Start a new milestone: questioning → research (optional) → requirements → roadmap.

Brownfield equivalent of new-project. Project exists, PROJECT.md has history. Gathers "what's next", updates PROJECT.md, then runs requirements → roadmap cycle.

Creates/Updates:

• .planning/PROJECT.md — updated with new milestone goals
• .planning/research/ — domain research (optional, NEW features only)
• .planning/REQUIREMENTS.md — scoped requirements for this milestone
• .planning/ROADMAP.md — phase structure (continues numbering)
• .planning/STATE.md — reset for new milestone

After: /gsd-plan-phase [N] to start execution.

<execution_context> @/.claude/gsd-core/workflows/new-milestone.md @/.claude/gsd-core/references/questioning.md @/.claude/gsd-core/references/ui-brand.md @/.claude/gsd-core/templates/project.md @~/.claude/gsd-core/templates/requirements.md </execution_context>

Milestone name: $ARGUMENTS (optional - will prompt if not provided)

Project and milestone context files are resolved inside the workflow (init new-milestone) and delegated via <required_reading> blocks where subagents are used.

Execute end-to-end. Preserve all workflow gates (validation, questioning, research, requirements, roadmap approval, commits).