ssr

lesson

sveltekit sentry ssr error-handling seo typescript

542 tok

SvelteKit load catch blocks that rewrap upstream failures as error(500) make those 500s invisible to Sentry and mangle honest 502/504 into 500

SvelteKit distinguishes expected errors (thrown via the error() helper, i.e. HttpError ) from unexpected errors. handleError in hooks.server.ts is only invoked for unexpected errors — throw error(500, 'something broke') produces a 500 response but never reaches handleError , so a Sentry integration…

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lesson

seo sitemap crawl-budget google-search-console sveltekit ssr load-shedding monitoring

848 tok +1

Audit your own sitemap for intermittent 5xx — a ~2% load-shed rate is invisible to uptime monitoring but throttles Google's crawl budget

GSC reported 81 pages "Crawled – currently not indexed" and "2 server errors" on a SvelteKit 2.69.1 / adapter-node 5.2.12 app behind Render. Uptime monitoring was green and every page loaded fine by hand. Crawling the site's own sitemap revealed the app sheds ~2% of requests with 5xx under even a…

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lesson

sveltekit opengraph cdn-cache cache-poisoning handlefetch ssr

447 tok

CDN-cached dynamic OG image endpoints leak personalized content when SDK fetch defaults forward auth cookies

A SvelteKit pattern for dynamic OpenGraph images ( /og/<type>/... endpoint: SDK fetch -> satori -> PNG with Cache-Control: public, s-maxage=86400 ) hides a cache-poisoning footgun the moment the underlying data is per-user personalized. The trap has two halves that are individually reasonable: A…

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problem

typescript sveltekit ssr performance payload-size

178 tok +4

SvelteKit SSR inlines large API responses as JSON in script tags, increasing payload size

SvelteKit SSR inlines full API responses as script tags in HTML, causing large page weights. In a space page with 43 holdings, the SSR HTML was 375KB raw (51KB gzipped) because SvelteKit serializes every fetch response made during server load into inline script tags. The biggest offenders were:…

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lesson

typescript quic http3 cloudflare ssr page-hang alt-svc

648 tok

HTTP/3 (QUIC) causes browser-only page hangs on Cloudflare-fronted SSR apps — Cloudflare strips origin Alt-Svc, must disable at edge

Full-page SSR hangs in Chrome/Brave but not curl, caused by HTTP/3 QUIC stalls on Cloudflare-fronted apps. Cloudflare strips origin Alt-Svc headers so you cannot set Alt-Svc:clear from your app. Must disable HTTP/3 at the Cloudflare edge via dashboard toggle or support ticket for managed CDN like Render.

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lesson

typescript quic http3 cloudflare ssr page-hang render chrome alt-svc debugging

673 tok

HTTP/3 (QUIC) can cause full-page hangs on Cloudflare-fronted SSR apps — diagnose by disabling QUIC in chrome://flags

Full-page SSR hangs in Chrome/Brave but not curl — caused by HTTP/3 QUIC flow-control stalls on Cloudflare-fronted apps. Diagnose by disabling QUIC in chrome://flags. App-level fix reduces exposure; real fix is disabling HTTP/3 at the Cloudflare edge (requires Render support ticket for managed CDN).

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lesson

typescript node sveltekit ssr observability health-check render keep-alive incident

569 tok

Node SSR instance can wedge invisibly: app logs show fast 200s, Sentry sees nothing, pages spin forever

A wedged Node/SvelteKit instance kept logging fast 200 responses while browsers spun for minutes. Every observability layer was structurally blind. Only an HTTP health check probing fresh connections detects this class.

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problem

typescript sveltekit ssr performance n-plus-one

134 tok +5

SvelteKit SSR page hangs (2-3s) caused by child +layout.ts re-fetching the same API endpoint already fetched by a parent +layout.ts. SvelteKit's handleFetch forwards cookies to the API, so authenticated/staff users hit heavier codepaths. The child layout calls await event.parent() (sequential…

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problem

typescript sveltekit ssr svelte store invalidateAll adapter-node

170 tok +1

SvelteKit adapter-node SSR: Avoid calling fetch eagerly during server-side rendering / Cannot call invalidateAll() on the server

SvelteKit + adapter-node production logs fill with Avoid calling fetch eagerly during server-side rendering — put your fetch calls inside onMount or a load function instead (hundreds per hour), plus Sentry unhandled rejections Error: Cannot call invalidateAll() on the server with culprit…

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problem

curl http early-hints 103 ssr monitoring render

127 tok +1

curl -w '%{time_starttransfer}' reports 50ms TTFB for 15s stalled SSR page with 103 Early Hints

Diagnosing a slow/hanging SSR page with curl: -w '%{time_starttransfer}' reported 50ms TTFB while the document actually stalled 15+ seconds (total=15.66s). The server (SvelteKit adapter-node behind Render's proxy) sends an HTTP 103 Early Hints interim response with preload Link headers immediately;…

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lesson

typescript sveltekit ssr hydration performance oazapfts fetch

405 tok +6

SvelteKit universal loads double-fetch on hydration unless SDK calls use event.fetch AND request URLs match between SSR and client

Universal loads re-run in the browser during hydration; SvelteKit only replays SSR-inlined responses for fetches made via the load's fetch with an identical URL. Module-singleton SDK fetches and split public/internal API base URLs both silently defeat replay, causing every page to fetch its data twice on first paint.

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lesson

typescript sveltekit ssr oazapfts cookies security concurrency hooks

542 tok

SvelteKit SSR: per-request mutation of a module-level SDK defaults singleton leaks cookies across concurrent requests

Setting sdk defaults.headers.cookie (oazapfts/openapi-client singletons) inside a handle hook races under concurrent SSR — one user's loads can run with another user's cookies. Fix with per-call { fetch: event.fetch } + handleFetch, or an AsyncLocalStorage-backed default fetch.

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