diff --git a/apps/backend-quarkus/pom.xml b/apps/backend-quarkus/pom.xml
index 0569e61..52f430f 100644
--- a/apps/backend-quarkus/pom.xml
+++ b/apps/backend-quarkus/pom.xml
@@ -38,7 +38,11 @@
         </dependency>
         <dependency>
             <groupId>io.quarkus</groupId>
-            <artifactId>quarkus-rest-jackson</artifactId>
+            <artifactId>quarkus-rest</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>io.quarkus</groupId>
+            <artifactId>quarkus-jackson</artifactId>
         </dependency>
         <dependency>
             <groupId>io.quarkus</groupId>
diff --git a/apps/backend-quarkus/src/main/java/app/playmancala/Backend.java b/apps/backend-quarkus/src/main/java/app/playmancala/Backend.java
new file mode 100644
index 0000000..2386982
--- /dev/null
+++ b/apps/backend-quarkus/src/main/java/app/playmancala/Backend.java
@@ -0,0 +1,186 @@
+package app.playmancala;
+
+import java.sql.SQLException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+
+import javax.sql.DataSource;
+
+import org.eclipse.microprofile.config.inject.ConfigProperty;
+import org.jboss.logging.Logger;
+
+import app.playmancala.auth.TokenVerifier;
+import app.playmancala.game.GameClockScheduler;
+import app.playmancala.game.GameManager;
+import app.playmancala.game.GameRecorder;
+import app.playmancala.game.GameStore;
+import app.playmancala.game.LiveGames;
+import app.playmancala.game.MatchMaker;
+import app.playmancala.history.GameHistoryService;
+import app.playmancala.history.Migrations;
+import app.playmancala.history.PostgresGameHistoryRepository;
+import app.playmancala.live.ValkeyCache;
+import app.playmancala.live.ValkeyLiveGameStore;
+import app.playmancala.loop.GameLoop;
+import app.playmancala.rtmt.RtmtServer;
+import io.quarkus.redis.datasource.RedisDataSource;
+import io.quarkus.runtime.Startup;
+import jakarta.annotation.PostConstruct;
+import jakarta.annotation.PreDestroy;
+import jakarta.enterprise.context.ApplicationScoped;
+import jakarta.inject.Inject;
+
+// Builds the backend from the infrastructure Quarkus provides and starts it (docs/specs/quarkus-backend, Startup).
+@Startup
+@ApplicationScoped
+public class Backend {
+    private static final Logger LOG = Logger.getLogger(Backend.class);
+    private static final long MIGRATION_RETRY_MS = 30_000;
+    private static final long VALKEY_STARTUP_WAIT_MS = 5_000;
+
+    @Inject
+    DataSource dataSource;
+
+    @Inject
+    RedisDataSource redis;
+
+    @Inject
+    TokenVerifier tokenVerifier;
+
+    @ConfigProperty(name = "mancala.auth.legacy-userkey-auth")
+    String legacyUserKeyAuth;
+
+    @ConfigProperty(name = "mancala.game-clock-ms")
+    long gameClockMs;
+
+    private GameLoop loop;
+    // Valkey writes, in order.
+    private ExecutorService storeWriter;
+    // Migrations and saving finished games.
+    private ScheduledExecutorService recorderIo;
+    // Token checks.
+    private ExecutorService verifier;
+
+    private RtmtServer rtmt;
+    private GameStore gameStore;
+    private GameManager gameManager;
+    private LiveGames liveGames;
+    private GameHistoryService history;
+    private GameClockScheduler clockScheduler;
+
+    @PostConstruct
+    void start() {
+        loop = new GameLoop();
+        storeWriter = Executors.newSingleThreadExecutor(Thread.ofPlatform().name("valkey-writer").daemon().factory());
+        recorderIo = Executors.newSingleThreadScheduledExecutor(Thread.ofPlatform().name("game-recorder").daemon().factory());
+        verifier = Executors.newVirtualThreadPerTaskExecutor();
+
+        ValkeyLiveGameStore liveGameStore = new ValkeyLiveGameStore(redis);
+        PostgresGameHistoryRepository repository = new PostgresGameHistoryRepository(dataSource);
+        GameRecorder recorder = new GameRecorder(repository, liveGameStore, recorderIo);
+        history = new GameHistoryService(repository, new ValkeyCache(redis), recorder::getUnsaved);
+        recorder.onSaved(history::onGameSaved);
+
+        // Exactly "true", like the Node backend.
+        rtmt = new RtmtServer(tokenVerifier, "true".equals(legacyUserKeyAuth), loop, verifier,
+                RtmtServer.DEFAULT_AUTH_TIMEOUT_MS);
+        gameStore = new GameStore();
+        gameManager = new GameManager(gameStore, rtmt, new MatchMaker(rtmt), liveGameStore, recorder, storeWriter,
+                loop::now, gameClockMs);
+        liveGames = new LiveGames(rtmt, gameStore, gameManager, loop);
+        clockScheduler = new GameClockScheduler(gameManager::endGamesOutOfTime, loop,
+                GameClockScheduler.CLOCK_CHECK_INTERVAL_MS);
+
+        // The game runs without the database; saved games wait in the recorder until migrations have run.
+        recorderIo.execute(this::migrate);
+        recorderIo.scheduleWithFixedDelay(() -> guarded(recorder::retryPending), 0, GameRecorder.RETRY_INTERVAL_MS,
+                TimeUnit.MILLISECONDS);
+        // The first task on the loop, so socket events queue behind it and reconnecting players find their games.
+        loop.execute(() -> {
+            waitForValkey();
+            gameManager.restoreLiveGames();
+            clockScheduler.start();
+            rtmt.start();
+        });
+    }
+
+    @PreDestroy
+    void stop() {
+        loop.close();
+        storeWriter.shutdown();
+        recorderIo.shutdownNow();
+        verifier.shutdownNow();
+    }
+
+    private void migrate() {
+        try {
+            Migrations.run(dataSource);
+        } catch (SQLException | RuntimeException e) {
+            LOG.errorf("(DB) Migrations failed, retrying in %ds: %s", MIGRATION_RETRY_MS / 1000, e.getMessage());
+            recorderIo.schedule(this::migrate, MIGRATION_RETRY_MS, TimeUnit.MILLISECONDS);
+        }
+    }
+
+    private void waitForValkey() {
+        long deadline = System.currentTimeMillis() + VALKEY_STARTUP_WAIT_MS;
+        while (true) {
+            try {
+                redis.execute("PING");
+                return;
+            } catch (RuntimeException e) {
+                if (System.currentTimeMillis() >= deadline) {
+                    LOG.error("(Valkey) Not reachable yet; starting without restoring live games");
+                    return;
+                }
+            }
+            try {
+                Thread.sleep(250);
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+                return;
+            }
+        }
+    }
+
+    private static void guarded(Runnable task) {
+        try {
+            task.run();
+        } catch (RuntimeException e) {
+            LOG.error("(Backend) Task failed", e);
+        }
+    }
+
+    public GameLoop loop() {
+        return loop;
+    }
+
+    public RtmtServer rtmt() {
+        return rtmt;
+    }
+
+    public GameStore gameStore() {
+        return gameStore;
+    }
+
+    public GameManager gameManager() {
+        return gameManager;
+    }
+
+    public LiveGames liveGames() {
+        return liveGames;
+    }
+
+    public GameHistoryService history() {
+        return history;
+    }
+
+    public TokenVerifier tokenVerifier() {
+        return tokenVerifier;
+    }
+
+    public boolean legacyUserKeyAuth() {
+        return "true".equals(legacyUserKeyAuth);
+    }
+}
diff --git a/apps/backend-quarkus/src/main/java/app/playmancala/auth/TokenVerifierProducer.java b/apps/backend-quarkus/src/main/java/app/playmancala/auth/TokenVerifierProducer.java
new file mode 100644
index 0000000..da25e80
--- /dev/null
+++ b/apps/backend-quarkus/src/main/java/app/playmancala/auth/TokenVerifierProducer.java
@@ -0,0 +1,23 @@
+package app.playmancala.auth;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Optional;
+
+import org.eclipse.microprofile.config.inject.ConfigProperty;
+
+import jakarta.enterprise.inject.Produces;
+import jakarta.inject.Singleton;
+
+public class TokenVerifierProducer {
+    @Produces
+    @Singleton
+    TokenVerifier tokenVerifier(
+            @ConfigProperty(name = "mancala.auth.issuer") String issuer,
+            @ConfigProperty(name = "mancala.auth.jwks-url") Optional<String> jwksUrl,
+            @ConfigProperty(name = "mancala.auth.client-ids") String clientIds) {
+        // Comma-separated; spaces and empty entries are ignored, as in the Node backend.
