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FetchSandbox
FetchSandbox

Posted on • Originally published at fetchsandbox.com

Stop mocking APIs. Use a stateful sandbox instead.

#webdev #api #testing #devops

Every developer has written this code:

// TODO: replace with real API call
const mockUser = { id: "usr_123", email: "test@test.com", status: "active" };
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Then forgotten about the TODO. Then shipped it. Then found out in production that the real API returns state not status, and the ID format is user_2xAbC not usr_123.

Mocks lie. Sandboxes don't.

What if you could test against real API behavior — without production credentials?

That's what FetchSandbox does. Give it any OpenAPI spec, and it provisions a stateful sandbox in seconds:

  • POST /users creates a user that persists
  • GET /users/{id} returns it
  • PATCH /users/{id} with { "banned": true } updates the state
  • Webhook events fire on every state change
  • Invalid transitions return proper error codes

No mock files. No Wiremock configs. No Postman collections to maintain.

We just added 5 new APIs

This week we onboarded five APIs developers actually struggle to test:

API Endpoints Why it's hard to test
Clerk 196 SSO flows need a real IdP, sessions expire
Resend 83 Actually sends emails, burns through quota
Cohere 41 API credits add up, RAG pipeline iteration is expensive
Svix 128 Webhook delivery fails to localhost
Glean 79 Enterprise search needs connected data sources

Each comes with curated workflows — real multi-step integration flows you can run instantly.

How it works in 30 seconds 

# 1. Create a sandbox
curl -X POST https://fetchsandbox.com/api/sandboxes \
  -H "Content-Type: application/json" \
  -d '{"spec_id":"clerk"}'

# Returns: { "id": "a1b2c3", "credentials": [{ "api_key": "sandbox_..." }] }

# 2. Use it like the real API
curl https://fetchsandbox.com/sandbox/a1b2c3/users \
  -H "Authorization: Bearer sandbox_..." \
  -H "Content-Type: application/json" \
  -d '{"email_address": ["dev@acme.com"], "first_name": "Jane"}'

# 3. State persists — fetch the user you just created
curl https://fetchsandbox.com/sandbox/a1b2c3/users/{id} \
  -H "Authorization: Bearer sandbox_..."
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That's it. Real stateful behavior, proper error codes, webhook events — all from the OpenAPI spec.

The agent-ready integration guide

Here's the part that surprised us. We generate "integration guides" — Markdown files that contain:

  • Every verified endpoint with exact request/response shapes
  • State machine transitions (what can happen to a subscription after it's created?)
  • Webhook events that fire at each step
  • Sandbox credentials for testing

When you hand this to a coding agent (Claude Code, Cursor, Copilot), the agent:

  1. Reads the guide
  2. Inspects your existing repo patterns
  3. Asks about scope before implementing
  4. Builds working integration code using only verified data

No hallucinated endpoints. No invented status codes. Every fact comes from sandbox execution.

We tested this with Paddle's subscription lifecycle. The agent built a complete billing dashboard — product catalog, checkout, subscription management with activate/pause/resume/cancel — from just the integration guide.

See the code →

49 APIs, 9,300+ endpoints

The full catalog includes Stripe, Paddle, PayPal, Twilio, GitHub, OpenAI, Shopify, DigitalOcean, Datadog, and 40 more.

Every spec gets:

  • Automatic docs portal
  • Workflow discovery
  • Stateful sandbox
  • Integration guides

Browse the catalog →

What's next

We're onboarding new APIs every week. If there's an API you're struggling to test, let us know — onboarding takes minutes if you have an OpenAPI spec.

Next up: Deepgram, AssemblyAI, Stytch, Replicate, and Lago.

FetchSandbox is free to try. No signup required. Sandboxes run in your browser.

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