This is a known limitation. The Task tool’s model parameter currently only supports "fast" for most users. Expanded model selection for subagents (allowing specific model IDs like claude-sonnet-4-6) is not yet generally available.
The model field in subagent frontmatter (.cursor/agents/*.md) is parsed, but if the specified model ID isn’t recognized, the system silently falls back to the parent model. This explains why your subagent runs on the parent’s model rather than the one configured in frontmatter.
A few related threads with the same report:
Our team is aware of this gap and it’s being tracked for the subagents feature. No ETA yet, but reports like yours help with prioritization.
In the meantime, the only model options for subagents are: omit the model parameter (inherits the parent model) or use "fast" (a lighter model optimized for quick tasks).