From af93e3382f23d67e40e9cb07aaf547a9974156af Mon Sep 17 00:00:00 2001
From: Lukasz Raczylo <lukasz@raczylo.com>
Date: Thu, 20 Aug 2026 02:40:51 +0100
Subject: [PATCH] fix(sse): gate event dispatch on data presence, not event:
 alone

---
 sse.go      | 17 +++++++++++------
 sse_test.go | 22 ++++++++++++++++++++++
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/sse.go b/sse.go
index b612488..eb315a4 100644
--- a/sse.go
+++ b/sse.go
@@ -51,15 +51,21 @@ func readSSE(r io.Reader, fn func(sseEvent) error) error {
 	var (
 		event     string
 		dataLines []string
-		pending   bool
+		hasData   bool
 	)
 
+	// dispatch fires fn on a blank line, but only when at least one
+	// "data:" line was accumulated — matching the EventSource spec, an
+	// "event:" field alone (no data) discards silently rather than
+	// producing a data-less event. It always resets the accumulated
+	// fields, dispatched or not.
 	dispatch := func() error {
-		if !pending {
+		fire := hasData
+		ev := sseEvent{event: event, data: []byte(strings.Join(dataLines, "\n"))}
+		event, dataLines, hasData = "", nil, false
+		if !fire {
 			return nil
 		}
-		ev := sseEvent{event: event, data: []byte(strings.Join(dataLines, "\n"))}
-		event, dataLines, pending = "", nil, false
 		return fn(ev)
 	}
 
@@ -80,10 +86,9 @@ func readSSE(r io.Reader, fn func(sseEvent) error) error {
 		switch field {
 		case "event":
 			event = value
-			pending = true
 		case "data":
 			dataLines = append(dataLines, value)
-			pending = true
+			hasData = true
 		case "id", "retry":
 			// recognized fields, intentionally ignored
 		}
diff --git a/sse_test.go b/sse_test.go
index 10b3046..113ff02 100644
--- a/sse_test.go
+++ b/sse_test.go
@@ -177,6 +177,28 @@ func TestReadSSE_BlankLinesWithoutFieldsDoNotDispatch(t *testing.T) {
 	}
 }
 
+// TestReadSSE_EventFieldAloneDoesNotDispatch proves an "event:" field with
+// no "data:" line does not produce a data-less event on the blank line
+// that follows it — matching the EventSource spec, which discards an
+// event whose data buffer is empty even when the event type was set.
+func TestReadSSE_EventFieldAloneDoesNotDispatch(t *testing.T) {
+	raw := "event: ping\n\ndata: real\n\n"
+
+	var got []sseEvent
+	err := readSSE(strings.NewReader(raw), func(ev sseEvent) error {
+		got = append(got, ev)
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("readSSE: %v", err)
+	}
+
+	want := []sseEvent{{event: "", data: []byte("real")}}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+}
+
 // TestNewSSEWriter_SetsHeaders proves newSSEWriter sets the three SSE
 // response headers on construction, before any body write.
 func TestNewSSEWriter_SetsHeaders(t *testing.T) {