Retry DeepSeek thinking-only turns so review does not 400 on U19.

Empty assistant content is still treated as unset; replay a placeholder and retry once with thinking disabled.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/services/llm/deepseek_provider.py

@@ -16,7 +16,7 @@ import json
 import logging
 from typing import Any
 
-from openai import AsyncOpenAI
+from openai import APIStatusError, AsyncOpenAI
 
 from backend.config import settings
 from backend.services.llm.base import LLMProvider, LLMSession
@@ -38,6 +38,10 @@ from backend.services.llm.types import (
 log = logging.getLogger(__name__)
 
 _VISION_HINT = "vision"
+# DeepSeek 400s when an assistant turn has reasoning only: content=null
+# and no tool_calls. Empty string is also treated as unset, so replay a
+# non-empty placeholder (the official SDK sample would send content=None).
+_EMPTY_ASSISTANT_CONTENT = " "
 
 
 def _is_vision_model(model: str) -> bool:
@@ -97,6 +101,26 @@ def _user_content_parts(blocks: list[ContentBlock], *, vision: bool) -> list[dic
     return parts
 
 
+def _repair_assistant_messages(messages: list[dict]) -> list[dict]:
+    """Ensure every assistant turn has content or tool_calls (DeepSeek 400)."""
+    out: list[dict] = []
+    changed = False
+    for m in messages:
+        if m.get("role") != "assistant":
+            out.append(m)
+            continue
+        tools = m.get("tool_calls") or []
+        content = m.get("content")
+        if tools or (isinstance(content, str) and content != ""):
+            out.append(m)
+            continue
+        fixed = dict(m)
+        fixed["content"] = _EMPTY_ASSISTANT_CONTENT
+        out.append(fixed)
+        changed = True
+    return out if changed else messages
+
+
 def messages_to_openai(messages: list[Message], *, vision: bool) -> list[dict]:
     """Convert unified messages into DeepSeek/OpenAI chat messages.
 
@@ -111,9 +135,6 @@ def messages_to_openai(messages: list[Message], *, vision: bool) -> list[dict]:
             tool_calls = [b for b in m.content if isinstance(b, ToolCall)]
             msg: dict[str, Any] = {"role": "assistant"}
             text = "".join(text_parts)
-            # DeepSeek 400s with "content or tool_calls must be set" when
-            # thinking-mode returns reasoning only (content=null, no tools).
-            # Tool turns may keep content=null; text-only turns need "".
             if tool_calls:
                 msg["content"] = text if text else None
                 msg["tool_calls"] = [
@@ -128,7 +149,7 @@ def messages_to_openai(messages: list[Message], *, vision: bool) -> list[dict]:
                     for tc in tool_calls
                 ]
             else:
-                msg["content"] = text
+                msg["content"] = text or _EMPTY_ASSISTANT_CONTENT
             reasoning = _reasoning_from_blocks(m.content)
             if reasoning:
                 msg["reasoning_content"] = reasoning
@@ -281,8 +302,44 @@ class DeepSeekSession(LLMSession):
             kwargs["tools"] = [_to_openai_tool(t) for t in tools]
             kwargs["tool_choice"] = _to_openai_tool_choice(tool_choice)
 
-        resp = await self._client.chat.completions.create(**kwargs)
-        return completion_from_openai(resp)
+        resp = await self._create(kwargs)
+        completion = completion_from_openai(resp)
+        # Thinking-only (no visible text, no tools) cannot be replayed on the
+        # next turn. Retry once with thinking off so the review loop gets a
+        # real content/tool_calls message instead of a 400 on the follow-up.
+        if (
+            thinking
+            and not (completion.text or "").strip()
+            and not completion.tool_calls
+        ):
+            log.warning(
+                "DeepSeek thinking-only turn (no content/tool_calls); "
+                "retrying with thinking disabled"
+            )
+            extra_body = {
+                "thinking": {"type": "disabled"},
+            }
+            kwargs = {**kwargs, "extra_body": extra_body}
+            resp = await self._create(kwargs)
+            completion = completion_from_openai(resp)
+        return completion
+
+    async def _create(self, kwargs: dict[str, Any]) -> Any:
+        try:
+            return await self._client.chat.completions.create(**kwargs)
+        except APIStatusError as exc:
+            body = str(exc)
+            if exc.status_code != 400 or "content or tool_calls" not in body:
+                raise
+            repaired = _repair_assistant_messages(list(kwargs["messages"]))
+            if repaired == kwargs["messages"]:
+                raise
+            log.warning(
+                "DeepSeek 400 (empty assistant content); retrying with "
+                "placeholder content"
+            )
+            kwargs = {**kwargs, "messages": repaired}
+            return await self._client.chat.completions.create(**kwargs)
 
     async def close(self) -> None:
         return None

tests/test_deepseek_provider.py

@@ -72,8 +72,10 @@ def test_messages_to_openai_pdf_becomes_text(sample_pdf: Path):
     assert "Extract the pin table" in blob
 
