--- title: "Developers" description: "Quickstart, endpoints, authentication, rate limits, error format and sandbox for the free ValueThen purchasing-power API." url: "https://valuethen.com/developers/" language: "en" site: "ValueThen" publisher: "Rising Throne" updated: "2026-09-02" license: "Data: World Bank CC BY 4.0, ECB, FRED" --- # Developers Quickstart, endpoints, authentication, rate limits, error format and sandbox for the free ValueThen purchasing-power API. # Developer portal Everything the calculator shows is available as a JSON API. There is no key, no sign-up and no paid tier: send a GET request and parse the response. The API is described by an OpenAPI 3.1 document so it can be imported into a client generator or handed to an agent as a tool definition. ## Quickstart One request converts an amount across years and currencies. Everything else is optional. ``` curl "https://api.valuethen.com/v1/convert?amount=100&from=USD&to=EUR&fromYear=1970&toYear=2026" ``` The response carries the chosen result, both calculation methods, the exchange rate used, the coverage range of each currency and the exact source series. Try any endpoint live in the sandbox below without writing code. ## Endpoints - `GET /v1/convert` — convert an amount between years and currencies. - `GET /v1/inflate` — same-currency inflation adjustment. - `GET /v1/series/{currency}` — the full annual price-index series. - `GET /v1/coverage` — every supported currency with its first and last year. - `GET /openapi.json` — the machine-readable description of all of the above. ## Authentication and API keys Public read access needs no credentials, which is deliberate: an agent should be able to use the data without a signup flow. If you need higher throughput for a first-party or partner integration, request an API key by email and send it in an `X-Api-Key` header. Keys raise the rate limit and identify your traffic; they never unlock different data. ## Commercial and high-volume use Personal, academic and low-volume use is free and always will be. If you are building the data into a product, need more than the anonymous budget, or want to redistribute the series, [email us with your expected volume](mailto:contact@therisingthrone.com?subject=valuethen.com%20%E2%80%94%20commercial%20%2F%20high-volume%20API%20use). Keys, higher limits and a commercial licence are arranged per integration; there is no self-serve billing yet. ## Embed the calculator Put a live result or a working calculator inside an article with one script tag. It adapts to light and dark, resizes itself, speaks seven languages and links back to the full calculator. Configure and copy the code on the [embed page](https://valuethen.com/embed/). ## Rate limits Anonymous callers get a fair-use budget per address. Every response carries the standard `RateLimit` headers, and a throttled request answers 429 with `Retry-After`, so a client can back off without guessing. Cache-friendly responses carry long `Cache-Control` lifetimes because historical data does not change. ## Errors Failures use RFC 9457 problem details with the `application/problem+json` media type: a stable `type` URI, a short `title`, the HTTP `status`, a human-readable `detail` and a machine-readable `code`. Coverage errors also return the valid year range so a client can correct itself in one retry. ## Versioning and deprecation The path carries the major version. Breaking changes ship under a new version rather than mutating the current one. When an endpoint is retired it first returns `Deprecation` and `Sunset` headers with a link to the replacement, and we leave at least six months between the first deprecation header and removal. The policy is published at [the API versioning page](https://api.valuethen.com/versioning). ## CLI and SDK An official command-line tool and JavaScript SDK are published on npm with no dependencies. Use the CLI for one-off answers in a shell and the SDK when the result feeds other code. ``` npx valuethen convert 100 USD 1970 ``` The [source repository](https://github.com/Rising-Throne/valuethen-cli) is MIT licensed and carries `AGENTS.md` agent rules, a Claude Code `plugin.json` manifest and the ValueThen skill, so a coding agent can pick up the conventions without reading this page. ## Batch and pagination Send up to 100 conversions in one `POST /v1/convert/batch`; each item succeeds or fails on its own. Lists are cursor-paginated through `/v1/currencies`. ## MCP and agent surfaces A Streamable HTTP MCP server at `api.valuethen.com/mcp` exposes the same operations as tools for agents. There is also a natural-language `/ask` endpoint with JSON or SSE, an agent guide at [llms.txt](https://valuethen.com/llms.txt), and a resource catalogue at [/.well-known/ard.json](https://valuethen.com/.well-known/ard.json). ## Sandbox The interactive reference runs every operation against live data from the browser. For a deterministic target while you build, `/v1/sandbox` mirrors the production request and response shapes with fixed sample data, no key and no rate limit. [Open the API sandbox](https://api.valuethen.com/docs)[OpenAPI description](https://api.valuethen.com/openapi.json)[Sandbox base URL](https://api.valuethen.com/v1/sandbox)[llms.txt](https://valuethen.com/llms.txt)[CLI on GitHub](https://github.com/Rising-Throne/valuethen-cli) Questions, or a key request: [contact@therisingthrone.com](mailto:contact@therisingthrone.com?subject=valuethen.com). ## Use this page programmatically The same figure from the free API, no key required: ```bash curl "https://api.valuethen.com/v1/convert?amount=100&from=USD&to=EUR&fromYear=1970&toYear=2026" ``` ```json { "result": 777.71, "method": "cpi-then-fx", "sources": ["World Bank FP.CPI.TOTL, 2010=100"] } ``` --- Data sources: World Bank FP.CPI.TOTL, ECB HICP and reference rates, FRED CPI-U. Free JSON API: https://api.valuethen.com/openapi.json · Agent guide: https://valuethen.com/llms.txt · MCP: https://api.valuethen.com/mcp