"API tools" is a crowded category, but the workflow underneath is stable. You build and send requests, you let the outside world reach your dev machine, you watch what is actually flowing, and you check that responses still have the shape everyone agreed on. Here are free tools for each job — including the terminal classics no GUI replaces.
1. An API client — build, send, repeat
The core tool. A good client keeps requests in collections, swaps environments (local / staging / production), and remembers history so yesterday's debugging session is not lost.
SchemaClient does this as a free native app for Windows, macOS and Linux with no account required. The other free options are covered honestly in the best free API testing tools — including where each beats us. If you are starting from zero, the how to test a REST API tutorial walks the whole loop.
2. A localhost tunnel — let the internet reach your laptop
The moment you integrate webhooks — Stripe payments, GitHub pushes, WhatsApp messages —
the provider needs a public URL, and localhost:3000 is not one. A tunnel gives your
dev server a temporary public address.
Most people bolt on ngrok for this. SchemaClient has tunneling built in, free, which means the request builder and the tunnel share one window — and there is a free ngrok alternative page if that is the specific itch.
3. A traffic monitor — see what actually arrived
Half of webhook debugging is discovering the payload is not what the docs promised. Watching requests live — method, path, headers, body, timing — beats adding print statements and redeploying. SchemaClient's traffic monitor shows every request hitting your tunnel in real time; the webhook debugging guide shows the workflow end to end.
4. A schema validator — catch the drift a 200 hides
Status-code assertions miss the failures that hurt: a 200 OK with the wrong shape underneath. Validating responses against JSON Schema — or the OpenAPI spec you already have — turns "looks fine" into "verified". This is SchemaClient's namesake feature: schema validation runs on every response.
5. curl and jq — the terminal stalwarts
No GUI replaces these two:
curl -s https://api.example.com/users/42 | jq '.email'
curl is everywhere, scriptable, and the lingua franca of bug reports. jq filters
and reshapes JSON faster than any viewer. Use them for one-offs and CI scripts; use a
client when you will send the request more than twice.
The one-app version
Tools 1–4 are literally one download: SchemaClient bundles the client, tunnel, monitor and validator, free with no usage limits. Add curl and jq from your package manager and the kit is complete — total spend, nothing. Download it here.