opencaselaw

# OpenCaseLaw

> An open, machine-readable corpus of Swiss federal and cantonal court
> decisions, federal and cantonal legislation, and scholarly commentary —
> with a verification architecture designed to eliminate AI hallucination
> of Swiss case references.  Data CC0, code MIT.  Updated daily.

OpenCaseLaw is the public infrastructure project at <https://opencaselaw.ch>.
It serves three audiences: legal practitioners who need verifiable
references, researchers who need a reproducible corpus, and AI systems
that need a trustworthy data source for Swiss-law questions.

The two canonical hosts:

  - opencaselaw.ch       — landing site, dataset card, public docs
  - mcp.opencaselaw.ch   — live MCP / REST endpoint, per-decision pages,
                            decision-export endpoints, scraper-health API

Everything below links to the most useful entry-points for a language
model that wants to answer a Swiss-law query, ground its answer in the
authoritative source, and emit clickable verification links to the user.

## Corpus & coverage

- [Hugging Face dataset (Parquet, CC0)](https://huggingface.co/datasets/voilaj/swiss-caselaw): 1,050,000+ decisions across all Swiss federal and cantonal courts, 1875–today, dreisprachig (DE/FR/IT). Daily refresh.
- [Per-court coverage transparency](https://opencaselaw.ch/coverage/): live scraper-health table — which sources updated when, what gaps remain, sortable + filterable.
- [Statistics endpoint](https://mcp.opencaselaw.ch/api/statistics): aggregate counts by court, canton, year.

## How to query the corpus

- [Live MCP endpoint](https://mcp.opencaselaw.ch): single URL — paste into Claude / ChatGPT / Cursor / Gemini / Copilot Studio. 38 specialised tools cover full-text search, citation graph traversal, statute lookup, doctrinal timelines, leading-case discovery, scholarly commentary, legislative materials, open-access scholarship search, exam-question generation.
- [REST API (OpenAPI 3.0.3)](https://mcp.opencaselaw.ch/api/docs): every MCP capability also exposed as REST. Useful for languages without an MCP client library.
- [Research CLI `ocl`](https://opencaselaw.ch/cli/): dependency-free command-line client over the REST API for scripts and agents. Checks the citations in a draft (resolved / missing / ambiguous, pinpoints verified) and saves the evidence behind a memo into a folder with a hashed manifest. Full guide: <https://github.com/jonashertner/opencaselaw/blob/main/docs/research-cli.md>. Typed research contract: `/api/research/openapi.json`.
- [How to connect](https://opencaselaw.ch/#connect): copy-paste setup instructions per AI client.

## Per-decision access (canonical)

Every decision has a stable canonical URL of the form:

`https://mcp.opencaselaw.ch/entscheid/{decision_id}`

— with `decision_id` like `bge_BGE_140_III_86`, `bger_4A_747_2012`, `bvger_A-1234_2024`. Each page carries Schema.org `LegalCase` markup and links to the original court source.

Per-decision exports for citation managers and word processors:

  - `/api/decisions/{decision_id}/export.docx` — Word document (citation, Regeste, structured Erwägungen)
  - `/api/decisions/{decision_id}/export.bib`  — BibTeX `@misc` entry
  - `/api/decisions/{decision_id}/export.ris`  — RIS bibliographic record (Zotero / EndNote / Mendeley)
  - `/api/atom/{court}.xml`                     — Atom feed of newest decisions per court

## Verification (anti-hallucination)

OpenCaseLaw exists in part because peer-reviewed measurement (Dahl et al., Stanford RegLab, 2024 — *Large Legal Fictions*) found that 58–88 % of legal queries to general-purpose LLMs produce at least one fabricated authority, and even commercial legal-RAG tools (Magesh et al., 2024 — *Hallucination-Free?*) show 17–33 %.

A six-layer verification architecture closes this gap when an LLM uses our MCP server:

  1. Corpus integrity (deterministic decision_id hashing, no fuzzy fallback)
  2. Server-built citations (every result carries a pre-baked `citation_string_de/fr/it` + canonical URL — never reconstructed by the LLM)
  3. SYSTEM_PROMPT operating contract (R1–R8: no hand-built citations, no quotes from memory, no statute claims from priors, every reference must be a clickable link)
  4. `check_claim_support` — independent Sonnet judge against verbatim Erwägung text
  5. `attest_response` — closing audit covering (a) case-citation existence + pinpoint, (b) statute reference resolves in Fedlex, (c) every quotation appears verbatim in cited source, (d) decision dates match, (e) opt-in proposition-grounding via batched Sonnet judge (catches Butler & Butler 2026's "reasoning error" class)
  6. Click-through verification — every reference is a Markdown link to mcp.opencaselaw.ch/entscheid/<id>; the user verifies in one click

Public explainer: [opencaselaw.ch/#verification](https://opencaselaw.ch/#verification).

## Benchmarks

- [Swiss Legal RAG Bench](https://huggingface.co/datasets/voilaj/swiss-legal-rag-bench): 10-question seed (DE/FR/IT, federal law) measuring correctness, groundedness, retrieval accuracy. Modelled on Butler & Butler, *Legal RAG Bench* (Isaacus, 2026). Baseline at 100 % correctness, 90 % groundedness, 70 % retrieval accuracy on the live OpenCaseLaw stack with Claude Sonnet 4.6.
- [Frozen offline retrieval benchmark](https://github.com/jonashertner/caselaw-repo-1/blob/main/benchmarks/search_benchmark_2026-03-19_offline_full.json): MRR=0.470, Hit@1=0.33 (100 queries, 15 legal domains).

## Source code & licences

- [GitHub repository (MIT)](https://github.com/jonashertner/caselaw-repo-1): scrapers, indexer, MCP server, REST API, verification logic, benchmark harness — all open.
- [Issue tracker](https://github.com/jonashertner/caselaw-repo-1/issues): coverage gaps, bugs, feature requests.
- [Dataset licence](https://creativecommons.org/publicdomain/zero/1.0/): CC0 1.0 (data).
- [Code licence](https://github.com/jonashertner/caselaw-repo-1/blob/main/LICENSE): MIT.

## Optional: deeper integration

- [Word add-in (BETA)](https://word.opencaselaw.ch/install.html) for legal practitioners — citation insertion, multi-citation clusters, citation styles.
- [Cantonal laws + commentary modules](https://opencaselaw.ch/#corpus) — local-first lookup for 19 cantonal portals, OnlineKommentar.ch (CC-BY-4.0).
- [LLM usage receipts](https://github.com/jonashertner/caselaw-repo-1/blob/main/scripts/llm_usage_report.py) — every Anthropic API call logged with token counts + cost; auditable spend dashboard.
- [Swiss playbook for Anthropic's Claude for Legal plugin](https://opencaselaw.ch/claude/legal.local.md): a `legal.local.md` with Swiss contract, NDA, data-protection and forum positions; every article verified through `get_law` on the date stated in the file; instructs the model to re-verify before stating a provision.

## What we collect when you use the hosted server

- [Privacy notice](https://opencaselaw.ch/datenschutz/) — the full, binding version in DE/FR/IT/RM/EN.
- Queries are recorded. Tool name, parameters **including your query text**, timestamp, client class and a rotating session identifier are logged, and query text is retained indefinitely in an access-restricted private research archive used to improve search and to train our own models.
- **No IP address is stored with a query**, there is no account and no login, and the session identifier changes with every connection, so queries cannot be tied to a person by us.
- IP addresses are stored separately and only for operations: 72 hours in the web-server log for abuse defence, and 90 days plus a private archive for the LLM-backed endpoints (`attest`, `verify-claim`, `check_claim_support`) to control cost and abuse.
- Word add-in document text is never stored, only its length.
- The collection code is open source: `mcp_server.py`, `scripts/collect_dev_data.py`.
- If you are sending queries on behalf of others, or your users would not expect this, run the corpus locally instead — the full CC0 dataset is at <https://huggingface.co/datasets/voilaj/swiss-caselaw> and nothing leaves your machine.

## Contact

- Email: <mailto:team@jonashertner.com>
- Maintainer: <https://jonashertner.com>
- LinkedIn: <https://www.linkedin.com/company/opencaselaw-ch/>