Feature request: expose advisor tool as agent/subagent frontmatter field

Summary

The Advisor Strategy (advisor_20260301) is available in the Messages API but not yet exposed in Claude Code's agent/subagent frontmatter. Adding it would let custom subagents use the advisor pattern without leaving Claude Code's agent system.

Proposed frontmatter

---
name: my-executor
model: haiku
advisor:
  model: claude-opus-4-6
  max_uses: 3
---

Why this matters

The advisor pattern is especially valuable for plan-then-execute workflows where a cheaper model (Haiku) executes detailed subtasks and only escalates to a more capable model (Opus) at difficult decision points.

Today, if a Haiku subagent hits an edge case the plan didn't anticipate, it has to stop and report back to the user. With advisor support in frontmatter, it could consult Opus in real-time and keep going — same cost control via max_uses, but fewer interruptions.

We've been running a three-tier version of this pattern in DevPlan (Opus plans → Haiku executes → Sonnet verifies) and the advisor API would close the gap between "follow the plan exactly" and "handle the unexpected."

Current workaround

None within Claude Code's agent system. The advisor tool works in direct API calls but can't be configured in .claude/agents/*.md frontmatter.

Relevant docs

• Create custom subagents — current frontmatter fields
• Advisor Strategy blog post — the feature itself