diff --git a/internal/server/server.go b/internal/server/server.go
index c7b965a..79d2ab0 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -253,6 +253,8 @@ func (s *Server) Handler() http.Handler {
 
 // ---- SSE：游标重放 + 在线订阅 + 终态对账由 Query 承担 ----
 
+const sseRetryMillis = 1000
+
 func (s *Server) events(w http.ResponseWriter, r *http.Request) {
 	conversationID := r.URL.Query().Get("conversationId")
 	isProject := false
@@ -291,6 +293,9 @@ func (s *Server) events(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 	}
+	// 浏览器 EventSource 断线后按这个间隔重连，并带上 Last-Event-ID。
+	fmt.Fprintf(w, "retry: %d\n\n", sseRetryMillis)
+	flusher.Flush()
 	since := int64(0)
 	if ids := r.Header.Get("Last-Event-ID"); ids != "" {
 		since, _ = strconv.ParseInt(ids, 10, 64)
diff --git a/internal/server/sse_retry_test.go b/internal/server/sse_retry_test.go
new file mode 100644
index 0000000..c8c6e93
--- /dev/null
+++ b/internal/server/sse_retry_test.go
@@ -0,0 +1,119 @@
+package server
+
+import (
+	"bufio"
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"agent-system-backend/internal/bus"
+	core "agent-system-core"
+)
+
+func TestSSERetryAndLiveDelivery(t *testing.T) {
+	srv, _ := newTestServer(t)
+	h := srv.Handler()
+	_, convID := setupConversation(t, h)
+	b := bus.New()
+	srv.Subscribe = func(cid string) (<-chan core.StreamEvent, func()) { return b.Subscribe(cid) }
+
+	ts := httptest.NewServer(h)
+	t.Cleanup(ts.Close)
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	t.Cleanup(cancel)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, ts.URL+"/api/events?conversationId="+convID, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { resp.Body.Close() })
+	reader := bufio.NewReader(resp.Body)
+	head := readUntil(t, reader, "retry: 1000")
+	if !strings.Contains(head, "retry: 1000") {
+		t.Fatalf("missing retry: %s", head)
+	}
+	b.Publish(core.StreamEvent{
+		Seq: 7, ConversationID: core.ID(convID), TS: time.Now().UTC().Format(time.RFC3339), V: 1,
+		Type: core.EvMessageDelta, Payload: []byte(`{"index":0,"text":"hi"}`),
+	})
+	body := readUntil(t, reader, "event: message.delta")
+	if !strings.Contains(body, "id: 7") {
+		t.Fatalf("live event: %s", body)
+	}
+}
+
+func TestSSEReconnectFillsOverflowGap(t *testing.T) {
+	srv, st := newTestServer(t)
+	h := srv.Handler()
+	_, convID := setupConversation(t, h)
+	evs := []core.StreamEvent{
+		{ConversationID: core.ID(convID), TS: time.Now().UTC().Format(time.RFC3339), V: 1, Type: core.EvMessageDelta, Payload: []byte(`{"index":0,"text":"a"}`)},
+		{ConversationID: core.ID(convID), TS: time.Now().UTC().Format(time.RFC3339), V: 1, Type: core.EvMessageDelta, Payload: []byte(`{"index":0,"text":"b"}`)},
+		{ConversationID: core.ID(convID), TS: time.Now().UTC().Format(time.RFC3339), V: 1, Type: core.EvMessageDelta, Payload: []byte(`{"index":0,"text":"c"}`)},
+	}
+	if _, err := st.AppendEvents(convID, evs); err != nil {
+		t.Fatal(err)
+	}
+	b := bus.NewWithBuffer(1)
+	ch, stop := b.Subscribe(convID)
+	b.Publish(evs[1])
+	b.Publish(evs[2])
+	closed := false
+	for i := 0; i < 3; i++ {
+		_, ok := <-ch
+		if !ok {
+			closed = true
+			break
+		}
+	}
+	if !closed {
+		t.Fatal("overflow must close the subscription")
+	}
+	stop()
+
+	ts := httptest.NewServer(h)
+	t.Cleanup(ts.Close)
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	t.Cleanup(cancel)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, ts.URL+"/api/events?conversationId="+convID, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req.Header.Set("Last-Event-ID", "1")
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { resp.Body.Close() })
+	reader := bufio.NewReader(resp.Body)
+	body := readUntil(t, reader, "id: 3")
+	if !strings.Contains(body, "id: 2") || !strings.Contains(body, "id: 3") {
+		t.Fatalf("gap not filled: %s", body)
+	}
+	if strings.Contains(body, "id: 1\n") {
+		t.Fatalf("replayed the event the client already had: %s", body)
+	}
+}
+
+func readUntil(t *testing.T, r *bufio.Reader, needle string) string {
+	t.Helper()
+	var b strings.Builder
+	for i := 0; i < 40; i++ {
+		line, err := r.ReadString('\n')
+		b.WriteString(line)
+		if strings.Contains(b.String(), needle) {
+			return b.String()
+		}
+		if err != nil {
+			t.Fatalf("read until %q: %v\n%s", needle, err, b.String())
+		}
+	}
+	t.Fatalf("gave up waiting for %q\n%s", needle, b.String())
+	return ""
+}