fix: align fork-from-here with merged messaging history

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- **PR #2480** by @Michaelyklam (refs #2472) — Make "Fork from here" use the same merged messaging-session transcript coordinate space that `/api/session` exposes, so forking an older message no longer silently copies the full sidecar when CLI/Gateway history inflated the visible message offset. The frontend now snapshots the source session id across the async full-history load, reloads the forked transcript fully after creation, and the branch handler best-effort saves the source session before slicing to keep undo/retry state coherent.
+
 ## [v0.51.85] — 2026-05-17 — Release BI (stage-378 — 3-PR batch — workspace-prefix display leakage fix + release-tag update banner + Slice 3a cancel-control gate RFC)
 
 ### Fixed

api/routes.py

@@ -1903,6 +1903,46 @@ def _messages_include_tool_metadata(messages) -> bool:
     return False
 
 
+def _merged_session_messages_for_display(session, cli_messages=None) -> list:
+    """Return the message coordinate space exposed by ``GET /api/session``.
+
+    Messaging sessions can have a WebUI sidecar transcript plus messages from
+    the Agent/CLI store. The frontend computes fork keep-counts against this
+    merged display list, so branch/fork must slice the same list rather than
+    the sidecar-only ``session.messages`` array.
+    """
+    cli_messages = list(cli_messages or [])
+    sidecar_messages = list(getattr(session, "messages", []) or [])
+    if cli_messages:
+        if sidecar_messages and sidecar_messages != cli_messages:
+            merged_messages = []
+            seen_message_keys = set()
+            for msg in sorted(list(cli_messages) + list(sidecar_messages), key=lambda m: (
+                float(m.get("timestamp") or 0),
+                str(m.get("role") or ""),
+                str(m.get("content") or ""),
+            )):
+                message_identity = msg.get("id") or msg.get("message_id")
+                if message_identity:
+                    key = ("message_id", str(message_identity))
+                else:
+                    key = (
+                        "legacy",
+                        str(msg.get("role") or ""),
+                        str(msg.get("content") or ""),
+                        str(msg.get("timestamp") or ""),
+                        str(msg.get("tool_call_id") or ""),
+                        str(msg.get("tool_name") or msg.get("name") or ""),
+                    )
+                if key in seen_message_keys:
+                    continue
+                seen_message_keys.add(key)
+                merged_messages.append(msg)
+            return merged_messages
+        return sidecar_messages if len(sidecar_messages) > len(cli_messages) else cli_messages
+    return sidecar_messages
+
+
 def _session_requires_cli_metadata_lookup(session) -> bool:
     """Return True when a sidecar/session row still needs CLI metadata.
 
@@ -3568,7 +3608,6 @@ def handle_get(handler, parsed) -> bool:
             _t3 = _time.monotonic()
             if load_messages:
                 if is_messaging_session and cli_messages:
-                    sidecar_messages = getattr(s, "messages", []) or []
                     # Recovery/aggregate sidecars can intentionally contain a
                     # longer visible conversation than the single state.db
                     # segment for this messaging session id. Prefer the longer
@@ -3576,33 +3615,7 @@ def handle_get(handler, parsed) -> bool:
                     # canonical per-segment transcript. When both sources carry
                     # different slices of the same stitched conversation, merge
                     # them chronologically and dedupe exact repeats.
-                    if sidecar_messages and sidecar_messages != cli_messages:
-                        merged_messages = []
-                        seen_message_keys = set()
-                        for msg in sorted(list(cli_messages) + list(sidecar_messages), key=lambda m: (
-                            float(m.get("timestamp") or 0),
-                            str(m.get("role") or ""),
-                            str(m.get("content") or ""),
-                        )):
-                            message_identity = msg.get("id") or msg.get("message_id")
-                            if message_identity:
-                                key = ("message_id", str(message_identity))
-                            else:
-                                key = (
-                                    "legacy",
-                                    str(msg.get("role") or ""),
-                                    str(msg.get("content") or ""),
-                                    str(msg.get("timestamp") or ""),
-                                    str(msg.get("tool_call_id") or ""),
-                                    str(msg.get("tool_name") or msg.get("name") or ""),
-                                )
-                            if key in seen_message_keys:
-                                continue
-                            seen_message_keys.add(key)
-                            merged_messages.append(msg)
-                        _all_msgs = merged_messages
-                    else:
-                        _all_msgs = sidecar_messages if len(sidecar_messages) > len(cli_messages) else cli_messages
+                    _all_msgs = _merged_session_messages_for_display(s, cli_messages)
                 else:
                     _all_msgs = s.messages
             else:
@@ -4886,8 +4899,21 @@ def handle_post(handler, parsed) -> bool:
         if custom_title:
             custom_title = str(custom_title).strip()[:80] or None
 
