diff --git a/agent/anthropic_adapter.py b/agent/anthropic_adapter.py
index 7815b1faca..fef0b007de 100644
--- a/agent/anthropic_adapter.py
+++ b/agent/anthropic_adapter.py
@@ -345,13 +345,18 @@ def _build_anthropic_client_with_bearer_hook(
 def _new_sdk_client(sdk, kwargs: Dict[str, Any], headers: Dict[str, str]):
     """``sdk.Anthropic(**kwargs)`` with ``headers`` attached. Bearer-only construction leaves
     ``api_key`` unset, so the SDK fills it from ANTHROPIC_API_KEY (loaded from ~/.hermes/.env) and
-    sends dual auth — X-Api-Key *and* Authorization: Bearer — on every Portal/MiniMax/OAuth/Entra
-    request; clear it whenever we intentionally authenticated via auth_token."""
+    sends dual auth — X-Api-Key *and* Authorization: Bearer *** on every Portal/MiniMax/OAuth/Entra
+    request; clear it whenever we intentionally authenticated via auth_token. Api-key-only
+    construction has the mirror problem: the SDK fills ``auth_token`` from ANTHROPIC_AUTH_TOKEN in
+    the environment and ships that Bearer credential to third-party Anthropic-compatible endpoints
+    alongside x-api-key — clear it whenever we intentionally authenticated via api_key."""
     if headers:
         kwargs["default_headers"] = headers
     client = sdk.Anthropic(**kwargs)
     if "auth_token" in kwargs and "api_key" not in kwargs:
         client.api_key = None
+    if "api_key" in kwargs and "auth_token" not in kwargs:
+        client.auth_token = None
     return client
 
 
diff --git a/tests/agent/test_anthropic_adapter.py b/tests/agent/test_anthropic_adapter.py
index 7ed804baa7..abc32ade74 100644
--- a/tests/agent/test_anthropic_adapter.py
+++ b/tests/agent/test_anthropic_adapter.py
@@ -1896,3 +1896,93 @@ class TestFinalPayloadHasNoBlankTextBlocks:
         )
         image_blocks = [b for b in tool_result_block["content"] if b.get("type") == "image"]
         assert len(image_blocks) == 1
+
+
+class TestApiKeyConstructionClearsEnvBearerToken:
+    """Api-key-style clients must not inherit ANTHROPIC_AUTH_TOKEN from the environment.
+
+    The Anthropic SDK fills an unset ``auth_token`` from ANTHROPIC_AUTH_TOKEN and then sends
+    ``Authorization: Bearer ***`` alongside ``x-api-key`` on every request, shipping a
+    foreign shell credential to third-party Anthropic-compatible endpoints.
+    """
+
+    def test_api_key_style_client_drops_env_derived_auth_token(self, monkeypatch):
+        anthropic_sdk = pytest.importorskip("anthropic")
+        monkeypatch.setenv("ANTHROPIC_AUTH_TOKEN", "sentinel-env-token-DO-NOT-SEND")
+        from agent.anthropic_adapter import _new_sdk_client
+
+        client = _new_sdk_client(
+            anthropic_sdk,
+            {"api_key": "provider-key", "base_url": "http://127.0.0.1:1"},
+            {},
+        )
+        assert client.api_key == "provider-key"
+        assert client.auth_token is None
+        assert "Authorization" not in client.auth_headers
+        assert client.auth_headers == {"X-Api-Key": "provider-key"}
+
+    def test_bearer_style_client_keeps_its_auth_token(self, monkeypatch):
+        anthropic_sdk = pytest.importorskip("anthropic")
+        monkeypatch.setenv("ANTHROPIC_API_KEY", "sentinel-env-key-DO-NOT-SEND")
+        from agent.anthropic_adapter import _new_sdk_client
+
+        client = _new_sdk_client(
+            anthropic_sdk,
+            {"auth_token": "bearer-secret", "base_url": "http://127.0.0.1:1"},
+            {},
+        )
+        assert client.auth_token == "bearer-secret"
+        assert client.api_key is None
+        assert client.auth_headers == {"Authorization": "Bearer bearer-secret"}
+
+    def test_third_party_request_carries_no_foreign_bearer(self, monkeypatch):
+        """End-to-end over a local header-capturing server: x-api-key intact, sentinel absent."""
+        anthropic_sdk = pytest.importorskip("anthropic")
+        http_server = pytest.importorskip("http.server")
+        monkeypatch.setenv("ANTHROPIC_AUTH_TOKEN", "sentinel-env-token-DO-NOT-SEND")
+
+        import json as _json
+        import threading
+
+        captured = {}
+
+        class _Handler(http_server.BaseHTTPRequestHandler):
+            def do_POST(self):
+                captured["headers"] = {k.lower(): v for k, v in self.headers.items()}
+                self.rfile.read(int(self.headers.get("content-length", 0)))
+                body = _json.dumps({
+                    "id": "msg_test",
+                    "type": "message",
+                    "role": "assistant",
+                    "content": [{"type": "text", "text": "ok"}],
+                    "model": "test",
+                    "stop_reason": "end_turn",
+                    "usage": {"input_tokens": 1, "output_tokens": 1},
+                }).encode()
+                self.send_response(200)
+                self.send_header("content-type", "application/json")
+                self.send_header("content-length", str(len(body)))
+                self.end_headers()
+                self.wfile.write(body)
+
+            def log_message(self, *args):
+                pass
+
+        server = http_server.HTTPServer(("127.0.0.1", 0), _Handler)
+        threading.Thread(target=server.serve_forever, daemon=True).start()
+        try:
+            client = build_anthropic_client(
+                "third-party-provider-key",
+                base_url=f"http://127.0.0.1:{server.server_port}",
+            )
+            client.messages.create(
+                model="test-model",
+                max_tokens=8,
+                messages=[{"role": "user", "content": "hi"}],
+            )
+        finally:
+            server.shutdown()
+
+        headers = captured["headers"]
+        assert headers.get("x-api-key") == "third-party-provider-key"
+        assert "sentinel-env-token-DO-NOT-SEND" not in headers.get("authorization", "")