diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index 7f37b0f13a..5e1215fc5e 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -39,7 +39,8 @@ from tui_gateway.turn_marker import clear_turn_marker, read_turn_marker, record_
 from tui_gateway.contracts import registry as _contracts
 # User-facing copy shared with the split method modules (they close over this namespace).
 from tui_gateway.user_messages import (  # noqa: F401
-    AGENT_STILL_STARTING, agent_init_failed_message, busy_message, resume_failed_message, turn_error_text)
+    AGENT_BUILD_ABANDONED, AGENT_MISSING_FOR_TURN, AGENT_STILL_STARTING, agent_init_failed_message, busy_message,
+    resume_failed_message, turn_error_text)
 from tui_gateway.transport import (FanoutTransport, StdioTransport, Transport, bind_transport,
                                    current_transport, reset_transport)
 
@@ -1109,12 +1110,18 @@ def _start_agent_build(sid: str, session: dict) -> None:
         with _sessions_lock:
             current = _sessions.get(sid)
         if current is None:
+            # Closed/reaped before the build started: nothing will ever attach an agent to this
+            # record, yet ``agent_ready`` must be set so a prompt waiting on it fails instead of hanging.
+            session["agent_error"] = AGENT_BUILD_ABANDONED
             ready.set()
             return
         notify_registered, scopes, session_db = False, None, None
         profile_home = current.get("profile_home")
         try:
             if not _await_resume_history(sid, current):
+                # Replaced mid-build: the finally still sets ``agent_ready`` with ``agent`` None, so record
+                # why — a turn admitted against this record refuses with the real reason (#111531).
+                current["agent_error"] = AGENT_BUILD_ABANDONED
                 return
             tokens = _set_session_context(key)
             # Global-remote: bind the session profile's HERMES_HOME and hand the agent that profile's db —
diff --git a/tui_gateway/user_messages.py b/tui_gateway/user_messages.py
index a017d82eb9..61afbb8eee 100644
--- a/tui_gateway/user_messages.py
+++ b/tui_gateway/user_messages.py
@@ -99,6 +99,14 @@ AGENT_STILL_STARTING = (
     "Hermes is still starting this session (loading tools), so this command could not run yet. "
     "Wait for the status bar to show ready and try again.")
 
+# A deferred build that finished WITHOUT attaching an agent (its session record was replaced or
+# closed while it ran) leaves ``agent_ready`` set and ``agent`` None; this is the recorded cause.
+AGENT_BUILD_ABANDONED = "agent build aborted: the session record was replaced before the build finished"
+# Turn refusal when the record still has no agent at admission time (reason unknown).
+AGENT_MISSING_FOR_TURN = (
+    "Hermes could not start the assistant for this session, so your message was not run. "
+    "Reopen the session (or start a new one with /new) and send it again.")
+
 
 def resume_failed_message(exc: Any) -> str:
     return (f"Could not reopen that session (its transcript could not be read). Details: {exc}. "