stunt

A stunt double for your APIs — test against real services, locally, without remote accounts.

stunt spins up stateful, realistic local stand-ins for the APIs you integrate (Stripe, Drive, Dropbox, gRPC services, GraphQL APIs, …) so you can develop and test without creating accounts, handling live credentials, burning money, hitting rate limits, or depending on the network. One static Go binary. Everything deterministic.

See the magic in 30 seconds:

stunt demo        # boots a stateful Stripe-style sim; prints copy-paste curl that creates a charge,
                  # lists it back (stateful!), captures it, and fires a webhook — all locally

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Why

Integrating remote APIs is painful for testing: you create accounts, juggle credentials, pay for usage, hit rate limits, depend on the network, and get non-deterministic results. Existing mock tools (WireMock, Prism, MSW, Microcks) are great at static and schema mocking, but they don't give you a stateful, runnable stand-in for an arbitrary service without hand-authoring everything.

stunt does. A Stripe-style adapter actually stores the charge you create — create it, list it, capture it, get the webhook — and it all resets on stunt clean.

How it's different

	stunt	WireMock	Prism	MSW	Mockoon
Stateful (create → list → mutate persists)	✅	partial	❌	❌	❌
Sandboxed adapter logic (safe to install strangers' mocks)	✅ Starlark	❌	❌	❌ JS	❌ JS
Protocols	REST, gRPC (+streaming), WebSocket, GraphQL	REST (+ limited)	REST, OpenAPI	REST, GraphQL	REST
Single static binary, no runtime deps	✅ Go	JVM	node	node	electron
Generate from real API descriptions	OpenAPI, HAR, proto	OpenAPI	OpenAPI	—	OpenAPI
Adapter ecosystem / catalog	✅ (git-distributed)	stubs	—	—	templates

(Each of those tools is excellent at what it does; this is a feature matrix, not a verdict. Corrections welcome.)

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Install

Go (any OS — Linux, macOS, Windows):

go install stuntapi.com/stunt/cmd/stunt@latest

macOS (Homebrew):

brew install --cask stuntapi/tap/stunt

Windows (winget):

# clone the manifest repo, then install from the local manifest dir
git clone --depth 1 https://github.com/stuntapi/winget
winget install --manifest winget/manifests/s/StuntAPI/Stunt/0.2.2

Once stunt is submitted to the default winget source, this becomes simply winget install StuntAPI.Stunt (no clone).

Pre-built binaries for every platform are also on the Releases page.

Dev-machine note: if go commands fail with a stdlib-version mismatch, prefix them with env -u GOROOT (a local toolchain quirk; not needed in CI/for end users).

Quickstart

stunt init     # writes a sample stunt.yaml
stunt plan     # validate + show what will run (warns on unloadable adapters)
stunt up       # serve all services (Ctrl-C to stop)  — logs every request
stunt down     # stop a backgrounded `stunt up`
stunt clean    # reset all adapter state to seed fixtures

Inline, declarative service (no adapter needed) — probabilistic faults, templates, conditions:

version: 1
rng_seed: 42
network: { mode: port, base_port: 8000 }
services:
  example:
    rules:
      - match: { method: GET, path: /hello }
        when: { chance: 20 }                       # 20% of replies error
        respond: { status: 503, body: { inline: { error: boom } } }
      - match: { method: GET, path: /hello }
        respond: { status: 200, body: { template: '{"message":"hi","id":"{{ faker.ID "k" }}"}' } }

Stateful adapter service (the bundled Stripe-style sim):

version: 1
network: { mode: port, base_port: 8000 }
services:
  stripe:
    adapter: ./adapters/stripe-style
    config:
      webhook_url: http://localhost:9090/webhook   # events_emit() delivers here

Then curl http://127.0.0.1:8000/v1/charges -H "Authorization: Bearer sk_test_demo" -d '{"amount":1000,"currency":"usd"}', list it back, capture it — state persists across requests and restarts.

