diff --git a/config.py b/config.py
index 7054ba0d..87946c82 100644
--- a/config.py
+++ b/config.py
@@ -2,6 +2,10 @@
 import os
 
 
+_ATLAS_CLOUD_PREFIX = "atlascloud/"
+_ATLAS_CLOUD_BASE_URL = "https://api.atlascloud.ai/v1"
+
+
 def get_default_model(fallback: str = "gpt-5.4"):
     """Return the configured default model for standard agents."""
     model = os.getenv("DEFAULT_MODEL", fallback)
@@ -12,18 +16,28 @@ def is_openai_provider() -> bool:
     """Return True when the configured provider is OpenAI (not LiteLLM).
 
     OpenAI model IDs never contain a slash (e.g. 'gpt-5.4', 'o3').
-    Any 'provider/model' string (e.g. 'anthropic/claude-sonnet-4-6',
-    'litellm/gemini/gemini-3-flash') is treated as a LiteLLM-routed model.
+    Any provider-prefixed model, including Atlas Cloud and LiteLLM routes,
+    must not receive Responses API-only reasoning or web-search settings.
     """
     return "/" not in os.getenv("DEFAULT_MODEL", "")
 
 
 def _resolve(model: str):
-    """Route 'provider/model' strings through LitellmModel.
+    """Resolve Atlas Cloud and LiteLLM provider-prefixed model strings."""
+    if model.startswith(_ATLAS_CLOUD_PREFIX):
+        model_id = model[len(_ATLAS_CLOUD_PREFIX) :]
+        if not model_id:
+            raise ValueError("Atlas Cloud model ID is required after 'atlascloud/'")
+        api_key = os.getenv("ATLASCLOUD_API_KEY")
+        if not api_key:
+            raise RuntimeError("ATLASCLOUD_API_KEY environment variable is required")
+
+        from agents import OpenAIChatCompletionsModel
+        from openai import AsyncOpenAI
+
+        client = AsyncOpenAI(api_key=api_key, base_url=_ATLAS_CLOUD_BASE_URL)
+        return OpenAIChatCompletionsModel(model=model_id, openai_client=client)
 
-    Handles both explicit 'litellm/<model>' and bare 'provider/model' forms.
-    OpenAI model IDs contain no slash, so they pass through unchanged.
-    """
     if "/" not in model:
         return model
     bare = model[len("litellm/"):] if model.startswith("litellm/") else model
diff --git a/tests/test_atlascloud_model.py b/tests/test_atlascloud_model.py
new file mode 100644
index 00000000..b48ff03c
--- /dev/null
+++ b/tests/test_atlascloud_model.py
@@ -0,0 +1,78 @@
+from __future__ import annotations
+
+import importlib
+import sys
+import types
+
+import pytest
+
+
+class FakeAsyncOpenAI:
+    def __init__(self, *, api_key, base_url):
+        self.api_key = api_key
+        self.base_url = base_url
+
+
+class FakeChatCompletionsModel:
+    def __init__(self, *, model, openai_client):
+        self.model = model
+        self.openai_client = openai_client
+
+
+class FakeLitellmModel:
+    def __init__(self, *, model):
+        self.model = model
+
+
+@pytest.fixture
+def config_module(monkeypatch):
+    agents = types.ModuleType("agents")
+    agents.OpenAIChatCompletionsModel = FakeChatCompletionsModel
+    openai = types.ModuleType("openai")
+    openai.AsyncOpenAI = FakeAsyncOpenAI
+    agency_swarm = types.ModuleType("agency_swarm")
+    agency_swarm.LitellmModel = FakeLitellmModel
+    monkeypatch.setitem(sys.modules, "agents", agents)
+    monkeypatch.setitem(sys.modules, "openai", openai)
+    monkeypatch.setitem(sys.modules, "agency_swarm", agency_swarm)
+    sys.modules.pop("config", None)
+    yield importlib.import_module("config")
+    sys.modules.pop("config", None)
+
+
+def test_atlascloud_model_uses_chat_completions_client(monkeypatch, config_module):
+    monkeypatch.setenv("ATLASCLOUD_API_KEY", "test-key")
+    monkeypatch.setenv("DEFAULT_MODEL", "atlascloud/deepseek-ai/deepseek-v4-pro")
+
+    model = config_module.get_default_model()
+
+    assert isinstance(model, FakeChatCompletionsModel)
+    assert model.model == "deepseek-ai/deepseek-v4-pro"
+    assert model.openai_client.api_key == "test-key"
+    assert model.openai_client.base_url == "https://api.atlascloud.ai/v1"
+    assert config_module.is_openai_provider() is False
+
+
+def test_atlascloud_model_requires_api_key(monkeypatch, config_module):
+    monkeypatch.delenv("ATLASCLOUD_API_KEY", raising=False)
+    monkeypatch.setenv("DEFAULT_MODEL", "atlascloud/deepseek-ai/deepseek-v4-pro")
+
+    with pytest.raises(RuntimeError, match="ATLASCLOUD_API_KEY"):
+        config_module.get_default_model()
+
+
+def test_atlascloud_model_requires_model_id(monkeypatch, config_module):
+    monkeypatch.setenv("ATLASCLOUD_API_KEY", "test-key")
+    monkeypatch.setenv("DEFAULT_MODEL", "atlascloud/")
+
+    with pytest.raises(ValueError, match="model ID"):
+        config_module.get_default_model()
+
+
+def test_existing_litellm_route_is_unchanged(monkeypatch, config_module):
+    monkeypatch.setenv("DEFAULT_MODEL", "anthropic/claude-sonnet")
+
+    model = config_module.get_default_model()
+
+    assert isinstance(model, FakeLitellmModel)
+    assert model.model == "anthropic/claude-sonnet"