[FEATURE] Allow customization of Plan Mode's planning phase (naming, format, instructions)

Problem Statement

Plan Mode bundles two concerns:

1. Planning — how the plan is written (naming, format, structure, process)
2. Transition — the "Clear context and implement?" prompt that manages context efficiently

Users who need control over the planning phase are forced to abandon Plan Mode entirely and lose the transition. There's no middle ground.

What users can't control in Plan Mode

• File naming: Plan Mode generates its own slug. Users with naming conventions (numbered IDs, categories, specific slugs) can't enforce them.
• Plan structure: No way to specify custom templates or required sections. Plan Mode uses its internal format.
• Planning process: Can't inject custom instructions like "ask clarifying questions first", "research the codebase before writing", or "follow this specific methodology".
• Plan content: The plan is written by Plan Mode's internal prompts. Users can only edit after the fact via option 4 ("Type here to tell Claude what to change").

The trade-off today

| | Plan Mode | Custom workflow |
|---|---|---|
| File naming control | ❌ | ✅ |
| Plan format/template | ❌ | ✅ |
| Planning process control | ❌ | ✅ |
| "Clear context + implement" transition | ✅ | ❌ |

Users shouldn't have to choose between control and convenience.

Proposed solution

Allow Plan Mode to accept user-defined configuration for its planning phase. This could work through:

Option A — Plan Mode settings in settings.json or CLAUDE.md:

{
  "planMode": {
    "namingConvention": "{id}-{slug}.md",
    "template": ".claude/templates/plan.md",
    "instructions": "Always ask clarifying questions before writing. Research the codebase first. Follow the propose workflow from my skills."
  }
}

Option B — Plan Mode hooks:

Leverage the existing hooks system to let users intercept plan creation:

• on_plan_create: Customize file naming and location
• on_plan_write: Transform plan content (apply templates, enforce format)
• on_plan_exit: Custom logic before the transition

Option C — Plan Mode skill override:

Let a skill define how Plan Mode's planning phase works, while Plan Mode retains control of the transition. The skill handles the "how to plan" and Plan Mode handles the "how to transition to implementation".

Why this matters

Power users build custom planning workflows because Plan Mode's defaults don't fit their needs. But they're forced into an all-or-nothing choice: use Plan Mode (lose control) or go custom (lose the transition). Making Plan Mode's planning phase configurable would let users have both.

This aligns with Claude Code's philosophy of extensibility through skills, hooks, and configuration — Plan Mode is currently the one major feature that isn't customizable.

Related issues

• #15800 — Tuning plan mode workflow with skills
• #19326 — Compact option when exiting plan mode
• #26061 — Plan mode state lost after compression

Environment

• Platform: darwin
• Version: latest