Evaluating an API
Choosing an API is a product decision
At some point, your engineering team will say: "We need a service for X." Could be sending SMS, processing payments, geocoding addresses, or generating images. There will be multiple options. And picking one isn't just a technical decision. It's a product decision.
The API you choose affects pricing, reliability, user experience, and how fast your team can ship. As a PM, you should know what to look for.
What to evaluate
Pricing model
APIs charge differently. Some charge per request (every API call costs money). Some charge per user or per seat. Some have a free tier that covers basic usage and charge once you exceed it.
This matters because it directly affects your unit economics. If you're sending 10,000 SMS per month, the difference between $0.0075 and $0.004 per message adds up fast.
Rate limits
Most APIs limit how many requests you can make in a given time window. Twilio might allow 100 requests per second. A free API might cap you at 10 per minute.
Why does this matter? If your app sends a marketing campaign to 50,000 users and the SMS API only allows 100 messages per second, that campaign takes over 8 minutes to send. Rate limits affect what your product can do, not just how your engineering team builds it.
Developer experience (DX)
You've heard of UX (user experience). DX is the same idea, but for developers. It's how easy and pleasant it is for your engineering team to work with the API.
Good DX means your engineers can integrate faster and with fewer mistakes. Bad DX means more guesswork, more bugs, more back-and-forth with the provider's support team. What makes a good DX:
- Documentation: clear explanations, real code examples, up-to-date reference pages.
- SDKs: pre-built libraries in popular languages (Python, JavaScript, etc.) so your team doesn't have to write raw HTTP requests. Stripe, for example, provides SDKs that turn a 15-line request into a single function call.
- Sandbox or test mode: a way to try the API without real consequences. Stripe's test mode uses fake credit cards. Twilio lets you send test SMS without charges.
- Error messages: when something goes wrong, does the API tell you what happened and how to fix it? A good error says "Invalid phone number format. Expected E.164 format like +33612345678." A bad one just says "Bad request."
Why should you, as a PM, care about DX? Because it directly affects how fast your team ships. An API with great DX might take a few days to integrate. The same feature with a poorly documented API could take weeks, plus ongoing maintenance headaches.
Reliability
If the SMS service goes down, your customers don't get their order confirmations. If the payment API has outages, you can't charge anyone.
Most API providers publish an SLA (Service Level Agreement), a guarantee of uptime. 99.9% uptime sounds great, but that still means about 8 hours of downtime per year. 99.99% means about 50 minutes. The difference matters when your revenue depends on it.
Webhooks and real-time support
Does the API send webhooks when things happen? Or do you need to poll (check repeatedly)?
If Twilio sends you a webhook when an SMS is delivered, your app knows immediately. If a competing service doesn't support webhooks, your server has to keep asking "is it delivered yet?" every few seconds. That's more work for your team and a worse experience for your users.
Side-by-side: Twilio vs Vonage for SMS
Let's compare two real SMS providers to see how these criteria play out:
| Criteria | Twilio | Vonage |
|---|---|---|
| Pricing (per SMS) | $0.0079 per message (US) | $0.0068 per message (US) |
| Free tier | Free trial with $15 credit | Free trial with EUR 2 credit |
| Rate limits | 100 messages/second default | 30 messages/second default |
| Developer experience | Extensive docs, SDKs in 7 languages, sandbox mode | Good docs, SDKs in 6 languages, fewer interactive examples |
| Webhooks | Yes, delivery status callbacks | Yes, delivery receipts via webhook |
| Uptime SLA | 99.95% | 99.99% |
| Support | Email, chat, phone (paid plans) | Email, chat (paid plans) |
Neither option is objectively "better." Vonage is cheaper per message but has lower default rate limits. Twilio has a stronger developer experience and a higher free credit, but costs slightly more. If you're sending high volumes, rate limits might matter more than per-message cost. If your team is small and needs to move fast, documentation quality could save days of engineering time.
The point: this is a tradeoff analysis, exactly the kind of decision PMs make every day.
The questions to ask before choosing
Before your team commits to an API provider, run through this checklist:
| Question | Why it matters |
|---|---|
| How much does it cost per request/user/month? | Affects your margins. Cheap at 1,000 users, expensive at 100,000. |
| Is there a free tier or sandbox for testing? | Lets your team test without cost and catch issues early. |
| What are the rate limits? | Determines what your product can do at scale (batch sends, peak traffic). |
| Does it support webhooks? | Real-time notifications vs. polling. Big impact on user experience and engineering effort. |
| What's the uptime SLA? | How much downtime per year. Matters if the API is in a critical path. |
| How's the developer experience? | Good DX (docs, SDKs, sandbox, error messages) = faster integration, fewer bugs. |
| What happens if we outgrow this provider? | Switching APIs is expensive. Check if the data format and patterns are standard. |
| Is there a competitor we should evaluate? | Always compare at least two options. Don't just pick the first one that works. |
You don't need to answer all of these yourself. But bringing them to the conversation with your engineering team shows that you understand what's at stake, and helps the team make a better decision together.
Key takeaways
- Choosing an API is a product decision, not just a technical one. It affects pricing, speed, reliability, and user experience.
- The main criteria: pricing model, rate limits, developer experience (DX), reliability (SLA), and webhook support.
- Always compare at least two options. The cheapest per-request API might have the worst rate limits or docs.
- Use the checklist above when your team evaluates a new service. You don't need to be the expert, but you should ask the right questions.