Playbook
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Step 04 of 10 2-3 weeks· advanced

Step 4: Design the API

Define the API contract before writing any backend code. Web realities (network latency, pagination, versioning) drive the design — not the WinForms patterns.

What you're doing in this step

Map every WinForms operation to a backend endpoint, but design with web realities in mind. Lock the contract as OpenAPI 3.1 before implementation so the frontend and backend can build in parallel.

Recommended prompts

Use one of these to do the work in your IDE

Open the template to read it in full. Click Copy prompt to grab it (with your stack values pre-filled where they apply) — then paste into Claude Code, Cursor, or wherever you build.

Primary recommendation 25 min

OpenAPI 3.1 Spec Generator

Generate a complete OpenAPI 3.1 specification from user stories or a feature description, ready for API-first development.

View template
Template· Template 1 day

API Contract Extraction from Legacy

Reverse-engineer API contracts from legacy code, traffic logs, or running systems to produce an OpenAPI spec for the new system.

Use this when: Easier to extract the contract from existing patterns (legacy stored procs / data calls) than to design from scratch

View template
Template· Template 30 min

REST API Scaffolder (Node.js)

Scaffold a production-ready REST API with Express, validation, error handling, and OpenAPI docs.

Use this when: You're picking Node + TypeScript instead of .NET for the API layer

nodejsexpresstypescript
View template
Template· Template 30 min

FastAPI REST API Scaffolder

Scaffold a production-ready FastAPI service with Pydantic v2, async SQLAlchemy, dependency injection, and OpenAPI docs.

Use this when: Picking Python / FastAPI instead of .NET

pythonfastapipostgres
View template
Recommended skills

Drop these into Claude Code for this phase

Skills auto-trigger on the right kind of request. Install once; they apply to every prompt that fits.

Skill· Skill 5 min setup

Spec-Driven Builder Skill

Tool-neutral skill that walks developers through PRD → stories → schema → API → tests for any new feature, producing real artifacts at each step. The methodology is identical on every supported tool.

claude-codecopilotcursor
View skill
Skill· Skill 5 min setup

Migration Planner Skill

Flagship migration skill that walks Claude Code through audit → strategy → slicing → cutover for any legacy system migration.

claude-code
View skill
Recommended MCP configs

Wire these tools into Claude Code first

MCP servers give Claude Code direct access to external systems (Jira, browsers, databases). Configure once.

MCP config· MCP config 5 min setup

Filesystem MCP for Evoke

Pre-configured filesystem MCP server for Claude Code — safe, scoped read/write access to project files.

claude-codemcp
View config
When you're done

Verify these in your own work before moving on

This is a checklist for you to mentally tick off in your repo and IDE — the site doesn't track it, you do.

  • OpenAPI 3.1 spec at /api/openapi.yaml
  • Every WinForms operation mapped to an endpoint
  • Pagination / filtering / sorting designed for grid replacements
  • Auth scheme documented (Bearer JWT)
  • Error format consistent across endpoints
  • Spec validated with openapi-cli lint
  • SDK clients can be generated
Common pitfalls

What goes wrong at this step

  • Chatty endpoints — WinForms code-behind makes 50 DB calls per form; don't make 50 API calls per page
  • No pagination — WinForms grids load 50K rows in memory; web grids need server-side pagination
  • Inconsistent error formats — every endpoint should return errors the same way
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