diff --git a/src/main/java/org/asamk/signal/http/HttpServerHandler.java b/src/main/java/org/asamk/signal/http/HttpServerHandler.java
index 1e62c3de..325b0eec 100644
--- a/src/main/java/org/asamk/signal/http/HttpServerHandler.java
+++ b/src/main/java/org/asamk/signal/http/HttpServerHandler.java
@@ -169,6 +169,12 @@ public class HttpServerHandler implements AutoCloseable {
             httpExchange.sendResponseHeaders(200, 0);
             final var sender = new ServerSentEventSender(httpExchange.getResponseBody());
 
+            // Flush HTTP response headers to the client immediately.
+            // Without this, the JVM HttpServer buffers everything until a later write
+            // in the keep-alive loop (15 s), causing clients with shorter timeouts
+            // (e.g. 10 s) to abort before receiving the initial response.
+            httpExchange.getResponseBody().flush();
+
             final var shouldStop = new AtomicBoolean(false);
             final var handlers = subscribeReceiveHandlers(managers, sender, () -> {
                 shouldStop.set(true);
diff --git a/src/test/java/org/asamk/signal/http/SseInitialFlushTest.java b/src/test/java/org/asamk/signal/http/SseInitialFlushTest.java
new file mode 100644
index 00000000..77e59dbd
--- /dev/null
+++ b/src/test/java/org/asamk/signal/http/SseInitialFlushTest.java
@@ -0,0 +1,537 @@
+package org.asamk.signal.http;
+
+import org.asamk.signal.manager.Manager;
+import org.asamk.signal.manager.api.CallInfo;
+import org.asamk.signal.manager.api.CallOffer;
+import org.asamk.signal.manager.api.Configuration;
+import org.asamk.signal.manager.api.Device;
+import org.asamk.signal.manager.api.DeviceLinkUrl;
+import org.asamk.signal.manager.api.Group;
+import org.asamk.signal.manager.api.GroupId;
+import org.asamk.signal.manager.api.GroupInviteLinkUrl;
+import org.asamk.signal.manager.api.Identity;
+import org.asamk.signal.manager.api.IdentityVerificationCode;
+import org.asamk.signal.manager.api.Message;
+import org.asamk.signal.manager.api.MessageEnvelope;
+import org.asamk.signal.manager.api.Pair;
+import org.asamk.signal.manager.api.ReceiveConfig;
+import org.asamk.signal.manager.api.Recipient;
+import org.asamk.signal.manager.api.RecipientIdentifier;
+import org.asamk.signal.manager.api.SendGroupMessageResults;
+import org.asamk.signal.manager.api.SendMessageResult;
+import org.asamk.signal.manager.api.SendMessageResults;
+import org.asamk.signal.manager.api.StickerPack;
+import org.asamk.signal.manager.api.StickerPackId;
+import org.asamk.signal.manager.api.StickerPackUrl;
+import org.asamk.signal.manager.api.TurnServer;
+import org.asamk.signal.manager.api.TypingAction;
+import org.asamk.signal.manager.api.UpdateGroup;
+import org.asamk.signal.manager.api.UpdateProfile;
+import org.asamk.signal.manager.api.UserStatus;
+import org.asamk.signal.manager.api.UsernameLinkUrl;
+import org.asamk.signal.manager.api.UsernameStatus;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.InputStream;
+import java.net.HttpURLConnection;
+import java.net.InetSocketAddress;
+import java.net.ServerSocket;
+import java.net.URL;
+import java.time.Duration;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Regression test for the SSE initial-flush bug:
+ * HttpServerHandler used to flush the initial SSE response only after a later
+ * write in the 15-second keep-alive loop, meaning the HTTP response headers
+ * were not flushed to the client until then.
+ * Clients with a shorter connection timeout (e.g. 10 s) would time out before
+ * receiving the initial response.
+ *
+ * This test verifies that the endpoint returns HTTP 200 within 2 seconds of
+ * connecting to GET /api/v1/events.