+        List<String> allowedClientIds = Arrays.stream(clientIds.split(",")).map(String::trim).filter(id -> !id.isEmpty()).toList();
+        return KeycloakTokenVerifier.create(issuer, allowedClientIds, jwksUrl.orElse(null));
+    }
+}
diff --git a/apps/backend-quarkus/src/main/java/app/playmancala/http/BearerAuth.java b/apps/backend-quarkus/src/main/java/app/playmancala/http/BearerAuth.java
new file mode 100644
index 0000000..cc69264
--- /dev/null
+++ b/apps/backend-quarkus/src/main/java/app/playmancala/http/BearerAuth.java
@@ -0,0 +1,35 @@
+package app.playmancala.http;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.jboss.logging.Logger;
+
+import app.playmancala.auth.InvalidTokenException;
+import app.playmancala.auth.TokenVerifier;
+import app.playmancala.model.AuthUser;
+import jakarta.ws.rs.WebApplicationException;
+import jakarta.ws.rs.core.Response;
+
+// Requires an "Authorization: Bearer <access token>" header with the same Keycloak token the WebSocket uses.
+final class BearerAuth {
+    private static final Logger LOG = Logger.getLogger(BearerAuth.class);
+    private static final Pattern BEARER = Pattern.compile("^Bearer (.+)$");
+
+    private BearerAuth() {
+    }
+
+    // The user, or a 401 response.
+    static AuthUser requireUser(TokenVerifier tokenVerifier, String authorization) {
+        Matcher match = BEARER.matcher(authorization == null ? "" : authorization);
+        if (!match.matches()) throw new WebApplicationException(Response.Status.UNAUTHORIZED);
+        try {
+            return tokenVerifier.verify(match.group(1));
+        } catch (InvalidTokenException e) {
+            throw new WebApplicationException(Response.Status.UNAUTHORIZED);
+        } catch (RuntimeException e) {
+            LOG.error("(Auth) Token verification failed", e);
+            throw new WebApplicationException(Response.Status.UNAUTHORIZED);
+        }
+    }
+}
diff --git a/apps/backend-quarkus/src/main/java/app/playmancala/http/ErrorMapper.java b/apps/backend-quarkus/src/main/java/app/playmancala/http/ErrorMapper.java
new file mode 100644
index 0000000..7b01516
--- /dev/null
+++ b/apps/backend-quarkus/src/main/java/app/playmancala/http/ErrorMapper.java
@@ -0,0 +1,20 @@
+package app.playmancala.http;
+
+import org.jboss.logging.Logger;
+import org.jboss.resteasy.reactive.server.ServerExceptionMapper;
+
+import io.vertx.core.http.HttpServerRequest;
+import jakarta.ws.rs.WebApplicationException;
+import jakarta.ws.rs.core.Response;
+
+// A failing dependency (the database, mostly) answers 503, as the Express error handler did.
+public class ErrorMapper {
+    private static final Logger LOG = Logger.getLogger(ErrorMapper.class);
+
+    @ServerExceptionMapper
+    public Response map(RuntimeException error, HttpServerRequest request) {
+        if (error instanceof WebApplicationException webError) return webError.getResponse();
+        LOG.errorf("(HTTP) %s %s failed: %s", request.method(), request.path(), error.getMessage());
+        return Response.status(Response.Status.SERVICE_UNAVAILABLE).build();
+    }
+}
diff --git a/apps/backend-quarkus/src/main/java/app/playmancala/http/GameResource.java b/apps/backend-quarkus/src/main/java/app/playmancala/http/GameResource.java
new file mode 100644
index 0000000..9addfc3
--- /dev/null
+++ b/apps/backend-quarkus/src/main/java/app/playmancala/http/GameResource.java
@@ -0,0 +1,125 @@
+package app.playmancala.http;
+
+import java.math.BigInteger;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.UUID;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import app.playmancala.Backend;
+import app.playmancala.game.BackendGame;
+import app.playmancala.game.LiveGames;
+import app.playmancala.history.GameHistoryService;
+import app.playmancala.model.AuthUser;
+import app.playmancala.model.GameRecord;
+import app.playmancala.model.Json;
+import app.playmancala.model.LiveGameSummary;
+import jakarta.inject.Inject;
+import jakarta.ws.rs.GET;
+import jakarta.ws.rs.HeaderParam;
+import jakarta.ws.rs.NotFoundException;
+import jakarta.ws.rs.Path;
+import jakarta.ws.rs.PathParam;
+import jakarta.ws.rs.Produces;
+import jakarta.ws.rs.QueryParam;
+import jakarta.ws.rs.core.HttpHeaders;
+import jakarta.ws.rs.core.MediaType;
+import jakarta.ws.rs.core.Response;
+
+// The HTTP API. Blocking: runs on worker threads, reads live games on the game loop and saved games itself.
+// Bodies are written with the shared mapper, so they are the same JSON as the WebSocket messages.
+@Path("/")
+public class GameResource {
+    private static final Pattern LEADING_INTEGER = Pattern.compile("^\\s*([+-]?\\d+)");
+
+    @Inject
+    Backend backend;
+
+    @GET
+    @Produces(MediaType.TEXT_PLAIN)
+    public String root() {
+        return "Server up and running! ci test1";
+    }
+
+    // Issues userKeys for legacy clients; token clients get their identity from Keycloak.
+    @GET
+    @Path("register{slash: /?}")
+    @Produces(MediaType.TEXT_PLAIN)
+    public String register() {
+        if (!backend.legacyUserKeyAuth()) throw new NotFoundException();
+        return UUID.randomUUID().toString();
+    }
+
+    // A live game as players receive it, or the saved record of a finished game.
+    @GET
+    @Path("game/{id}")
+    public Response game(@PathParam("id") String id) {
+        // Serialised on the loop, because the client game refers to the live engine objects.
+        String live = backend.loop().call(() -> {
+            BackendGame game = backend.gameStore().get(id);
+            return game == null ? null : Json.write(backend.gameManager().toClientGame(game));
+        });
+        if (live != null) return json(live);
+        GameRecord record = backend.history().get(id);
+        if (record == null) throw new NotFoundException();
+        Map<String, Object> response = new LinkedHashMap<>();
+        response.put("status", "finished");
+        response.put("record", record);
+        return json(response);
+    }
+
+    // Live games if any are being played, otherwise the most spectated finished games. With kind=finished always the
+    // finished games, for clients that get live games over the WebSocket.
+    @GET
+    @Path("games/popular")
+    public Response popular(@QueryParam("limit") String limitParam, @QueryParam("kind") String kind) {
+        int limit = parseLimit(limitParam, 5);
+        List<LiveGameSummary> live = "finished".equals(kind) ? List.of()
+                : backend.loop().call(() -> backend.liveGames().getTop(limit));
+        Map<String, Object> response = new LinkedHashMap<>();
+        if (!live.isEmpty()) {
+            response.put("kind", "live");
+            response.put("games", live);
+        } else {
+            response.put("kind", "finished");
+            response.put("games", backend.history().getPopular(limit));
+        }
+        return json(response);
+    }
+
+    @GET
+    @Path("games/me/active")
+    public Response activeGame(@HeaderParam(HttpHeaders.AUTHORIZATION) String authorization) {
+        AuthUser user = BearerAuth.requireUser(backend.tokenVerifier(), authorization);
+        LiveGameSummary game = backend.loop().call(() -> {
+            BackendGame active = backend.gameStore().getGameByUser(user.id());
+            return active == null ? null : LiveGames.toLiveGameSummary(active);
+        });
+        Map<String, Object> response = new HashMap<>();
+        response.put("game", game);
+        return json(response);
+    }
+
+    @GET
+    @Path("games/me")
+    public Response myGames(@HeaderParam(HttpHeaders.AUTHORIZATION) String authorization,
+            @QueryParam("limit") String limit) {
+        AuthUser user = BearerAuth.requireUser(backend.tokenVerifier(), authorization);
+        return json(Map.of("games", backend.history().getRecentForUser(user.id(), parseLimit(limit, 3))));
+    }
+
+    private static Response json(Object body) {
+        return Response.ok(body instanceof String text ? text : Json.write(body), MediaType.APPLICATION_JSON).build();
+    }
+
+    // Like parseInt in the Node backend: the leading integer, clamped to 1..MAX_LIST_LIMIT.
