Clarification around tool_result blocks after tool_use

[joshling1919]

The documentation for handling tool use and tool results (https://docs.anthropic.com/en/docs/build-with-claude/tool-use#handling-tool-use-and-tool-result-content-blocks) suggests that sending tool results is optional:

[optional] Continue the conversation by sending a new message with the role of user, and a content block containing the tool_result type...

However, in practice, attempting to continue the conversation with a user message after a tool_use response without first providing the corresponding tool_result block results in an error:

Did not find 1 tool_result block(s) at the beginning of this message. Messages following tool_use blocks must begin with a matching number of tool_result blocks.

To clarify, are tool_results not actually optional? How should we think about the role of tool_result in the cases where we are using tool_use mainly to get structured outputs?