Webhook vs API: What Your Agent Wired Up When It Added a Webhook

Your agent said it "added a webhook for payments," the checkout worked, and you moved on to the next feature. Here is the part nobody told you: you just opened a door on the public internet that an outside service knocks on, and the agent almost certainly did not add the one check that makes sure the knock is real.

That is a different kind of gap than a bug. A bug is code that does the wrong thing. This is code that does exactly what it was asked, where the important part was never asked for. A webhook is not the same as the API you already have, and the difference is not academic trivia. One of them is a door you open when you want something. The other is a door someone else opens whenever they want, and if you do not check who is knocking, anyone can knock.

If you have been shipping with Cursor, Claude Code, Lovable, or Replit and "webhook" has always just slid past you as one more word the agent used, this one is for you. The hypothesis: an API is a door you knock on to pull data when you want it, a webhook is a door another service knocks on to push you news, and the check that a webhook is genuine is an acceptance criterion you specify, not a default the agent picks.

#The plain difference: pull versus push

Start with the version the vendor pages bury under diagrams.

An API is the set of doors your app knocks on to get things done. When your app wants a user's data, it calls the API and asks for it. When it wants to charge a card, it calls the payment API and tells it to. The pattern is always the same: your app initiates, on your schedule, because it wants something right now. Zapier's explainer puts the mechanics plainly: APIs are "request" driven, while webhooks are "event-driven" data transfers. You pull with an API. You knock, and the other side answers.

A webhook flips the direction. Instead of your app asking, an outside service tells you the moment something happens. Stripe finishes charging a card, so it sends a message to a URL you gave it: "this payment succeeded." You did not ask at that moment. You set up a door ahead of time and handed the address to Stripe, and now Stripe knocks on it whenever there is news. RudderStack's comparison names the split cleanly: APIs use a pull approach where your application actively requests information, while webhooks employ a push mechanism where data is sent automatically when an event fires.

Here is the mental picture that survives contact with a real app. An API is you calling the pizza place to ask if your order is ready. A webhook is the pizza place texting you the second it comes out of the oven. In the first, you decide when to ask, and you can ask a hundred times. In the second, they decide when to tell you, and you just have to be ready to receive the text. Both move the same information. The difference is who starts the conversation, and that difference is the whole thing.

The confusion is so common that the top organic result for this exact search, six years running, is a Reddit thread of developers trying to pin it down, and a fresh one landed last week asking how a webhook differs from a push API. If professional developers keep re-asking this, the answer is not obvious, and you were never behind for not knowing it.

#Why AI builders trip over this one specifically

Search "webhook vs api" and you get mParticle, Akamai, Twilio, Zapier, SuperTokens, Strapi. Every page is technically correct and written for someone who decided, on purpose, to wire up a webhook and needed to understand it to do so. None of them is written for your situation: you did not decide to add a webhook, you cannot fully read the handler the agent wrote, and you only know it exists because the agent mentioned it in a sentence you scrolled past on the way to seeing the payment go through.

That is the actual shift, and it is bigger than one integration. In the old world, the person who set up a webhook understood it, because setting it up required understanding it. You could not register a Stripe webhook and write the handler without learning what a webhook was and what could go wrong. The knowledge and the code arrived together. AI cut that link. Now the webhook can exist, and route real payment events into your database, in the hands of someone who has never heard the term. The integration gets built. The understanding of what it should guard against does not come bundled in.

This is the capability-test version of the problem, the one that gets worse as the models get better, not better. Andrej Karpathy named the failure mode when he coined vibe coding: you "fully give in to the vibes" and "forget that the code even exists." Forgetting the code exists is harmless for a weekend toy. But a payment webhook is not code you can afford to forget. It is a public URL that an outside service calls to tell your app that money moved, and a handler that believes every knock is a handler that will believe a forged one. The more your agent can wire up on its own, the more public doors exist that you never decided the rules for. Autonomy does not remove the decision about who is allowed to knock. It just makes that decision silently, on your behalf, in the direction of whatever demos fastest.

Consider the same feature asked two ways.

Vague: "Add a webhook so we mark the order as paid when Stripe confirms the payment."

Specified: "Add a webhook endpoint at /api/webhooks/stripe that listens for the payment_intent.succeeded event. Before trusting any incoming request, verify the Stripe signature against our webhook signing secret and reject with a 400 if it fails. Only then mark the matching order as paid. Ignore duplicate events for an order that is already paid."

