An optimistic update is a promise the server can break

Updating the UI before the server answers makes an app feel fast. It also means the UI claims something the server has not agreed to yet. The failure path, where the request fails and the UI has to take the claim back, is the one nobody clicks through, because on a working backend it never happens.

Reticle can make it happen 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: failing a request needs a browser Reticle owns

reticle_network_mock works in a leased tab (reticle_run({ tool: "reticle_lease", args: { action: "acquire", url } })) or a driven one, and not through the always-on SDK, which answers { ok: false, reason: "no-cdp-provider" }. Without one, do step 4 (the success path) and report the rollback as unknown. Driving the happy path and reporting that rollback works is the one thing you may not do.

Confirm the session can observe first: reticle_session({ action: "list" }). A hidden or throttled tab can accept a click and render nothing.

1. Make the save fail, slowly

reticle_run({ tool: "reticle_network_mock", sessionId, args: {
  mocks: [{ urlContains: "/api/todos", method: "PATCH", status: 500, delayMs: 1500 }],
}})

The delay is what makes the optimistic state observable. Without it the failure can land before anything is on screen, and you cannot tell "rolled back" from "never updated".

2. Act, and prove the optimistic state appears

Get the control's ref with reticle_look({ action: "page", sessionId, mode: "interactive" }), then name the optimistic state before you click:

reticle_act_and_wait({ sessionId, ref: "<rename or toggle>", action: "click", until:
  { kind: "text", contains: "Buy oat milk" }
})

This proves the UI updated before the server answered. It is the claim the next step checks is taken back.

3. Prove it reverts, and says why

Once the request has failed, the new value must be gone, the old one back, and the user told. Name the app's real failure message, scoped to its alert, because any other alert already on the page would satisfy a bare role check:

reticle_assert({ sessionId, timeout_ms: 5000, predicate: { kind: "allOf", predicates: [
  { kind: "net", urlContains: "/api/todos", method: "PATCH", status: 500 },
  { kind: "text", contains: "Buy oat milk", absent: true },
  { kind: "text", contains: "Buy milk" },
  { kind: "text", contains: "Could not save", scope: "[role=alert]" },
  { kind: "console", level: "error", absent: true },
]}})

If the app keeps its state in a registered store, check the store too, because a UI can roll back on screen while the store keeps the optimistic value:

reticle_assert({ sessionId, predicate: { kind: "state", path: "todos.0.title", equals: "Buy milk" } })

4. Remove the mock, and prove success sticks

Clear the mock, then make the same change again from the state the rollback left. The ref from step 2 may be stale after a re-render, and a rename needs its new value entered again, so read the page first:

reticle_run({ tool: "reticle_network_mock", sessionId, args: { clear: true } })
reticle_look({ action: "page", sessionId, mode: "interactive" })

Redo whatever set up the edit (for a rename, reticle_act the field with the new value), then save and name the outcome:

reticle_act_and_wait({ sessionId, ref: "<save or toggle>", action: "click", until: { kind: "allOf", predicates: [
  { kind: "net", urlContains: "/api/todos", method: "PATCH", ok: true },
  { kind: "text", contains: "Buy oat milk" },
]}})

Then reload and prove the server kept it, which is what "saved" means. Use a real reload and wait for the new page, not a navigate to the URL you are already on:

reticle_navigate({ sessionId, reload: true })

It must answer confirmed: true with a sessionId. Assert on that session, which is the reloaded page:

reticle_assert({ sessionId: "<sessionId from the reload>", predicate: { kind: "text", contains: "Buy oat milk" } })

If the reload is not confirmed, report persistence as unknown.

Honesty

Only verified: "yes" is a pass. A verdict taken under a mock is about the mocked condition, so say which endpoint you failed and with what status. Clear every mock before handing back, or the next session sees a backend that fails every save.

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