Technical Specification and API Design (OpenAPI)
I write the build-ready specification for your app or API: endpoints, request and response shapes, errors, auth, webhooks, workflows and admin screens, as an OpenAPI 3.1 contract plus plain-English docs. It is for founders and teams handing work to developers. Prices start from $650 ($500 founding), and a spec takes about 1–3 weeks.
See a sample report, proposal and weekly update- Price
- from $650
- Founding
- from $500
- Timeline
- ~1–2 weeks
- Format
- Fixed scope, fixed price
- Hours
- US Eastern Time, async
Who this is for.
A good fit if
- Teams hiring developers (or an agency) who want a contract to build and test against.
- Frontend and backend developers working in parallel who need an agreed API.
- Founders who want quotes from developers that can be compared.
Common triggers
- The frontend and backend teams keep disagreeing about payloads.
- An outsourced build with no written spec.
- A mobile app or partner will call your API.
Not a fit if
- You need docs for a public developer audience. See Public/Partner API & Developer Docs.
- The idea isn’t settled yet. Start with a Discovery Sprint.
What I do, and what you get.
What I do
- Turn the features into endpoints, resources and workflows.
- Define request/response schemas, validation, pagination and error format.
- Design auth (sessions, tokens, API keys) and permissions per endpoint.
- Design webhooks: event types, signing, retries and idempotency.
- Specify admin screens and background jobs (Standard/Plus).
- Lint the OpenAPI file and produce a mock server your frontend can use on day one.
What you get
- An OpenAPI 3.1 file that is linted and versioned in your repo.
- A written spec covering flows, business rules, states and edge cases.
- An error catalog and auth/permissions matrix.
- A webhook spec with a signature verification example.
- A mock server command, and handover notes.
Your part
- An agreed feature list (or a Discovery Sprint report).
- A decision maker for business rules.
- Existing code or API, if any.
Included
- OpenAPI 3.1 contract
- Written spec and rules
- Auth, errors, webhooks
- Mock server setup
- One revision round
Not included
- Implementing the API
- UI design or wireframes
- Public developer portal (see Partner API & Developer Docs)
- SDKs in several languages
- Extra revision rounds (quoted)
Fixed prices, agreed before work starts.
Up to 15 endpoints, auth, errors, OpenAPI + written spec
Up to 40 endpoints, workflows, webhooks, admin screens
Up to 70 endpoints or several services, events between them, versioning plan. Billed in 2 monthly milestones of $910 ($700 founding).
Build your scope
- Timeline
- ~1–2 weeks
- Payment
- Paid in full before work starts
- Earliest start
- Oct 5
Founding price: 3 of 7 spots left, until Mar 31, 2027 — one-off work, in return for a testimonial. How it works
Prices are in USD, paid by bank transfer against an invoice. Work under $1,000 is paid upfront; larger work is 50% to start and 50% before handover, or monthly milestones. How working together works
Variations
- New API
- Designed contract-first.
- Existing API
- Documented as-is first, then the fixes are proposed.
- Laravel / Go / FastAPI
- The notes map the spec onto that framework’s validation and routing.
How it runs.
- 01
Email the template
Email the template below, and I reply within 1 US business day.
- 02
Proposal and invoice
I send a fixed proposal and invoice.
- 03
Resource and flow outline
I send a resource and flow outline for your review.
- 04
Full spec and revision
I write the full spec and OpenAPI, then we do one revision round.
- 05
Handover
Handover with a mock server and a walkthrough.
Where I have done this before.
Licensing & Entitlement Platform
I designed a documented provisioning API with signed, timestamped webhooks and replay safety, running in production.
Read the case study →Healthcare · Patient booking · Bilingual webHospital Website & Serial Booking System
A written requirements spec and a role/permission model with about 48 permissions (built, not yet live).
Read the case study →Questions buyers ask.
Why OpenAPI?
It’s the common format that docs tools, mock servers, test tools and code generators read.
REST or GraphQL?
REST by default. GraphQL only when there’s a clear reason.
Can you implement it?
Yes, as a separate project (Backend APIs).
Who owns the spec?
You do, in full.
How many revisions?
One round is included, and more are quoted.
Ask about Technical Spec & API Design.
Tell me what you have and where it hurts. I reply within one US business day with either a fixed quote or the entry step that makes a fixed quote possible.
Helpful to include
- Product
- Feature list or discovery report
- Who calls the API (web, mobile, partners)
- Stack
- Rough endpoint count
- Deadline
- Budget range
More in Product & API Design
Partner API & Developer Docs
I get your API ready for outside developers: API keys, rate limits, versioning, signed webhooks, and a docs site partners can build against without emailing you. It is for SaaS teams opening their product to partners, marketplaces or customers. Prices start from $780 ($600 founding), and the work takes about 2–5 weeks.
AI Agent Interface (MCP Server)
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