Details:
- Allows users to provide custom instructions for the LLM-backed user simulator via the `custom_instructions` field in `LlmBackedUserSimulatorConfig`.
- The custom instructions must include placeholders for the stop signal, conversation plan, and conversation history. A pydantic validator ensures these placeholders are present.
- If no custom instructions are provided, the current default template is used.
Co-authored-by: Keyur Joshi <keyurj@google.com>
PiperOrigin-RevId: 850471448
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