Covers 3 building blocks:

• Built-in Plan mode using extension contribution points Built-in Plan mode vscode-copilot-chat#1382
• Chat modes can define handoffs to the next mode/prompt: Allow chaining sequential workflows with chat modes and handoffs #272211
• Isolated subagent loops for read-only research Enable runSubagent tool for spawning isolated agent loops by default #270841

How this matters:

• Built-in planning is important
• Planning a an area that requires deep personalization for most

Backlog:

• Make planning mode default or automatic for coding task prompts
• Make planning easier to customize, setting specific read-only tools (MCP servers, CLI commands) and plan templates (PRD, ADR, etc)
• Iterative plan mode in editor docs for heavier spec-driven development

Key scenarios the building blocks enable:

1. Plan before implementing: Plan implementation (iterate on plan) → Implement step by step (iterate on code) → ✅
2. Spec-Driven: Product Spec (iterate on user stories) ↺ Design Plan (iterate on design decisions) ↺ Implementation Plan (iterate on sequencing) ↺ Implement (iterate on code) → ✅
3. Deep Architect: Draft Architecture (iterate on high-level design) ↺ Review Draft (iterate on critique) ↺ Implement (iterate on code) → ✅
4. Test-driven development: Plan ↺ Red Phase (iterate on failing tests) → Green Phase (iterating on implementation) → Refactor Phase (iterating on implementation polish) → ✅
5. Low-code mockups for UX ideation: Brainstorm Solutions → Prototype Solutions → Iterate → Converge → ✅