# Network testing for AI coding agents with MCP | Lagsmith

> Let Claude Code, Cursor and Codex test your app on bad networks. Lagsmith’s MCP server throttles the app under test without slowing down the AI agent.

Source: https://lagsmith.com/ai-agents-network-testing

# 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](https://lagsmith.com/download)

```
> 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 s
     Executed 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 ms
     POST https://api.pantryhq.app/v1/payments 503 injected by Lagsmith 1 ms
     POST 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](https://lagsmith.com/download) 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](https://lagsmith.com/docs/mcp).
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](https://lagsmith.com/download) [Compare with Network Link Conditioner](https://lagsmith.com/network-link-conditioner-alternative)