+    static int parseLimit(String value, int fallback) {
+        Matcher match = LEADING_INTEGER.matcher(value == null ? "" : value);
+        if (!match.find()) return fallback;
+        BigInteger limit = new BigInteger(match.group(1));
+        return limit.max(BigInteger.ONE).min(BigInteger.valueOf(GameHistoryService.MAX_LIST_LIMIT)).intValue();
+    }
+}
diff --git a/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtEndpoint.java b/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtEndpoint.java
new file mode 100644
index 0000000..de16978
--- /dev/null
+++ b/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtEndpoint.java
@@ -0,0 +1,93 @@
+package app.playmancala.rtmt;
+
+import java.net.URLDecoder;
+import java.nio.charset.StandardCharsets;
+
+import org.jboss.logging.Logger;
+
+import app.playmancala.Backend;
+import io.quarkus.websockets.next.CloseReason;
+import io.quarkus.websockets.next.OnClose;
+import io.quarkus.websockets.next.OnError;
+import io.quarkus.websockets.next.OnOpen;
+import io.quarkus.websockets.next.OnTextMessage;
+import io.quarkus.websockets.next.WebSocket;
+import io.quarkus.websockets.next.WebSocketConnection;
+import io.smallrye.common.annotation.NonBlocking;
+import jakarta.inject.Inject;
+
+// Clients connect to the root path, as they did to the Node backend. Events run on the connection's event loop, in
+// order, and are only handed to the game loop, where RtmtServer handles them.
+@WebSocket(path = "/")
+public class RtmtEndpoint {
+    private static final Logger LOG = Logger.getLogger(RtmtEndpoint.class);
+
+    @Inject
+    Backend backend;
+
+    @OnOpen
+    @NonBlocking
+    public void onOpen(WebSocketConnection connection) {
+        RtmtSocket socket = new ConnectionSocket(connection);
+        backend.loop().execute(() -> backend.rtmt().onOpen(socket));
+    }
+
+    @OnTextMessage
+    @NonBlocking
+    public void onTextMessage(WebSocketConnection connection, String text) {
+        RtmtSocket socket = new ConnectionSocket(connection);
+        backend.loop().execute(() -> backend.rtmt().onText(socket, text));
+    }
+
+    @OnClose
+    @NonBlocking
+    public void onClose(WebSocketConnection connection) {
+        RtmtSocket socket = new ConnectionSocket(connection);
+        backend.loop().execute(() -> backend.rtmt().onClose(socket));
+    }
+
+    @OnError
+    @NonBlocking
+    public void onError(WebSocketConnection connection, Throwable error) {
+        // A close event follows, which cleans up.
+        LOG.debugf("(RTMT) WebSocket error: %s", error.getMessage());
+    }
+
+    private record ConnectionSocket(WebSocketConnection connection) implements RtmtSocket {
+        @Override
+        public String id() {
+            return connection.id();
+        }
+
+        @Override
+        public String queryParam(String name) {
+            String query = connection.handshakeRequest().query();
+            if (query == null || query.isEmpty()) return null;
+            for (String pair : query.split("&")) {
+                int equals = pair.indexOf('=');
+                String key = equals < 0 ? pair : pair.substring(0, equals);
+                if (URLDecoder.decode(key, StandardCharsets.UTF_8).equals(name)) {
+                    return equals < 0 ? "" : URLDecoder.decode(pair.substring(equals + 1), StandardCharsets.UTF_8);
+                }
+            }
+            return null;
+        }
+
+        @Override
+        public boolean isOpen() {
+            return connection.isOpen();
+        }
+
+        @Override
+        public void send(String text) {
+            connection.sendText(text).subscribe().with(ok -> { },
+                    error -> LOG.debugf("(RTMT) Could not send: %s", error.getMessage()));
+        }
+
+        @Override
+        public void close(int code, String reason) {
+            connection.close(new CloseReason(code, reason)).subscribe().with(ok -> { },
+                    error -> LOG.debugf("(RTMT) Could not close: %s", error.getMessage()));
+        }
+    }
+}
diff --git a/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtServer.java b/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtServer.java
new file mode 100644
index 0000000..50eaa73
--- /dev/null
+++ b/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtServer.java
@@ -0,0 +1,256 @@
+package app.playmancala.rtmt;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.Executor;
+import java.util.function.BiConsumer;
+import java.util.regex.Pattern;
+
+import org.jboss.logging.Logger;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.node.NullNode;
+
+import app.playmancala.auth.InvalidTokenException;
+import app.playmancala.auth.TokenVerifier;
+import app.playmancala.loop.Cancellable;
+import app.playmancala.loop.Scheduler;
+import app.playmancala.model.AuthUser;
+import app.playmancala.model.Channels;
+import app.playmancala.model.Json;
+
+// The WebSocket protocol: authentication, routing messages by channel, users with several sockets and the ping
+// keep-alive. Messages are JSON objects { channel, message }. Everything here runs on the game loop; the endpoint
+// forwards socket events to it, and tokens are checked on `verifier` and the outcome handled back on the loop.
+public class RtmtServer implements Rtmt {
+    public static final long PING_INTERVAL_MS = 2_000;
+    public static final long DEFAULT_AUTH_TIMEOUT_MS = 5_000;
+    private static final Logger LOG = Logger.getLogger(RtmtServer.class);
+    private static final Pattern LEGACY_USER_KEY_PATTERN =
+            Pattern.compile("^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$", Pattern.CASE_INSENSITIVE);
+
+    private record Envelope(String channel, Object message) {
+    }
+
+    private static final class Client {
+        final RtmtSocket socket;
+        boolean alive = true;
+        // Set once the socket is authenticated.
+        String userId;
+        Cancellable authTimeout;
+
+        Client(RtmtSocket socket) {
+            this.socket = socket;
+        }
+    }
+
+    private final TokenVerifier tokenVerifier;
+    private final boolean legacyUserKeyAuth;
+    private final Scheduler loop;
+    private final Executor verifier;
+    private final long authTimeoutMs;
+
+    private final Map<String, BiConsumer<String, JsonNode>> messageChannels = new HashMap<>();
+    // By socket id.
+    private final Map<String, Client> clients = new HashMap<>();
+    // A user can be connected from several devices or tabs at once.
+    private final Map<String, Set<Client>> sockets = new HashMap<>();
+    private final Map<String, AuthUser> users = new HashMap<>();
+    private final List<BiConsumer<String, Boolean>> connectionChangeListeners = new ArrayList<>();
+    private Cancellable pingTimer;
+
+    public RtmtServer(TokenVerifier tokenVerifier, boolean legacyUserKeyAuth, Scheduler loop, Executor verifier,
+            long authTimeoutMs) {
+        this.tokenVerifier = tokenVerifier;
+        this.legacyUserKeyAuth = legacyUserKeyAuth;
+        this.loop = loop;
+        this.verifier = verifier;
+        this.authTimeoutMs = authTimeoutMs;
+    }
+
+    public void start() {
+        stop();
+        pingTimer = loop.scheduleAtFixedRate(this::ping, PING_INTERVAL_MS);
+    }
+
+    public void stop() {
+        if (pingTimer != null) pingTimer.cancel();
+        pingTimer = null;
+    }
+
+    public void onOpen(RtmtSocket socket) {
+        LOG.debug("(RTMT) Client connected");
+        Client client = new Client(socket);
+        clients.put(socket.id(), client);
+
+        String legacyUserKey = socket.queryParam("userKey");
+        if (legacyUserKey != null) {
+            if (legacyUserKeyAuth && LEGACY_USER_KEY_PATTERN.matcher(legacyUserKey).matches()) {
+                bindSocket(client, new AuthUser(legacyUserKey, null, true));
+            } else {
+                rejectSocket(client, "userKey login is not accepted");
+            }
+            return;
+        }
+
+        client.authTimeout = loop.schedule(() -> {
+            if (client.userId == null) rejectSocket(client, "authentication timeout");
+        }, authTimeoutMs);
+    }
+
+    public void onClose(RtmtSocket socket) {
+        Client client = clients.remove(socket.id());
+        if (client == null) return;
+        LOG.debug("(RTMT) Client disconnected");
+        if (client.authTimeout != null) client.authTimeout.cancel();
+        unbindSocket(client);
+    }
+
+    public void onText(RtmtSocket socket, String text) {
+        Client client = clients.get(socket.id());
+        if (client == null) return;
+        JsonNode envelope;
+        try {
+            envelope = Json.parse(text);
+        } catch (Exception e) {
+            LOG.debug("(RTMT) Ignoring malformed message");
+            return;
+        }
+        if (envelope == null || !envelope.path("channel").isTextual()) return;
+        String channel = envelope.get("channel").asText();
+        JsonNode message = envelope.has("message") ? envelope.get("message") : NullNode.getInstance();
+
+        if (channel.equals(Channels.PONG)) {
+            client.alive = true;
+            return;
+        }
+        if (channel.equals(Channels.AUTH)) {
+            authenticate(client, message);
+            return;
+        }
+        if (client.userId == null) {
+            LOG.debugf("(RTMT) Ignoring message from unauthenticated client on channel %s", channel);
+            return;
+        }
+        BiConsumer<String, JsonNode> callback = messageChannels.get(channel);
+        if (callback != null) {
+            callback.accept(client.userId, message);
+        } else {
+            LOG.debugf("(RTMT) Channel callback not found! %s", channel);
+        }
+    }
+
+    private void authenticate(Client client, JsonNode message) {
+        JsonNode token = message.path("token");
+        if (!token.isTextual()) {
+            rejectSocket(client, "missing token");
+            return;
+        }
+        verifier.execute(() -> {
+            try {
+                AuthUser user = tokenVerifier.verify(token.asText());
+                loop.execute(() -> onAuthenticated(client, user));
+            } catch (InvalidTokenException e) {
+                loop.execute(() -> rejectSocket(client, "invalid token"));
+            } catch (RuntimeException e) {
+                LOG.error("(RTMT) Token verification failed", e);
+                loop.execute(() -> rejectSocket(client, "authentication failed"));
+            }
+        });
+    }
+
+    private void onAuthenticated(Client client, AuthUser user) {
+        if (!client.socket.isOpen() || clients.get(client.socket.id()) != client) return;
+        if (client.userId != null && !client.userId.equals(user.id())) {
+            rejectSocket(client, "token belongs to another user");
+            return;
+        }
+        if (client.authTimeout != null) client.authTimeout.cancel();
+        if (client.userId != null) {
+            // Token refresh on an already authenticated socket.
