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caldays vs ImprovMX API

Same instrument, two spec sheets — measured, not claimed.

APIcaldaysNEW—ImprovMX APINEW—
Uptime · 30d
Uptime · 90d—%—%
Uptime · 30d—%—%
P50 · ms——
P95 · ms——
AuthnoneapiKey
CORSyesyes
HTTPSyesyes
Card requirednono
Commercial useunclearunclear
Data licenseUnverifiedUnverified
Free tierFree — limits not publishedFree tier — API key required
Rate limitUnpublished301 req/window · 300 remaining · resets 1783526520
In directory since2026-07-052026-07-05
operationalpartialdownno data

caldays vs ImprovMX API: common questions

Which is more reliable, caldays or ImprovMX API?

On our scheduled checks, ImprovMX API leads on measured uptime — caldays at —% versus ImprovMX API at —% over 90 days. These are our own probe results, not provider claims; the uptime bars above show the day-by-day record for both.

Do caldays and ImprovMX API need an API key?

caldays needs no key, while ImprovMX API requires a free API key. If you want to start calling without signup, reach for caldays first.

Can I call caldays and ImprovMX API from the browser?

Yes — both caldays and ImprovMX API send CORS headers over HTTPS, so front-end code can fetch either directly with no backend proxy. That makes them easy to swap in a client-side app while you compare responses.

Are caldays and ImprovMX API free for commercial use?

caldays has unclear commercial terms, and ImprovMX API has unclear commercial terms. We track service terms and the data license as separate fields — see the Commercial use and Data license rows above, and confirm both before shipping either in a paid product.