Claude Code subagent frontmatter tested: which model settings actually take effect

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A developer running Claude Code 2.1.278 on macOS tested whether YAML frontmatter settings in custom subagent files — specifically model and effort fields — are actually honored at runtime. Three subagents were examined: a bulk reader pinned to Sonnet, a deep reviewer pinned to Opus, and a preflight reviewer set to inherit its model. Testing was conducted on September 22, 2026, using seven CLI invocations in an isolated throwaway directory to avoid contamination from existing config files. The experiment tracked which model actually responded to each subagent's first API request, whether effort levels survived a parent session running at a different setting, and how Claude Code handled an unrecognized frontmatter key. Findings revealed measurable per-launch token costs and showed that the validator passed through at least one frontmatter field it did not recognize without raising an error.

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