diff --git a/src/agent_loop.py b/src/agent_loop.py
index 5effc54b5..45570a90f 100644
--- a/src/agent_loop.py
+++ b/src/agent_loop.py
@@ -1948,6 +1948,10 @@ async def stream_agent_loop(
     # and can override this list for users who know their setup.
     _model_no_tools = any(kw in _model_lc for kw in (
         "deepseek-r1",
+        # Open-weight GPT-OSS models are commonly served through llama.cpp /
+        # llama-cpp-python. Their names contain "gpt-o", but they do not use
+        # OpenAI's native tool-call channel unless the endpoint opts in.
+        "gpt-oss",
     ))
     # Native Ollama endpoints (/api/chat) handle tool schemas differently from
     # the OpenAI-compat path. Models like gemma4, qwen3.5, ministral respond to
diff --git a/tests/test_tool_support_heuristic.py b/tests/test_tool_support_heuristic.py
index ed2dbc76d..9294fc740 100644
--- a/tests/test_tool_support_heuristic.py
+++ b/tests/test_tool_support_heuristic.py
@@ -25,6 +25,7 @@ def _compute_is_api_model(model: str, endpoint_url: str, endpoint_supports=None)
     ))
     model_no_tools = any(kw in model_lc for kw in (
         "deepseek-r1",
+        "gpt-oss",
     ))
 
     if endpoint_supports is True:
@@ -72,6 +73,11 @@ class TestDeepSeekToolSupport:
             "gemma4:e4b", "http://host.docker.internal:11434/v1"
         ) is False
 
+    def test_gpt_oss_local_openai_compat_defaults_to_fenced_tools(self):
+        assert _compute_is_api_model(
+            "gpt-oss-20b", "http://localhost:8000/v1"
+        ) is False
+
     def test_qwen_native_ollama_defaults_to_fenced_tools(self):
         assert _compute_is_api_model(
             "qwen3.5:4b", "http://localhost:11434/api/chat"
@@ -117,6 +123,12 @@ class TestDeepSeekToolSupport:
         )
         assert result is True
 
+    def test_endpoint_supports_true_overrides_gpt_oss_default(self):
+        result = _compute_is_api_model(
+            "gpt-oss-20b", "http://localhost:8000/v1", endpoint_supports=True
+        )
+        assert result is True
+
     def test_endpoint_supports_false_overrides_cloud(self):
         """supports_tools=False on an endpoint gates even cloud APIs."""
         result = _compute_is_api_model(