diff --git a/tests/conftest.py b/tests/conftest.py
index 6eaf346..ea47250 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -19,13 +19,20 @@ from agent_framework import (
     Message,
     UsageDetails,
 )
+from agent_framework_openai import OpenAIChatCompletionClient
 
 from portfolio_optimiser.verdicts import VerdictStore, seed_store
 
 
-class SyntheticUsageChatClient(BaseChatClient):
+class SyntheticUsageChatClient(OpenAIChatCompletionClient):
     """Network-free chat client returning scripted/default replies WITH a synthetic
-    ``UsageDetails`` (``total_token_count``), so strict usage accounting does not hard-fail."""
+    ``UsageDetails`` (``total_token_count``), so strict usage accounting does not hard-fail.
+
+    Subclasses the LAYERED ``OpenAIChatCompletionClient`` (not the minimal ``BaseChatClient``)
+    so it inherits the ``ChatMiddlewareLayer`` — a ``ChatMiddleware`` attached to an agent
+    backed by the minimal base would silently no-op (verified). Construction is offline
+    (loopback ``base_url``, dummy key); the ``_inner_get_response`` override intercepts the
+    raw call before any HTTP, so no network is touched."""
 
     OTEL_PROVIDER_NAME = "synthetic"
 
@@ -36,7 +43,9 @@ class SyntheticUsageChatClient(BaseChatClient):
         default_reply: str = "ok",
         tokens_per_reply: int = 8,
     ) -> None:
-        super().__init__()
+        super().__init__(
+            model="synthetic", api_key="synthetic", base_url="http://127.0.0.1:9/v1"
+        )
         self._scripted = list(scripted or [])
         self._idx = 0
         self._default = default_reply