diff --git a/src/strands/models/bedrock.py b/src/strands/models/bedrock.py
index b1628d817..8a6d5116f 100644
--- a/src/strands/models/bedrock.py
+++ b/src/strands/models/bedrock.py
@@ -180,7 +180,7 @@ def get_config(self) -> BedrockConfig:
     def _should_include_tool_result_status(self) -> bool:
         """Determine whether to include tool result status based on current config."""
         include_status = self.config.get("include_tool_result_status", "auto")
-        
+
         if include_status is True:
             return True
         elif include_status is False:
@@ -275,6 +275,7 @@ def _format_bedrock_messages(self, messages: Messages) -> Messages:
         """Format messages for Bedrock API compatibility.
 
         This function ensures messages conform to Bedrock's expected format by:
+        - Filtering out SDK_UNKNOWN_MEMBER content blocks
         - Cleaning tool result content blocks by removing additional fields that may be
           useful for retaining information in hooks but would cause Bedrock validation
           exceptions when presented with unexpected fields
@@ -292,11 +293,17 @@ def _format_bedrock_messages(self, messages: Messages) -> Messages:
             https://docs.aws.amazon.com/bedrock/latest/APIReference/API_runtime_ToolResultBlock.html
         """
         cleaned_messages = []
+        filtered_unknown_members = False
 
         for message in messages:
             cleaned_content: list[ContentBlock] = []
 
             for content_block in message["content"]:
+                # Filter out SDK_UNKNOWN_MEMBER content blocks
+                if "SDK_UNKNOWN_MEMBER" in content_block:
+                    filtered_unknown_members = True
+                    continue
+
                 if "toolResult" in content_block:
                     # Create a new content block with only the cleaned toolResult
                     tool_result: ToolResult = content_block["toolResult"]
@@ -323,6 +330,12 @@ def _format_bedrock_messages(self, messages: Messages) -> Messages:
             # Create new message with cleaned content
             cleaned_message: Message = Message(content=cleaned_content, role=message["role"])
             cleaned_messages.append(cleaned_message)
+
+        if filtered_unknown_members:
+            logger.warning(
+                "Filtered out SDK_UNKNOWN_MEMBER content blocks from messages, consider upgrading boto3 version"
+            )
+
         return cleaned_messages
 
     def _has_blocked_guardrail(self, guardrail_data: dict[str, Any]) -> bool:
diff --git a/tests/strands/models/test_bedrock.py b/tests/strands/models/test_bedrock.py
index e0f7879c0..13918b6ea 100644
--- a/tests/strands/models/test_bedrock.py
+++ b/tests/strands/models/test_bedrock.py
@@ -1331,7 +1331,7 @@ def test_format_request_removes_status_field_when_configured(model, model_id):
 def test_auto_behavior_anthropic_vs_non_anthropic(bedrock_client):
     model_anthropic = BedrockModel(model_id="us.anthropic.claude-sonnet-4-20250514-v1:0")
     assert model_anthropic.get_config()["include_tool_result_status"] == "auto"
-    
+
     model_non_anthropic = BedrockModel(model_id="amazon.titan-text-v1")
     assert model_non_anthropic.get_config()["include_tool_result_status"] == "auto"
 
@@ -1339,7 +1339,7 @@ def test_auto_behavior_anthropic_vs_non_anthropic(bedrock_client):
 def test_explicit_boolean_values_preserved(bedrock_client):
     model = BedrockModel(model_id="us.anthropic.claude-sonnet-4-20250514-v1:0", include_tool_result_status=True)
     assert model.get_config()["include_tool_result_status"] is True
-    
+
     model2 = BedrockModel(model_id="amazon.titan-text-v1", include_tool_result_status=False)
     assert model2.get_config()["include_tool_result_status"] is False
     """Test that format_request keeps status field by default for anthropic.claude models."""
@@ -1368,3 +1368,27 @@ def test_explicit_boolean_values_preserved(bedrock_client):
     expected = {"content": [{"text": "Tool output"}], "toolUseId": "tool123", "status": "success"}
     assert tool_result == expected
     assert "status" in tool_result
+
+
+def test_format_request_filters_sdk_unknown_member_content_blocks(model, model_id, caplog):
+    """Test that format_request filters out SDK_UNKNOWN_MEMBER content blocks."""
+    messages = [
+        {
+            "role": "assistant",
+            "content": [
+                {"text": "Hello"},
+                {"SDK_UNKNOWN_MEMBER": {"name": "reasoningContent"}},
+                {"text": "World"},
+            ],
+        }
+    ]
+
+    formatted_request = model.format_request(messages)
+
+    content = formatted_request["messages"][0]["content"]
+    assert len(content) == 2
+    assert content[0] == {"text": "Hello"}
+    assert content[1] == {"text": "World"}
+
+    for block in content:
+        assert "SDK_UNKNOWN_MEMBER" not in block