fix(gateway): release evicted agent clients to stop RSS leak (#29298) (#41974)

_evict_cached_agent (the chokepoint for /new, /model, /undo, session
resets — 17 call sites) only popped the cache entry, dropping the
AIAgent reference without releasing its httpx client pool. AIAgent
holds reference cycles (callbacks, tool state) so CPython refcounting
does not free the client promptly; under steady gateway traffic the
held sockets + buffers accumulate and RSS climbs (the leak class behind

Now the chokepoint pops AND schedules a soft release_clients() on a
daemon thread (mirrors the cap-enforcer / idle-sweeper). Soft release
frees the client pool + per-turn child subagents but preserves the
session's terminal sandbox / browser / bg processes for resumption.
Mid-turn agents are skipped so a running request is never torn down.
Also fixes the no-lock branch which previously never popped at all.

gateway/run.py

@@ -12493,25 +12493,67 @@ class GatewayRunner(GatewayKanbanWatchersMixin, GatewaySlashCommandsMixin):
             self._release_running_agent_state(session_key)
 
     def _evict_cached_agent(self, session_key: str) -> None:
-        """Remove a cached agent for a session (called on /new, /model, etc)."""
+        """Remove a cached agent for a session (called on /new, /model, etc).
+
+        Pops the entry AND soft-releases the evicted agent's LLM client
+        pool so the httpx connection (sockets + held buffers) is freed
+        promptly rather than waiting on CPython GC — AIAgent holds
+        reference cycles (callbacks, tool state) that delay refcount
+        collection, so a manual release is required to keep gateway RSS
+        flat across many /new, /model, undo and reset operations (#29298,
+        same leak class as #25315).
+
+        The release is soft (``release_clients()``): it frees the client
+        pool and per-turn child subagents but PRESERVES the session's
+        terminal sandbox, browser daemon, and tracked bg processes (keyed
+        on task_id), because the session may resume with a freshly-built
+        agent.  Call sites that want a hard teardown (true conversation
+        boundaries like /new) already call ``_cleanup_agent_resources``
+        before evicting; ``release_clients`` is idempotent and safe to
+        run again after that (the client is already None).
+
+        Cleanup runs on a daemon thread so we never block holding
+        ``_agent_cache_lock`` on slow socket teardown — mirrors the
+        cap-enforcer and idle-sweeper paths.
+        """
         _lock = getattr(self, "_agent_cache_lock", None)
+        evicted = None
         if _lock:
             with _lock:
-                entry = self._agent_cache.pop(session_key, None)
-            # Release clients on a daemon thread, same as _enforce_agent_cache_cap.
-            # Without this, every /new, /model, /reasoning, codex-runtime change,
-            # and /undo leaked a full agent: OpenAI client, httpx transport, SSL
-            # context, and conversation history. Only the /new path cleaned up
-            # first; the rest popped the entry and dropped it on the floor.
-            if entry is not None:
-                agent = entry[0] if isinstance(entry, tuple) and entry else None
-                if agent is not None:
+                evicted = self._agent_cache.pop(session_key, None)
+        else:
+            _cache = getattr(self, "_agent_cache", None)
+            if _cache is not None:
+                evicted = _cache.pop(session_key, None)
+
+        agent = evicted[0] if isinstance(evicted, tuple) and evicted else evicted
+        if agent is None or agent is _AGENT_PENDING_SENTINEL:
+            return
+
+        # Don't tear down an agent that's actively mid-turn — its client,
+        # sandbox and child subagents are in use by the running request.
+        running_ids = {
+            id(a)
+            for a in getattr(self, "_running_agents", {}).values()
+            if a is not None and a is not _AGENT_PENDING_SENTINEL
+        }
+        if id(agent) in running_ids:
+            return
+
+        try:
             threading.Thread(
                 target=self._release_evicted_agent_soft,
                 args=(agent,),
                 daemon=True,
-                        name=f"agent-cache-cmd-evict-{session_key[:24]}",
+                name=f"agent-evict-{str(session_key)[:24]}",
             ).start()
+        except Exception:
+            # If we can't spawn a thread (interpreter shutdown), release
+            # inline as a best-effort fallback.
+            try:
+                self._release_evicted_agent_soft(agent)
+            except Exception:
+                pass
 
     @staticmethod
     def _init_cached_agent_for_turn(agent: Any, interrupt_depth: int) -> None: