frontend-optimistic-mutations

Frontend Optimistic Mutations (the write path)

When to Use

Use this skill when you need a portable, framework-agnostic discipline for the write path of any React or React Native app using a query/cache layer. Codifies the optimistic-update lifecycle (cancel in-flight queries → snapshot every affected cache → patch instantly → roll back verbatim on error → invalidate on...

Portable skill — readable by Claude Code, OpenCode, Codex, Cursor, Windsurf, and others. This skill describes the discipline of the write path — optimistic updates, rollback, idempotency, cache coherence — not a UI library or a styling system. It builds directly on the frontend-data-contracts skill (writes go through the typed client) and the frontend-architecture skill (mutations live in modules/{feature}/hooks/, keyed by a factory).

The goal: a write feels instant (the UI reflects it before the server confirms), is safe (a failure restores the exact prior state, and a retry never double-charges), and leaves the cache coherent (the detail view and every list page agree). All three at once — that's the craft.

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0. The five core ideas

1. The optimistic lifecycle is fixed. cancel → snapshot → patch → (error: roll back) → (settle: invalidate). Every optimistic mutation follows the same five beats.
2. Roll back verbatim. On failure, restore the exact snapshot taken before the patch — not a "best guess" re-derivation. Keep the snapshot in mutation context.
3. Idempotency is generated once, not per attempt. The key is created at form init (or first intent), so a network retry replays the original server response instead of performing the action twice.
4. Caches move in lock-step. A status change patches the detail cache and every list page that contains the entity, so badges never disagree across surfaces.
5. Server state never enters the client store. Optimistic state lives in the query cache, not Zustand/Redux. The cache is the single source of truth for server data (per frontend-architecture §4).

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1. When to be optimistic (and when not)

Situation	Strategy
High-confidence, low-conflict write (toggle status, like, mark-paid, reorder)	Optimistic — patch immediately, roll back on error.
Create that returns a server-generated id/number/total	Pending state, then setQueryData from the server response. A temporary optimistic row is optional; reconcile on success.
Destructive or hard-to-reverse write (delete with cascade, send money)	Confirm first, then optimistic or pending — never silent-optimistic.
Write whose result the user can't see yet (background job)	Pending + toast, invalidate when done. No optimistic patch.

Optimism is a UX tool for writes you're confident will succeed. If failure is common or expensive to undo, prefer a pending state.

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2. The optimistic lifecycle (TanStack Query)

The canonical shape. Each beat has a job; skipping one breaks correctness.

// modules/invoice/hooks/useInvoiceMutations.ts
interface MarkPaidContext {
  previousInvoice: Invoice | undefined; // detail snapshot
  previousLists: Array<[readonly unknown[], InvoiceListResponse]>; // every list page snapshot
}

export function useMarkInvoicePaid() {
  const queryClient = useQueryClient();
  const notifyError = useApiErrorToast();

  return useMutation<Invoice, ApiError, { id: InvoiceId }, MarkPaidContext>({
    mutationFn: ({ id }) => apiClient.post<Invoice>(INVOICE_API.markPaid(id)),

    // 1 + 2 + 3: cancel in-flight reads, snapshot, patch
    onMutate: async ({ id }) => {
      await queryClient.cancelQueries({ queryKey: invoiceKeys.all }); // (1) no late refetch clobber

      const detailKey = invoiceKeys.detail(id);
      const previousInvoice = queryClient.getQueryData<Invoice>(detailKey); // (2) snapshot detail
      if (previousInvoice) {
        queryClient.setQueryData<Invoice>(detailKey, {
          // (3) patch detail
          ...previousInvoice,
          status: InvoiceStatus.Paid,
        });
      }

      const previousLists: MarkPaidContext["previousLists"] = [];
      for (const [key, list] of queryClient.getQueriesData<InvoiceListResponse>(
        {
          queryKey: invoiceKeys.lists(),
        },
      )) {
        if (!list) continue;
        previousLists.push([key, list]); // (2) snapshot each page
        if (!list.invoices.some((i) => i.id === id)) continue;
        queryClient.setQueryData<InvoiceListResponse>(key, {
          // (3) patch matching row
          ...list,
          invoices: list.invoices.map((i) =>
            i.id === id ? { ...i, status: InvoiceStatus.Paid } : i,
          ),
        });
      }
      return { previousInvoice, previousLists };
    },

    // 4: roll back verbatim
    onError: (error, { id }, ctx) => {
      if (ctx?.previousInvoice)
        queryClient.setQueryData(invoiceKeys.detail(id), ctx.previousInvoice);
      for (const [key, list] of ctx?.previousLists ?? [])
        queryClient.setQueryData(key, list);
      notifyError(error);
    },

    // 5: invalidate so authoritative server state (paidAt, aggregates) refetches
    onSettled: (_d, _e, { id }) => {
      void queryClient.invalidateQueries({ queryKey: invoiceKeys.detail(id) });
      void queryClient.invalidateQueries({ queryKey: invoiceKeys.lists() });
    },
  });
}

Why each beat:

• cancel — without it, a query that was already in flight can resolve after your patch and overwrite the optimistic state.
• snapshot — the only safe rollback source; never reconstruct prior state by hand.
• patch — the instant UX; mutate detail and lists together (§4).
• roll back — restore snapshots verbatim, then surface the typed ApiError.
• invalidate on settle — success or failure, refetch so server-computed fields (timestamps, totals) are authoritative. Settle, not just success: a failed write may still have changed server state.

---

3. Non-optimistic writes: create with server-owned fields

A create that returns an id/number/total can't be fully optimistic. Run it as a pending mutation and seed the cache from the response.

export function useCreateInvoice() {
  const queryClient = useQueryClient();
  return useMutation<Invoice, ApiError, CreateInvoiceInput>({
    mutationFn: ({ document, idempotencyKey }) =>
      apiClient.post<Invoice>("/invoices", document, { idempotencyKey }),
    onSuccess: (invoice) => {
      queryClient.setQueryData(invoiceKeys.detail(invoice.id), invoice); // seed detail
      void queryClient.invalidateQueries({ queryKey: invoiceKeys.lists() }); // refresh lists
    },
    onError: (error) => notifyError(error),
  });
}

---

4. Cache coherence (lock-step detail + lists)

A single entity appears in many caches: its detail, and every filtered/paginated list page. An optimistic patch must touch all of them or surfaces disagree. Use a hierarchical key factory (from frontend-architecture §4.3) so you can target precisely.

export const invoiceKeys = {
  all: ["invoices"] as const,
  lists: () => [...invoiceKeys.all, "list"] as const,
  list: (p: IListParams) => [...invoiceKeys.lists(), p] as const,
  detail: (id: InvoiceId) => [...invoiceKeys.all, "detail", id] as const,
} as const;

• getQueriesData({ queryKey: invoiceKeys.lists() }) enumerates every cached list page so you can patch each.
• invalidateQueries({ queryKey: invoiceKeys.lists() }) refreshes them all on settle.
• detail(id) targets exactly one entity.

Snapshot each page you touch (keyed by its exact query key) so rollback restores every page verbatim, not just the one currently on screen.

---

5. Idempotency (safe retries on money-moving writes)

A retried POST must not perform the action twice. Generate the key once, at form init (or first user intent), carry it through retries, and let the client send it as a header. The server replays the original response for a repeated key within its window.

// at form initialisation — stable for the lifetime of this attempt
const idempotencyKey = useMemo(() => crypto.randomUUID(), []);

// mutation forwards it; the typed client puts it on the header
apiClient.post<Invoice>("/invoices", document, { idempotencyKey });

Hard rules:

• Generate the key at intent time, not inside mutationFn (which re-runs per retry → defeats the purpose).
• The data client auto-detects financial routes and injects the header; an explicit key always wins so retries replay.
• Pair idempotency with disabled-while-pending UI so the user can't fire a second distinct write.

---

6. Retry policy

• Reads: retry a few times with backoff (default in most query libs) — safe and idempotent.
• Writes: do not auto-retry non-idempotent mutations. Retry only when an idempotency key guarantees replay, or only on network errors (status 0), never on 4xx.
• Conflicts (409): don't retry — surface the typed error, invalidate, and let the user re-decide on fresh data.

useMutation({
  retry: (count, error: ApiError) => error.isNetworkError && count < 2, // network-only, bounded
});

---

7. Library adapters

The five-beat lifecycle is the same; the hooks differ.

Library	Optimistic mechanism
TanStack Query	onMutate (cancel + snapshot + patch) → onError (rollback) → onSettled (invalidate). The reference shape above.
RTK Query	onQueryStarted: updateQueryData returns a patchResult; await queryFulfilled and call patchResult.undo() in catch. invalidatesTags on settle.
SWR	mutate(key, optimisticData, { rollbackOnError: true, populateCache, revalidate: true }) — optimistic data + automatic rollback + revalidate.

For React Native, all three libraries work unchanged; the cache is the source of truth on native too. Keep mutation hooks DOM-free so they're shareable across web and native.

---

8. Conventions checklist (enforce in review)

• Optimistic mutations follow cancel → snapshot → patch → rollback → invalidate.
• onMutate cancels in-flight queries before patching.
• Rollback restores the exact snapshot from context, not a re-derivation.
• Detail and every affected list page are patched and snapshotted together.
• onSettled invalidates so server-computed fields are refetched (on success and error).
• Idempotency key is generated at intent time and replayed across retries, not regenerated per attempt.
• Money-moving / destructive writes confirm first and disable the trigger while pending.
• Non-idempotent writes don't auto-retry; 409s surface rather than retry.
• Server state stays in the query cache — never copied into a client store.
• Query keys come from a hierarchical factory; invalidation is scoped, not global-blunt.

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9. How to apply this skill

Adding an optimistic mutation: decide it's safe to be optimistic (§1). Write the five beats (§2). Identify every cache the entity lives in and patch/snapshot all of them (§4).

Making a write safe to retry: generate an idempotency key at form init, thread it through the mutation, confirm the client sends it (§5), and set a network-only bounded retry (§6).

Debugging a flicker / wrong-state-after-write: check that onMutate cancels queries (late refetch clobber) and that onSettled invalidates (stale server-computed fields). Check that all list pages were patched, not just the visible one.

Reviewing the write path: run the checklist in §8. The highest-value catches are missing cancelQueries (race clobber), partial cache patches (detail/list disagreement), and idempotency keys generated inside mutationFn (no longer protect retries).

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Publishing / installing this skill

This skill follows the Anthropic SKILL.md format and is portable across agents.

1. Keep it under skills/frontend-optimistic-mutations/SKILL.md in a public GitHub repo.
2. Keep the frontmatter name and high-signal description — discovery indexes match against it.
3. Install with: npx skills add <org>/<repo> --skill "frontend-optimistic-mutations".
4. Non-SKILL.md agents can be pointed here from AGENTS.md / CLAUDE.md; Kiro can mirror it as a steering file.

Limitations

• Use this skill only when the task clearly matches its upstream source and local project context.
• Verify commands, generated code, dependencies, credentials, and external service behavior before applying changes.
• Do not treat examples as a substitute for environment-specific tests, security review, or user approval for destructive or costly actions.