+            users.put(user.id(), user);
+        } else {
+            bindSocket(client, user);
+        }
+        send(client, Channels.ON_AUTH, Map.of("user", user));
+    }
+
+    private void rejectSocket(Client client, String reason) {
+        if (!client.socket.isOpen()) return;
+        send(client, Channels.ON_AUTH_ERROR, Map.of("reason", reason));
+        client.socket.close(Channels.CLOSE_AUTH_FAILED, reason);
+    }
+
+    private void bindSocket(Client client, AuthUser user) {
+        client.userId = user.id();
+        users.put(user.id(), user);
+        sockets.computeIfAbsent(user.id(), id -> new LinkedHashSet<>()).add(client);
+        // Fired for every new socket so each device gets the current game state.
+        List.copyOf(connectionChangeListeners).forEach(listener -> listener.accept(user.id(), true));
+    }
+
+    private void unbindSocket(Client client) {
+        String userId = client.userId;
+        if (userId == null) return;
+        Set<Client> userSockets = sockets.get(userId);
+        if (userSockets == null || !userSockets.remove(client)) return;
+        if (userSockets.isEmpty()) {
+            sockets.remove(userId);
+            users.remove(userId);
+            List.copyOf(connectionChangeListeners).forEach(listener -> listener.accept(userId, false));
+        }
+    }
+
+    private void ping() {
+        for (Client client : List.copyOf(clients.values())) {
+            if (!client.alive) {
+                LOG.debug("(RTMT) Closing a socket that did not answer the ping");
+                client.socket.close(1001, "ping timeout");
+                continue;
+            }
+            client.alive = false;
+            send(client, Channels.PING, Map.of());
+        }
+    }
+
+    @Override
+    public void sendMessage(String clientId, String channel, Object message) {
+        Set<Client> userSockets = sockets.get(clientId);
+        if (userSockets == null) {
+            LOG.debugf("(RTMT) Client not connected: %s", clientId);
+            return;
+        }
+        String text = Json.write(new Envelope(channel, message));
+        userSockets.forEach(client -> {
+            if (client.socket.isOpen()) client.socket.send(text);
+        });
+    }
+
+    private void send(Client client, String channel, Object message) {
+        if (client.socket.isOpen()) client.socket.send(Json.write(new Envelope(channel, message)));
+    }
+
+    @Override
+    public void listenMessage(String channel, BiConsumer<String, JsonNode> callback) {
+        messageChannels.put(channel, callback);
+    }
+
+    @Override
+    public boolean isClientOnline(String clientId) {
+        return sockets.containsKey(clientId);
+    }
+
+    @Override
+    public AuthUser getUser(String clientId) {
+        return users.get(clientId);
+    }
+
+    @Override
+    public void listenOnClientConnectionChange(BiConsumer<String, Boolean> listener) {
+        connectionChangeListeners.add(listener);
+    }
+}
diff --git a/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtSocket.java b/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtSocket.java
new file mode 100644
index 0000000..f8af2a2
--- /dev/null
+++ b/apps/backend-quarkus/src/main/java/app/playmancala/rtmt/RtmtSocket.java
@@ -0,0 +1,15 @@
+package app.playmancala.rtmt;
+
+// A client's WebSocket as RtmtServer sees it. Sends and closes do not block.
+public interface RtmtSocket {
+    String id();
+
+    // A query parameter of the URL the client connected to, or null.
+    String queryParam(String name);
+
+    boolean isOpen();
+
+    void send(String text);
+
+    void close(int code, String reason);
+}
diff --git a/apps/backend-quarkus/src/main/resources/application.properties b/apps/backend-quarkus/src/main/resources/application.properties
index 4875640..b55fd58 100644
--- a/apps/backend-quarkus/src/main/resources/application.properties
+++ b/apps/backend-quarkus/src/main/resources/application.properties
@@ -35,7 +35,7 @@ quarkus.datasource.devservices.image-name=postgres:17.6
 # Valkey. Tests start one with Dev Services. Short timeouts, so callers fall back quickly while it is down.
 %dev,prod.quarkus.redis.hosts=${VALKEY_URL:redis://localhost:6379}
 quarkus.redis.timeout=2S
-quarkus.redis.tcp.connect-timeout=1S
+quarkus.redis.tcp.connection-timeout=1S
 quarkus.redis.devservices.image-name=valkey/valkey:9.1.2
 # Nothing listens on port 1; for the tests of an unreachable Valkey.
 %test.quarkus.redis.offline.hosts=redis://127.0.0.1:1
diff --git a/apps/backend-quarkus/src/test/java/app/playmancala/fakes/InMemoryCache.java b/apps/backend-quarkus/src/test/java/app/playmancala/fakes/InMemoryCache.java
new file mode 100644
index 0000000..f5a8148
--- /dev/null
+++ b/apps/backend-quarkus/src/test/java/app/playmancala/fakes/InMemoryCache.java
@@ -0,0 +1,35 @@
+package app.playmancala.fakes;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Optional;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+
+import app.playmancala.live.Cache;
+import app.playmancala.model.Json;
+
+public class InMemoryCache implements Cache {
+    public final Map<String, String> values = new HashMap<>();
+
+    @Override
+    public <T> Optional<T> get(String key, TypeReference<T> type) {
+        String value = values.get(key);
+        if (value == null) return Optional.empty();
+        try {
+            return Optional.of(Json.MAPPER.readValue(value, type));
+        } catch (Exception e) {
+            return Optional.empty();
+        }
+    }
+
+    @Override
+    public void set(String key, Object value, long ttlSeconds) {
+        values.put(key, Json.write(value));
+    }
+
+    @Override
+    public void delete(String... keys) {
+        for (String key : keys) values.remove(key);
+    }
+}
diff --git a/apps/backend-quarkus/src/test/java/app/playmancala/history/GameHistoryServiceTest.java b/apps/backend-quarkus/src/test/java/app/playmancala/history/GameHistoryServiceTest.java
new file mode 100644
index 0000000..cbc75f7
--- /dev/null
+++ b/apps/backend-quarkus/src/test/java/app/playmancala/history/GameHistoryServiceTest.java
@@ -0,0 +1,102 @@
+package app.playmancala.history;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.UUID;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import app.playmancala.fakes.InMemoryCache;
+import app.playmancala.fakes.InMemoryGameHistoryRepository;
+import app.playmancala.model.EndReason;
+import app.playmancala.model.FinishedGameSummary;
+import app.playmancala.model.GameRecord;
+import app.playmancala.model.GameResult;
+import app.playmancala.model.PlayerSummary;
+
+// The list behaviour the Node backend's HTTP API tests checked: the cache, unsaved games and a database that is down.
