fix(databricks): only inject a reasoning block when content follows it

Databricks rejects a reasoning block in the final position of an assistant
message. That is exactly a thinking-plus-tool-call turn: the call moves to
message-level tool_calls, leaving the reasoning block alone and therefore
final, which returns "The final block in an assistant message cannot be
thinking".

Verified against a live endpoint. The keys are still stripped when the block
cannot be placed, so neither the original nor this failure can occur.

litellm/llms/databricks/chat/transformation.py

@@ -481,10 +481,13 @@ class DatabricksConfig(DatabricksBase, OpenAILikeChatConfig, AnthropicConfig):
 
         Databricks rejects both message-level keys outright, and constrains the replacement to
         exactly one reasoning block holding exactly one summary entry, whose signature is
-        required. Blocks without a signature cannot be replayed, and a message already carrying
-        a reasoning block must not gain a second one, so in both cases the keys are dropped
-        rather than sent in a form the API refuses. reasoning_content is a plain-text mirror of
-        the same thinking, so it carries nothing the signed summary does not.
+        required. It also rejects a reasoning block as the final block of an assistant message,
+        which happens when the turn was thinking plus a tool call: the call moves to
+        message-level tool_calls and leaves nothing behind it. So the block is only injected
+        when other content follows it, and a message already carrying a reasoning block never
+        gains a second one. Otherwise the keys are dropped rather than sent in a form the API
+        refuses. reasoning_content is a plain-text mirror of the same thinking, so it carries
+        nothing the signed summary does not.
         """
         dropped: Final = ("thinking_blocks", "reasoning_content")
         stripped: Final = {k: v for k, v in message.items() if k not in dropped}  # mutable-ok: outbound provider JSON
@@ -497,7 +500,7 @@ class DatabricksConfig(DatabricksBase, OpenAILikeChatConfig, AnthropicConfig):
             None,
         )
         holds_reasoning: Final = any(isinstance(b, dict) and b.get("type") == "reasoning" for b in existing)
-        replace: Final = signed is not None and not holds_reasoning
+        replace: Final = signed is not None and not holds_reasoning and bool(existing)
         text: Final = (signed.get("thinking") or "") if signed is not None else ""
         signature: Final = signed["signature"] if signed is not None else ""
         entry: Final = {"type": "summary_text", "text": text, "signature": signature}  # mutable-ok: provider JSON

tests/test_litellm/llms/databricks/chat/test_databricks_chat_transformation.py

@@ -626,3 +626,35 @@ def test_existing_reasoning_block_is_not_duplicated():
 
     assert "thinking_blocks" not in result
     assert [b for b in result["content"] if b.get("type") == "reasoning"] == [already_converted]
+
+
+def test_reasoning_block_is_not_emitted_as_the_only_content_block():
+    """Databricks rejects a reasoning block as the final block of an assistant message. That
+    is exactly a thinking-plus-tool-call turn, where the call moves to message-level
+    tool_calls and leaves nothing after the reasoning block, so the block is dropped."""
+    result = DatabricksConfig()._move_reasoning_into_content_block(
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [{"id": "call_1", "type": "function", "function": {"name": "bash", "arguments": "{}"}}],
+            "thinking_blocks": [{"type": "thinking", "thinking": "", "signature": "sig-abc"}],
+        }
+    )
+
+    assert "thinking_blocks" not in result
+    assert not any(isinstance(b, dict) and b.get("type") == "reasoning" for b in result.get("content") or [])
+    assert result["tool_calls"][0]["id"] == "call_1"
+
+
+def test_reasoning_block_still_emitted_when_text_follows():
+    """The normal case must keep working: text after the reasoning block is valid."""
+    result = DatabricksConfig()._move_reasoning_into_content_block(
+        {
+            "role": "assistant",
+            "content": "391",
+            "thinking_blocks": [{"type": "thinking", "thinking": "t", "signature": "sig-abc"}],
+        }
+    )
+
+    assert result["content"][0]["type"] == "reasoning"
+    assert result["content"][1] == {"type": "text", "text": "391"}