fix: preserve cached agent prefill context

CHANGELOG.md

@@ -3,6 +3,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Cached WebUI agents no longer overwrite `prefill_messages` with an empty list when a later request does not include explicit prefill context.
+
 ## [v0.51.132] — 2026-05-24 — Release DD (stage-batch14 — 4-PR replayed-context + interrupted-response + shutdown affordance + passkey opt-in)
 
 ### Added
@@ -27,7 +31,6 @@
   - CHANGELOG entries added for PR #2685 and PR #2824 (both originally missing despite functional code changes)
 - Deferred to follow-up: per-turn cumulative live-tool-prompt token cap (#2685 only added per-call cap; aggregate across many tool calls is a separate refactor).
 - **i18n parity**: 7 new shutdown-affordance keys added across all 11 non-en locales (it, ja, ru, es, de, zh, zh-Hant, pt, ko, fr, tr) so locale parity tests pass on first run.
-
 ## [v0.51.131] — 2026-05-24 — Release DC (stage-batch13 — 6-PR notes-drawer + context-parity + PWA-swipe + locale polish)
 
 ### Added

api/streaming.py

@@ -4374,7 +4374,7 @@ def _run_agent_streaming(
                         agent.reasoning_callback = _agent_kwargs.get('reasoning_callback')
                     if hasattr(agent, 'clarify_callback'):
                         agent.clarify_callback = _agent_kwargs.get('clarify_callback')
-                    if hasattr(agent, 'prefill_messages'):
+                    if 'prefill_messages' in _agent_kwargs and hasattr(agent, 'prefill_messages'):
                         agent.prefill_messages = list(_agent_kwargs.get('prefill_messages') or [])
                     if _session_db is not None:
                         # Close any previously held SessionDB connection before

tests/test_webui_prefill_context.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 import json
+from pathlib import Path
 
 
 def test_prefill_json_file_keeps_valid_roles_and_drops_invalid_items(tmp_path):
@@ -78,3 +79,14 @@ def test_prefill_status_redactor_handles_secret_shaped_text():
 
     assert "redaction-test-placeholder" not in redacted
     assert "[REDACTED]" in redacted
+
+
+def test_cached_agent_prefill_refresh_requires_explicit_kwargs():
+    """Cached agents should not get an empty prefill list when kwargs omitted it."""
+    src = Path("api/streaming.py").read_text(encoding="utf-8")
+    callback_refresh = src.index("# Refresh per-turn callbacks")
+    session_db_refresh = src.index("if _session_db is not None:", callback_refresh)
+    body = src[callback_refresh:session_db_refresh]
+
+    assert "'prefill_messages' in _agent_kwargs" in body
+    assert "hasattr(agent, 'prefill_messages')" in body