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For product teams

An API your partners can build on without calling you

To build an API for a SaaS product, agree who will use it and for what, design the contract before the code, then add authentication, rate limits, versioning, documentation and contract tests. Partners integrate from the docs, and your releases stop breaking their work.

Free first conversation · A person replies within one business day · You own the code and accounts

  • A phase of work
  • A point where you decide to stop or go on
See the routes →

01 Outcomes

What changes for you

  • Integrations that survive releases

    Contract tests run on every change, so a release that would break a partner's integration is stopped before it ships rather than reported by the partner.

  • Partners who start on their own

    Clear reference docs, examples and a sandbox mean a partner's developer can make a first working call without a meeting with your engineers.

  • Room to change the product

    A versioning and deprecation policy lets your team improve the product behind the API while existing integrations keep their promised behavior.

Your route

Choose where your API work starts

API work usually starts from one of three places. Pick yours to see the phases in order, the review points that hold a release back, and what each phase leaves you with.

Where are you starting from?

Starting from Partners keep asking for an API we do not have

  1. Agree users and uses

    We talk to the partners and customers asking for it, list what they need to read and change, and decide what the first version will leave out.

  2. Draft the contract

    The endpoints, fields, errors and permissions are written as a specification first, so partners can review it before any code exists.

    Stop or go decision

  3. Build behind the guards

    We implement it with authentication, scoped permissions, rate limits and clear error messages, tested against the agreed contract.

  4. Document and release

    Reference docs, examples and a sandbox go live with the first partners, and we watch real usage before opening it wider.

    Stop or go decision

Starting from An internal API now needs to be public

  1. Audit what it exposes

    We check the current endpoints for internal fields, loose permissions and behavior nobody documented, the things outsiders would find first.

  2. Decide the public surface

    Together we choose what partners may see and do. Everything else stays internal, behind a separate, narrower contract.

    Stop or go decision

  3. Harden and version

    Authentication, rate limits, consistent errors and a version in every path or header, so later changes do not surprise anyone.

  4. Publish with docs

    We release the public version with its reference docs and a changelog, and keep the internal API free to change.

    Stop or go decision

Starting from Our integrations break whenever we release

  1. Map who calls what

    We list every integration, the endpoints it depends on and the fields it reads, so the risk of each change is visible.

  2. Add contract tests

    Tests that encode each promise run on every change. A release that breaks a contract stops in the pipeline, not at a partner.

    Stop or go decision

  3. Set a deprecation policy

    New versions arrive alongside old ones, with a notice period and a changelog, so partners move on a timetable they know.

Talk this through with us →

02 How it starts

How it starts

  1. A call with an engineer

    You show us the product, who is asking for access and what has broken before. We ask about your stack, your release process and your partners.

  2. A draft contract and plan

    We sketch the first version of the API contract and a delivery plan, with the review point where partners see it before the build.

  3. The first endpoints, tested

    We build a small set of endpoints end to end, with docs and contract tests, so you can judge the approach on working code.

The usual engagement model: Fixed-scope implementation

03 Start

Show us the API your product needs

Book a call with an engineer. Share the partner requests or the integration that keeps breaking, and we will sketch the first version with you.

What happens next

  1. You pick a time, or send us a message.
  2. A person at Nexstella reads what you shared before we talk.
  3. You leave with a clear next step, even when that step is to wait.

Or write to [email protected]. Send a brief instead

Meet

Pick a time with an engineer

What is your work email? The calendar invite and the video link go there.

The invite goes to the calendar you work from. Personal address? Use the Contact tab . If you stop before picking a time, we send one reminder to this address, and nothing else.

30 minutes, video call with an engineer · Times show in your time zone

What happens next

  1. You pick a time, or send us a message.
  2. A person at Nexstella reads what you shared before we talk.
  3. You leave with a clear next step, even when that step is to wait.

Contact

Send us a message

Pick the topic that fits and tell us what you are working on. A person replies within one business day.

A person replies from a real address.

Phone (optional)

Choose the country code, then type the number without it.

The topic leads the subject line, so the right person picks it up first.

A few sentences is plenty. You do not need to know which technology you need.

We use what you send to reply and, if it leads somewhere, to scope the work. Read the privacy notice, or write to [email protected].

Want the full picture? Read Digital product engineering and app development.

Also written for you: Connecting business systems , Scaling SaaS platforms .

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