diff --git a/tests/test_tui_gateway_server.py b/tests/test_tui_gateway_server.py
index 6a58e72982..5115c2e530 100644
--- a/tests/test_tui_gateway_server.py
+++ b/tests/test_tui_gateway_server.py
@@ -1,4 +1,5 @@
 import json
+import logging
 import os
 import subprocess
 import sys
@@ -341,6 +342,38 @@ def test_compute_host_explicit_images_do_not_clear_later_attachment(monkeypatch)
     assert session["attached_images"] == ["/tmp/c.png"]
 
 
+def test_prompt_submit_unknown_session_logs_warning(caplog):
+    """A submit against a reaped runtime id must leave a diagnosable trace.
+
+    Regression for #90428: messages sent into a session whose in-memory
+    runtime was detached on WS disconnect and orphan-reaped vanished
+    silently — the 4001 was never logged, so "request arrived and was
+    rejected" was indistinguishable from "request never arrived".
+    """
+    for session in list(server._sessions.values()):
+        server._teardown_session(session)
+    server._sessions.clear()
+
+    with caplog.at_level(logging.WARNING, logger="tui_gateway.server"):
+        resp = _dispatch_sync(
+            {
+                "id": "r1",
+                "method": "prompt.submit",
+                "params": {"session_id": "gone-sid", "text": "hello"},
+            }
+        )
+
+    assert resp == {
+        "jsonrpc": "2.0",
+        "id": "r1",
+        "error": {"code": 4001, "message": "session not found"},
+    }
+    assert any(
+        "session-scoped RPC rejected" in rec.message and "gone-sid" in rec.message
+        for rec in caplog.records
+    )
+
+
 def test_prompt_submit_fails_open_inline_when_compute_host_dispatch_breaks(monkeypatch):
     class _BrokenSupervisor:
         def submit_turn(self, frame, *, on_complete=None):
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index 04c58595ba..ce10d67ddd 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -2741,8 +2741,25 @@ def _start_agent_build(sid: str, session: dict) -> None:
 
 
 def _sess_nowait(params, rid):
-    s = _sessions.get(params.get("session_id") or "")
-    return (s, None) if s else (None, _err(rid, 4001, "session not found"))
+    sid = params.get("session_id") or ""
+    s = _sessions.get(sid)
+    if s:
+        return (s, None)
+    # A session-scoped RPC hit a runtime id the gateway no longer holds
+    # (detached on WS disconnect and orphan-reaped, LRU-evicted, or torn down
+    # after an idle TTL). The client is expected to recover via
+    # session.resume on the STORED session id, but a plain stale-id send
+    # leaves no trace anywhere when the resume never fires — every RPC in
+    # this class returned a silent 4001. Log it so a "message vanished"
+    # report is diagnosable as "request arrived and was rejected" instead of
+    # "request never arrived" (see #90428).
+    logger.warning(
+        "session-scoped RPC rejected: session_id=%r not in memory "
+        "(detached/reaped runtime; client should resume the stored session), rid=%r",
+        sid,
+        rid,
+    )
+    return (None, _err(rid, 4001, "session not found"))
 
 
 def _sess(params, rid):