Case Study: Persisted TanStack Query Mutations

A case study comparing Codex GPT-5.5 runs with and without GitHits found that GitHits reduced token usage by 66% and tool calls by 46% while fixing a persisted TanStack Query mutation issue in a React checkout flow. The bug occurred because a persisted paused mutation lacked a default mutation function after reload, which was resolved by registering the mutation function on the QueryClient.

Back to blog /blog/ June 15, 2026 · 3 min read Case Study: Persisted TanStack Query Mutations A measured Codex run fixing a persisted TanStack Query mutation by finding the default mutation function rule. The fixture is a React checkout flow using TanStack Query persistence. The user places an order while offline. TanStack Query pauses the mutation. The app persists the query cache. After reload, the app restores the cache, goes online, and calls resumePausedMutations . Expected result: the order is submitted once after reload. Actual result: the order is never submitted. Both runs used Codex GPT-5.5 against the same fixture. The prompt was: Fix this TypeScript React fixture so npm test and npm run typecheck succeed, preserving queued checkout writes across app reloads. The target packages were @tanstack/react-query 5.101.0 and @tanstack/react-query-persist-client 5.101.0 . Case study replay TanStack Query persisted checkout outbox model Codex GPT-5.5Fix this TypeScript React fixture so npm test and npm run typecheck succeed, preserving queued checkout writes across app reloads. Without GitHits - tokens - 0 - time - 0s / 126s - Ready. Click "Watch Replay" to start. - Reached the same mutation-default fix, but first installed dependencies and reconstructed the persistence path from local TanStack Query internals. With GitHits - tokens - 0 - time - 0s / 79s - Ready. Click "Watch Replay" to start. - Used TanStack Query persistence docs to register a keyed mutation default, allowing hydrated paused checkout writes to resume after reload. Result | Run | Time | Tokens | Tools | |---|---|---|---| | With GitHits | 79s | 350,965 | 19 | | Without GitHits | 126s | 1,031,661 | 35 | Both runs produced a passing patch. The GitHits run used 680,696 fewer processed tokens and 16 fewer tool calls. Failure The fixture created a QueryClient without mutation defaults: export function createCheckoutClient api?: CheckoutApi : QueryClient { return new QueryClient { defaultOptions: { queries: { retry: false }, mutations: { retry: false }, }, } ; } The checkout component passed the mutation function to useMutation : js const submitOrder = useMutation { mutationKey: submitOrderMutationKey, mutationFn: api.submitOrder, } ; That function exists only in the mounted component. After persistence and reload, the restored mutation has its key and state. It does not have the component closure. The relevant TanStack Query rule: a persisted paused mutation needs a default mutation function registered for its mutation key before it can resume. Fix Register the checkout submit function on the client: js export function createCheckoutClient api?: CheckoutApi : QueryClient { const client = new QueryClient { defaultOptions: { queries: { retry: false }, mutations: { retry: false }, }, } ; if api { client.setMutationDefaults submitOrderMutationKey, { mutationFn: api.submitOrder, } ; } return client; } The test restores an offline write into a fresh client, goes online, resumes paused mutations, and checks that the checkout API receives the original order once. Trace The GitHits run found the rule with docs search: search target=npm:@tanstack/react-query@5.101.0 source=docs query=persistQueryClient resumePausedMutations mutation defaults offline persistence reload limit=5 docs read page id=20025 start line=1 end line=120 docs read page id=20025 start line=340 end line=416 It then patched createCheckoutClient and ran: npm test npm run typecheck The no-GitHits run installed dependencies and searched local package source for the same behavior. It read TanStack Query hydration, mutation cache, query client defaults, and default mutation option code before editing. In this run, docs search found the package contract directly. The agent did not need to inspect source internals to discover it.