+class GameHistoryServiceTest {
+    static final PlayerSummary ALICE = new PlayerSummary("kc:alice", "alice", false);
+    static final PlayerSummary BOB = new PlayerSummary("kc:bob", "bob", false);
+    static final PlayerSummary CAROL = new PlayerSummary("kc:carol", "carol", false);
+
+    InMemoryGameHistoryRepository repository;
+    InMemoryCache cache;
+    List<GameRecord> unsaved;
+    GameHistoryService history;
+    long endedAt = 1_000_000;
+
+    @BeforeEach
+    void setUp() {
+        repository = new InMemoryGameHistoryRepository();
+        cache = new InMemoryCache();
+        unsaved = new ArrayList<>();
+        history = new GameHistoryService(repository, cache, () -> List.copyOf(unsaved));
+    }
+
+    GameRecord record(PlayerSummary player1, PlayerSummary player2, int peakSpectators) {
+        endedAt += 60_000;
+        return new GameRecord(UUID.randomUUID().toString(), player1, player2, List.of(), GameResult.PLAYER1,
+                EndReason.LEFT, null, endedAt - 1000, endedAt, 0, peakSpectators, 300_000L, List.of(), peakSpectators);
+    }
+
+    static List<String> ids(List<FinishedGameSummary> games) {
+        return games.stream().map(FinishedGameSummary::id).toList();
+    }
+
+    @Test
+    void returnsTheUsersLatestGamesAndShowsANewOneRightAwayDespiteTheCache() {
+        GameRecord first = record(ALICE, BOB, 0);
+        repository.save(first);
+        repository.save(record(CAROL, BOB, 0));
+        assertEquals(List.of(first.id()), ids(history.getRecentForUser(ALICE.userId(), 3)));
+
+        GameRecord second = record(BOB, ALICE, 0);
+        repository.save(second);
+        // Still cached.
+        assertEquals(List.of(first.id()), ids(history.getRecentForUser(ALICE.userId(), 3)));
+        history.onGameSaved(second);
+        assertEquals(List.of(second.id()), ids(history.getRecentForUser(ALICE.userId(), 1)));
+    }
+
+    @Test
+    void returnsAnEmptyListWhileTheDatabaseIsDown() {
+        repository.failing = true;
+        assertEquals(List.of(), history.getRecentForUser(ALICE.userId(), 3));
+        assertEquals(List.of(), history.getPopular(5));
+    }
+
+    @Test
+    void includesGamesThatEndedButAreNotSavedYet() {
+        repository.failing = true;
+        GameRecord game = record(ALICE, BOB, 0);
+        unsaved.add(game);
+        assertEquals(List.of(game.id()), ids(history.getRecentForUser(ALICE.userId(), 3)));
+        assertEquals(List.of(), ids(history.getRecentForUser(CAROL.userId(), 3)));
+        assertEquals(List.of(game.id()), ids(history.getPopular(5)));
+        assertSame(game, history.get(game.id()));
+    }
+
+    @Test
+    void listsPopularGamesBySpectatorsThenNewestWithoutDuplicates() {
+        GameRecord quiet = record(ALICE, BOB, 0);
+        GameRecord watched = record(ALICE, BOB, 3);
+        GameRecord newerQuiet = record(ALICE, BOB, 0);
+        List.of(quiet, watched).forEach(repository::save);
+        unsaved.add(newerQuiet);
+        unsaved.add(watched); // saved meanwhile
+        assertEquals(List.of(watched.id(), newerQuiet.id(), quiet.id()), ids(history.getPopular(5)));
+        assertEquals(List.of(watched.id()), ids(history.getPopular(1)));
+    }
+
+    @Test
+    void readingASavedGameFailsWhileTheDatabaseIsDown() {
+        repository.failing = true;
+        assertThrows(IllegalStateException.class, () -> history.get(UUID.randomUUID().toString()));
+    }
+}
diff --git a/apps/backend-quarkus/src/test/java/app/playmancala/http/ApiTest.java b/apps/backend-quarkus/src/test/java/app/playmancala/http/ApiTest.java
new file mode 100644
index 0000000..8bb82bf
--- /dev/null
+++ b/apps/backend-quarkus/src/test/java/app/playmancala/http/ApiTest.java
@@ -0,0 +1,221 @@
+package app.playmancala.http;
+
+import static io.restassured.RestAssured.given;
+import static org.awaitility.Awaitility.await;
+import static org.hamcrest.Matchers.matchesPattern;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.net.URI;
+import java.time.Duration;
+import java.util.List;
+import java.util.Map;
+import java.util.UUID;
+import java.util.concurrent.TimeUnit;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.databind.JsonNode;
+
+import app.playmancala.game.PublicIds;
+import app.playmancala.model.Channels;
+import app.playmancala.model.Json;
+import app.playmancala.support.TestClient;
+import io.quarkus.test.common.http.TestHTTPResource;
+import io.quarkus.test.junit.QuarkusTest;
+
+// The WebSocket and the HTTP API end to end, with Postgres and Valkey in Docker and a fake token verifier.
+// Ported from the Node backend's tests/server/ExpressApp.test.ts and tests/rtmt/rtmt_websocket.test.ts.
+@QuarkusTest
+class ApiTest {
+    @TestHTTPResource("/")
+    URI base;
+
+    URI webSocketUri(String query) {
+        return URI.create(base.toString().replaceFirst("^http", "ws") + query);
+    }
+
+    static String unique(String name) {
+        return "kc:" + name + "-" + UUID.randomUUID().toString().substring(0, 8);
+    }
+
+    static JsonNode get(String path, String userId) {
+        var request = given();
+        if (userId != null) request = request.header("Authorization", "Bearer valid:" + userId);
+        String body = request.when().get(path).then().statusCode(200).extract().asString();
+        try {
+            return Json.parse(body);
+        } catch (Exception e) {
+            throw new IllegalStateException(body, e);
+        }
+    }
+
+    static List<String> ids(JsonNode games) {
+        return games.valueStream().map(game -> game.get("id").asText()).toList();
+    }
+
+    // Both ask for a game and get paired; returns the on_game_start message.
+    static JsonNode startGame(TestClient first, TestClient second) {
+        first.send(Channels.NEW_GAME, Map.of());
+        second.send(Channels.NEW_GAME, Map.of());
+        JsonNode start = first.waitFor(Channels.ON_GAME_START);
+        assertEquals(start.get("id"), second.waitFor(Channels.ON_GAME_START).get("id"));
+        return start;
+    }
+
+    @Test
+    void answersOnTheRootPathAndIssuesLegacyUserKeys() {
+        given().when().get("/").then().statusCode(200);
+        String uuid = "[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}";
+        given().when().get("/register/").then().statusCode(200).body(matchesPattern(uuid));
+        given().when().get("/register").then().statusCode(200).body(matchesPattern(uuid));
+    }
+
+    @Test
+    void allowsEveryOrigin() {
+        given().header("Origin", "https://playmancala.app").when().get("/games/popular").then().statusCode(200)
+                .header("Access-Control-Allow-Origin", matchesPattern("\\*|https://playmancala\\.app"));
+    }
+
+    @Test
+    void aGameFromStartToTheSavedRecord() {
+        String alice = unique("alice");
+        String bob = unique("bob");
+        String carol = unique("carol");
+        try (TestClient aliceClient = TestClient.connectAs(webSocketUri(""), alice);
+                TestClient bobClient = TestClient.connectAs(webSocketUri(""), bob);
+                TestClient carolClient = TestClient.connectAs(webSocketUri(""), carol)) {
+            JsonNode start = startGame(aliceClient, bobClient);
+            String gameId = start.get("id").asText();
+            assertEquals("initial", start.at("/mancalaGame/state").asText());
+            assertEquals(300_000, start.at("/clock/initialMs").asLong());
+
+            // A spectator, seen by the API but without spectator ids.
+            carolClient.send(Channels.LISTEN_GAME_EVENTS, gameId);
+            await().atMost(Duration.ofSeconds(5)).until(() -> get("/game/" + gameId, null).get("spectatorCount").asInt() == 1);
+            JsonNode live = get("/game/" + gameId, null);
+            assertEquals(gameId, live.get("id").asText());
+            assertFalse(live.has("spectatorIds"));
+            assertTrue(ids(get("/games/popular?limit=20", null).get("games")).contains(gameId));
+            assertEquals(gameId, get("/games/me/active", alice).at("/game/id").asText());
+            assertTrue(get("/games/me/active", carol).get("game").isNull());
+
+            // A move reaches the players and the spectator.
