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Choosing API-First Software: Why It Matters

"Does it have an API?" rarely ranks high on the requirements list when buying business software. It is a technical question, and the people making the decision are usually not developers. Yet the answer determines three things you will all run into within two years: whether you can connect it to the rest of your stack, whether you can automate repetitive work, and whether you can ever get out. This article makes the question concrete enough to ask it yourself.

What API-first actually means

An API (application programming interface) is the door through which other software talks to yours. API-first means that door was not bolted on afterwards, but that the product itself goes through it: the screens you use talk to the same interface an external integration does.

That distinction is not academic. With an API added later, half of it is typically missing: you can read organisations but not write hours, or create invoices but not see whether they were paid. With API-first the coverage is complete, because a gap in the API would also be a gap in the product.

A practical rule of thumb: check whether the vendor publishes its API documentation openly, without a login. Vendors who take their API seriously use it as a selling point. See, for example, the public API reference covering all 48 endpoints in the Horixa API.

Three moments when it suddenly matters

The integration moment. You want a won deal in your marketing tool to become a project, hours from a specialist tool to flow in, or your invoices to reach your accountant. Without an API, only one integration remains: a person retyping.

The automation moment. The same task every month: assembling a report, creating projects from a template, syncing client data. With an API that is a thirty-line script. Without one it is permanent work for a human.

The exit moment. This is the most important and the least discussed. Your data lives in your vendor's system. Can you get all of it out, including the relationships between hours, projects, services and invoices? If not, switching is not a choice but a move with data loss. What you need is covered in switching project management software.

That last point has a pleasant side effect: a vendor you can easily leave has to win you over every year. That is exactly the incentive you want.

Seven criteria for a usable API

  1. No surcharge, no enterprise gate. An API available only on the most expensive plan is not an open API but a sales instrument. Ask explicitly whether it is included in your plan and whether limits apply.
  2. Full coverage. Can you reach every core entity (contacts, projects, services, hours, leave, quotes, invoices, costs), and can you both read and write where that makes sense?
  3. Real authentication with permissions. API keys with scopes, so an integration that only reads hours cannot also reach your revenue figures. A key that "can do everything" is a risk you cannot contain.
  4. Webhooks. Without them, every integration has to ask every few minutes whether anything changed. That is slow, wastes calls and runs into limits. With webhooks you get a message at the moment itself. Horixa sends nine, including hours.created, invoice.paid, project.updated and opportunity.won, from a transactional outbox so an event is never lost if the receiver is briefly offline.
  5. A machine-readable specification. An OpenAPI file means a developer generates a client in minutes instead of reading documentation for days. It is also the most honest quality signal there is: a specification generated from the code cannot go stale.
  6. Versioning. An API living at /v2 tells you the vendor has thought about what happens when something has to change. Without a version in the path, a Tuesday morning update can break your integration.
  7. Clear limits. Not because limits are bad, but because you need to know where they are before you build on them.

The hidden cost of a closed API

Software you cannot get into costs money in places where no invoice appears.

  • Duplicate data entry. One employee spending two hours a week retyping data costs, at a fully loaded rate of €85 an hour, over €8,800 a year. That is more than most agencies spend on their business software in total.
  • Buying integrations from third parties. Without an API you end up with off-the-shelf connectors that cost money monthly and do exactly what their builder had in mind.
  • A blockade at the exit. If migration is only possible through manual exports, the relationships between entities are lost. That is why agencies stay for years in software they outgrew.

The new layer: AI clients via MCP

Since 2025 there is one more reason to look at the API. The Model Context Protocol (MCP) is an open standard that lets an AI client connect directly to a data source. In practice: you ask questions about your own projects and invoices in the AI environment you already work in, rather than in a chat window your vendor built.

An MCP server is essentially an API with a description a language model understands. Vendors who already have one were almost without exception able to build it because their API was already in order. Horixa runs one at /api/mcp, with the same scopes as regular API keys and fully read-only. What it is and which questions to ask a vendor is covered in what an AI assistant in business software is.

The 30-minute test

You do not need to be a developer to assess an API. These five steps take half an hour and tell you more than a demo:

  1. Find the documentation without logging in. If you cannot in two minutes, that is your answer.
  2. Find an endpoint for hours. Can you both read and write hours, including project, service and hour type?
  3. Find the list of webhooks. If there is none, every integration becomes a polling integration.
  4. Find how authentication works. Can you create a key with limited permissions, and can you revoke it?
  5. Ask the vendor for a sample invoice response. Do you see the invoice lines, the VAT per rate and the status? Or just a total?

At Horixa the full REST API with webhooks is included in every plan, including the entry plan at €19.95 per user per month plus base licence. You create keys yourself with scopes per area. What each plan includes is on the pricing page; the technical side is on the developer page.

Frequently asked questions

What does API-first mean? That the software itself runs through its own API, rather than the API having been added afterwards. The practical difference is coverage: with API-first you can reach everything you can reach in the screens, because a gap in the API would be a gap in the product.

Do I need a developer to use an API? For a one-off integration usually yes, though it is often a job of a few hours. For simple automations you can also use an integration platform, but those work through your vendor's API too, so the requirement is the same.

What is the difference between an API and webhooks? With an API you request something when you want it. With a webhook the software sends you a message as soon as something happens, for instance when an invoice is paid. You need both: the API to read and write, webhooks to know when.

Can a vendor charge extra for API access? They can, but it tells you something. An API behind a paywall means exchanging your own data is treated as an add-on service. Include that cost in your comparison, and bear in mind it can rise at the next contract renewal.

How do I know I can get my data back out? Ask for an export of one project including its hours, costs and invoices, and check whether the references between those parts survive. Separate spreadsheets per entity without references are not a usable export.

Is an API worth it for a small firm? Yes, but for a different reason. At five employees you probably will not integrate anything immediately. The value is in the exit: you establish that you can always retrieve your own data in full, and that costs you nothing while you do not need it.

Want to see how far an API reaches? The full API reference is published openly, and with a free trial account you create your first key within a minute.