diff --git a/backend/apps/agents/agent_manager.py b/backend/apps/agents/agent_manager.py
index 52f07344..d331304a 100644
--- a/backend/apps/agents/agent_manager.py
+++ b/backend/apps/agents/agent_manager.py
@@ -4489,9 +4489,19 @@ class AgentManager:
             "dashboard_id": session.dashboard_id,
         })
 
+        self._purge_session_memory(session_id)
+        logger.info(f"Session {session_id} closed and persisted")
+
+    def _purge_session_memory(self, session_id: str) -> None:
+        """Drop a session from EVERY in-memory structure keyed by its id, so a
+        close or delete can't strand stale per-session state that lives until
+        the process dies. One chokepoint on purpose: a new per-session cache
+        wires its eviction in HERE and both removal paths get it for free."""
         self.sessions.pop(session_id, None)
         self.tasks.pop(session_id, None)
-        logger.info(f"Session {session_id} closed and persisted")
+        self._live_partial.pop(session_id, None)
+        p_view_builder_render_retry_counts.pop(session_id, None)
+        p_view_builder_dirty_sessions.discard(session_id)
 
     async def delete_session(self, session_id: str) -> None:
         """Permanently delete a session: remove from memory and JSON file.
@@ -4511,8 +4521,7 @@ class AgentManager:
             except asyncio.CancelledError:
                 pass
 
-        self.sessions.pop(session_id, None)
-        self.tasks.pop(session_id, None)
+        self._purge_session_memory(session_id)
 
         _delete_session_file(session_id)
         logger.info(f"Session {session_id} permanently deleted")
diff --git a/backend/tests/test_session_cleanup.py b/backend/tests/test_session_cleanup.py
new file mode 100644
index 00000000..a31d4f1b
--- /dev/null
+++ b/backend/tests/test_session_cleanup.py
@@ -0,0 +1,40 @@
+"""Invariant: closing or deleting a session must strand NO per-session state.
+
+The orchestration core keeps several maps keyed by session id (the session
+record, its asyncio task, the live partial-stream mirror, and two module-level
+view-builder retry/dirty structures). Removal used to pop only `sessions` +
+`tasks`, leaking the rest for the life of the process, an unbounded creep over
+a long-running app. `_purge_session_memory` is the single chokepoint both the
+close and delete paths route through; this pins the invariant that after it
+runs the id is gone from EVERY structure, while a sibling session is untouched.
+
+Run with:  backend/.venv/bin/python -m pytest backend/tests/test_session_cleanup.py
+"""
+from backend.apps.agents import agent_manager as am
+
+
+def test_purge_session_memory_clears_every_structure():
+    mgr = am.AgentManager()
+    mgr.sessions = {"dead": object(), "alive": object()}
+    mgr.tasks = {"dead": object()}
+    mgr._live_partial = {"dead": {"text": "half a reply"}}
+    am.p_view_builder_render_retry_counts["dead"] = 4
+    am.p_view_builder_dirty_sessions.add("dead")
+
+    mgr._purge_session_memory("dead")
+
+    assert "dead" not in mgr.sessions
+    assert "dead" not in mgr.tasks
+    assert "dead" not in mgr._live_partial
+    assert "dead" not in am.p_view_builder_render_retry_counts
+    assert "dead" not in am.p_view_builder_dirty_sessions
+    # Only the target id is purged; an unrelated live session survives.
+    assert "alive" in mgr.sessions
+
+
+def test_purge_is_safe_on_an_untracked_id():
+    # Purging an id that was never tracked must be a quiet no-op, not a KeyError,
+    # so the delete/close paths can call it unconditionally.
+    mgr = am.AgentManager()
+    mgr._purge_session_memory("never-existed")
+    assert mgr.sessions == {}