Same webhook. The first version hands every one of those decisions, is this call genuine, what event does it handle, what if it arrives twice, to the agent's default, which is the configuration that gets the happy-path demo working with the least friction: accept the request, trust it, mark the order paid. The second version turns each rule into something you can actually check without reading the handler. Send a request with a bad signature and confirm you get a 400. Send the same valid event twice and confirm the order is not charged forward twice. That is a test anyone can run. "Is the webhook set up correctly?" is not a question you can answer by looking. "Does a forged webhook call get rejected?" is a question you can answer in ten seconds.

#The one check the agent skipped: is this knock real?

Here is the thing that separates a webhook from the rest of your API, and it is the thing worth carrying out of this whole post. When your app calls an API, your app is the one initiating, so your app controls the conversation. When a webhook fires, an outside party initiates, which means the request arrives at a public URL that anyone on the internet can also find and call. Nothing about the address stops a stranger from sending your webhook endpoint a message that says "payment succeeded" for an order that was never paid.

That is why real webhook providers sign their requests. Stripe, GitHub, and every serious sender attach a signature computed from a secret only you and they share, and the deal is simple: your handler recomputes that signature and, if it does not match, throws the request away. A Stack Overflow answer in the "is a webhook just an HTTP request" thread gets the mechanics right, a webhook is just a specific kind of HTTP request, but that framing hides the danger. Because it is just an HTTP request, anyone can send one. The signature check is the entire difference between "a message from Stripe" and "a message claiming to be from Stripe."

An agent building for a passing demo has no reason to add that check. The demo works without it. Stripe sends a real event, the handler trusts it, the order is marked paid, the video looks great. The signature verification only matters the day someone who is not Stripe sends the same shape of request to your public URL and your app, trusting every knock, marks their order paid for free. Nothing in the demo would ever surface that. It is a hole that is invisible until it is exploited, which is exactly the kind of hole that never makes it into a spec nobody wrote.

There is a real trade-off here, and pretending otherwise would be dishonest. Not every webhook guards money, and locking down a low-stakes internal notification the way you would lock a payment handler is wasted effort and a source of false alarms when the signing secret rotates and legitimate calls start bouncing. And webhooks bring their own operational headaches the pull-based API does not: if your endpoint is down when the event fires, you may miss it entirely, so anything critical needs the sender's retries turned on or a periodic reconciliation against the API as a backstop. The skill is not "verify and armor every webhook to the maximum." It is knowing which incoming doors carry consequences, money, access, destructive actions, and specifying the checks for those precisely, while leaving the trivial ones simple. Telling those apart is judgment. Judgment is the part the agent does not do for you.

#The reframe: the check is a criterion, not a default

This is the same move we made writing about what an API endpoint is and what an API gateway is: the holes that keep showing up in AI-built apps are not coding mistakes, they are unspecified rules. An unverified webhook is exactly that. Its job was never just "receive the event and update the database." Its full job is "receive the event, confirm it genuinely came from the real sender, ignore it if it is a duplicate, and only then update the database." That second half is the part that never gets written down, so the agent ships the first half and the door stands unlatched.

Reframing it this way changes what you are on the hook for. You are not responsible for writing the signature-verification code; the agent can do that, and do it well, once it is told to. You are responsible for stating what the door must check before it trusts a knock, and then confirming that it does.

A webhook has a small, answerable set of questions attached to it, and every one is a plain-language decision. Does this handler verify the request genuinely came from the sender before acting on it? What happens to a request that fails that check, a clean rejection, or a silent pass-through that updates your data anyway? What happens if the same event arrives twice, does the order get processed twice? And is the receipt logged, so that when a payment does not reconcile you can see what actually arrived? Answer those, and you have written the acceptance criteria for one of the most exposed surfaces in your app: a public door an outsider gets to open.

#Where this fits in the loop

This is the gap BrainGrid is built to close. You describe the feature in plain language, "mark the order paid when the payment succeeds," and the Planning Agent turns it into a requirement with explicit acceptance criteria, including the ones you would otherwise leave in your head: verify the sender's signature, reject anything that fails it, handle duplicate events, log every receipt. "A webhook request with an invalid signature is rejected with a 400 and changes nothing" becomes part of the standard the Builder Agent builds against, whether that build runs in BrainGrid Cloud with a live preview or in your own GitHub repo through Claude Code, Cursor, or Codex over MCP.

