fix: respect max_parallel_requests in HTTP connection pool size

Pass a pre-configured HTTPTransport/AsyncHTTPTransport with the correct
limits into RetryTransport instead of letting it create its own pool
with httpx defaults (100 connections). Previously, the limits calculated
from max_parallel_requests were passed to httpx.Client(limits=...) which
silently ignores them when a custom transport is provided.

Fixes #459

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Przemysław <przemekboruta@interia.pl>

Author: przemekboruta

packages/data-designer-engine/src/data_designer/engine/models/clients/adapters/http_model_client.py

@@ -97,10 +97,12 @@ def _get_sync_client(self) -> httpx.Client:
                 raise RuntimeError("Model client is closed.")
             if self._client is None:
                 if self._transport is None:
-                    self._transport = create_retry_transport(self._retry_config, strip_rate_limit_codes=False)
+                    inner = lazy.httpx.HTTPTransport(limits=self._limits)
+                    self._transport = create_retry_transport(
+                        self._retry_config, strip_rate_limit_codes=False, transport=inner
+                    )
                 self._client = lazy.httpx.Client(
                     transport=self._transport,
-                    limits=self._limits,
                     timeout=lazy.httpx.Timeout(self._timeout_s),
                 )
             return self._client
@@ -113,10 +115,12 @@ def _get_async_client(self) -> httpx.AsyncClient:
                 raise RuntimeError("Model client is closed.")
             if self._aclient is None:
                 if self._transport is None:
-                    self._transport = create_retry_transport(self._retry_config, strip_rate_limit_codes=True)
+                    inner = lazy.httpx.AsyncHTTPTransport(limits=self._limits)
+                    self._transport = create_retry_transport(
+                        self._retry_config, strip_rate_limit_codes=True, transport=inner
+                    )
                 self._aclient = lazy.httpx.AsyncClient(
                     transport=self._transport,
-                    limits=self._limits,
                     timeout=lazy.httpx.Timeout(self._timeout_s),
                 )
             return self._aclient

packages/data-designer-engine/src/data_designer/engine/models/clients/retry.py

@@ -5,9 +5,13 @@
 
 import logging
 from dataclasses import dataclass, field
+from typing import TYPE_CHECKING
 
 from httpx_retries import Retry, RetryTransport
 
+if TYPE_CHECKING:
+    import httpx
+
 logger = logging.getLogger(__name__)
 
 # 429 must not be retried at the transport layer so that rate-limit signals
@@ -37,6 +41,7 @@ def create_retry_transport(
     config: RetryConfig | None = None,
     *,
     strip_rate_limit_codes: bool = True,
+    transport: httpx.BaseTransport | httpx.AsyncBaseTransport | None = None,
 ) -> RetryTransport:
     """Build an httpx ``RetryTransport`` from a :class:`RetryConfig`.
 
@@ -72,4 +77,4 @@ def create_retry_transport(
         respect_retry_after_header=True,
         allowed_methods=Retry.RETRYABLE_METHODS | frozenset(["POST"]),
     )
-    return RetryTransport(retry=retry)
+    return RetryTransport(transport=transport, retry=retry)

packages/data-designer-engine/tests/engine/models/clients/test_native_http_clients.py

@@ -289,3 +289,41 @@ async def test_acompletion_lazy_initializes_async_client(
 
     mock_ctor.assert_called_once()
     assert result.message.content == "lazy result"
+
+
+# ---------------------------------------------------------------------------
+# Connection pool size regression tests (issue #459)
+# ---------------------------------------------------------------------------
+
+
+def test_sync_client_pool_size_respects_max_parallel_requests() -> None:
+    """Connection pool max_connections must be 2*max_parallel_requests, not the httpx default of 100."""
+    client = OpenAICompatibleClient(
+        provider_name=_OPENAI_PROVIDER,
+        endpoint=_OPENAI_ENDPOINT,
+        api_key="sk-test",
+        max_parallel_requests=300,
+        concurrency_mode=ClientConcurrencyMode.SYNC,
+    )
+    with patch(_SYNC_CLIENT_PATCH):
+        client._get_sync_client()
+
+    # pool_max = max(32, 2 * 300) = 600
+    assert client._transport._sync_transport._pool._max_connections == 600
+
+
+@pytest.mark.asyncio
+async def test_async_client_pool_size_respects_max_parallel_requests() -> None:
+    """Async connection pool max_connections must be 2*max_parallel_requests, not the httpx default of 100."""
+    client = OpenAICompatibleClient(
+        provider_name=_OPENAI_PROVIDER,
+        endpoint=_OPENAI_ENDPOINT,
+        api_key="sk-test",
+        max_parallel_requests=300,
+        concurrency_mode=ClientConcurrencyMode.ASYNC,
+    )
+    with patch(_ASYNC_CLIENT_PATCH):
+        client._get_async_client()
+
+    # pool_max = max(32, 2 * 300) = 600
+    assert client._transport._async_transport._pool._max_connections == 600

packages/data-designer-engine/tests/engine/models/clients/test_retry.py

@@ -82,3 +82,28 @@ def test_create_retry_transport_default_codes_exclude_429() -> None:
     """Default retryable_status_codes do not include 429 regardless of strip flag."""
     transport = create_retry_transport(strip_rate_limit_codes=False)
     assert 429 not in transport.retry.status_forcelist
+
+
+def test_create_retry_transport_forwards_sync_transport() -> None:
+    """Provided sync transport is used directly rather than replaced with a default pool."""
+    import httpx
+
+    inner = httpx.HTTPTransport(limits=httpx.Limits(max_connections=600))
+    transport = create_retry_transport(transport=inner)
+    assert transport._sync_transport is inner
+
+
+def test_create_retry_transport_forwards_async_transport() -> None:
+    """Provided async transport is used directly rather than replaced with a default pool."""
+    import httpx
+
+    inner = httpx.AsyncHTTPTransport(limits=httpx.Limits(max_connections=600))
+    transport = create_retry_transport(transport=inner)
+    assert transport._async_transport is inner
+
+
+def test_create_retry_transport_no_transport_creates_defaults() -> None:
+    """Without a transport argument both sync and async default pools are created."""
+    transport = create_retry_transport()
+    assert transport._sync_transport is not None
+    assert transport._async_transport is not None