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PostalCodes vs SLF

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

APIPostalCodesNEW—SLFNEW—
Uptime · 30d
Uptime · 90d—%—%
Uptime · 30d—%—%
P50 · ms——
P95 · ms——
Authnonenone
CORSnoyes
HTTPSyesyes
Card requirednono
Commercial useunclearunclear
Data licenseUnverifiedUnverified
Free tierFree — limits not publishedFree — static file, no key
Rate limitUnpublishedUnpublished
In directory since2026-07-052026-07-05
operationalpartialdownno data

PostalCodes vs SLF: common questions

Which is more reliable, PostalCodes or SLF?

On our scheduled checks, SLF leads on measured uptime — PostalCodes at —% versus SLF 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 PostalCodes and SLF need an API key?

Neither needs a paid key — PostalCodes is callable with no signup, and SLF is callable with no signup. Both are quick to prototype with; rate limits still apply.

Can I call PostalCodes and SLF from the browser?

Only SLF is browser-friendly — it returns CORS headers over HTTPS. PostalCodes needs a server-side call or proxy, so factor that into which one fits a front-end project.

Are PostalCodes and SLF free for commercial use?

PostalCodes has unclear commercial terms, and SLF 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.