fix(deepseek): add empty-string fallback for reasoning_content in cross-provider scenarios (#192)

EvoScientist/llm/patches.py

@@ -481,8 +481,18 @@ def _patch_deepseek_reasoning_passback(model: Any) -> None:
     message to carry its reasoning_content as a top-level field (sibling to
     content / tool_calls).  Without this, multi-turn requests fail with 400.
 
-    For deepseek-reasoner models, also injects an empty reasoning_content
-    when none is present (DeepSeek API requires the field even if empty).
+    For assistant messages where no reasoning_content was captured (e.g.
+    history left over from another provider, from DeepSeek Flash, or from an
+    older EvoSci version that ran before the capture patch landed), we
+    inject an empty string.  This satisfies DeepSeek's format requirement
+    in thinking mode.  Non-thinking DeepSeek endpoints are believed to
+    accept the extra field without complaint based on observed behavior,
+    but this has not been independently audited; if a future DeepSeek
+    release rejects empty reasoning_content on non-thinking models, this
+    fallback would need a per-call thinking-mode check instead of a blanket
+    inject.  The check is intentionally not gated on model name: this
+    function is only mounted when provider == "deepseek" (see
+    EvoScientist/llm/models.py), so all callers are DeepSeek endpoints.
 
     Args:
         model: A langchain-openai ChatOpenAI instance configured for DeepSeek.
@@ -523,7 +533,6 @@ def _patch_deepseek_reasoning_passback(model: Any) -> None:
         if not isinstance(msgs, list):
             return payload
 
-        is_reasoner = "deepseek-reasoner" in str(getattr(model, "model_name", ""))
         ai_idx = 0
         for msg in msgs:
             if not isinstance(msg, dict) or msg.get("role") != "assistant":
@@ -531,7 +540,13 @@ def _patch_deepseek_reasoning_passback(model: Any) -> None:
             rc = ai_rcs[ai_idx] if ai_idx < len(ai_rcs) else None
             if rc:
                 msg["reasoning_content"] = rc
-            elif is_reasoner and "reasoning_content" not in msg:
+            elif "reasoning_content" not in msg:
+                # Empty-string fallback for ALL DeepSeek models (not just
+                # reasoner). Required when history contains AI messages that
+                # came from a different provider (Anthropic / OpenAI /
+                # DeepSeek Flash) or from an older EvoSci that didn't capture
+                # reasoning_content. Empirically tolerated by non-thinking
+                # DeepSeek endpoints; see docstring for the audit caveat.
                 msg["reasoning_content"] = ""
             ai_idx += 1
 

tests/test_llm.py

@@ -879,7 +879,7 @@ class TestPatchDeepseekReasoningPassback:
 
         assert payload["messages"][1]["reasoning_content"] == ""
 
-    def test_no_injection_for_non_reasoner_without_rc(self):
+    def test_empty_fallback_for_non_reasoner_without_rc(self):
         from langchain_core.messages import AIMessage, HumanMessage
 
         from EvoScientist.llm.patches import _patch_deepseek_reasoning_passback
@@ -894,7 +894,9 @@ class TestPatchDeepseekReasoningPassback:
         ]
         payload = model._get_request_payload(messages)
 
-        assert "reasoning_content" not in payload["messages"][1]
+        # Empty-string fallback applies to ALL DeepSeek models (not just
+        # reasoner) so cross-provider history doesn't trigger 400.
+        assert payload["messages"][1]["reasoning_content"] == ""
 
     def test_handles_multiple_ai_messages(self):
         from langchain_core.messages import AIMessage, HumanMessage
@@ -986,7 +988,7 @@ class TestPatchDeepseekReasoningPassback:
         from EvoScientist.llm.patches import _patch_deepseek_reasoning_passback
 
         model = self._make_model(
-            model_name="deepseek-v4-pro",  # NOT reasoner — empty fallback off
+            model_name="deepseek-v4-pro",
             payload_messages=[
                 {"role": "user", "content": "q1"},
                 {"role": "assistant", "content": "a1"},  # no rc
@@ -1009,8 +1011,8 @@ class TestPatchDeepseekReasoningPassback:
         ]
         payload = model._get_request_payload(messages)
 
-        # First AI msg: no rc → not injected (V4 Pro doesn't get empty fallback)
-        assert "reasoning_content" not in payload["messages"][1]
+        # First AI msg: no rc → empty-string fallback (covers cross-model switch)
+        assert payload["messages"][1]["reasoning_content"] == ""
         # Second AI msg: has rc → injected
         assert payload["messages"][3]["reasoning_content"] == "rc2"
 
@@ -1045,6 +1047,37 @@ class TestPatchDeepseekReasoningPassback:
         assert "input" in payload
         assert "messages" not in payload
 
+    def test_cross_provider_switch_history(self):
+        """User chats with Anthropic/OpenAI then switches to DeepSeek V4 Pro.
+
+        Historical AI messages have no reasoning_content (the previous
+        provider never produced it). The patch must inject an empty-string
+        fallback so DeepSeek doesn't 400 on
+        "reasoning_content must be passed back to the API".
+        """
+        from langchain_core.messages import AIMessage, HumanMessage
+
+        from EvoScientist.llm.patches import _patch_deepseek_reasoning_passback
+
+        model = self._make_model(
+            model_name="deepseek-v4-pro",
+            payload_messages=[
+                {"role": "user", "content": "earlier question to anthropic"},
+                {"role": "assistant", "content": "anthropic answer"},
+                {"role": "user", "content": "now ask deepseek pro"},
+            ],
+        )
+        _patch_deepseek_reasoning_passback(model)
+
+        messages = [
+            HumanMessage("earlier question to anthropic"),
+            AIMessage(content="anthropic answer"),  # no reasoning_content
+            HumanMessage("now ask deepseek pro"),
+        ]
+        payload = model._get_request_payload(messages)
+
+        assert payload["messages"][1]["reasoning_content"] == ""
+
 
 # =============================================================================
 # Test _patch_openai_capture_reasoning_content (module-level monkey-patch)