SandboxAPIs vs. mocking libraries

01 / What mocking libraries are good at

Read this part first.

These are good tools and for a large share of API testing they are the right answer. They do one thing we cannot do at all: they serve whatever endpoint you can describe, on any API, including your own and including one that does not exist yet.

WireMock has been at this since 2011 and ships request matching, response templating, record and playback, and fault injection in a single Apache-2.0 package. Prism turns an OpenAPI document into a running server in one command. MSW intercepts at the network level, so the code under test is not modified to accommodate the test. Mockoon runs locally with no account at all.

If what you need is control over the response, use one of them and stop reading. The difference below is structural, not a matter of degree, and it only matters when what you need is data you did not write.

WireMock

Apache-2.0, started in 2011 as a Java library, and now also a standalone server, a Docker image and an npm package. Request matching, response templating, record and playback, and fault and delay injection in one tool.

Prism

Apache-2.0, from Stoplight. Points at an OpenAPI 2, 3.0 or 3.1 document (or a Postman collection) and runs a mock server from it, plus a validation proxy for contract testing.

Mockoon

MIT, and in its own words the easiest and quickest way to run mock APIs locally, with no remote deployment and no account. A desktop app, a CLI and a serverless package that share the same features.

MSW (Mock Service Worker)

MIT, and describes itself as industry standard API mocking for JavaScript. Intercepts in the browser through the Service Worker API and in Node by extending the client classes, so your application code is untouched.

Postman mock servers

Hosted. Built from a collection: Postman matches an incoming request to a saved example and replies with that example's data. Available on every Postman plan.

02 / Side by side

10 dimensions, both columns honest.

Every cell below describes what each approach does by definition, not what any one product happens to ship this quarter. Our column says where we lose: we are read-only, we replicate a fixed set of services rather than every API, and a pinned host never changes while the live hosts re-roll every day. Check any vendor's own docs against the sources at the foot of this page.

Dimension	Mocking libraries	SandboxAPIs
Setup time	Minutes to a running server. Then as long as it takes to author a response for every path your code walks, which is the part that is never in the quickstart.	Our winOne base URL. No install, no account, no seed script, nothing to author. The data set is already there.
Realism of the data	Exactly as realistic as you make it. Prism can generate values from a schema with faker, which are type-correct and mean nothing; hand-written stubs mean whatever you took the time to write.	Our winA simulated company generated over 90 days of modelled time: people with tenure, review threads that argue, commits with parents, timestamps that run forward. No placeholder values anywhere, ever.
Coherence across services	Out of category by design. Each stub is independent, so two stubs agree only for as long as somebody keeps editing both.	Our winOne canon compiled to 21 dialects. The same incident is a Slack thread, a Jira ticket, a GitHub issue and a GitLab merge request, with the same head SHA on both git hosts.
Determinism over time	BothYes, and this is not a small thing. A hand-authored stub is static by construction; there is nothing in a file that can drift.	BothYes, across a whole generated data set rather than one file. A pinned host stores no bytes: it regenerates them from a recipe, and CI proves it by generating, deleting and regenerating.
Write support	Their winYes. Stub a POST, script a sequence, branch per call, keep state between requests. This is a real capability and we do not have it.	No. Every write is refused with the vendor's own error status and error body, naming what was attempted. Your client's error path gets exercised; your POST does not get stored.
Rate-limit and error realism	BothYou author the headers, so they are right until the vendor changes and nothing tells you. For synthetic failure they are stronger than us: arbitrary latency, truncated bodies, 503 storms on demand.	BothThe vendor's real pagination and rate-limit headers, validated against the vendor's own published spec on every CI run. A header lets a caller ask any host for a rate-limit, server-error, timeout or latency response on demand.
Cost	BothWireMock, Prism, Mockoon and MSW are free and open source (Apache-2.0 or MIT), and self-hosted they cost only the machine. Hosted tiers are priced per user: Postman lists $0, $9, $19 and $49 per user per month.	BothFree with no account at 60 requests an hour, free with a key at 600. Paid plans start at $5 a month for one developer.
Maintenance burden	Yours. Every stub is a file your team owns, and when the vendor changes the shape, your stub keeps returning the old one and the test keeps passing.	Our winOurs. Every covered response is validated against the vendor's vendored, sha-pinned spec on every CI run, and the coverage manifest's status column is generated from that result rather than typed.
Works for AI-agent tests	For a scripted path, yes. An agent that decides to look at something you did not anticipate gets a miss, and a stub server's miss is not the vendor's 404.	Our winThe whole read surface answers: an agent can wander. Where we do not cover something it gets the vendor's own 404 plus a coverage header, never a silent null and never an invented value.
Works in CI	BothYes, and better in one specific way: it runs in your process with no network at all, so an air-gapped runner is fine and nobody is depending on a service somebody else operates.	BothYes, against a pinned host that returns identical bytes in a year. It is a network call to a service we run, which is a real dependency, and the honest mitigation is the pin rather than a promise.

