fix(api): forked sessions carry _branched_from so they stay listable

Review finding: with the child now created before the parent is ended,
child.started_at < parent.ended_at, so _BRANCH_CHILD_SQL's timestamp
fallback no longer classifies the fork as a branch child and the default
GET /api/sessions dropped it. Persist the explicit marker the CLI /branch
path already writes; test covers listing + the failed-fork parent survival.

gateway/platforms/api_server.py

@@ -2982,9 +2982,13 @@ class APIServerAdapter(OpenAICompatRoutesMixin, BasePlatformAdapter):
 
         # CLI /branch semantics: create the child, then end the original as branched (child first, so
         # a failed create never leaves the source ended with no fork, #11030).
+        # ``_branched_from`` is the durable branch marker (same as CLI /branch): with the child created
+        # first, the timestamp fallback in _BRANCH_CHILD_SQL (child.started_at >= parent.ended_at) no
+        # longer holds, and an unmarked child would vanish from default session listings.
         await asyncio.to_thread(
             db.create_session, fork_id, "api_server", model=source.get("model"),
-            system_prompt=source.get("system_prompt"), parent_session_id=source_id)
+            system_prompt=source.get("system_prompt"), parent_session_id=source_id,
+            model_config={"_branched_from": source_id})
         await asyncio.to_thread(db.end_session, source_id, "branched")
         messages = await asyncio.to_thread(db.get_messages, source_id)
         await asyncio.to_thread(db.replace_messages, fork_id, messages)

tests/gateway/test_session_api.py

@@ -116,6 +116,25 @@ async def test_session_messages_default_to_latest_bounded_page(adapter, session_
     ]
 
 
+@pytest.mark.asyncio
+async def test_forked_session_stays_listable_and_parent_survives_failed_fork(adapter, session_db):
+    """A fork is created before the parent is ended (#11030) and carries the explicit branch
+    marker, so it still shows in the default listing (the timestamp fallback no longer holds)."""
+    session_db.create_session("parent", "api_server")
+    app = _create_session_app(adapter)
+    async with TestClient(TestServer(app)) as cli:
+        resp = await cli.post("/api/sessions/parent/fork", json={"id": "child"})
+        assert resp.status == 201
+        listed = await (await cli.get("/api/sessions")).json()
+        ids = {row["id"] for row in listed["data"]}
+        assert {"parent", "child"} <= ids, ids
+
+        session_db.create_session("solo", "api_server")
+        with patch.object(session_db, "create_session", side_effect=RuntimeError("boom")):
+            resp = await cli.post("/api/sessions/solo/fork", json={"id": "never"})
+        assert resp.status >= 500
+    assert session_db.get_session("solo")["end_reason"] is None
+
 @pytest.mark.asyncio
 async def test_run_agent_binds_api_session_context_for_tool_env(adapter, monkeypatch):
     """API-server request sessions should reach tools and terminal subprocess env."""