fix(llm): apply DeepSeek capabilities to OpenRouter-namespaced models

- OpenRouter exposes DeepSeek as deepseek/<id>, which matched neither the exact
  IDs nor the patterns, so a thinking model like deepseek/deepseek-v4-flash fell
  through to _DEFAULT and had object-form tool_choice forced on it
- strip the official deepseek/ namespace before lookup so it reuses the native
  quirks; deepseek/deepseek-chat still keeps tool_choice, and third-party
  finetunes on other publishers stay on _DEFAULT #1199

tests/test_capabilities.py

@@ -116,6 +116,37 @@ class TestDefault:
         assert caps.supports_tool_choice is True
 
 
+@pytest.mark.unit
+class TestOpenRouterDeepSeekNamespace:
+    """OpenRouter namespaces DeepSeek as ``deepseek/<id>``; strip it so the
+    same quirks apply as the native provider (#1199)."""
+
+    def test_prefixed_v4_flash_suppresses_tool_choice(self):
+        # Was falling through to _DEFAULT (tool_choice on) -> slow object-form call.
+        assert get_capabilities("deepseek/deepseek-v4-flash").supports_tool_choice is False
+
+    def test_prefixed_reasoner_suppresses_tool_choice(self):
+        assert get_capabilities("deepseek/deepseek-reasoner").supports_tool_choice is False
+
+    def test_prefixed_chat_selects_deepseek_chat_not_default(self):
+        # Must resolve to _DEEPSEEK_CHAT, not _DEFAULT: supports_json_schema=False
+        # is what distinguishes them (both keep tool_choice).
+        caps = get_capabilities("deepseek/deepseek-chat")
+        assert caps.supports_tool_choice is True
+        assert caps.supports_json_schema is False  # _DEEPSEEK_CHAT, not _DEFAULT
+
+    def test_only_official_namespace_is_stripped(self):
+        # A third-party publisher whose model name WOULD match a deepseek pattern
+        # must stay _DEFAULT: proves we strip only "deepseek/", not any "*/".
+        caps = get_capabilities("tngtech/deepseek-v4-flash")
+        assert caps.supports_tool_choice is True         # not thinking
+        assert caps.supports_json_schema is True          # _DEFAULT
+
+    def test_native_ids_unchanged(self):
+        assert get_capabilities("deepseek-v4-flash").supports_tool_choice is False
+        assert get_capabilities("deepseek-chat").supports_tool_choice is True
+
+
 @pytest.mark.unit
 def test_capabilities_dataclass_is_frozen():
     """Capability rows are immutable so they can be safely shared."""

tradingagents/llm_clients/capabilities.py

@@ -118,6 +118,15 @@ _BY_PATTERN: list[tuple[re.Pattern[str], ModelCapabilities]] = [
 
 def get_capabilities(model_name: str) -> ModelCapabilities:
     """Resolve capabilities by exact ID, then pattern, then default."""
+    # OpenRouter namespaces official DeepSeek models as ``deepseek/<id>``, so
+    # strip that prefix to reuse the same quirks as the native provider — e.g.
+    # ``deepseek/deepseek-v4-flash`` must suppress tool_choice like
+    # ``deepseek-v4-flash`` does, not fall through to _DEFAULT (#1199). Only the
+    # official namespace is stripped; third-party finetunes on other publishers
+    # (e.g. ``tngtech/deepseek-...``) keep _DEFAULT, since their quirks are unknown.
+    if model_name.startswith("deepseek/"):
+        model_name = model_name.removeprefix("deepseek/")
+
     if model_name in _BY_ID:
         return _BY_ID[model_name]
     for pattern, caps in _BY_PATTERN: