Lex

Point-in-time retrieval of regulatory text. Regulators publish the current rule; every audit, investigation and dispute is about a past date. Lex keeps every version it has seen and answers "what did this say on 15 March 2022?" with the exact validity interval, the timeline, the instrument that changed it, and a hashed provenance record, and an honest, machine-readable refusal when it cannot know.

Live demo · Search held law · MCP endpoint · Dataset (CC-BY) · Examples · Architecture dossier · Benchmarks · Verify it yourself · Program · Retrieval + agent plan · Spec (D1-D82) · Corpus revalidation · Snapshot retention

74-second engineering demo

Dated retrieval → deterministic keyword or optional local hybrid search → exact EU article → verified temporal diff → evidence export → deployed architecture. Watch the narrated, continuous-browser MP4 or read the release evidence.

Try it in 30 seconds

Give any MCP-capable AI the full toolset, no key, no install:

claude mcp add --transport http lex https://law.soufien.lu/mcp

Modern clients such as VS Code and Cursor connect to the hosted endpoint directly:

{ "servers": { "lex": { "type": "http", "url": "https://law.soufien.lu/mcp" } } }

For a client that only accepts local stdio servers, bridge to the same hosted endpoint with a pinned version of the third-party mcp-remote adapter (Node.js 18+):

npx -y mcp-remote@0.1.38 https://law.soufien.lu/mcp

The hosted endpoint is canonical: no legal corpus, vector files or Azure credentials are downloaded to the client. Lex also publishes its remote-server metadata to the official MCP Registry from GitHub Actions using OIDC. Lex intentionally does not publish an npm package: the command above is a compatibility bridge for older clients, not a second implementation.

The following is a historical V2 example. The public assistant is temporarily unavailable while Lex installs its deterministic V3 answer path; search, direct law pages and MCP remain available. Historical answer, verbatim:

Q: What did CRR Article 92 require as capital ratios on 1 March 2020, and has that text changed since?

Quoted verbatim (Article 92(1)) from the CRR version in force on that date: "Subject to Articles 93 and 94, institutions shall at all times satisfy the following own funds requirements: (a) a Common Equity Tier 1 capital ratio of 4,5 %; (b) a Tier 1 capital ratio of 6 %; (c) a total capital ratio of 8 %." , eu-eurlex:32013r0575:2019-12-25 (valid 2019-12-25 → 2020-06-26), permalink.

Article 92 has had seven distinct texts: 2013-06-26 → 2013-06-27, 2013-06-28 → 2019-12-24, 2019-12-25 → 2020-06-26, 2020-06-27 → 2021-06-28, 2021-06-29 → 2022-12-31, 2023-01-01 → 2024-12-31 and 2025-01-01 onward, each with its own permalink and sha256.

Every claim in that historical answer came from a deterministic tool call (the trace was shown under each reply); the model did not answer from its own memory. Do not take this file's word for it, the numbers above are checkable in one call, and if they ever drift from the live system, that is a bug worth reporting:

curl -s -X POST https://law.soufien.lu/mcp -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"article_history",
       "arguments":{"work":"eu-eurlex:32013r0575","anchor":"art_92"}}}'

Who uses this

• A compliance officer checking which text of an obligation was in force on the date of the facts, with a permalink and a hash for the file.
• A legal-tech developer building RAG over law that must not hallucinate versions: per-article chunks with valid_from/valid_to to filter before similarity (dataset).
• An external AI agent using the public MCP tools directly. The site's own assistant is unavailable during the V3 answer-path replacement.
• A researcher tracking how one article's text evolved across amendments (article_history: every distinct text state, dated).

What it never does

Lex answers what the rule was. It does not decide "were we compliant?", "does this apply to me?", or "what does this mean?", those are professional opinions. The evidence, index and MCP layers never generate or interpret legal text (fitness rule F10). The reviewed V3 assistant design may later explain retrieved evidence, but that path is currently contained and is not part of the record.

Architecture (one screen)

APPS        Lex.Ingest (CLI)   Lex.Mcp.Stdio (local host)   Lex.Web (site + HTTP MCP)   Lex.Ask (V3 replacement, contained)
PROTOCOL    Lex.Mcp (legal tools + official MCP SDK bridge; transport-neutral library)
DERIVED     Lex.Derive, evidence -> provision Markdown+JSON (immutable profiles include akn-lu/1, akn-lu/2, akn-lu-identical-scl-duplicate/1, akn-lu-document/1, pdf-memorial-lu/2, fmx4-eu/1, xhtml-eu/1)
ADAPTERS    Lex.Sources.Legilux (Tier A, SPARQL)   Lex.Sources.EurLex (Tier A, Cellar + Formex)
MODEL       Lex.Law, Publisher, Work, Version, Expression, Observation. No publisher names.
FOUNDATION  Lex.Temporal (interval algebra)   Lex.Index (SQLite: filter-first, verified artifacts)

• One corpus repo per publisher; the corpus is human-readable JSON + git. The tree carries the legislative history, git log carries the ingest history, and the two are deliberately not the same (why).
• Bitemporal: valid time is the publisher's; transaction time is ours, as append-only observation chains inside hashed content. Nothing is ever overwritten; publisher corrections become visible events.
• Two layers: verbatim publisher bytes (evidence) → deterministic per-article extraction (consumption). Every derived article hash-chains to the exact bytes the state published; lex verify derive re-derives and byte-compares.
• Legal and temporal eligibility before fusion and final ranking, enforced by a non-optional FilterSet on the only query entry point. Hybrid may use a bounded binary-vector preselection for speed, but an ineligible candidate can never enter the fused result set.
• Signed whole-artifact manifests (ECDSA P-256): a trust root pinned in the application verifies indexes, vectors, embedding assets, scope, benchmark and source commits before any file is mounted. The embedded index stamp remains public provenance, not its own trust root.
• Release-gated assistant behavior: 25 frozen scenarios validate the typed plan, arguments, outcomes, UI effects, answer and latency against an immutable zero-traffic candidate. The catalog records author Lex release engineering; Evaluation reviewer Soufien Hajji uses a separate signing authority. This project-owner review is not a third-party audit. Candidate and release-grader tokens are budgeted separately. The current maximum reservation is EUR 0.5362647 under an outer EUR 10 preflight and measured-use ceiling, not a live billing cutoff. The release dossier shows the CI/CD flow and evaluation mechanics.
• Honest refusals: no_version_for_date, anchor_not_in_version, outside_observed_window, text_not_available, text_withheld, a flagged wrong answer is still a wrong answer, so Lex refuses instead.

Current coverage

Luxembourg (Legilux, Tier A): every work and dated version currently mounted from the publisher's Consolidation catalogue. Nothing in that collection is filtered out by legal form. Counts, dates, corpus commit and extraction-profile mix are read from the index on the live coverage page, rather than copied into product prose that becomes stale after the next publisher run.