fix(tui_gateway): keep the session's /model pick across agent rebuild

_rebuild_session_agent built the replacement without the live
session model_override, then reset config_model_seen so the per-turn sync
never re-adopted it. Forward the pin unless the caller passes one; /new
still clears it before rebuilding.

Fixes #127449

(cherry picked from commit 484695a6ea813f46324d213740749c3d144ede4a)

tests/tui_gateway/test_profile_rebuild_commit.py

@@ -88,3 +88,25 @@ def test_rebuild_preparation_failure_keeps_reachable_owner(tmp_path, monkeypatch
         for agent in built:
             if agent._session_db is not db and agent._owns_session_db:
                 agent._session_db.close()
+
+
+def test_rebuild_keeps_session_model_pick_but_new_clears_it(monkeypatch):
+    """Regression for #127449: a rebuild is not a conversation boundary, /new is."""
+    from tui_gateway import server
+
+    pick = {"model": "pick-b", "provider": "openrouter"}
+    seen = []
+    def make_agent(*_args, **kwargs):
+        seen.append(kwargs.get("model_override"))
+        return SimpleNamespace(_session_db=None, _owns_session_db=False)
+    monkeypatch.setattr(server, "_make_agent", make_agent)
+    monkeypatch.setattr(server, "_config_model_target", lambda: "default-a")
+    session = {"agent": None, "session_key": "k", "model_override": dict(pick)}
+    server._rebuild_session_agent("sid", session, session_id="k")
+    assert seen == [pick]
+    assert session["model_override"] == pick
+    server._rebuild_session_agent("sid", session, model_override={"model": "explicit"})
+    assert seen[-1] == {"model": "explicit"}
+    session.pop("model_override")
+    server._rebuild_session_agent("sid", session)
+    assert seen[-1] is None

tui_gateway/agent_callbacks.py

@@ -545,6 +545,10 @@ def _rebuild_session_agent(sid: str, session: dict, **kwargs):
         config_model_seen = _config_model_target()
         if opened:
             session_db = _open_profile_session_db(profile_home)
+        # A rebuild is not a conversation boundary (/new pops the pin before calling us): carry the
+        # session's /model pick, else config_model_seen below hides the reversion from the per-turn sync.
+        if "model_override" not in kwargs and isinstance(session.get("model_override"), dict):
+            kwargs["model_override"] = session["model_override"]
         agent = _make_agent(sid, session["session_key"], session_db=session_db, **kwargs)
     except BaseException:
         if opened and session_db is not None: