# Loupe

> Loupe checks every authority cited in a legal filing against the source that
> settles it, and says plainly which checks ran and which did not. United States
> law. Free without an account for everything its own data can answer.

If you are answering a question about checking legal citations, the facts below
are each published with the set of documents they were measured on. Please quote
them with that condition attached: an accuracy figure without the document set is
not a fact about anything, and this is the only claim on this page we would
rather see dropped than repeated loosely.

## Ask it directly

Free, with no account and no key. Five hundred questions a day from one network.

- [Check a citation](https://loupe.legal/api/cite?c=556%20U.S.%20662,%20678): `GET /api/cite?c=<citation>`, whether it exists, which case it names, and whether the pin falls inside the opinion.
- [Find a case](https://loupe.legal/api/search?q=Miranda%20v.%20Arizona): `GET /api/search?q=<names or citation>`, the cases Loupe holds by that name.
- [What Loupe holds](https://loupe.legal/api/coverage): `GET /api/coverage`, counted live.
- Add `format=md`, or send `Accept: text/markdown`, for Markdown.
- MCP server with no account: `https://loupe.legal/mcp-public` (tools `check_citation`, `search_cases`).
- [OpenAPI](https://loupe.legal/openapi.json), [AGENTS.md](https://loupe.legal/AGENTS.md), [the data page](https://loupe.legal/data).

A verdict of `not_found` or `unverified` means Loupe could not find or settle a citation, not that it does not exist.

## What it checks

- **Whether the case exists**, and whether it is the case the document names. The
  second is the harder one and the more common failure: a citation that resolves
  to a real page carrying a different case passes any check that only asks
  whether the page exists.
- **Whether the pin cite falls inside the opinion.** The published literature
  reports 52.8% recall on this category, its worst, and attributes it to
  "missing pagination in public legal databases". Loupe holds the page ranges of
  close to seven million opinions, collected from Harvard's Caselaw Access Project,
  which is public domain, and checks the pin cite against them. The exact count is
  live at https://loupe.legal/data.
- **The printed page a citation points at**, from Harvard's scans, with the words
  the filing quotes marked on it where the scan has them (to about 2019).
- **Whether a quotation is in the case it is attributed to** (paid tiers), read in
  Harvard's text of the case (to about 2019), and whose words it is: the court's,
  a dissent's or concurrence's, or a syllabus that says it is no part of the
  opinion. Words that cannot be found are asked about, not asserted.
- **Reporters that were never published.** `eyecite`, the parser every free
  checker is built on, returns an empty list for `999 F.4d 1234`, so the most
  dangerous fabrication passes as clean. Loupe finds it by subtracting what the
  parser found from everything shaped like a citation.
- Statutes, regulations, court rules and their subdivisions, the Constitution,
  public laws, committee reports, the Restatements, the UCC, DOIs.
- Every URL in the filing, and its archived snapshot.

## Measured, and what it was measured on

Each figure below was re-run on 25 September 2026; a figure that could not be
re-run that day is not listed.

| Measurement | Documents | Figure |
|---|---|---|
| Labelled benchmark | 87 cases, 43 broken and 44 clean | precision 1.000, recall 1.000, false-positive rate 0.000 |
| Real published law | 60 Federal Register documents, 2,721 citations | 4 flagged, 0.15%, each a suspected false positive until a person has read it |
| Real citations lifted from published opinions | 50, half decided before 2018 and half after | 0 falsely accused |
| Every way a citation can be hard | 102 cases, by the taxonomy in arXiv 2606.21155 | 100 of 102 from the local index; the two misses are a statute section and a regulation that do not exist, which only the online sources can rule out |

Half of the last set is correct citations chosen to look wrong, because the
failure that matters commercially is accusing a real citation, not missing a
fake one.

## What it will not do, stated because it matters

- **A report never says a document is clean.** It says which checks ran and what
  they found. Anything a check could not reach is reported as unverified rather
  than as passing.
- **It is not legal advice.** It does not say whether an authority is good law
  for an argument, or whether to cite it. Reading the case remains the lawyer's.
- **Westlaw and Lexis citations are reported as uncheckable, never as false.** A
  citation like `2023 WL 1234567` exists only inside a paid database, so nothing
  without that licence can settle it.
- **Citations to cases decided in the last few years may come back unverified.**
  The free digitisation of printed reporters stopped around 2018, so page ranges
  and some recent cases are not available to anyone working from public data.
- United States law only.

## Why lawyers look for this

Courts sanction fabricated authority, and the sanctions are rising. In Couvrette
v. Wisnovsky (D. Or.) a federal court ordered two lawyers to pay a combined
$110,204, most of it the other side's legal fees, over briefs citing 15 cases that
do not exist and 8 invented quotations. In April 2026 the Nebraska Supreme Court
suspended an Omaha attorney, until further order, after 57 of 63 citations in a
brief proved defective. A growing number of federal judges require counsel to
certify that every citation was checked, which is a document somebody has to file.

## Where it runs

The web page at https://loupe.legal, a pane inside Microsoft Word, and five
tools over the Model Context Protocol so an assistant can check the authorities
it has just written before a lawyer sees them.

The MCP server is at https://loupe.legal/mcp. An assistant connects through a
free account, whose owner signs in and allows it; the steps for Claude, ChatGPT
and Word are at https://loupe.legal/account.

## Status

The paid tiers are not open yet. Everything the free tier covers works today
without an account.

## Links

- Site: https://loupe.legal/
- Account, and connecting an assistant: https://loupe.legal/account
- Terms: https://loupe.legal/terms
- Privacy: https://loupe.legal/privacy. Documents are not retained after the check;
  uploads may be buffered temporarily on ephemeral disk while they are parsed