03 / Run it yourself

The difference, as a command.

The difference in three commands. Nobody wrote these three responses; they are three views of one generated event, and each one names the next. Try authoring the equivalent by hand and the third stub is where it stops being worth it.

references-resolve.sh

GH=https://gh-2026-03-g6.snap.sandboxapis.dev

# 1. A merged pull request. Note the author, and the head SHA.
curl -s "$GH/repos/olympus-labs/parthenon/pulls/47" \
  | jq '{number, title, merged, user: .user.login, head: .head.ref, sha: .head.sha}'

# 2. That author, as a user resource. A stub would 404 here.
curl -s "$GH/users/clio" | jq '{login, name, company, type}'

# 3. That SHA, as a commit. Same person, one day earlier.
curl -s "$GH/repos/olympus-labs/parthenon/commits/d2aa02c65f1951b0851807e798a845d888f7112d" \
  | jq '{sha, author: .author.login, date: .commit.author.date, message: .commit.message}'

what it prints

{
  "number": 47,
  "title": "Fix a leak in the HTTP client's keep-alive pool",
  "merged": true,
  "user": "clio",
  "head": "clio/leak-http-client-keep-alive-pool",
  "sha": "d2aa02c65f1951b0851807e798a845d888f7112d"
}
{
  "login": "clio",
  "name": "Clio",
  "company": "@olympus-labs",
  "type": "User"
}
{
  "sha": "d2aa02c65f1951b0851807e798a845d888f7112d",
  "author": "clio",
  "date": "2026-06-15T23:05:48Z",
  "message": "Fix off-by-one on the last page of pagination"
}

Three requests, no key, no account. The pull request names an author, the author is a fetchable user, the head SHA is a commit, and the commit belongs to the same person a day before the merge. That chain is what a stub cannot give you: not because stubbing is hard, but because the second and third responses would be two more files for somebody to keep true.

That host is a pinned snapshot, so those bytes are the bytes you get next year too. The live hosts re-roll daily with new activity, which is the right choice for a demo and the wrong one for an assertion. See versioning and pinning.

04 / Picking one

Which should you actually use?

When to choose the other thing

Choose a mocking library

The API is yours, or it is a third party we do not serve.
The endpoint does not exist yet and the client cannot wait for it.
You need to assert on the requests your code sent, not only on what came back.
You are rehearsing failure: timeouts, truncated bodies, a slow first byte, a 503 storm.
You have an OpenAPI document and want a server from it in one command.
Your CI runners have no outbound network, or a third-party dependency in the test path is not acceptable.

When to choose SandboxAPIs

Choose SandboxAPIs

You are integrating with one of the 21 services we replicate, and you want its real surface rather than the slice you remembered to stub.
Your code walks references: a pull request's author has to be a user you can fetch, its commits have to exist at the SHAs it claims.
You need the same story on more than one service, and you do not want to hand-maintain the agreement.
You are testing an agent, which will ask for things you did not anticipate.
You would otherwise point tests, CI or an agent at a real account with a real token in it.

Two minutes, no signup

Run the commands, then decide.

Read the quickstart See the coverage manifest

Mocking libraries vs. SandboxAPIs

Read this part first.

10 dimensions, both columns honest.

The difference, as a command.

Which should you actually use?

Run the commands, then decide.

Every claim about somebody else, with the page it came from.

The other comparisons.