Task tool model parameter is ignored for built-in subagent types

Description

The Task tool model parameter does not work when spawning built-in subagent types like general-purpose. Subagents inherit the parent model regardless of what model is specified.

Steps to Reproduce

1. Run Claude Code with Opus as the parent model
2. Spawn a subagent with explicit model parameter:

1. Check billing/usage dashboard

Expected Behavior

Subagent should use Sonnet model as specified.

Actual Behavior

Subagent uses Opus (parent model). Billing shows zero Sonnet usage despite multiple agents spawned with model: sonnet.

Workarounds Attempted

• Custom agent file in .claude/agents/ with model: sonnet - not recognized without restart
• Environment variable CLAUDE_CODE_SUBAGENT_MODEL - requires session restart, defeats purpose of runtime parameter

Impact

• Users cannot control subagent costs at runtime
• Documentation implies this should work
• Significant cost difference between Opus and Sonnet for batch operations