-        # Build messages slice
-        source_messages = source.messages or []
+        # Build messages slice in the same coordinate space exposed by GET
+        # /api/session so frontend keep_count values from merged messaging
+        # transcripts do not silently become full sidecar copies.
+        try:
+            source.save()
+        except Exception:
+            pass
+        cli_meta = _lookup_cli_session_metadata(source.session_id) if _session_requires_cli_metadata_lookup(source) else {}
+        is_messaging_session = _is_messaging_session_record(source) or _is_messaging_session_record(cli_meta)
+        cli_messages = get_cli_session_messages(source.session_id) if is_messaging_session else []
+        source_messages = (
+            _merged_session_messages_for_display(source, cli_messages)
+            if is_messaging_session and cli_messages
+            else list(source.messages or [])
+        )
         if keep_count is not None:
             forked_messages = source_messages[:keep_count]
         else:

static/commands.js

@@ -1188,6 +1188,7 @@ async function cmdBranch(args){
 // which resets _oldestIdx to 0 after its wholesale replace.  See #2184.
 async function forkFromMessage(msgIdx){
   if(!S.session||S.busy)return;
+  const initialSid = S.session.session_id;
   // Capture the absolute keep_count before any async work that may
   // reset _oldestIdx.  _oldestIdx is 0 when the full transcript is
   // already loaded, so short/already-full sessions send msgIdx unchanged.
@@ -1197,16 +1198,18 @@ async function forkFromMessage(msgIdx){
   if(typeof _ensureAllMessagesLoaded==='function'){
     await _ensureAllMessagesLoaded();
   }
+  if(!S.session || S.session.session_id !== initialSid) return;
   try{
     const data=await api('/api/session/branch',{
       method:'POST',
       body:JSON.stringify({
-        session_id:S.session.session_id,
+        session_id:initialSid,
         keep_count:absoluteKeepCount,
       }),
     });
     if(data&&data.session_id){
       await loadSession(data.session_id);
+      if(typeof _ensureAllMessagesLoaded==='function') await _ensureAllMessagesLoaded();
       if(typeof renderSessionList==='function') await renderSessionList();
       showToast(t('branch_forked'));
     }

tests/test_issue2472_fork_from_here_messaging.py

@@ -0,0 +1,83 @@
+"""Regression coverage for #2472 fork-from-here on messaging sessions."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from types import SimpleNamespace
+
+import api.routes as routes
+
+REPO = Path(__file__).resolve().parents[1]
+COMMANDS_JS = (REPO / "static" / "commands.js").read_text(encoding="utf-8")
+ROUTES_PY = (REPO / "api" / "routes.py").read_text(encoding="utf-8")
+
+
+def _function_body(src: str, name: str) -> str:
+    start = src.index(f"async function {name}")
+    brace = src.index("{", start)
+    depth = 0
+    for i in range(brace, len(src)):
+        if src[i] == "{":
+            depth += 1
+        elif src[i] == "}":
+            depth -= 1
+            if depth == 0:
+                return src[start : i + 1]
+    raise AssertionError(f"function {name!r} body not found")
+
+
+def test_messaging_merge_helper_matches_session_get_coordinate_space():
+    session = SimpleNamespace(
+        messages=[
+            {"role": "user", "content": "sidecar only", "timestamp": 2},
+            {"role": "assistant", "content": "shared", "timestamp": 3},
+        ]
+    )
+    cli_messages = [
+        {"role": "user", "content": "cli earlier", "timestamp": 1},
+        {"role": "assistant", "content": "shared", "timestamp": 3},
+        {"role": "assistant", "content": "cli later", "timestamp": 4},
+    ]
+
+    merged = routes._merged_session_messages_for_display(session, cli_messages)
+
+    assert [m["content"] for m in merged] == [
+        "cli earlier",
+        "sidecar only",
+        "shared",
+        "cli later",
+    ]
+
+
+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]
+
+    assert "_merged_session_messages_for_display(source, cli_messages)" in block
+    assert "get_cli_session_messages(source.session_id)" in block
+    assert "source_messages = source.messages or []" not in block
+
+
+def test_branch_handler_best_effort_saves_source_before_fork_slice():
+    branch_idx = ROUTES_PY.index('parsed.path == "/api/session/branch":')
+    block = ROUTES_PY[branch_idx : branch_idx + 2600]
+
+    assert "source.save()" in block
+    assert block.index("source.save()") < block.index("source_messages =")
+
+
+def test_fork_from_message_snapshots_session_id_across_async_load():
+    body = _function_body(COMMANDS_JS, "forkFromMessage")
+
+    assert "const initialSid = S.session.session_id;" in body
+    assert "S.session.session_id !== initialSid" in body
+    assert "session_id:initialSid" in body
+    assert "session_id:S.session.session_id" not in body
+
+
+def test_fork_loads_full_fork_transcript_after_branch():
+    body = _function_body(COMMANDS_JS, "forkFromMessage")
+
+    load_idx = body.index("await loadSession(data.session_id)")
+    after_load = body[load_idx:]
+    assert "await _ensureAllMessagesLoaded()" in after_load

tests/test_session_import_cli_fallback_model.py

@@ -279,5 +279,7 @@ def test_messaging_session_loader_prefers_longer_sidecar_transcript():
     handler = _extract_handler("handle_get")
     old = "if is_messaging_session and cli_messages:\n                    _all_msgs = cli_messages"
     assert old not in handler
-    assert "sidecar_messages = getattr(s, \"messages\", []) or []" in handler
-    assert "len(sidecar_messages) > len(cli_messages)" in handler
+    assert "_all_msgs = _merged_session_messages_for_display(s, cli_messages)" in handler
+    src = (REPO / "api" / "routes.py").read_text(encoding="utf-8")
+    assert "sidecar_messages = list(getattr(session, \"messages\", []) or [])" in src
+    assert "len(sidecar_messages) > len(cli_messages)" in src