PRD to Plan

Break a PRD into a phased implementation plan using vertical slices (tracer bullets). Output is a Markdown file in ./plans/.

Process

1. Confirm the PRD is in context

The PRD should already be in the conversation. If it isn't, ask the user to paste it or point you to the file.

2. Explore the codebase

If you have not already explored the codebase, do so to understand the current architecture, existing patterns, and integration layers.

3. Identify durable architectural decisions

Before slicing, identify high-level decisions that are unlikely to change throughout implementation:

• Route structures / URL patterns
• Database schema shape
• Key data models
• Authentication / authorization approach
• Third-party service boundaries

These go in the plan header so every phase can reference them.

4. Draft vertical slices

Break the PRD into tracer bullet phases. Each phase is a thin vertical slice that cuts through ALL integration layers end-to-end, NOT a horizontal slice of one layer.

When a phase has significant unknowns, include an explicit prototyping milestone to de-risk before full implementation. Prototypes are additive, testable, and clearly labeled. State criteria for promoting or discarding the prototype.

<vertical-slice-rules>

• Each slice delivers a narrow but COMPLETE path through every layer (schema, API, UI, tests)
• A completed slice is demoable or verifiable on its own
• Prefer many thin slices over few thick ones
• Do NOT include specific file names, function names, or implementation details that are likely to change as later phases are built
• DO include durable decisions: route paths, schema shapes, data model names
• Each phase MUST have observable validation — exact commands and expected output, not just "code changes"
• Prefer additive changes followed by subtractions that keep tests passing

</vertical-slice-rules>

5. Quiz the user

Present the proposed breakdown as a numbered list. For each phase show:

• Title: short descriptive name
• User stories covered: which user stories from the PRD this addresses

Ask the user:

• Does the granularity feel right? (too coarse / too fine)
• Should any phases be merged or split further?

Iterate until the user approves the breakdown.

6. Write the plan file

Create ./plans/ if it doesn't exist. Write the plan as a Markdown file named after the feature (e.g. ./plans/user-onboarding.md). Use the template below.

<plan-template>

Plan: <Feature Name>

Source PRD: <brief identifier or link>

This plan is a living document. The Progress, Surprises & Discoveries, Decision Log, and Outcomes sections MUST be kept up to date as work proceeds. This plan must be fully self-contained — a novice with only this file should be able to implement the feature end-to-end.

Purpose / Big Picture

Explain in a few sentences what someone gains after this change and how they can see it working. State the user-visible behavior you will enable.

Architectural decisions

Durable decisions that apply across all phases:

• Routes: ...
• Schema: ...
• Key models: ...
• Interfaces & Dependencies: Name libraries, modules, types, traits/interfaces, function signatures that must exist. Be prescriptive.
• (add/remove sections as appropriate)

Context and Orientation

Describe the current state relevant to this task as if the reader knows nothing. Name key files and modules by full path. Define any non-obvious terms.

Phase 1: <Title>

User stories: <list from PRD>

What to build

A concise description of this vertical slice. Describe the end-to-end behavior, not layer-by-layer implementation.

Validation and Acceptance

Phrase acceptance as observable behavior a human can verify — exact commands to run, expected output, what to observe. Not just "code compiles" but "running X produces Y". If tests are involved: "run <test command> — the new test <name> fails before the change and passes after."

Idempotence and Recovery

Can these steps be run multiple times safely? If a step is risky, provide retry/rollback path.

Phase 2: <Title>

User stories: <list from PRD>

What to build

...

Validation and Acceptance

...

<!-- Repeat for each phase -->

Progress

Use checkboxes with timestamps. Every stopping point must be documented, even if it requires splitting partial work into "done" vs "remaining."

• Phase 1: ...
• Phase 2: ...

Surprises & Discoveries

Document unexpected behaviors, bugs, performance tradeoffs, or insights discovered during implementation. Include concise evidence (test output is ideal).

• (none yet)

Decision Log

Record every decision made while working on the plan:

• (none yet)

Outcomes & Retrospective

Summarize outcomes, gaps, and lessons learned at major milestones or at completion. Compare result against original purpose.

• (to be filled at completion)

</plan-template>