+            String turnPlayerId = start.at("/mancalaGame/turnPlayerId").asText();
+            (turnPlayerId.equals(alice) ? aliceClient : bobClient).send(Channels.GAME_MOVE, Map.of("index", 0));
+            for (TestClient client : List.of(aliceClient, bobClient, carolClient)) {
+                assertEquals(1, client.waitFor(Channels.ON_GAME_UPDATE).at("/mancalaGame/history").size());
+            }
+            assertEquals(1, get("/game/" + gameId, null).at("/mancalaGame/history").size());
+
+            // Alice leaves: Bob wins, and the game is saved.
+            aliceClient.send(Channels.LEAVE_GAME, Map.of());
+            JsonNode end = carolClient.waitFor(Channels.ON_GAME_END);
+            assertEquals("left", end.get("endReason").asText());
+            assertEquals(bob, end.get("winnerId").asText());
+            assertEquals(alice, bobClient.waitFor(Channels.ON_GAME_USER_LEAVE).asText());
+
+            JsonNode finished = get("/game/" + gameId, null);
+            assertEquals("finished", finished.get("status").asText());
+            assertEquals(1, finished.at("/record/moves").size());
+            assertEquals(alice, finished.at("/record/leftUserId").asText());
+            assertTrue(get("/games/me/active", alice).get("game").isNull());
+            await().atMost(Duration.ofSeconds(10)).until(() -> ids(get("/games/me", alice).get("games")).contains(gameId));
+            assertEquals(List.of(gameId), ids(get("/games/me?limit=1", bob).get("games")));
+            assertFalse(get("/games/me", bob).get("games").get(0).has("moves"));
+            assertTrue(ids(get("/games/popular?kind=finished&limit=20", null).get("games")).contains(gameId));
+        }
+    }
+
+    @Test
+    void rejectsANewGameWhileOneIsInProgress() {
+        try (TestClient alice = TestClient.connectAs(webSocketUri(""), unique("alice"));
+                TestClient bob = TestClient.connectAs(webSocketUri(""), unique("bob"))) {
+            String gameId = startGame(alice, bob).get("id").asText();
+            alice.send(Channels.NEW_GAME, Map.of());
+            assertEquals(gameId, alice.waitFor(Channels.ON_NEW_GAME_REJECTED).get("gameId").asText());
+            alice.send(Channels.LEAVE_GAME, Map.of());
+            bob.waitFor(Channels.ON_GAME_END);
+        }
+    }
+
+    @Test
+    void aReconnectingPlayerGetsTheGameAgain() {
+        String alice = unique("alice");
+        try (TestClient aliceClient = TestClient.connectAs(webSocketUri(""), alice);
+                TestClient bob = TestClient.connectAs(webSocketUri(""), unique("bob"))) {
+            String gameId = startGame(aliceClient, bob).get("id").asText();
+            aliceClient.close();
+            JsonNode offline = bob.waitFor(Channels.ON_USER_CONNECTION_CHANGE, 2).getLast();
+            assertTrue(offline.toString().contains("\"isOnline\":false"), offline.toString());
+
+            try (TestClient again = TestClient.connectAs(webSocketUri(""), alice)) {
+                assertEquals(gameId, again.waitFor(Channels.ON_GAME_UPDATE).get("id").asText());
+                again.send(Channels.LEAVE_GAME, Map.of());
+                bob.waitFor(Channels.ON_GAME_END);
+            }
+        }
+    }
+
+    @Test
+    void sendsLiveGamePreviewsToSubscribers() {
+        try (TestClient viewer = TestClient.connectAs(webSocketUri(""), unique("viewer"));
+                TestClient alice = TestClient.connectAs(webSocketUri(""), unique("alice"));
+                TestClient bob = TestClient.connectAs(webSocketUri(""), unique("bob"))) {
+            viewer.send(Channels.SUBSCRIBE_LIVE_GAMES, Map.of());
+            viewer.waitFor(Channels.ON_LIVE_GAMES);
+            String gameId = startGame(alice, bob).get("id").asText();
+            await().atMost(Duration.ofSeconds(5)).until(() -> viewer.messages(Channels.ON_LIVE_GAMES).stream()
+                    .anyMatch(push -> ids(push.get("games")).contains(gameId)));
+            alice.send(Channels.LEAVE_GAME, Map.of());
+            bob.waitFor(Channels.ON_GAME_END);
+        }
+    }
+
+    @Test
+    void requiresAValidTokenForTheUsersGames() {
+        given().when().get("/games/me/active").then().statusCode(401);
+        given().header("Authorization", "Bearer forged").when().get("/games/me/active").then().statusCode(401);
+        given().when().get("/games/me").then().statusCode(401);
+    }
+
+    @Test
+    void answers404ForUnknownGames() {
+        given().when().get("/game/unknown").then().statusCode(404);
+        given().when().get("/game/" + UUID.randomUUID()).then().statusCode(404);
+    }
+
+    @Test
+    void closesASocketWithAnInvalidToken() throws Exception {
+        TestClient client = new TestClient(webSocketUri(""));
+        client.send(Channels.AUTH, Map.of("token", "forged"));
+        assertEquals(Json.tree(Map.of("reason", "invalid token")), client.waitFor(Channels.ON_AUTH_ERROR));
+        assertEquals(Channels.CLOSE_AUTH_FAILED, client.closed.get(5, TimeUnit.SECONDS));
+    }
+
+    @Test
+    void playsWithLegacyUserKeyClients() {
+        String key1 = UUID.randomUUID().toString();
+        String key2 = UUID.randomUUID().toString();
+        try (TestClient player1 = new TestClient(webSocketUri("?userKey=" + key1));
+                TestClient player2 = new TestClient(webSocketUri("?userKey=" + key2))) {
+            String gameId = startGame(player1, player2).get("id").asText();
+            JsonNode preview = get("/games/popular?limit=20", null).get("games").valueStream()
+                    .filter(game -> game.get("id").asText().equals(gameId)).findFirst().orElseThrow();
+            assertFalse(preview.toString().contains(key1));
+            assertTrue(preview.toString().contains(PublicIds.publicUserId(key1)));
+            player1.send(Channels.LEAVE_GAME, Map.of());
+            player2.waitFor(Channels.ON_GAME_END);
+        }
+    }
+
+    @Test
+    void parsesLimitsLikeParseInt() {
+        assertEquals(3, GameResource.parseLimit(null, 3));
+        assertEquals(3, GameResource.parseLimit("abc", 3));
+        assertEquals(7, GameResource.parseLimit("7abc", 3));
+        assertEquals(1, GameResource.parseLimit("-4", 3));
+        assertEquals(20, GameResource.parseLimit("99999999999999999999", 3));
+    }
+}
diff --git a/apps/backend-quarkus/src/test/java/app/playmancala/rtmt/RtmtServerTest.java b/apps/backend-quarkus/src/test/java/app/playmancala/rtmt/RtmtServerTest.java
new file mode 100644
index 0000000..0d3d29f
--- /dev/null
+++ b/apps/backend-quarkus/src/test/java/app/playmancala/rtmt/RtmtServerTest.java
@@ -0,0 +1,324 @@
+package app.playmancala.rtmt;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.databind.JsonNode;
+
+import app.playmancala.auth.InvalidTokenException;
+import app.playmancala.auth.TokenVerifier;
+import app.playmancala.loop.ManualScheduler;
+import app.playmancala.model.AuthUser;
+import app.playmancala.model.Channels;
+import app.playmancala.model.Json;
+
+// Ported from the Node backend's tests/rtmt/rtmt_websocket.test.ts, with fake sockets instead of a server.
+class RtmtServerTest {
+    static final String LEGACY_KEY = "0b9f6d1e-7c1a-4f7e-9a51-3f0c2b6d8e11";
+    static final long AUTH_TIMEOUT_MS = 300;
+
+    // Accepts tokens of the form "valid:<id>".
+    static final TokenVerifier FAKE_VERIFIER = token -> {
+        if (!token.startsWith("valid:")) throw new InvalidTokenException("bad token");
+        String id = token.substring("valid:".length());
+        return new AuthUser(id, "name-of-" + id, false);
+    };
+
+    final ManualScheduler loop = new ManualScheduler(0);
+    // Token checks wait here until the test runs them, as they would on a worker thread.
