diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3351ace3..ce8e7d23 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,8 @@
 
 - Messaging/session display merges now preserve sidecar transcript order when the sidecar already contains at least as many rows as the mirrored state store, avoiding role/content fallback sorting when timestamp precision collapses.
 
+- Gateway-backed turns and compacted/reconciled message batches now keep subsecond timestamp ordering instead of assigning the same integer-second timestamp to multiple transcript rows.
+
 ## [v0.51.153] — 2026-05-28 — Release DY (stage-batch35 — 11-PR low-risk cleanup: title-language + clarify SSE + upload filename + discoverability + SSE reconnect + gateway image + docker docs)
 
 ### Changed
diff --git a/api/gateway_chat.py b/api/gateway_chat.py
index d3a0aad0..25818084 100644
--- a/api/gateway_chat.py
+++ b/api/gateway_chat.py
@@ -297,11 +297,16 @@ def _run_gateway_chat_streaming(
             s = get_session(session_id)
             if not _stream_writeback_is_current(s, stream_id):
                 return
-            now = int(time.time())
+            now = time.time()
+            # Preserve subsecond ordering for gateway-backed turns. Using an
+            # integer seconds timestamp gives the user and assistant rows the
+            # same sort key; later transcript merges can then fall back to
+            # role/content ordering instead of turn order.
+            assistant_ts = now + 0.000001
             user_msg = {"role": "user", "content": str(msg_text or ""), "timestamp": now}
             if attachments:
                 user_msg["attachments"] = list(attachments)
-            assistant_msg = {"role": "assistant", "content": assistant_text, "timestamp": now}
+            assistant_msg = {"role": "assistant", "content": assistant_text, "timestamp": assistant_ts}
             previous_context = list(getattr(s, "context_messages", None) or getattr(s, "messages", None) or [])
             s.context_messages = previous_context + [user_msg, assistant_msg]
             display = list(getattr(s, "messages", None) or [])
diff --git a/api/streaming.py b/api/streaming.py
index 669b6e55..df515511 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -2982,6 +2982,22 @@ def _merge_display_messages_after_agent_result(previous_display, previous_contex
     return merged
 
 
+def _stamp_missing_message_timestamps(messages, *, now: float | None = None) -> int:
+    """Stamp missing message timestamps without collapsing transcript order.
+
+    Compacted/reconciled rows can arrive without timestamps. Assigning one
+    integer seconds value to the whole batch makes later timestamp-based display
+    merges unstable; use a subsecond sequence instead.
+    """
+    base = time.time() if now is None else float(now)
+    stamped = 0
+    for msg in messages or []:
+        if isinstance(msg, dict) and not msg.get('timestamp') and not msg.get('_ts'):
+            msg['timestamp'] = base + (stamped * 0.000001)
+            stamped += 1
+    return stamped
+
+
 def _assistant_reply_added_after_current_turn(result_messages, previous_context, msg_text) -> bool:
     """Return True only when the just-finished turn produced assistant text."""
     result_messages = list(result_messages or [])
@@ -5298,11 +5314,9 @@ def _run_agent_streaming(
                         'usage': _live_usage_snapshot(),
                     })
 
-                # Stamp 'timestamp' on any messages that don't have one yet
-                _now = time.time()
-                for _m in s.messages:
-                    if isinstance(_m, dict) and not _m.get('timestamp') and not _m.get('_ts'):
-                        _m['timestamp'] = int(_now)
+                # Stamp 'timestamp' on any messages that don't have one yet,
+                # preserving transcript order across compacted/reconciled batches.
+                _stamp_missing_message_timestamps(s.messages)
                 # Only auto-generate title when still default; preserves user renames
                 if s.title == 'Untitled' or s.title == 'New Chat' or not s.title:
                     s.title = title_from(s.messages, s.title)
diff --git a/tests/test_message_timestamp_stamping.py b/tests/test_message_timestamp_stamping.py
new file mode 100644
index 00000000..1a4b50f6
--- /dev/null
+++ b/tests/test_message_timestamp_stamping.py
@@ -0,0 +1,29 @@
+from api.streaming import _stamp_missing_message_timestamps
+
+
+def test_stamp_missing_message_timestamps_uses_subsecond_sequence():
+    messages = [
+        {"role": "user", "content": "one"},
+        {"role": "assistant", "content": "two"},
+        {"role": "user", "content": "three"},
+    ]
+
+    stamped = _stamp_missing_message_timestamps(messages, now=1000.0)
+
+    assert stamped == 3
+    assert [m["timestamp"] for m in messages] == [1000.0, 1000.000001, 1000.000002]
+
+
+def test_stamp_missing_message_timestamps_preserves_existing_timestamp_metadata():
+    messages = [
+        {"role": "user", "content": "old", "timestamp": 900.0},
+        {"role": "assistant", "content": "synthetic", "_ts": 901.0},
+        {"role": "user", "content": "new"},
+    ]
+
+    stamped = _stamp_missing_message_timestamps(messages, now=1000.0)
+
+    assert stamped == 1
+    assert messages[0]["timestamp"] == 900.0
+    assert "timestamp" not in messages[1]
+    assert messages[2]["timestamp"] == 1000.0
diff --git a/tests/test_webui_gateway_chat_backend.py b/tests/test_webui_gateway_chat_backend.py
index 29b55288..4b4fa414 100644
--- a/tests/test_webui_gateway_chat_backend.py
+++ b/tests/test_webui_gateway_chat_backend.py
@@ -224,6 +224,9 @@ def test_gateway_chat_worker_translates_sse_and_persists_session(tmp_path, monke
     saved = models.get_session(s.session_id)
     assert [m["role"] for m in saved.messages] == ["user", "assistant"]
     assert saved.messages[-1]["content"] == "hello"
+    assert isinstance(saved.messages[0]["timestamp"], float)
+    assert isinstance(saved.messages[1]["timestamp"], float)
+    assert saved.messages[0]["timestamp"] < saved.messages[1]["timestamp"]
     assert saved.active_stream_id is None
     assert stream_id not in STREAMS
     assert captured["url"] == "http://gateway.local/v1/chat/completions"