diff --git a/plugins/model-providers/deepseek/__init__.py b/plugins/model-providers/deepseek/__init__.py
index f67146df113..525766f87eb 100644
--- a/plugins/model-providers/deepseek/__init__.py
+++ b/plugins/model-providers/deepseek/__init__.py
@@ -94,6 +94,7 @@ deepseek = DeepSeekProfile(
         "deepseek-reasoner",
     ),
     base_url="https://api.deepseek.com/v1",
+    default_aux_model="deepseek-chat",
 )
 
 register_provider(deepseek)
diff --git a/tests/plugins/model_providers/test_deepseek_profile.py b/tests/plugins/model_providers/test_deepseek_profile.py
index c53e70070a8..8c316a38086 100644
--- a/tests/plugins/model_providers/test_deepseek_profile.py
+++ b/tests/plugins/model_providers/test_deepseek_profile.py
@@ -182,3 +182,26 @@ class TestDeepSeekFullKwargsIntegration:
         )
         assert "reasoning_effort" not in kwargs
         assert "extra_body" not in kwargs or "thinking" not in kwargs.get("extra_body", {})
+
+
+class TestDeepSeekAuxModel:
+    """DeepSeek aux model is set on the profile so users stop seeing the
+    bogus 'No auxiliary LLM provider configured' warning (#26924).
+
+    Pinned at the profile layer rather than the legacy
+    `_API_KEY_PROVIDER_AUX_MODELS_FALLBACK` dict — new providers are
+    expected to set `default_aux_model` on `ProviderProfile`, and the
+    fallback dict only exists for providers that predate the profiles
+    system.
+    """
+
+    def test_profile_advertises_deepseek_chat(self, deepseek_profile):
+        assert deepseek_profile.default_aux_model == "deepseek-chat"
+
+    def test_consumer_api_returns_deepseek_chat(self):
+        from agent.auxiliary_client import _get_aux_model_for_provider
+        assert _get_aux_model_for_provider("deepseek") == "deepseek-chat"
+
+    def test_consumer_api_returns_non_empty(self):
+        from agent.auxiliary_client import _get_aux_model_for_provider
+        assert _get_aux_model_for_provider("deepseek") != ""