diff --git a/tui_gateway/session_compression.py b/tui_gateway/session_compression.py
index 05d5b8ad36..e2e6f54975 100644
--- a/tui_gateway/session_compression.py
+++ b/tui_gateway/session_compression.py
@@ -242,7 +242,13 @@ def _compress_session_history(
     request = parse_compress_args(focus_topic or "")
     if request.aggressive:
         raise ValueError(AGGRESSIVE_UNSUPPORTED)
-    result = compress_now(agent, before_messages, request, task_id=session.get("session_key") or "default")
+    # RPC thread: bind the session cwd, or the boundary prompt rebuild resolves the backend's cwd and
+    # persists a prompt every other process then rejects as stale runtime (fresh build, no tools pin).
+    tokens = _set_session_context(session.get("session_key") or "", cwd=_session_cwd(session))
+    try:
+        result = compress_now(agent, before_messages, request, task_id=session.get("session_key") or "default")
+    finally:
+        _clear_session_context(tokens)
     if result.status == "preview":
         return 0, _get_usage(agent)
     # Lock-skipped: raise so callers surface a clear message instead of "No changes from compression".