase = System.getenv("TESTCASE")?.trim().orEmpty()
+    val requests = System.getenv("REQUESTS")?.trim().orEmpty()
+
+    System.err.println("== quic-interop client ==")
+    System.err.println("testcase: $testcase")
+    System.err.println("requests: $requests")
+
+    val code =
+        when (testcase) {
+            "handshake" -> runHandshakeTest(requests)
+            else -> EXIT_UNSUPPORTED
+        }
+    exitProcess(code)
+}
+
+private fun runHandshakeTest(requests: String): Int {
+    val target =
+        parseFirstTarget(requests) ?: run {
+            System.err.println("no parseable target in REQUESTS")
+            return EXIT_FAIL
+        }
+    val (host, port) = target
+    System.err.println("handshake target: $host:$port")
+
+    val scope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
+    val outcome =
+        runBlocking {
+            val socket =
+                try {
+                    UdpSocket.connect(host, port)
+                } catch (t: Throwable) {
+                    return@runBlocking "udp_failed: ${t.message ?: t::class.simpleName}"
+                }
+            val conn =
+                QuicConnection(
+                    serverName = host,
+                    config = QuicConnectionConfig(),
+                    tlsCertificateValidator = PermissiveCertificateValidator(),
+                )
+            val driver = QuicConnectionDriver(conn, socket, scope)
+            driver.start()
+
+            val handshake =
+                withTimeoutOrNull(HANDSHAKE_TIMEOUT_SEC * 1_000L) {
+                    runCatching { conn.awaitHandshake() }
+                }
+            val result =
+                when {
+                    handshake == null -> "timeout"
+                    handshake.isSuccess -> "ok"
+                    else -> "failed: ${handshake.exceptionOrNull()?.message ?: "?"}"
+                }
+            runCatching { driver.close() }
+            delay(50)
+            result
+        }
+    scope.cancel()
+
+    return if (outcome == "ok") {
+        System.err.println("handshake ok")
+        EXIT_OK
+    } else {
+        System.err.println("handshake $outcome")
+        EXIT_FAIL
+    }
+}
+
+private fun parseFirstTarget(requests: String): Pair<String, Int>? {
+    val first = requests.split(Regex("\\s+")).firstOrNull { it.isNotBlank() } ?: return null
+    val uri =
+        try {
+            URI(first)
+        } catch (_: Throwable) {
+            return null
+        }
+    val host = uri.host ?: return null
+    val port = uri.port.takeIf { it > 0 } ?: 443
+    return host to port
+}
diff --git a/settings.gradle b/settings.gradle
index 38a632ba1..df2b7aaf1 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -40,3 +40,5 @@ include ':quic'
 include ':nestsClient'
 include ':desktopApp'
 include ':cli'
+include ':quic-interop'
+project(':quic-interop').projectDir = file('quic/interop')