+    final List<Runnable> pendingChecks = new ArrayList<>();
+    final List<String> connectionChanges = new ArrayList<>();
+    RtmtServer server;
+    int socketCount;
+
+    class FakeSocket implements RtmtSocket {
+        final String id = "socket-" + ++socketCount;
+        final String query;
+        final List<JsonNode> received = new ArrayList<>();
+        boolean open = true;
+        Integer closeCode;
+
+        FakeSocket(String query) {
+            this.query = query;
+        }
+
+        @Override
+        public String id() {
+            return id;
+        }
+
+        @Override
+        public String queryParam(String name) {
+            return query != null && query.startsWith(name + "=") ? query.substring(name.length() + 1) : null;
+        }
+
+        @Override
+        public boolean isOpen() {
+            return open;
+        }
+
+        @Override
+        public void send(String text) {
+            try {
+                received.add(Json.parse(text));
+            } catch (Exception e) {
+                throw new IllegalStateException(e);
+            }
+        }
+
+        // Closing also delivers the close event, as the endpoint would.
+        @Override
+        public void close(int code, String reason) {
+            if (!open) return;
+            open = false;
+            closeCode = code;
+            server.onClose(this);
+        }
+
+        void sendText(String text) {
+            server.onText(this, text);
+            runChecks();
+        }
+
+        void send(String channel, Object message) {
+            sendText(Json.write(Map.of("channel", channel, "message", message)));
+        }
+
+        List<JsonNode> messages(String channel) {
+            return received.stream().filter(m -> m.get("channel").asText().equals(channel)).map(m -> m.get("message")).toList();
+        }
+    }
+
+    void start(boolean legacyUserKeyAuth) {
+        server = new RtmtServer(FAKE_VERIFIER, legacyUserKeyAuth, loop, pendingChecks::add, AUTH_TIMEOUT_MS);
+        server.listenOnClientConnectionChange((id, isOnline) -> connectionChanges.add(id + (isOnline ? " online" : " offline")));
+        server.start();
+    }
+
+    void runChecks() {
+        List<Runnable> checks = List.copyOf(pendingChecks);
+        pendingChecks.clear();
+        checks.forEach(Runnable::run);
+    }
+
+    FakeSocket connect(String query) {
+        FakeSocket socket = new FakeSocket(query);
+        server.onOpen(socket);
+        return socket;
+    }
+
+    FakeSocket connectAs(String id) {
+        FakeSocket socket = connect(null);
+        socket.send(Channels.AUTH, Map.of("token", "valid:" + id));
+        assertEquals(1, socket.messages(Channels.ON_AUTH).size());
+        return socket;
+    }
+
+    void disconnect(FakeSocket socket) {
+        socket.open = false;
+        server.onClose(socket);
+    }
+
+    @Test
+    void authenticatesASocketAndRoutesItsMessagesAsThatUser() {
+        start(false);
+        List<String> received = new ArrayList<>();
+        server.listenMessage("hello", (id, message) -> received.add(id + " " + message));
+
+        FakeSocket socket = connect(null);
+        socket.send(Channels.AUTH, Map.of("token", "valid:alice"));
+        assertEquals(List.of(Json.tree(Map.of("user", Map.of("id", "alice", "name", "name-of-alice", "isGuest", false)))),
+                socket.messages(Channels.ON_AUTH));
+
+        socket.send("hello", Map.of("n", 1));
+        assertEquals(List.of("alice {\"n\":1}"), received);
+        assertTrue(server.isClientOnline("alice"));
+        assertEquals("name-of-alice", server.getUser("alice").name());
+        assertEquals(List.of("alice online"), connectionChanges);
+    }
+
+    @Test
+    void sendsMessagesAsChannelAndMessage() {
+        start(false);
+        FakeSocket socket = connectAs("alice");
+        server.sendMessage("alice", "news", "text");
+        assertEquals(Json.tree(Map.of("channel", "news", "message", "text")), socket.received.getLast());
+        assertEquals(List.of("channel", "message"), socket.received.getLast().propertyStream().map(Map.Entry::getKey).toList());
+    }
+
+    @Test
+    void ignoresMessagesBeforeAuthentication() {
+        start(false);
+        List<String> received = new ArrayList<>();
+        server.listenMessage("hello", (id, message) -> received.add(id));
+        connect(null).send("hello", Map.of());
+        assertEquals(List.of(), received);
+    }
+
+    @Test
+    void closesTheSocketOnAnInvalidToken() {
+        start(false);
+        FakeSocket socket = connect(null);
+        socket.send(Channels.AUTH, Map.of("token", "forged"));
+        assertEquals(List.of(Json.tree(Map.of("reason", "invalid token"))), socket.messages(Channels.ON_AUTH_ERROR));
+        assertEquals(Channels.CLOSE_AUTH_FAILED, socket.closeCode);
+    }
+
+    @Test
+    void closesTheSocketWhenATokenIsMissing() {
+        start(false);
+        FakeSocket socket = connect(null);
+        socket.send(Channels.AUTH, Map.of());
+        assertEquals(List.of(Json.tree(Map.of("reason", "missing token"))), socket.messages(Channels.ON_AUTH_ERROR));
+        assertEquals(Channels.CLOSE_AUTH_FAILED, socket.closeCode);
+    }
+
+    @Test
+    void closesTheSocketWhenNoTokenArrivesInTime() {
+        start(false);
+        FakeSocket socket = connect(null);
+        loop.advance(AUTH_TIMEOUT_MS - 1);
+        assertTrue(socket.open);
+        loop.advance(1);
+        assertEquals(List.of(Json.tree(Map.of("reason", "authentication timeout"))), socket.messages(Channels.ON_AUTH_ERROR));
+        assertEquals(Channels.CLOSE_AUTH_FAILED, socket.closeCode);
+    }
+
+    @Test
+    void keepsAnAuthenticatedSocketPastTheTimeout() {
+        start(false);
+        FakeSocket socket = connectAs("alice");
+        loop.advance(AUTH_TIMEOUT_MS * 2);
+        assertNull(socket.closeCode);
+    }
+
+    @Test
+    void acceptsARefreshedTokenForTheSameUserButNotForAnotherUser() {
+        start(false);
+        FakeSocket socket = connectAs("alice");
+        socket.send(Channels.AUTH, Map.of("token", "valid:alice"));
+        assertEquals(2, socket.messages(Channels.ON_AUTH).size());
+        assertEquals(List.of("alice online"), connectionChanges);
+
+        socket.send(Channels.AUTH, Map.of("token", "valid:mallory"));
+        assertEquals(List.of(Json.tree(Map.of("reason", "token belongs to another user"))),
+                socket.messages(Channels.ON_AUTH_ERROR));
+        assertEquals(Channels.CLOSE_AUTH_FAILED, socket.closeCode);
+        assertFalse(server.isClientOnline("alice"));
+    }
+
+    @Test
+    void ignoresATokenCheckThatEndsAfterTheSocketClosed() {
+        start(false);
+        FakeSocket socket = connect(null);
+        server.onText(socket, Json.write(Map.of("channel", Channels.AUTH, "message", Map.of("token", "valid:alice"))));
+        disconnect(socket);
+        runChecks();
+        assertFalse(server.isClientOnline("alice"));
+        assertEquals(List.of(), connectionChanges);
+    }
+
+    @Test
+    void rejectsUserKeyConnectionsWhenLegacyAuthIsOff() {
+        start(false);
+        FakeSocket socket = connect("userKey=" + LEGACY_KEY);
+        assertEquals(Channels.CLOSE_AUTH_FAILED, socket.closeCode);
+    }
+
+    @Test
+    void survivesMalformedMessages() {
+        start(false);
+        FakeSocket socket = connectAs("alice");
+        socket.sendText("{not json");
+        socket.sendText("null");
+        socket.sendText("[1]");
+        socket.sendText("{\"channel\":5}");
+        socket.send(Channels.AUTH, Map.of("token", "valid:alice"));
+        assertEquals(2, socket.messages(Channels.ON_AUTH).size());
+    }
+
+    @Test
+    void deliversMessagesToEverySocketOfTheUser() {
+        start(false);
+        FakeSocket phone = connectAs("alice");
+        FakeSocket laptop = connectAs("alice");
+        server.sendMessage("alice", "news", Map.of("n", 1));
+        assertEquals(List.of(Json.tree(Map.of("n", 1))), phone.messages("news"));
+        assertEquals(List.of(Json.tree(Map.of("n", 1))), laptop.messages("news"));
+        assertEquals(List.of("alice online", "alice online"), connectionChanges);
+    }
+
+    @Test
+    void keepsTheUserOnlineUntilTheLastSocketCloses() {
+        start(false);
+        FakeSocket phone = connectAs("alice");
+        FakeSocket laptop = connectAs("alice");
+
+        disconnect(phone);
+        assertTrue(server.isClientOnline("alice"));
+        assertFalse(connectionChanges.contains("alice offline"));
+
+        disconnect(laptop);
+        assertFalse(server.isClientOnline("alice"));
+        assertNull(server.getUser("alice"));
+        assertTrue(connectionChanges.contains("alice offline"));
+    }
+
+    @Test
+    void doesNotDropAReconnectedSocketWhenTheOldOneClosesLate() {
+        start(false);
+        FakeSocket oldSocket = connectAs("alice");
+        FakeSocket newSocket = connectAs("alice");
+        disconnect(oldSocket);
+        server.sendMessage("alice", "news", Map.of());
+        assertEquals(List.of(Json.tree(Map.of())), newSocket.messages("news"));
+    }
+
+    @Test
+    void pingsEverySocketAndClosesThoseThatDoNotAnswer() {
+        start(false);
+        FakeSocket answering = connectAs("alice");
+        FakeSocket silent = connectAs("bob");
+        loop.advance(RtmtServer.PING_INTERVAL_MS);
+        assertEquals(1, answering.messages(Channels.PING).size());
+        assertEquals(Json.tree(Map.of()), silent.messages(Channels.PING).getFirst());
+
+        answering.send(Channels.PONG, Map.of());
+        loop.advance(RtmtServer.PING_INTERVAL_MS);
+        assertNull(answering.closeCode);
+        assertEquals(1001, silent.closeCode);
+        assertFalse(server.isClientOnline("bob"));
+    }
+
+    @Test
+    void acceptsAUuidUserKey() {
+        start(true);
+        connect("userKey=" + LEGACY_KEY);
+        assertTrue(server.isClientOnline(LEGACY_KEY));
+        assertEquals(new AuthUser(LEGACY_KEY, null, true), server.getUser(LEGACY_KEY));
+    }
+
+    @Test
+    void aUserKeyCannotClaimAKeycloakUsersId() {
+        start(true);
+        FakeSocket socket = connect("userKey=kc:" + LEGACY_KEY);
+        assertEquals(Channels.CLOSE_AUTH_FAILED, socket.closeCode);
+    }
+
+    @Test
+    void aLegacySocketCannotSwitchToATokenIdentity() {
+        start(true);
+        FakeSocket socket = connect("userKey=" + LEGACY_KEY);
+        socket.send(Channels.AUTH, Map.of("token", "valid:kc:alice"));
+        assertEquals(Channels.CLOSE_AUTH_FAILED, socket.closeCode);
+        assertFalse(server.isClientOnline(LEGACY_KEY));
+    }
+}
diff --git a/apps/backend-quarkus/src/test/java/app/playmancala/support/FakeTokenVerifier.java b/apps/backend-quarkus/src/test/java/app/playmancala/support/FakeTokenVerifier.java
new file mode 100644
index 0000000..797682c
--- /dev/null
+++ b/apps/backend-quarkus/src/test/java/app/playmancala/support/FakeTokenVerifier.java
@@ -0,0 +1,21 @@
+package app.playmancala.support;
+
+import app.playmancala.auth.InvalidTokenException;
+import app.playmancala.auth.TokenVerifier;
+import app.playmancala.model.AuthUser;
+import jakarta.annotation.Priority;
+import jakarta.enterprise.inject.Alternative;
+import jakarta.inject.Singleton;
+
+// Replaces Keycloak in @QuarkusTests: accepts tokens of the form "valid:<user id>".
+@Alternative
+@Priority(1)
+@Singleton
+public class FakeTokenVerifier implements TokenVerifier {
+    @Override
+    public AuthUser verify(String token) throws InvalidTokenException {
+        if (!token.startsWith("valid:")) throw new InvalidTokenException("bad token");
+        String id = token.substring("valid:".length());
+        return new AuthUser(id, "name-of-" + id, false);
+    }
+}
diff --git a/apps/backend-quarkus/src/test/java/app/playmancala/support/TestClient.java b/apps/backend-quarkus/src/test/java/app/playmancala/support/TestClient.java
new file mode 100644
index 0000000..89c362a
--- /dev/null
+++ b/apps/backend-quarkus/src/test/java/app/playmancala/support/TestClient.java
@@ -0,0 +1,96 @@
+package app.playmancala.support;
+
+import static org.awaitility.Awaitility.await;
+
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.WebSocket;
+import java.time.Duration;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionStage;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import com.fasterxml.jackson.databind.JsonNode;
+
+import app.playmancala.model.Channels;
+import app.playmancala.model.Json;
+
+// A WebSocket client speaking the game protocol; answers pings like the real clients.
+public class TestClient implements WebSocket.Listener, AutoCloseable {
+    public final List<JsonNode> messages = new CopyOnWriteArrayList<>();
+    public final CompletableFuture<Integer> closed = new CompletableFuture<>();
+    private final StringBuilder partial = new StringBuilder();
+    private final WebSocket webSocket;
+
+    public TestClient(URI uri) {
+        webSocket = HttpClient.newHttpClient().newWebSocketBuilder().buildAsync(uri, this).join();
+    }
+
+    // Connects and logs in as the user, with a token the FakeTokenVerifier accepts.
+    public static TestClient connectAs(URI uri, String userId) {
+        TestClient client = new TestClient(uri);
+        client.send(Channels.AUTH, Map.of("token", "valid:" + userId));
+        client.waitFor(Channels.ON_AUTH);
+        return client;
+    }
+
+    public void send(String channel, Object message) {
+        webSocket.sendText(Json.write(Map.of("channel", channel, "message", message)), true).join();
+    }
+
+    public List<JsonNode> messages(String channel) {
+        return messages.stream().filter(m -> m.get("channel").asText().equals(channel)).map(m -> m.get("message")).toList();
+    }
+
+    // The first message on the channel, waiting up to 10 seconds for it.
+    public JsonNode waitFor(String channel) {
+        return waitFor(channel, 1).getFirst();
+    }
+
+    public List<JsonNode> waitFor(String channel, int count) {
+        await().atMost(Duration.ofSeconds(10)).until(() -> messages(channel).size() >= count);
+        return messages(channel);
+    }
+
+    @Override
+    public void onOpen(WebSocket webSocket) {
+        webSocket.request(1);
+    }
+
+    @Override
+    public CompletionStage<?> onText(WebSocket webSocket, CharSequence data, boolean last) {
+        partial.append(data);
+        if (last) {
+            try {
+                JsonNode message = Json.parse(partial.toString());
+                if (message.get("channel").asText().equals(Channels.PING)) {
+                    webSocket.sendText(Json.write(Map.of("channel", Channels.PONG, "message", Map.of())), true);
+                }
+                messages.add(message);
+            } catch (Exception e) {
+                throw new IllegalStateException(e);
+            }
+            partial.setLength(0);
+        }
+        webSocket.request(1);
+        return null;
+    }
+
+    @Override
+    public CompletionStage<?> onClose(WebSocket webSocket, int statusCode, String reason) {
+        closed.complete(statusCode);
+        return null;
+    }
+
+    @Override
+    public void onError(WebSocket webSocket, Throwable error) {
+        closed.completeExceptionally(error);
+    }
+
+    @Override
+    public void close() {
+        if (!webSocket.isOutputClosed()) webSocket.sendClose(WebSocket.NORMAL_CLOSURE, "").join();
+    }
+}
diff --git a/docs/specs/quarkus-backend/tasks.md b/docs/specs/quarkus-backend/tasks.md
index 653c7ec..d30c6a5 100644
--- a/docs/specs/quarkus-backend/tasks.md
+++ b/docs/specs/quarkus-backend/tasks.md
@@ -12,7 +12,7 @@ Work through the tasks in order; each ends with its tests passing and a commit.
       unit tests.
 - [x] 5. **Storage**: migrations and their runner, `PostgresGameHistoryRepository`, `GameHistoryService`,
       `ValkeyLiveGameStore`, `ValkeyCache`; integration tests with Dev Services.
-- [ ] 6. **WebSocket and HTTP**: `RtmtServer` with unit tests, `RtmtEndpoint`, the HTTP resources, `Backend`
+- [x] 6. **WebSocket and HTTP**: `RtmtServer` with unit tests, `RtmtEndpoint`, the HTTP resources, `Backend`
       wiring and startup, configuration; `@QuarkusTest`s for the API and the WebSocket.
 - [ ] 7. **Replace the Node backend**: move the project to `apps/backend`, Dockerfile, `.dockerignore`, compose,
       remove the Node sources; build the image.