501api reference

A read-only JSON API over public-domain U.S. IRS tax-exempt organization data — the EO BMF registry, the SOI financial extract, and a parsed e-file 990 slice. Every figure is source-tagged so you always know its provenance. Currently in v1 public beta.

Base URL

Authentication

None required to try — the API is anonymously readable, rate-limited per IP. A free beta key (coming soon) raises your limits.

$ curl https://api.501api.org/v1/orgs?q=animals

Rate limits

Anonymous requests are limited per IP. A free beta key (coming soon) will raise the limit and add per-key usage stats.

Response envelope

Every response is { data, meta }. meta always carries the API version and the no-warranty notice; list endpoints add a pagination block.

{
  "data": [
    "…"
  ],
  "meta": {
    "api_version": "v1",
    "warranty": "…provided AS IS, verify against IRS.",
    "pagination": {
      "total": 1843,
      "page": 1,
      "per_page": 25
    }
  }
}

Machine-readable & AI agents

Building a client (or pointing an AI agent at this API)? Start here — these are self-contained and JS-free.

• /llms.txt — one-screen orientation (the llms.txt convention).
• /llms-full.txt — the entire reference as Markdown, in one fetch.
• /v1/openapi — OpenAPI 3 spec (JSON) for codegen. · Swagger UI

Endpoints

Errors

Errors use standard HTTP status codes with a consistent body. An unknown EIN returns 404:

{
  "errors": {
    "detail": "Not Found"
  }
}