# HarnessContentBlockStart
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The start payload for a content block.

## Contents
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**Important**  
This data type is a UNION, so only one of the following members can be specified when used or returned.

 ** toolResult **   <a name="BedrockAgentCore-Type-HarnessContentBlockStart-toolResult"></a>
Start of a tool result content block.  
Type: [HarnessToolResultBlockStart](API_HarnessToolResultBlockStart.md) object  
Required: No

 ** toolUse **   <a name="BedrockAgentCore-Type-HarnessContentBlockStart-toolUse"></a>
Start of a tool use content block.  
Type: [HarnessToolUseBlockStart](API_HarnessToolUseBlockStart.md) object  
Required: No

## See Also
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For more information about using this API in one of the language-specific AWS SDKs, see the following:
+  [AWS SDK for C\$1\$1](https://docs.aws.amazon.com/goto/SdkForCpp/bedrock-agentcore-2024-02-28/HarnessContentBlockStart) 
+  [AWS SDK for Java V2](https://docs.aws.amazon.com/goto/SdkForJavaV2/bedrock-agentcore-2024-02-28/HarnessContentBlockStart) 
+  [AWS SDK for Ruby V3](https://docs.aws.amazon.com/goto/SdkForRubyV3/bedrock-agentcore-2024-02-28/HarnessContentBlockStart)