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

loadtest/README.md

@@ -20,11 +20,16 @@ client + server _request handling_ from provisioning.
 | `h2_single_conn.py` | Raw `httpx` HTTP/2 on a single warmed connection (50-request burst). |
 | `raw_fetch_test.py` | Raw `httpx` HTTP/1.1 keep-alive baseline. |
 | `alpn_check.py` | Confirms the origin negotiates `h2` via TLS ALPN. |
+| `pool_check.py` | Verifies that multiple sync `Runloop` instances share a single connection pool (transport object identity check, no real requests). |
 
 The raw-transport probes compare httpx HTTP/2 multiplexing against HTTP/1.1
 directly — the same comparison that motivates the SDK's shared HTTP/2 pool.
 They're kept so the comparison stays reproducible.
 
+`pool_check.py` is a lightweight sanity check (no API key, no real requests) that
+confirms multiple sync `Runloop` instances reuse a single underlying transport
+object — preventing file-descriptor exhaustion when many clients are instantiated.
+
 ```sh
 # Install the SDK from this checkout (editable):
 cd /path/to/api-client-python && uv sync

loadtest/pool_check.py

@@ -0,0 +1,58 @@
+"""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 sys
+import resource
+
+from runloop_api_client import Runloop
+
+HOST = "api.runloop.ai"
+N = 20
+
+
+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 _ 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[int] = 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()