+ */
+class SseInitialFlushTest {
+
+    private HttpServerHandler handler;
+    private int port;
+
+    /** Finds a free local port. */
+    private static int freePort() throws Exception {
+        try (var ss = new ServerSocket(0)) {
+            ss.setReuseAddress(true);
+            return ss.getLocalPort();
+        }
+    }
+
+    @BeforeEach
+    void setUp() throws Exception {
+        port = freePort();
+        handler = new HttpServerHandler(new InetSocketAddress("127.0.0.1", port), new MinimalStubManager());
+        handler.init();
+    }
+
+    @AfterEach
+    void tearDown() {
+        if (handler != null) {
+            handler.close();
+        }
+    }
+
+    /**
+     * The SSE endpoint MUST flush the initial HTTP response immediately upon
+     * connection, before the 15-second keep-alive loop fires. A read timeout of
+     * 2 000 ms is used — well below the 15-second wait interval but generous
+     * enough to survive any CI scheduling jitter.
+     */
+    @Test
+    void sseEndpointReturnsHeadersWithinTwoSeconds() {
+        assertDoesNotThrow(() -> {
+            var url = new URL("http://127.0.0.1:" + port + "/api/v1/events");
+            var conn = (HttpURLConnection) url.openConnection();
+            conn.setRequestProperty("Accept", "text/event-stream");
+            conn.setReadTimeout(2_000);   // 2 s — fails before fix (15 s flush), passes after
+            conn.setConnectTimeout(2_000);
+            try {
+                conn.connect();
+                assertEquals(200, conn.getResponseCode());
+            } finally {
+                conn.disconnect();
+            }
+        }, "SSE endpoint did not return the initial response within 2 seconds");
+    }
+
+    // -------------------------------------------------------------------------
+    // Minimal Manager stub — only receive-handler methods need real behaviour;
+    // everything else is a no-op stub.
+    // -------------------------------------------------------------------------
+
+    private static final class MinimalStubManager implements Manager {
+
+        @Override
+        public String getSelfNumber() {
+            return "+10000000000";
+        }
+
+        @Override
+        public void addReceiveHandler(ReceiveMessageHandler handler, boolean isWeakListener) {
+            // no-op
+        }
+
+        @Override
+        public void removeReceiveHandler(ReceiveMessageHandler handler) {
+            // no-op
+        }
+
+        @Override
+        public boolean isReceiving() {
+            return false;
+        }
+
+        @Override
+        public void receiveMessages(Optional<Duration> timeout, Optional<Integer> maxMessages, ReceiveMessageHandler handler) {
+        }
+
+        @Override
+        public void stopReceiveMessages() {
+        }
+
+        @Override
+        public void setReceiveConfig(ReceiveConfig receiveConfig) {
+        }
+
+        @Override
+        public Map<String, UserStatus> getUserStatus(Set<String> numbers) {
+            return Map.of();
+        }
+
+        @Override
+        public Map<String, UsernameStatus> getUsernameStatus(Set<String> usernames) {
+            return Map.of();
+        }
+
+        @Override
+        public void updateAccountAttributes(String deviceName, Boolean unidentifiedDeliveryIndicators, Boolean discoverableByNumber, Boolean numberSharing) {
+        }
+
+        @Override
+        public Configuration getConfiguration() {
+            return null;
+        }
+
+        @Override
+        public void updateConfiguration(Configuration configuration) {
+        }
+
+        @Override
+        public void updateProfile(UpdateProfile updateProfile) {
+        }
+
+        @Override
+        public String getUsername() {
+            return null;
+        }
+
+        @Override
+        public UsernameLinkUrl getUsernameLink() {
+            return null;
+        }
+
+        @Override
+        public void setUsername(String username) {
+        }
+
+        @Override
+        public void deleteUsername() {
+        }
+
+        @Override
+        public void startChangeNumber(String newNumber, boolean voiceVerification, String captcha) {
+        }
+
+        @Override
+        public void finishChangeNumber(String newNumber, String verificationCode, String pin) {
+        }
+
+        @Override
+        public void unregister() {
+        }
+
+        @Override
+        public void deleteAccount() {
+        }
+
+        @Override
+        public void submitRateLimitRecaptchaChallenge(String challenge, String captcha) {
+        }
+
+        @Override
+        public List<Device> getLinkedDevices() {
