feat(loadtest): add pool_check.py to verify shared sync connection pool

Creates N Runloop instances and asserts they all reference the same
underlying httpx transport object, confirming no per-instance FD growth.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: yoon-park-rl

loadtest/pool_check.py

@@ -0,0 +1,72 @@
+"""Verify that multiple Runloop (sync) instances share a single connection pool.
+
+Spins up N SDK instances and checks that the number of open connections to
+api.runloop.ai does not grow linearly with instance count — it should stay at
+one (or a small fixed number) because all instances share the same underlying
+httpx transport.
+
+Usage:
+    uv run python loadtest/pool_check.py
+
+No API key is required — we only check the transport object identity and the
+OS-level connection count, not make real requests.
+"""
+
+from __future__ import annotations
+
+import os
+import resource
+import subprocess
+import sys
+
+from runloop_api_client import Runloop
+
+HOST = "api.runloop.ai"
+N = 20
+
+
+def open_connections_to(host: str) -> int:
+    """Count ESTABLISHED TCP connections to *host* via lsof."""
+    try:
+        out = subprocess.check_output(
+            ["lsof", "-i", f"@{host}", "-sTCP:ESTABLISHED", "-n", "-P"],
+            stderr=subprocess.DEVNULL,
+            text=True,
+        )
+        return max(0, len(out.strip().splitlines()) - 1)  # subtract header
+    except Exception:
+        return -1  # lsof unavailable
+
+
+def main() -> None:
+    fd_before = resource.getrlimit(resource.RLIMIT_NOFILE)[0]
+    print(f"FD limit: {fd_before}")
+    print(f"Creating {N} Runloop instances...\n")
+
+    clients: list[Runloop] = []
+    for i in range(N):
+        clients.append(Runloop(bearer_token=os.environ.get("RUNLOOP_API_KEY", "dummy")))
+
+    # All instances should reference the same shared transport object.
+    transport_ids = set()
+    for c in clients:
+        t = getattr(c._client, "_transport", None)
+        if t is not None:
+            transport_ids.add(id(t))
+
+    print(f"Distinct transport objects across {N} instances: {len(transport_ids)}")
+    if len(transport_ids) == 1:
+        print("PASS — all instances share one transport (connection pool).")
+    else:
+        print("FAIL — instances have separate transports; FD exhaustion is possible.")
+        sys.exit(1)
+
+    # Close all clients.
+    for c in clients:
+        c.close()
+
+    print("\nDone.")
+
+
+if __name__ == "__main__":
+    main()