modelId was deprecated when v0 switched from specific models to the v0 Agent, which calls different models under the hood. Later, v0 rolled out 3 different agents (mini, pro, and max) which are primarily based on haiku, sonnet, and opus respectively. When this happened, the API was not updated to restore the modelId functionality
It currently corresponds to pro, which uses sonnet
I will ping the v0 team and see if we can get the option restored