Lagsmith

Network testing for AI coding agents

Run your end-to-end tests on 3G without your AI agent’s own connection slowing down. Lagsmith’s built-in MCP server lets Claude Code, Cursor, Codex, Claude Desktop and Windsurf control the network of the app under test.

Download for Mac
> Run the checkout UI tests on 3G with the payments API returning 503s, and tell me what breaks.
I’ll find the app in the simulator first.
lagsmith - list_targets (MCP)
simulators: iPhone 17 Pro (booted)apps: com.pantryhq.Pantry, com.apple.mobilesafari, …presets: Edge, 3G, DSL, LTE, WiFi, Very Bad Network, 100% Loss
lagsmith - set_network (MCP)(app: "com.pantryhq.Pantry", simulator: "booted", preset: "3G")
{"ok": true, "rule": "Pantry", "preset": "3G", "conditions": {"down": 780, "up": 330, "latency": 200}}
lagsmith - fail_endpoint (MCP)(app: "com.pantryhq.Pantry", simulator: "booted", pattern: "api.pantryhq.app/v1/payments", kind: "status", value: 503)
{"ok": true, "endpoints": [{"pattern": "api.pantryhq.app/v1/payments", "kind": "status", "value": 503}]}
lagsmith - relaunch_app (MCP)(app: "com.pantryhq.Pantry", simulator: "booted")
{"ok": true, "relaunched": ["Pantry on iPhone 17 Pro"]}
Bash(xcodebuild test -scheme PantryUITests -only-testing:PantryUITests/CheckoutTests -destination 'platform=iOS Simulator,name=iPhone 17 Pro')
Test Suite 'CheckoutTests' started✓ testCartLoads (4.8 s)✓ testApplyCoupon (6.1 s)✗ testPlaceOrder (30.0 s): Failed to find "Order confirmed" within 30 sExecuted 3 tests, with 1 failure
lagsmith - get_requests (MCP)(app: "com.pantryhq.Pantry", simulator: "booted", filter: "host:api.pantryhq.app is:failed", include_bodies: true)
POST https://api.pantryhq.app/v1/payments 503 injected by Lagsmith 2 msPOST https://api.pantryhq.app/v1/payments 503 injected by Lagsmith 1 msPOST https://api.pantryhq.app/v1/payments 503 injected by Lagsmith 1 ms(3 requests, 0.4 s apart)
lagsmith - reset_network (MCP)(app: "com.pantryhq.Pantry", simulator: "booted")
{"ok": true, "preset": "No Throttling"}
Found it. On 3G the cart and coupon flows are fine, but when payments returns 503: 1. PaymentService retries 3 times with no backoff (0.4 s apart), then gives up silently. 2. The checkout screen keeps its spinner forever: there’s no error state for a failed payment, so testPlaceOrder times out waiting for "Order confirmed". Want me to add exponential backoff and an error state with a "Try again" button in CheckoutView, then rerun the tests under the same conditions?
Illustrative Claude Code session using Lagsmith’s MCP server. Your agent stays at full speed; only Pantry is on 3G.

The problem with throttling the whole Mac

Your AI agent talks to its model over the network. Turn on a whole-Mac throttle like Network Link Conditioner and the agent slows to a crawl along with the app it is testing. Lagsmith slows down only the app under test.

What the agent can do

  • List simulators and their apps
  • Set and reset an app’s network conditions
  • Relaunch an app with request monitoring
  • Read the app’s requests
  • Fail or mock endpoints

So you can ask things like: “Relaunch the app on 3G, log in, and tell me which requests took longer than two seconds,” or “Make /v1/feed return a 500 and check the app shows the error state.”

Set it up

  1. Install Lagsmith and allow its network extension.
  2. Add Lagsmith’s MCP server to your agent. For Claude Code, run: 
    claude mcp add --scope user lagsmith -- "/Applications/Lagsmith.app/Contents/MacOS/lagsmith-mcp"
    Using Cursor, Codex, Claude Desktop or Windsurf? See the setup for each.
  3. Ask your agent to run your tests on a slow or failing network.

Frequently asked questions

Which AI agents does Lagsmith work with?

Any agent that supports MCP, including Claude Code, Cursor, Codex, Claude Desktop and Windsurf.

Will throttling the app slow down my agent?

No. Lagsmith only slows down the app under test. The agent’s own connection stays at full speed.

Does the MCP server need a separate install?

No. It is built into the Lagsmith app.

Is the MCP server included in the free version?

Automation, including MCP for AI agents, is part of Pro. Every install starts with a 14-day Pro trial.

Slow down one app, not your whole Mac

Free to download, with a 14-day Pro trial. No account, no card. Requires macOS 13 Ventura or later.

Download for Mac Compare with Network Link Conditioner