 
-def test_thinking_only_assistant_sends_empty_content():
-    """Thinking with no visible text and no tools must still set content."""
+def test_thinking_only_assistant_sends_placeholder_content():
+    """Empty string is treated as unset; replay a non-empty placeholder."""
+    from backend.services.llm.deepseek_provider import _EMPTY_ASSISTANT_CONTENT
+
     out = messages_to_openai(
         [Message("assistant", [
             TextBlock("", reasoning_content="Need to inspect pin 3 first."),
@@ -81,11 +83,19 @@ def test_thinking_only_assistant_sends_empty_content():
         vision=False,
     )
     assert out[0]["role"] == "assistant"
-    assert out[0]["content"] == ""
+    assert out[0]["content"] == _EMPTY_ASSISTANT_CONTENT
+    assert out[0]["content"]
     assert out[0]["reasoning_content"] == "Need to inspect pin 3 first."
     assert "tool_calls" not in out[0]
 
 
+def test_empty_assistant_blocks_still_set_content():
+    from backend.services.llm.deepseek_provider import _EMPTY_ASSISTANT_CONTENT
+
+    out = messages_to_openai([Message("assistant", [])], vision=False)
+    assert out[0]["content"] == _EMPTY_ASSISTANT_CONTENT
+
+
 def test_tool_call_assistant_may_have_null_content():
     out = messages_to_openai(
         [Message("assistant", [
@@ -165,6 +175,66 @@ def test_forced_tool_choice_disables_thinking():
     assert captured["tool_choice"]["function"]["name"] == "save_resolved_specs"
 
 
+def test_thinking_only_retries_with_thinking_disabled():
+    calls: list[dict] = []
+
+    class FakeCompletions:
+        async def create(self, **kwargs):
+            calls.append({
+                "thinking": kwargs["extra_body"]["thinking"]["type"],
+            })
+            usage = SimpleNamespace(
+                prompt_tokens=10, completion_tokens=5,
+                prompt_cache_hit_tokens=0, prompt_tokens_details=None,
+            )
+            if kwargs["extra_body"]["thinking"]["type"] == "enabled":
+                msg = SimpleNamespace(
+                    content=None,
+                    reasoning_content="pondering the schematic",
+                    tool_calls=None,
+                )
+                return SimpleNamespace(
+                    choices=[SimpleNamespace(message=msg, finish_reason="stop")],
+                    usage=usage,
+                )
+            fn = SimpleNamespace(name="get_pintable", arguments='{"ref":"U19"}')
+            tc = SimpleNamespace(id="c1", function=fn)
+            msg = SimpleNamespace(content=None, reasoning_content=None, tool_calls=[tc])
+            return SimpleNamespace(
+                choices=[SimpleNamespace(message=msg, finish_reason="tool_calls")],
+                usage=usage,
+            )
+
+    class FakeClient:
+        def __init__(self):
+            self.chat = SimpleNamespace(completions=FakeCompletions())
+
+    from backend.services.llm.deepseek_provider import DeepSeekSession
+    from backend.services.llm.types import Message, TextBlock, ToolSchema
+    import asyncio
+
+    session = DeepSeekSession(
+        client=FakeClient(),
+        model="deepseek-v4-pro",
+        system="sys",
+        max_tokens=256,
+        thinking=True,
+        reasoning_effort="medium",
+    )
+    completion = asyncio.run(session.complete(
+        messages=[Message("user", [TextBlock("review U19")])],
+        tools=[ToolSchema(
+            name="get_pintable", description="x",
+            input_schema={"type": "object"},
+        )],
+        tool_choice="auto",
+    ))
+    assert len(calls) == 2
+    assert calls[0]["thinking"] == "enabled"
+    assert calls[1]["thinking"] == "disabled"
+    assert completion.tool_calls[0].name == "get_pintable"
+
+
 def test_tool_schema_and_choice():
     schema = ToolSchema(
         name="save_pintable",