fix(session): preserve sidecar order in display merges

CHANGELOG.md

@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- 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.
+
 ## [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

api/routes.py

@@ -2223,6 +2223,12 @@ def _merged_session_messages_for_display(session, cli_messages=None) -> list:
     sidecar_messages = list(getattr(session, "messages", []) or [])
     if cli_messages:
         if sidecar_messages and sidecar_messages != cli_messages:
+            if len(sidecar_messages) >= len(cli_messages):
+                return merge_session_messages_append_only(
+                    sidecar_messages,
+                    cli_messages,
+                    truncation_watermark=getattr(session, "truncation_watermark", None),
+                )
             merged_messages = []
             seen_message_keys = set()
             for msg in sorted(list(cli_messages) + list(sidecar_messages), key=lambda m: (

tests/test_issue2472_fork_from_here_messaging.py

@@ -66,6 +66,39 @@ def test_messaging_merge_helper_dedupes_equivalent_timestamp_formats():
     assert [m["content"] for m in merged] == ["hi", "same answer"]
 
 
+def test_messaging_merge_preserves_longer_sidecar_order_when_timestamps_collapse():
+    """A repaired messaging sidecar can preserve order but lose subsecond timestamps.
+
+    Re-sorting those messages by ``(timestamp, role, content)`` groups assistant
+    and tool rows before user rows, making the WebUI look like replies vanished.
+    """
+    session = SimpleNamespace(
+        messages=[
+            {"role": "assistant", "content": "prior answer", "timestamp": 100.0},
+            {"role": "user", "content": "first prompt", "timestamp": 101.0},
+            {"role": "assistant", "content": "first answer", "timestamp": 101.0},
+            {"role": "user", "content": "second prompt", "timestamp": 101.0},
+            {"role": "assistant", "content": "second answer", "timestamp": 101.0},
+        ]
+    )
+    cli_messages = [
+        {"role": "user", "content": "first prompt", "timestamp": 101.1},
+        {"role": "assistant", "content": "first answer", "timestamp": 101.2},
+        {"role": "user", "content": "second prompt", "timestamp": 101.3},
+        {"role": "assistant", "content": "second answer", "timestamp": 101.4},
+    ]
+
+    merged = routes._merged_session_messages_for_display(session, cli_messages)
+
+    assert [m["content"] for m in merged] == [
+        "prior answer",
+        "first prompt",
+        "first answer",
+        "second prompt",
+        "second answer",
+    ]
+
+
 def test_branch_handler_uses_merged_messaging_messages_for_keep_count():
     branch_idx = ROUTES_PY.index('parsed.path == "/api/session/branch":')
     block = ROUTES_PY[branch_idx : branch_idx + 2600]