Is it possible to update the assistant system prompt during a call? Do dynamic variables get updated every turn of the conversation? Thanks!

Dynamic variables like

{{now}}

,

{{date}}

, and

{{time}}

are automatically updated every turn of the conversation. For example, if you use

{{now}}

in your prompt, it will always reflect the current UTC time at each step:

text
The current time is {{now}}.

Custom variables (like

{{name}}

) only update if you provide new values in your API request. For more details, see the [dynamic variables documentation](https://docs.vapi.ai/assistants/dynamic-variables). Source: - [Dynamic Variables documentation](https://docs.vapi.ai/assistants/dynamic-variables)

Yes, you can update the system prompt during a conversation, but you can’t modify the original assistant’s system prompt directly while a run is active. Instead, you update it by adding a new message of type system to the thread. Every time you add a system message, the model treats it as updated instructions for all following turns. So the recommended approach is: keep the assistant’s base prompt static, and whenever you need dynamic rules or variables, append a new system message to the thread before creating the next run. This way the assistant always receives the latest instructions without needing to recreate it.

Hi dudleykong, It seems like your request came through without specific details. If you have questions about setting up or managing your Voice AI assistant, feel free to provide more information, and I'll be happy to assist you. You can also check out our [Voice AI Prompting Guide](https://docs.vapi.ai/prompting-guide) for tips on crafting effective prompts for your AI assistant.