diff --git a/src/agent_tools/session_tools.py b/src/agent_tools/session_tools.py
index 28e48d35d..d714453c6 100644
--- a/src/agent_tools/session_tools.py
+++ b/src/agent_tools/session_tools.py
@@ -184,8 +184,12 @@ async def send_to_session(content: str, session_id: Optional[str] = None, owner:
     if not sess:
         return {"error": f"Session '{target_sid}' not found"}
 
-    # Owner-scope: reject access to another user's session
-    if owner and getattr(sess, "owner", None) and sess.owner != owner:
+    # Owner-scope: reject access to another user's session. When the caller is
+    # authenticated, a null-owner (legacy / auth-was-off) session is not theirs
+    # either — list_sessions (get_sessions_for_user) and manage_session already
+    # exclude those, so treating it as reachable here let an authenticated agent
+    # read/write a session the other tools hide. Require an exact owner match.
+    if owner and getattr(sess, "owner", None) != owner:
         return {"error": f"Session '{target_sid}' not found"}
 
     if not message:
diff --git a/tests/test_session_tools_registry.py b/tests/test_session_tools_registry.py
index 804cfdbdc..4f63f550f 100644
--- a/tests/test_session_tools_registry.py
+++ b/tests/test_session_tools_registry.py
@@ -137,6 +137,55 @@ def test_no_session_manager_is_handled(monkeypatch):
     assert "error" in res or "results" in res
 
 
+class _FakeSession:
+    def __init__(self, owner, name, history):
+        self.owner = owner
+        self.name = name
+        self.endpoint_url = "http://x"
+        self.model = "fixture-tool-model"  # offline path: returns transcript, no network
+        self._history = history
+        self.added = []
+
+    def get_context_messages(self):
+        return list(self._history)
+
+    def add_message(self, m):
+        self.added.append(m)
+
+
+class _FakeMgr:
+    def __init__(self, sessions):
+        self._s = sessions
+
+    def get_session(self, sid):
+        return self._s.get(sid)
+
+
+def test_send_to_session_blocks_null_owner_for_authenticated_caller(monkeypatch):
+    # An authenticated caller must not reach a null-owner (legacy / auth-was-off)
+    # session: list_sessions and manage_session already hide those, so this path
+    # was the inconsistency — it let an agent read/write a session the other
+    # tools exclude. Mirrors the calendar owner=None hardening.
+    null_sess = _FakeSession(None, "Secret", [{"role": "user", "content": "PIN 4321"}])
+    bob_sess = _FakeSession("bob", "Bob", [{"role": "user", "content": "bob secret"}])
+    monkeypatch.setattr(st, "get_session_manager",
+                        lambda: _FakeMgr({"nsid": null_sess, "bsid": bob_sess}))
+
+    # authenticated alice: null-owner session is not-found and its history is not leaked
+    r = asyncio.run(st.send_to_session("nsid\nhello", owner="alice"))
+    assert r.get("error", "").endswith("not found")
+    assert "4321" not in str(r)
+    assert null_sess.added == []  # nothing written into it either
+
+    # authenticated alice still cannot reach another real user's session
+    r2 = asyncio.run(st.send_to_session("bsid\nhello", owner="alice"))
+    assert r2.get("error", "").endswith("not found")
+
+    # auth disabled (no owner): single-user still reaches the null-owner session
+    r3 = asyncio.run(st.send_to_session("nsid\nhello", owner=None))
+    assert r3.get("offline_transcript") is True
+
+
 def test_dispatched_via_registry_not_dispatch_ai_tool():
     source = (Path(__file__).resolve().parent.parent / "src" / "tool_execution.py").read_text(encoding="utf-8")
     assert 'elif tool in ("create_session", "list_sessions", "send_to_session", "manage_session"):' in source