Adapters

An adapter is a directory describing how to simulate one API: adapter.yaml + Starlark handlers + fixtures/schemas. Logic runs in a sandboxed Starlark VM (no host I/O — that's why strangers' adapters are safe to install) backed by stateful primitives (Collection / KV / Blob / Identity / Events). Build your own:

stunt adapter new myapi-style                 # scaffold (synthetic data)
stunt adapter import openapi spec.yaml        # generate from an OpenAPI doc
stunt adapter import har session.har          # infer endpoints + synthetic fixtures
stunt adapter import proto api.proto          # scaffold a gRPC adapter (descriptor + handlers)
stunt adapter lint ./myapi-style              # enforce SYNTHETIC data only (the safety guard)
stunt adapter test ./myapi-style              # conformance vs your local real traces
stunt catalog search stripe                   # browse the adapter registry

Reference adapters in this repo (all unofficial, synthetic-data-only, with a DISCLAIMER):

Adapter	Simulates	Backing
stripe-style	payments — charges (create/retrieve/list/capture/refund), customers (CRUD), balance, auth + webhooks	Collection + Starlark
drive-style	files API — upload/get/download/list/patch/delete, folders, about/quota	Blob + Collection
dropbox-style	files API (RPC-style) — upload/download/list_folder/get_metadata	Blob + Collection
twitter-style	mock OAuth, tweets (CRUD), users, timeline	Collection (pure-mock)
echo-style	gRPC service (unary + streaming) + WebSocket — multi-transport reference	Collection + KV
blog-style	GraphQL blog API — users/posts/comments, nested relations, mutations	Collection + Starlark

Full adapter authoring reference (the adapter.yaml schema, Starlark builtins reference with exact signatures, gRPC/WebSocket/GraphQL sections): adapters/README.md.

Transports & primitives

• Transports: REST, gRPC (unary and streaming), WebSocket, GraphQL (full introspection + DoS limits).
• Rules engine: first-match-wins; match (method/path globs/headers), when.chance (probabilistic), when.expr (boolean over request.*), respond (status/headers/latency/timeout + body.inline|file|template).
• Primitives: Collection (SQLite), KV, Blob (FS), Identity (HMAC tokens), Events (webhooks w/ retry), Clock+scheduler (deterministic), Generator, Validator (JSON-Schema). State persists in .stunt/state/.
• Networking: optional portless.dev-style TLS proxy on *.localhost (HTTP/2, local CA). The privileged listener forwards to an unprivileged engine, so adapter code never runs as root. WSS passthrough verified.

Observability dashboard

Every running server serves its own localhost dashboard — a live request inspector (HTTP/gRPC/WebSocket, bodies, copy-as-curl, replay), a state browser (the collections/kv/blobs your tests created), snapshot/restore for deterministic runs, and an instance manager. A matching CLI (--json) backs every feature.

$ stunt up
  dashboard:  http://127.0.0.1:54321   (token: 9f3c…)
$ stunt ui          # open it
$ stunt requests --follow    # live feed in the terminal
$ stunt ps          # list running servers across all manifests

Loopback-only, token-authed, DNS-rebinding-guarded; sensitive headers redacted; logging is async (never backpressures requests). Full guide with screenshots: docs/dashboard.md.

Safety & trust

The defining property: a community adapter is safe to install — adapter logic is sandboxed Starlark with no host I/O, bounded by execution-step limits; all file reads an adapter can trigger are path-containment-guarded; stunt adapter lint enforces synthetic-data-only. See SECURITY.md for the full threat model.

Contributing

Contributions are welcome — especially adapters. See CONTRIBUTING.md for the workflow and quality gates (just ci = build + test -race + vet + gofmt + mod-tidy + lint-adapters). Quick path: stunt adapter new myapi-style → edit → stunt adapter lint → PR.

Status & roadmap

v0.2.2 — observability dashboard. Core plus a full per-server dashboard (live request inspector, state browser, snapshot/restore, instance manager) are built, self-tested (just ci green), and dogfooded. On the roadmap: a public catalog (today's stunt catalog is offline/bundled + git refs), stunt setup privileged-path hardening, and broader adapter coverage. Not planned for v1: GraphQL subscriptions, npm adapter distribution.

Found a security issue? See SECURITY.md — do not open a public issue.

Project layout

cmd/stunt (CLI) · internal/{rules,manifest,engine,adapter(+runtime),starlark,grpcsim,graphqlsim} · internal/primitives{,/blob,/clock,/events,/gen,/identity,/kv,/validator} · internal/netutil{,/proxy} · internal/contrib{,/openapi,/har,/lint,/conform,/proto,/scaffold} · internal/catalog · internal/cli · internal/adapterdist · adapters/.

Reference & contributing

• Operating guide: AGENTS.md (or run stunt llm for the in-binary reference) — the full manifest schema, CLI reference, and the complete Starlark handler API.
• Contributing: see CONTRIBUTING.md.