diff --git a/src/strands/models/openai.py b/src/strands/models/openai.py
index 6e32c5bdd..764cb8519 100644
--- a/src/strands/models/openai.py
+++ b/src/strands/models/openai.py
@@ -18,6 +18,7 @@ class Client(Protocol):
     """Protocol defining the OpenAI-compatible interface for the underlying provider client."""
 
     @property
+    # pragma: no cover
     def chat(self) -> Any:
         """Chat completions interface."""
         ...
diff --git a/src/strands/types/models/openai.py b/src/strands/types/models/openai.py
index 8f5ffab32..c00a7774b 100644
--- a/src/strands/types/models/openai.py
+++ b/src/strands/types/models/openai.py
@@ -206,7 +206,9 @@ def format_chunk(self, event: dict[str, Any]) -> StreamEvent:
 
             case "content_delta":
                 if event["data_type"] == "tool":
-                    return {"contentBlockDelta": {"delta": {"toolUse": {"input": event["data"].function.arguments}}}}
+                    return {
+                        "contentBlockDelta": {"delta": {"toolUse": {"input": event["data"].function.arguments or ""}}}
+                    }
 
                 return {"contentBlockDelta": {"delta": {"text": event["data"]}}}
 
diff --git a/tests/strands/types/models/test_openai.py b/tests/strands/types/models/test_openai.py
index 97a0882ab..2657c334e 100644
--- a/tests/strands/types/models/test_openai.py
+++ b/tests/strands/types/models/test_openai.py
@@ -246,12 +246,12 @@ def test_format_request(model, messages, tool_specs, system_prompt):
 @pytest.mark.parametrize(
     ("event", "exp_chunk"),
     [
-        # Case 1: Message start
+        # Message start
         (
             {"chunk_type": "message_start"},
             {"messageStart": {"role": "assistant"}},
         ),
-        # Case 2: Content Start - Tool Use
+        # Content Start - Tool Use
         (
             {
                 "chunk_type": "content_start",
@@ -260,12 +260,12 @@ def test_format_request(model, messages, tool_specs, system_prompt):
             },
             {"contentBlockStart": {"start": {"toolUse": {"name": "calculator", "toolUseId": "c1"}}}},
         ),
-        # Case 3: Content Start - Text
+        # Content Start - Text
         (
             {"chunk_type": "content_start", "data_type": "text"},
             {"contentBlockStart": {"start": {}}},
         ),
-        # Case 4: Content Delta - Tool Use
+        # Content Delta - Tool Use
         (
             {
                 "chunk_type": "content_delta",
@@ -274,32 +274,41 @@ def test_format_request(model, messages, tool_specs, system_prompt):
             },
             {"contentBlockDelta": {"delta": {"toolUse": {"input": '{"expression": "2+2"}'}}}},
         ),
-        # Case 5: Content Delta - Text
+        # Content Delta - Tool Use - None
+        (
+            {
+                "chunk_type": "content_delta",
+                "data_type": "tool",
+                "data": unittest.mock.Mock(function=unittest.mock.Mock(arguments=None)),
+            },
+            {"contentBlockDelta": {"delta": {"toolUse": {"input": ""}}}},
+        ),
+        # Content Delta - Text
         (
             {"chunk_type": "content_delta", "data_type": "text", "data": "hello"},
             {"contentBlockDelta": {"delta": {"text": "hello"}}},
         ),
-        # Case 6: Content Stop
+        # Content Stop
         (
             {"chunk_type": "content_stop"},
             {"contentBlockStop": {}},
         ),
-        # Case 7: Message Stop - Tool Use
+        # Message Stop - Tool Use
         (
             {"chunk_type": "message_stop", "data": "tool_calls"},
             {"messageStop": {"stopReason": "tool_use"}},
         ),
-        # Case 8: Message Stop - Max Tokens
+        # Message Stop - Max Tokens
         (
             {"chunk_type": "message_stop", "data": "length"},
             {"messageStop": {"stopReason": "max_tokens"}},
         ),
-        # Case 9: Message Stop - End Turn
+        # Message Stop - End Turn
         (
             {"chunk_type": "message_stop", "data": "stop"},
             {"messageStop": {"stopReason": "end_turn"}},
         ),
-        # Case 10: Metadata
+        # Metadata
         (
             {
                 "chunk_type": "metadata",