FetchSandbox

Turn any OpenAPI spec into a working sandbox your AI agent can use, right from your IDE.

This is the Model Context Protocol (MCP) server for FetchSandbox. It exposes three tools that let any MCP-compatible agent ingest an OpenAPI spec, list its workflows, and run them — with realistic, schema-validated responses for every endpoint.

⭐ If FetchSandbox saves you a debugging session, star this repo. It helps people find the project and helps us prioritize what to build next.

Why

Agents read raw OpenAPI specs and hallucinate. They guess field names, invent IDs that won't exist, and produce broken curl commands. FetchSandbox turns the spec into a stateful, AJV-validated sandbox so the agent can actually call the API and see real-shaped responses.

Plug it into your IDE once, and any time you ask your agent "let me try the Stripe API" or "show me the GitHub issue lifecycle," it can do that — for real, end-to-end.

Install — by agent

The MCP runs as a stdio process spawned by your IDE. There's nothing to install globally — npx runs the published version on demand. We recommend pinning to @latest so each session auto-upgrades to the current release; otherwise npm caches the first version it saw and silently drifts behind.

Pick your tool below, paste the snippet, restart.

Claude Desktop

File: ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%\Claude\claude_desktop_config.json (Windows)

{
  "mcpServers": {
    "fetchsandbox": {
      "command": "npx",
      "args": ["-y", "fetchsandbox-mcp@latest"]
    }
  }
}

Quit and reopen Claude Desktop (Cmd+Q, then reopen — not just close window).

Claude Code

User-level (all projects): ~/.claude/settings.json. Or project-level: .mcp.json in the repo root.

{
  "mcpServers": {
    "fetchsandbox": {
      "command": "npx",
      "args": ["-y", "fetchsandbox-mcp@latest"]
    }
  }
}

Restart the Claude Code session.

Cursor

File: ~/.cursor/mcp.json (global) or .cursor/mcp.json (project)

{
  "mcpServers": {
    "fetchsandbox": {
      "command": "npx",
      "args": ["-y", "fetchsandbox-mcp@latest"]
    }
  }
}

Restart Cursor.

Cline (VS Code extension)

Open the Cline panel → settings cog → MCP Servers → add a new server with:

• Command: npx
• Args: -y fetchsandbox-mcp@latest

Reload the VS Code window.

Continue.dev

File: ~/.continue/config.yaml

mcpServers:
  - name: fetchsandbox
    command: npx
    args:
      - -y
      - fetchsandbox-mcp@latest

Restart your IDE.

Codex CLI (OpenAI)

File: ~/.codex/config.toml

[mcp_servers.fetchsandbox]
command = "npx"
args = ["-y", "fetchsandbox-mcp@latest"]

Restart Codex.

Zed

File: ~/.config/zed/settings.json

{
  "context_servers": {
    "fetchsandbox": {
      "command": {
        "path": "npx",
        "args": ["-y", "fetchsandbox-mcp@latest"]
      }
    }
  }
}

GitHub Copilot

GitHub Copilot doesn't currently support the Model Context Protocol. Track github/copilot#feedback for updates. In the meantime, run any MCP-compatible chat (Claude Code, Cursor, Cline) alongside Copilot.

Anything else (Roo, Goose, etc.)

If your agent speaks MCP, it accepts a stdio command. Use:

• Command: npx
• Args: ["-y", "fetchsandbox-mcp@latest"]

Try it now

After restarting your agent, paste any of these prompts. Each hits a hand-curated workflow with realistic IDs and real state transitions.

Stripe — accept a payment

Use fetchsandbox to import the Stripe spec from https://raw.githubusercontent.com/stripe/openapi/master/openapi/spec3.json and run the accept_payment workflow. Show me the trace.

The agent imports 587 endpoints, matches the bundled curated Stripe sandbox, and runs a 6-step workflow: create customer (cus_…) → create PaymentIntent (pi_…, $49.99 USD, requires_payment_method) → confirm (requires_capture) → capture (succeeded) → retrieve → verify webhooks (payment_intent.created, payment_intent.succeeded).

Twilio — send an SMS

Use fetchsandbox to import the Twilio Messaging spec from https://raw.githubusercontent.com/twilio/twilio-oai/main/spec/yaml/twilio_messaging_v1.yaml and run the send_sms workflow.

The agent imports the messaging API and runs a curated send-and-verify flow with realistic Twilio-formatted message SIDs (SM…).

GitHub — issue lifecycle

Use fetchsandbox to import the GitHub REST API from https://raw.githubusercontent.com/github/rest-api-description/main/descriptions/api.github.com/api.github.com.json and run the issue_lifecycle workflow.

The agent walks the create → comment → close → reopen flow against a real-shaped GitHub sandbox.

Paddle — paste-content variant

If a vendor doesn't publish their spec at a stable URL (Paddle, Notion, Linear), paste the content directly:

Here's the Paddle Billing OpenAPI spec — <paste JSON or YAML>. Use fetchsandbox to import it and run the subscriptions_canceled workflow.

Same engine path; same curated quality if the spec's info.title matches a bundled config.

Any other API

Use fetchsandbox to import <your OpenAPI URL> — list the workflows and tell me which is most interesting.

For specs we don't have curated configs for, the engine auto-enumerates create + verify workflows for every detected resource. Honest about what it shows: UUIDs instead of vendor-style IDs, generic enum values instead of API-specific ones — but the request/response shape and template substitution between steps still work.

Tools

import_spec

Ingest an OpenAPI 3.x spec and get a sandbox you can call. Pass either a public URL or pasted content.

url:     "https://raw.githubusercontent.com/stripe/openapi/master/openapi/spec3.json"
content: "<paste OpenAPI JSON or YAML here>"
name:    "Optional friendly name"

Returns spec_id, sandbox_id, base_url (proxy that serves real-shaped responses), workflows_preview (first 10), matched_bundled (true if we matched a curated config), and a dashboard_url to view everything in the browser.

list_workflows

List the named, runnable workflows the engine inferred or curated for an imported spec.

spec_id: "<id from import_spec>"

run_workflow

Execute one workflow and return the step-by-step request/response trace. Template variables ({{step1.id}}) are resolved automatically between steps. The response now includes a share_url per run — a public receipt URL you can paste into a PR or share with a teammate.

sandbox_id:    "<id from import_spec>"
workflow_name: "<id or name from list_workflows>"

Configuration

Env var	Default	Purpose
FETCHSANDBOX_BASE_URL	https://fetchsandbox.com	Override for stage testing or self-hosted backends.
FETCHSANDBOX_TELEMETRY	(on)	Set to 0 to disable anonymous usage telemetry.

What we record

When telemetry is on, each tool call records: an opaque per-machine session id (random UUID stored at ~/.fetchsandbox/session.json), the tool name, latency, success/failure, and the spec URL or "pasted". We do not record spec content, request bodies, or credentials. We use this to count daily-active sessions and learn which APIs people are bringing to the platform.

To opt out:

export FETCHSANDBOX_TELEMETRY=0

Want to see it catch real bugs?

Try the FetchSandbox Playground — five small brownfield apps with planted bugs in real API integrations (Stripe webhook dedup, Resend bounce drops, Clerk JWT verification, AgentMail attachment handling, Surge opt-out). Clone, run, point your agent at one, and see whether FetchSandbox catches the bug. PRs with your session findings welcome.

License

MIT — see LICENSE.

Links

• FetchSandbox — main site, docs, dashboard
• Playground — try it on planted bugs
• npm package — npx fetchsandbox-mcp@latest
• Issues — bug reports, feature asks