The error state has never run

Every app has a catch block nobody has executed and an empty state nobody has seen. They are written from imagination, shipped untested, and discovered by a user on a bad day.

Reticle can force those conditions in the running app. Not installed? RETICLE_INSTALL_SOURCE=npx_skill npx @reticlehq/server@latest init, then the install-and-verify skill.

Read this before you start: network mocking needs a driven or leased browser

reticle_network_mock intercepts requests in a browser Reticle owns, and the always-on SDK cannot do it. A connected tab with neither reticle drive nor a lease still returns { ok: false, reason: "no-cdp-provider" }.

Your route is a leased Playwright tab (reticle_lease acquire) or RETICLE_CDP_URL pointed at a Chrome started with remote debugging:

# macOS — the user runs this once, in their own Chrome
/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --remote-debugging-port=9222

RETICLE_CDP_URL=http://localhost:9222 npx @reticlehq/server@latest mcp

If you have neither a lease nor a driven browser, say so in one line and offer the clock half of this skill anyway: reticle_clock needs none of it. What you may not do is drive the happy path, watch it pass, and report that error handling works.

Force a failure

reticle_run({ tool: "reticle_network_mock", sessionId, args: {
  mocks: [{ urlContains: "/api/deploys", status: 500 }],
}})

Then drive the flow and name the recovery you expect before you act:

reticle_act_and_wait({ sessionId, ref, action: "click", until: { kind: "allOf", predicates: [
  { kind: "element", query: { testid: "error-banner" } },
  { kind: "console", level: "error", absent: true },
]}})

Note the second predicate. A UI that "handles" an error by logging an uncaught exception has not handled it. Clear mocks with { clear: true } when you are done, or every later check runs against a lie.

Worth forcing, in rough order of how often they are broken: 500, a 4xx with a real error body, an empty 200 ([], the empty state), a malformed payload, and a request that never resolves (the spinner that spins forever).

Skip time instead of sleeping

reticle_run({ tool: "reticle_clock", args: { sessionId, freeze: true } })
reticle_run({ tool: "reticle_clock", args: { sessionId, advanceMs: 5000 } })
reticle_run({ tool: "reticle_clock", args: { sessionId, reset: true } })

Toasts that auto-dismiss, debounced search, polling, session timeouts, retry backoff. All of these are normally verified by sleeping, which is slow and flaky in equal measure. A timing assertion is a statement about the machine, so it passes on your laptop and fails in CI.

Freeze, advance by exactly the interval, assert the consequence. Same result on a fast laptop and a loaded runner. Always reset when you finish, or a frozen clock silently breaks everything that runs after you.

What to assert

The recovery, not the absence of a crash:

1. The error is shown to the user: a specific element, not just "the page did not blank".
2. The app stayed usable: retry works, the form still has its input, navigation is not stuck.
3. State is honest: reticle_state shows the failure, not a half-applied optimistic update. A UI that rolled back visually while the store kept the optimistic value is the classic bug here, and only the store read finds it.
4. No uncaught error in the console.

Honesty

Mocking changes the app's world, so a verdict taken under a mock is a statement about the mocked condition and nothing else. Say which mock was active when you report a pass, and clear every mock and reset the clock before handing back. An audit that leaves a 500 pinned on /api/deploys breaks the next person's session and looks like a real outage.

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Capability reference: curl https://docs.reticle.sh/capabilities.md. Everything else: curl https://docs.reticle.sh/llms.txt.