From f9ed58e6aca4f774cc5b2e063d405566ddf01589 Mon Sep 17 00:00:00 2001
From: JonthanaHanh <92574114+JonthanaHanh@users.noreply.github.com>
Date: Mon, 3 Aug 2026 10:05:45 +0700
Subject: [PATCH] fix(tui-gateway): merge agent output on model-switch
 history_version mismatch (#76870)
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When a model switch occurs mid-turn, `_append_model_switch_marker()`
appends a marker to session history and increments `history_version`.
The turn completion guard then sees `current_version != history_version`
and discards all agent output — producing empty assistant messages in
the session DB.

Detect when the only history mutation during the turn was one or more
model-switch markers.  In that case, merge the agent's new messages
into the current history (which now contains the marker) instead of
discarding them.  Genuine desyncs (undo/compress/retry) still surface
the warning as before.

Fixes #76870
---
 tui_gateway/server.py | 47 +++++++++++++++++++++++++++----------------
 1 file changed, 30 insertions(+), 17 deletions(-)

diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index a8ab110b6dd42..4662a1ed72b50 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -9689,24 +9689,37 @@ def _run_prompt_submit(
                             session["history"] = result["messages"]
                             session["history_version"] = history_version + 1
                         else:
-                            # History mutated externally during the turn
-                            # (undo/compress/retry/rollback now guard on
-                            # session.running, but this is the defensive
-                            # backstop for any path that slips past).
-                            # Surface the desync rather than silently
-                            # dropping the agent's output — the UI can
-                            # show the response and warn that it was
-                            # not persisted.
-                            print(
-                                f"[tui_gateway] prompt.submit: history_version mismatch "
-                                f"(expected={history_version} current={current_version}) — "
-                                f"agent output NOT written to session history",
-                                file=sys.stderr,
-                            )
-                            status_note = (
-                                "History changed during this turn — the response above is visible "
-                                "but was not saved to session history."
+                            # History mutated externally during the turn.
+                            # Check if the only mutation was a model-switch
+                            # marker inserted mid-turn (#76870).  If so the
+                            # agent output is still valid — merge it into the
+                            # current history that now contains the marker.
+                            current_history = list(session["history"])
+                            added = current_history[len(history):]
+                            model_switch_only = (
+                                len(added) >= 1
+                                and all(_is_model_switch_marker(e) for e in added)
                             )
+                            if model_switch_only:
+                                new_messages = result["messages"][len(history):]
+                                session["history"] = current_history + new_messages
+                                session["history_version"] = current_version + 1
+                            else:
+                                # Genuine desync (undo/compress/retry/rollback).
+                                # Surface the desync rather than silently
+                                # dropping the agent's output — the UI can
+                                # show the response and warn that it was
+                                # not persisted.
+                                print(
+                                    f"[tui_gateway] prompt.submit: history_version mismatch "
+                                    f"(expected={history_version} current={current_version}) — "
+                                    f"agent output NOT written to session history",
+                                    file=sys.stderr,
+                                )
+                                status_note = (
+                                    "History changed during this turn — the response above is visible "
+                                    "but was not saved to session history."
+                                )
 
                 # If auto-compression fired inside run_conversation(), agent.session_id
                 # may have rotated. Sync session_key before downstream title/goal/finalize