+            return List.of();
+        }
+
+        @Override
+        public void updateLinkedDevice(int deviceId, String name) {
+        }
+
+        @Override
+        public void removeLinkedDevices(int deviceId) {
+        }
+
+        @Override
+        public void addDeviceLink(DeviceLinkUrl deviceLinkUrl) {
+        }
+
+        @Override
+        public void setRegistrationLockPin(Optional<String> pin) {
+        }
+
+        @Override
+        public List<Group> getGroups() {
+            return List.of();
+        }
+
+        @Override
+        public List<Group> getGroups(Collection<GroupId> groupIds) {
+            return List.of();
+        }
+
+        @Override
+        public SendGroupMessageResults quitGroup(GroupId groupId, Set<RecipientIdentifier.Single> administrators) {
+            return null;
+        }
+
+        @Override
+        public void deleteGroup(GroupId groupId) {
+        }
+
+        @Override
+        public Pair<GroupId, SendGroupMessageResults> createGroup(String name, Set<RecipientIdentifier.Single> members, String avatarFile) {
+            return null;
+        }
+
+        @Override
+        public SendGroupMessageResults updateGroup(GroupId groupId, UpdateGroup updateGroup) {
+            return null;
+        }
+
+        @Override
+        public Pair<GroupId, SendGroupMessageResults> joinGroup(GroupInviteLinkUrl inviteLinkUrl) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendTypingMessage(TypingAction action, Set<RecipientIdentifier> recipients) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendReadReceipt(RecipientIdentifier.Single sender, List<Long> messageIds) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendViewedReceipt(RecipientIdentifier.Single sender, List<Long> messageIds) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendMessage(Message message, Set<RecipientIdentifier> recipients, boolean notifySelf) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendEditMessage(Message message, Set<RecipientIdentifier> recipients, long targetSentTimestamp) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendRemoteDeleteMessage(long targetSentTimestamp, Set<RecipientIdentifier> recipients) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendMessageReaction(String emoji, boolean remove, RecipientIdentifier.Single targetAuthor, long targetSentTimestamp, Set<RecipientIdentifier> recipients, boolean notifySelf, boolean story) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendAdminDelete(RecipientIdentifier.Single targetAuthor, long targetSentTimestamp, Set<RecipientIdentifier.Group> recipients, boolean notifySelf, boolean story) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendPinMessage(int duration, RecipientIdentifier.Single targetAuthor, long targetSentTimestamp, Set<RecipientIdentifier> recipients, boolean notifySelf, boolean story) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendUnpinMessage(RecipientIdentifier.Single targetAuthor, long targetSentTimestamp, Set<RecipientIdentifier> recipients, boolean notifySelf, boolean story) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendPaymentNotificationMessage(byte[] receipt, String note, RecipientIdentifier.Single recipient) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendEndSessionMessage(Set<RecipientIdentifier.Single> recipients) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendMessageRequestResponse(MessageEnvelope.Sync.MessageRequestResponse.Type type, Set<RecipientIdentifier> recipients) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendPollCreateMessage(String question, boolean multipleChoice, List<String> options, Set<RecipientIdentifier> recipients, boolean notifySelf) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendPollVoteMessage(RecipientIdentifier.Single author, long timestamp, List<Integer> optionIds, int version, Set<RecipientIdentifier> recipients, boolean notifySelf) {
+            return null;
+        }
+
+        @Override
+        public SendMessageResults sendPollTerminateMessage(long timestamp, Set<RecipientIdentifier> recipients, boolean notifySelf) {
+            return null;
+        }
+
+        @Override
+        public void hideRecipient(RecipientIdentifier.Single recipient) {
+        }
+
+        @Override
+        public void deleteRecipient(RecipientIdentifier.Single recipient) {
+        }
+
+        @Override
+        public void deleteContact(RecipientIdentifier.Single recipient) {
+        }
+
+        @Override
+        public void setContactName(RecipientIdentifier.Single recipient, String givenName, String familyName, String newGivenName, String newFamilyName, String nick) {
+        }
+
+        @Override
+        public void setContactsBlocked(Collection<RecipientIdentifier.Single> recipients, boolean blocked) {
+        }
+
+        @Override
+        public void setGroupsBlocked(Collection<GroupId> groupIds, boolean blocked) {
+        }
+
+        @Override
+        public void setExpirationTimer(RecipientIdentifier.Single recipient, int messageExpirationTimer) {
+        }
+
+        @Override
+        public StickerPackUrl uploadStickerPack(File path) {
+            return null;
+        }
+
+        @Override
+        public void installStickerPack(StickerPackUrl url) {
+        }
+
+        @Override
+        public List<StickerPack> getStickerPacks() {
+            return List.of();
+        }
+
+        @Override
+        public void requestAllSyncData() {
+        }
+
+        @Override
+        public boolean isContactBlocked(RecipientIdentifier.Single recipient) {
+            return false;
+        }
+
+        @Override
+        public void sendContacts() {
+        }
+
+        @Override
+        public List<Recipient> getRecipients(boolean onlyWithProfile, Optional<Boolean> blocked, Collection<RecipientIdentifier.Single> addresses, Optional<String> name) {
+            return List.of();
+        }
+
+        @Override
+        public String getContactOrProfileName(RecipientIdentifier.Single recipient) {
+            return null;
+        }
+
+        @Override
+        public Group getGroup(GroupId groupId) {
+            return null;
+        }
+
+        @Override
+        public List<Identity> getIdentities() {
+            return List.of();
+        }
+
+        @Override
+        public List<Identity> getIdentities(RecipientIdentifier.Single recipient) {
+            return List.of();
+        }
+
+        @Override
+        public boolean trustIdentityVerified(RecipientIdentifier.Single recipient, IdentityVerificationCode verificationCode) {
+            return false;
+        }
+
+        @Override
+        public boolean trustIdentityAllKeys(RecipientIdentifier.Single recipient) {
+            return false;
+        }
+
+        @Override
+        public void addAddressChangedListener(Runnable listener) {
+        }
+
+        @Override
+        public void addClosedListener(Runnable listener) {
+        }
+
+        @Override
+        public InputStream retrieveAttachment(String id) {
+            return null;
+        }
+
+        @Override
+        public InputStream retrieveContactAvatar(RecipientIdentifier.Single recipient) {
+            return null;
+        }
+
+        @Override
+        public InputStream retrieveProfileAvatar(RecipientIdentifier.Single recipient) {
+            return null;
+        }
+
+        @Override
+        public InputStream retrieveGroupAvatar(GroupId groupId) {
+            return null;
+        }
+
+        @Override
+        public InputStream retrieveSticker(StickerPackId stickerPackId, int stickerId) {
+            return null;
+        }
+
+        @Override
+        public CallInfo startCall(RecipientIdentifier.Single recipient) {
+            return null;
+        }
+
+        @Override
+        public CallInfo acceptCall(long callId) {
+            return null;
+        }
+
+        @Override
+        public void hangupCall(long callId) {
+        }
+
+        @Override
+        public SendMessageResult rejectCall(long callId) {
+            return null;
+        }
+
+        @Override
+        public List<CallInfo> listActiveCalls() {
+            return List.of();
+        }
+
+        @Override
+        public void sendCallOffer(RecipientIdentifier.Single recipient, CallOffer callOffer) {
+        }
+
+        @Override
+        public void sendCallAnswer(RecipientIdentifier.Single recipient, long callId, byte[] answer) {
+        }
+
+        @Override
+        public void sendIceUpdate(RecipientIdentifier.Single recipient, long callId, List<byte[]> iceCandidates) {
+        }
+
+        @Override
+        public void sendHangup(RecipientIdentifier.Single recipient, long callId, MessageEnvelope.Call.Hangup.Type type) {
+        }
+
+        @Override
+        public void sendBusy(RecipientIdentifier.Single recipient, long callId) {
+        }
+
+        @Override
+        public List<TurnServer> getTurnServerInfo() {
+            return List.of();
+        }
+
+        @Override
+        public void close() {
+        }
+
+        @Override
+        public void addCallEventListener(CallEventListener listener) {
+        }
+
+        @Override
+        public void removeCallEventListener(CallEventListener listener) {
+        }
+    }
+}