MCP for AI agents

Lagsmith has an MCP server built in. AI coding agents that support MCP, including Claude Code, Cursor, Codex, Claude Desktop and Windsurf, can use it to:

list simulators and apps,
set and reset an app’s network,
relaunch an app with request monitoring,
read its requests,
fail or mock endpoints.

Because Lagsmith only slows down the app under test, the agent’s own connection stays at full speed.

The MCP server ships inside the app at /Applications/Lagsmith.app/Contents/MacOS/lagsmith-mcp, so Lagsmith needs to be in your Applications folder. Automation, including MCP, is part of Pro and included in the 14-day trial.

Claude Code

Run this once in Terminal:

claude mcp add --scope user lagsmith -- "/Applications/Lagsmith.app/Contents/MacOS/lagsmith-mcp"

--scope user makes Lagsmith available in every project. Start a new Claude Code session and run /mcp to check it’s connected.

Cursor

Add this to ~/.cursor/mcp.json (or .cursor/mcp.json in a project):

~/.cursor/mcp.json

{
  "mcpServers": {
    "lagsmith": {
      "command": "/Applications/Lagsmith.app/Contents/MacOS/lagsmith-mcp"
    }
  }
}

Codex

Add this to ~/.codex/config.toml:

~/.codex/config.toml

[mcp_servers.lagsmith]
command = "/Applications/Lagsmith.app/Contents/MacOS/lagsmith-mcp"

Claude Desktop

In Claude Desktop, open Settings › Developer › Edit Config and add:

~/Library/Application Support/Claude/claude_desktop_config.json

{
  "mcpServers": {
    "lagsmith": {
      "command": "/Applications/Lagsmith.app/Contents/MacOS/lagsmith-mcp"
    }
  }
}

Then restart Claude Desktop.

Windsurf

Add this to Windsurf’s MCP config:

~/.codeium/windsurf/mcp_config.json

{
  "mcpServers": {
    "lagsmith": {
      "command": "/Applications/Lagsmith.app/Contents/MacOS/lagsmith-mcp"
    }
  }
}

If your config file already has other servers, add the lagsmith entry inside the existing mcpServers section rather than replacing the file.

Tools

Lagsmith’s MCP server gives agents 10 tools:

Tool	What it does	Main parameters
`list_targets`	Running simulators and their apps, running Mac apps, presets and current rules	none
`status`	Every rule, what it’s doing, and whether the proxy is running	none
`set_network`	Slow down or break one app’s network	app, simulator; preset, or down/up (kbps), latency/jitter (ms), loss (%); failure (airplane, refuse, timeout, drop, outages); hosts, hosts_mode
`reset_network`	Back to full speed; the app stays watched	app, simulator
`stop_watching`	Turn the app’s rule off entirely	app, simulator
`relaunch_app`	Relaunch a simulator app with request monitoring	app, simulator
`get_requests`	The app’s recent HTTP requests, newest first	app, simulator, limit, filter (e.g. is:failed, status:5xx), include_bodies
`fail_endpoint`	Make one endpoint fail while the rest works	app, simulator, pattern, kind (offline, timeout, status, delay), value
`mock_endpoint`	Answer one endpoint with a made-up response	app, simulator, pattern, status, body, content_type, delay
`clear_endpoints`	Remove endpoint failures and mocks	app, simulator, pattern

What a session looks like

> 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.

Example prompts

“Relaunch the app on 3G with monitoring, sign in, and list requests slower than two seconds.”
“Put the app in Airplane Mode and check that the offline banner appears.”
“Make POST /v1/orders return 500 and check the app shows a retry button.”

MCP for AI agents

Claude Code

Cursor

Codex

Claude Desktop

Windsurf

Tools

What a session looks like

Example prompts

Slow down one app, not your whole Mac