That is the loop: Plan the door's checks alongside the feature, Build against them, Verify each one with evidence, Repeat. A webhook that accepts a forged call does not quietly pass, because "only genuine, signature-verified requests are acted on" is written down as a criterion, and a feature is not done until every criterion is checked with evidence. Code review can tell you the handler is tidy. Verification tells you a forged request actually bounces. Those are different questions, and for a public door that touches real money, only the second one keeps you safe.

#What this means if you are building right now

If your agent added or touched a webhook this week, do one thing before you ship. Find out what the webhook does, and ask whether it touches money, access, or anything you cannot undo. If it does, write the rule down as a criterion, "only requests with a valid signature from the real sender are trusted, everything else is rejected," and then verify it the only way that counts. Send the endpoint a request with a bad signature and confirm it refuses. Send a real event twice and confirm the work happens once. If a forged knock gets in, or a duplicate double-processes, you found the unlatched door before someone else did.

The uncomfortable truth underneath this is the same one that runs through every honest conversation about building with AI. The agent is not wiring up your webhook badly. It is wiring it up silently, toward whatever demos fastest, every time you leave the checks out of the spec. An unverified webhook is not a mysterious flaw buried in code you cannot read. It is a public door whose lock you never asked for. Ask for the lock, make it a criterion, and check it. For the doors behind this one, we wrote about what an API endpoint is and how to write acceptance criteria an AI agent can actually verify.

#FAQ

#What is the difference between a webhook and an API?

An API is a door your app knocks on to pull data or trigger an action when you want it: your app initiates the request, on your schedule. A webhook flips the direction: it is a door you set up so an outside service can push you news the moment an event happens, like a payment succeeding or a file finishing upload. With an API, you ask and wait for an answer. With a webhook, you register a URL ahead of time and the other service calls it whenever there is something to report. A useful shorthand: APIs pull, webhooks push. Technically a webhook is delivered as an API call, so it is fair to call it a specific, event-driven kind of API rather than a separate technology.

#Is a webhook just an HTTP request?

Yes, mechanically a webhook is an HTTP request, usually a POST, sent to a URL you provided. That is exactly why it needs care. Because it is an ordinary HTTP request, anyone who knows or guesses your webhook URL can send one that looks just like the real thing. The safeguard is that legitimate senders like Stripe and GitHub sign their requests with a secret only you and they share, and your handler is supposed to verify that signature before trusting the request. So "it's just an HTTP request" is true, and it is the reason the signature check is not optional for any webhook that touches money or access.

#Is a webhook better than an API?

Neither is better; they solve opposite problems. Use an API when your app needs to fetch data or perform an action on demand, when you are the one who decides the timing. Use a webhook when you need to react the instant something happens on another service and you do not want to poll it over and over asking "did it happen yet?" In practice most real apps use both: a webhook tells you an event occurred, and then your code often calls that service's API to fetch the full up-to-date details. The webhook is the notification; the API is how you ask follow-up questions.

#What are webhooks used for?

Webhooks are used anywhere your app needs to know about an event on another service the moment it happens, without constantly checking. The classic example is payments: Stripe sends a webhook when a charge succeeds or fails so you can fulfill the order. Others include a CI service telling you a build finished, a form tool notifying you of a new submission, a chat platform pushing you new messages, or a file service reporting an upload completed. The common thread is real-time reaction to someone else's event. Instead of your app asking "anything new?" every few seconds, the other service tells you, which is faster and far less wasteful.

#What are the disadvantages of webhooks?

Webhooks have three main downsides. First, security: because the endpoint is a public URL an outsider can call, you must verify each request is genuinely from the real sender, or a forged call can trick your app. Second, delivery reliability: if your endpoint is down or slow when the event fires, you can miss it, so critical webhooks need the sender's retry feature enabled or a periodic reconciliation against the API as a backstop. Third, duplicates and ordering: the same event can arrive more than once or out of order, so your handler needs to be safe to run twice without doing the work twice. None of these are reasons to avoid webhooks; they are the reasons a webhook that touches anything important needs a real specification, not just a handler that trusts every knock.

BrainGrid is the AI Product Planner that turns your idea into a requirement with acceptance criteria, including the checks a public door has to pass before your app trusts it, so your agent ships a latched door instead of an open one. Try it at braingrid.ai.