diff --git a/backend/services/llm/deepseek_provider.py b/backend/services/llm/deepseek_provider.py
index aa3d32b..80026f4 100644
--- a/backend/services/llm/deepseek_provider.py
+++ b/backend/services/llm/deepseek_provider.py
@@ -111,8 +111,11 @@ 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)
-            msg["content"] = text if text else None
+            # 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"] = [
                     {
                         "id": tc.id,
@@ -124,6 +127,8 @@ def messages_to_openai(messages: list[Message], *, vision: bool) -> list[dict]:
                     }
                     for tc in tool_calls
                 ]
+            else:
+                msg["content"] = text
             reasoning = _reasoning_from_blocks(m.content)
             if reasoning:
                 msg["reasoning_content"] = reasoning
diff --git a/tests/test_deepseek_provider.py b/tests/test_deepseek_provider.py
index 026d8b7..831721b 100644
--- a/tests/test_deepseek_provider.py
+++ b/tests/test_deepseek_provider.py
@@ -72,6 +72,33 @@ 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."""
+    out = messages_to_openai(
+        [Message("assistant", [
+            TextBlock("", reasoning_content="Need to inspect pin 3 first."),
+        ])],
+        vision=False,
+    )
+    assert out[0]["role"] == "assistant"
+    assert out[0]["content"] == ""
+    assert out[0]["reasoning_content"] == "Need to inspect pin 3 first."
+    assert "tool_calls" not in out[0]
+
+
+def test_tool_call_assistant_may_have_null_content():
+    out = messages_to_openai(
+        [Message("assistant", [
+            ToolCall(id="c1", name="get_pintable", input={"ref": "U2"},
+                     reasoning_content="look up pins"),
+        ])],
+        vision=False,
+    )
+    assert out[0]["content"] is None
+    assert out[0]["tool_calls"][0]["function"]["name"] == "get_pintable"
+    assert out[0]["reasoning_content"] == "look up pins"
+
+
 def test_messages_to_openai_tool_roundtrip():
     messages = [
         Message("assistant", [