diff --git a/backend/services/llm/deepseek_provider.py b/backend/services/llm/deepseek_provider.py
index b81efaa..927a1e9 100644
--- a/backend/services/llm/deepseek_provider.py
+++ b/backend/services/llm/deepseek_provider.py
@@ -39,10 +39,6 @@ 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:
@@ -103,20 +99,23 @@ def _user_content_parts(blocks: list[ContentBlock], *, vision: bool) -> list[dic
 
 
 def _repair_assistant_messages(messages: list[dict]) -> list[dict]:
-    """Ensure every assistant turn has content or tool_calls (DeepSeek 400)."""
+    """Ensure every assistant turn has a string ``content`` (DeepSeek 400).
+
+    ``content: null`` / omitted content is rejected even when ``tool_calls``
+    is present. Empty string is accepted.
+    """
     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 != ""):
+        if isinstance(content, str):
             out.append(m)
             continue
         fixed = dict(m)
-        fixed["content"] = _EMPTY_ASSISTANT_CONTENT
+        fixed["content"] = content if isinstance(content, str) else ""
         out.append(fixed)
         changed = True
     return out if changed else messages
@@ -136,8 +135,10 @@ 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 rejects content=null and a missing content key, even
+            # when tool_calls is set. Empty string is accepted.
+            msg["content"] = text
             if tool_calls:
-                msg["content"] = text if text else None
                 msg["tool_calls"] = [
                     {
                         "id": tc.id,
@@ -149,8 +150,6 @@ def messages_to_openai(messages: list[Message], *, vision: bool) -> list[dict]:
                     }
                     for tc in tool_calls
                 ]
-            else:
-                msg["content"] = text or _EMPTY_ASSISTANT_CONTENT
             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 0214643..95c157d 100644
--- a/tests/test_deepseek_provider.py
+++ b/tests/test_deepseek_provider.py
@@ -12,6 +12,7 @@ from backend.config import settings
 from backend.services.llm.pdf_ingest import extract_pdf_text, make_text_pdf
 from backend.services.llm.deepseek_provider import (
     _is_vision_model,
+    _repair_assistant_messages,
     _to_openai_tool,
     _to_openai_tool_choice,
     completion_from_openai,
@@ -72,10 +73,8 @@ def test_messages_to_openai_pdf_becomes_text(sample_pdf: Path):
     assert "Extract the pin table" in blob
 
 
-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
-
+def test_thinking_only_assistant_sends_empty_string_content():
+    """DeepSeek accepts content=\"\" and rejects content=null / omitted."""
     out = messages_to_openai(
         [Message("assistant", [
             TextBlock("", reasoning_content="Need to inspect pin 3 first."),
@@ -83,20 +82,18 @@ def test_thinking_only_assistant_sends_placeholder_content():
         vision=False,
     )
     assert out[0]["role"] == "assistant"
-    assert out[0]["content"] == _EMPTY_ASSISTANT_CONTENT
-    assert out[0]["content"]
+    assert out[0]["content"] == ""
+    assert "content" in out[0]
     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
+    assert out[0]["content"] == ""
 
 
-def test_tool_call_assistant_may_have_null_content():
+def test_tool_call_assistant_sends_empty_string_content():
     out = messages_to_openai(
         [Message("assistant", [
             ToolCall(id="c1", name="get_pintable", input={"ref": "U2"},
@@ -104,11 +101,20 @@ def test_tool_call_assistant_may_have_null_content():
         ])],
         vision=False,
     )
-    assert out[0]["content"] is None
+    assert out[0]["content"] == ""
     assert out[0]["tool_calls"][0]["function"]["name"] == "get_pintable"
     assert out[0]["reasoning_content"] == "look up pins"
 
 
+def test_repair_null_content_even_when_tool_calls_present():
+    repaired = _repair_assistant_messages([
+        {"role": "assistant", "content": None, "tool_calls": [{"id": "c1"}]},
+        {"role": "user", "content": "ok"},
+    ])
+    assert repaired[0]["content"] == ""
+    assert repaired[0]["tool_calls"][0]["id"] == "c1"
+
+
 def test_messages_to_openai_tool_roundtrip():
     messages = [
         Message("assistant", [