🚧 Update topic explorer

backend/app/routers/topics.py

@@ -116,7 +116,10 @@ async def stream(session_id: str, user: OperatorRole):
         raise HTTPException(status.HTTP_404_NOT_FOUND, "session not found") from exc
 
     async def event_stream():
-        index = 0
+        # Start BELOW the first index: messages_since uses strictly-
+        # greater semantics, and a fresh session's first message has
+        # index 0 — a cursor starting at 0 would drop it.
+        index = -1
         heartbeat = 0.0
         try:
             while True:

backend/tests/conftest.py

@@ -256,6 +256,7 @@ def client(db_session, monkeypatch):
             self.id = id
             self.username = username
             self.topic_filter = topic_filter
+            self.buffer = []  # list of message dicts, index-tagged
 
     class FakeSessions:
         def __init__(self):
@@ -281,7 +282,8 @@ def client(db_session, monkeypatch):
     monkeypatch.setattr(explorer_module, "sessions", fake_sessions)
 
     def fake_messages_since(session_id, index):
-        return []
+        session = fake_sessions.get(session_id)
+        return [dict(m) for m in session.buffer if m["index"] > index]
 
     def fake_list_retained(max_results=200, timeout=3.0):
         return [

backend/tests/test_api.py

@@ -283,6 +283,58 @@ def test_topic_explorer_flow(admin_client):
     assert any(m["topic"] == "site/config/level" for m in retained.json())
 
 
+def test_topic_explorer_stream_delivers_first_message(admin_client, monkeypatch):
+    """Regression test: the SSE cursor must include a session's FIRST
+    buffered message (index 0). messages_since is strictly-greater, so
+    a cursor starting at 0 dropped it — subscribe, publish {"cmd":"start"}
+    then {"cmd":"stop"}, and the stream must deliver BOTH."""
+    from app import explorer as explorer_module
+
+    sub = admin_client.post("/api/v1/topics/subscribe", json={"topic_filter": "e2e/#"})
+    assert sub.status_code == 200
+    session_id = sub.json()["session_id"]
+
+    session = explorer_module.sessions.get(session_id)
+    # Simulate the broker delivering both payloads (as _on_message would).
+    session.buffer = [
+        {
+            "index": 0,
+            "topic": "e2e/diff/device1",
+            "qos": 1,
+            "retain": False,
+            "received_at": 1.0,
+            "payload_b64": "eyJjbWQiOiAic3RhcnQifQ==",
+            "payload_text": '{"cmd": "start"}',
+            "properties": None,
+        },
+        {
+            "index": 1,
+            "topic": "e2e/diff/device1",
+            "qos": 1,
+            "retain": False,
+            "received_at": 2.0,
+            "payload_b64": "eyJjbWQiOiAic3RvcCJ9",
+            "payload_text": '{"cmd": "stop"}',
+            "properties": None,
+        },
+    ]
+
+    with admin_client.stream("GET", f"/api/v1/topics/stream/{session_id}") as response:
+        assert response.status_code == 200
+        assert response.headers["content-type"].startswith("text/event-stream")
+        received = []
+        for line in response.iter_lines():
+            if line.startswith("data:"):
+                received.append(json.loads(line[len("data:"):].strip()))
+            if len(received) == 2:
+                break
+
+    assert [m["payload_text"] for m in received] == [
+        '{"cmd": "start"}',
+        '{"cmd": "stop"}',
+    ], "stream must deliver the session's first message (index 0), not drop it"
+
+
 def test_topic_explorer_publish_audited(admin_client, db_session):
     """Publishes through the test console are audited (spec section 35),
     and wildcards are rejected."""