An open-source CLI and SDK on npm. No API key, no sign-up and no dependencies.
npx valuethen convert 100 USD 1970
$100.00 in 1970 = $853.01 in 2026
Price change: +753.0%
Sources: World Bank FP.CPI.TOTL, 2010=100; recent years chained from CPI-U (FRED CPIAUCNS)
Run npx valuethen convert 100 USD 1970 and the answer comes back with the series it was computed from. The valuethen package on npm wraps the free ValueThen API in a command-line tool and a small JavaScript SDK.
The SDK covers single conversions, full annual series, coverage lookups and batches of up to 100 conversions with an idempotency key, so a retry never double-counts. Errors are typed: ask for a year outside a currency’s coverage and you get the valid range back.
For AI assistants there is an MCP server at api.valuethen.com/mcp and an agent skill, so an assistant can convert historical amounts and name the series it used.
The source is public on GitHub under the Rising Throne organisation.