For machines

Public API

Everything on this site is available as JSON, under the same measurement rules. Anonymous requests work immediately at a low rate limit so the docs are explorable without signing up.

Interactive docs ↗ OpenAPI 3.1 document ↗

Read this before you sum anything

A word count never appears in a response as a bare number. It is always an object:

{
    "words": 104642,
    "status": "rolled_up",
    "reason": null,
    "method": "descendant_sum"
  }

When words is null the value was not measured. It is not zero. status and reason say why. Filter on status before any arithmetic — writing total += row.words is exactly the mistake this project was rebuilt to stop making, and the envelope exists so the API cannot help you make it.

Endpoints

All 16 of them. This table is generated at build time from the route definitions in the API's own source, so it cannot drift from what the service actually serves.

Method	Path	Returns
Agencies
GET	/v1/agencies	List agencies with their word-count rollups
GET	/v1/agencies/{slug}	One agency, with every scope it claims
Amendments
GET	/v1/amendments	Section-level amendment history
Diff
GET	/v1/diff	Compare one CFR section between two eCFR issue dates
Meta
GET	/v1/meta	Freshness, corpus totals, and tier limits
Parts
GET	/v1/parts/{citation}	One CFR part
Search
GET	/v1/search	Find an agency, title, chapter, or part by name or citation
Shared jurisdiction
GET	/v1/overlap	CFR scopes claimed by more than one agency
Titles
GET	/v1/titles	All 50 CFR titles
GET	/v1/titles/{n}/structure	The table of contents for a title
Word counts
GET	/v1/word-counts	Published word counts, attributed and deduplicated
Account
GET	/v1/account/keys	List your keys
POST	/v1/account/keys	Mint another key
DELETE	/v1/account/keys/{id}	Revoke a key
POST	/v1/account/register	Request an API key
POST	/v1/account/verify	Verify an email and receive the first key

Paths are shown as mounted; every one is relative to https://ecfr.fixit.works. Parameters, response schemas and error shapes are in the OpenAPI document, which is the contract and is generated from the same schemas the responses are validated against. Outside /v1 the Worker also serves GET /openapi.json, GET /docs and GET /health.

Rate limits and keys

Anonymous. No key, low daily ceiling, per-IP. Enough to read the docs and try every endpoint; not enough to mirror the corpus.
Registered. A free key, issued after email verification, with a much higher daily quota. Send the key as Authorization: Bearer <key>.
Elevated. For research or public-interest projects that need bulk access. Ask, and say what you are building.

Quota state comes back on every authenticated response, and a 429 tells you when the window resets rather than leaving you to guess.

Terms of use

The data is public-domain federal material; this project's measurements and code are open source. Attribution is appreciated and, for the derived counts, expected — cite them as eCFR Atlas measurements, not as government figures.
Do not use this API as a proxy for eCFR. If you need the corpus, fetch it from ecfr.gov directly and be a good citizen about it: sustained request rates above roughly 8 per second get throttled.
Keys are per-project, not per-person. Do not embed one in a public client.

Read methodology for what the numbers mean, and data quality for what is missing from them.