# japan-law-util-mcp-server

An MCP server for [e-Gov法令検索](https://laws.e-gov.go.jp/)'s
[法令API Version 2](https://laws.e-gov.go.jp/api/2/swagger-ui), the Digital Agency's
API for Japanese national law: find a law, read its articles, search the text of every
law, and pin any point in time.

e-Gov法令検索の法令API（Version 2）を使って、日本の法令の検索・条文の閲覧・全文の
キーワード検索・施行時点の指定ができる MCP サーバーです。

> **Unofficial.** This project is not affiliated with or endorsed by the Digital Agency
> (デジタル庁), which operates e-Gov法令検索. It is written against the
> [published API specification](https://laws.e-gov.go.jp/api/2/swagger-ui).

## Tools

| Tool | What it does |
|---|---|
| `load_guideline` | The usage guideline for the whole tool set: how to find a law, how to read it by `elm`, the keyword search expressions, how to pin a point in time, what e-Gov法令検索 covers, and the conditions of use. It takes no arguments and always returns the whole guideline. |
| `search_laws` | Find laws by title, law number, type, subject category or promulgation date; paged. |
| `search_law_text` | Search the text of every law for a keyword or search expression; returns the laws it appears in with the matching sentences. |
| `list_law_revisions` | One law's amendment history: enforcement dates, amending laws, revision ids, and whether each revision is in force, not yet in force or repealed. |
| `get_law_outline` | One law's table of contents — parts, chapters, sections, articles with their captions, supplementary provisions and appendices — each with the `elm` that reads it. |
| `read_law_text` | A law's text as plain text, whole or the one part `elm` names, in pieces with `next_offset` to continue. |

The server follows a *skill-style* layout. Its MCP `instructions` are one sentence
saying what the tools are for and to call `load_guideline` first. Everything that
concerns the tool set as a whole — including the conditions of use — lives in the
guideline, and each tool's description covers only that tool. Agents that load many MCP
servers keep only a short entry per server in their prompt, and read the guideline when
they need it.

### Reading a law

A law's text is addressed with the API's `elm` paths. `get_law_outline` returns the one
to use for each entry, so the agent never has to build one:

```
MainProvision-Article_17            本則第十七条
MainProvision-Article_3_2           本則第三条の二
MainProvision-Chapter_4-Section_2   本則第四章第二節
SupplProvision[1]-Article_1         附則第一条
AppdxTable[1]                       別表第一
```

The law text arrives as the API's light JSON (`json_format=light`) and is rendered to
plain text: captions, article and paragraph numbers, items, tables as `|`-separated
rows, 振り仮名 resolved to the word itself. A whole act is tens of thousands of
characters, so `read_law_text` returns it in pieces and the fetched law is cached for
ten minutes.

The supplementary provisions of the laws that amended this one are left out unless
`include_amendment_supplements` is true — an act can carry twenty of them. That choice
decides which `SupplProvision[n]` each index names, so `get_law_outline` takes the same
argument and the two have to agree.

## Run

Python 3.11 or later.

```bash
pip install -e .
japan-law-util-mcp-server --host 127.0.0.1 --port 8000      # Streamable HTTP at /mcp
japan-law-util-mcp-server --transport stdio                 # for clients that start it themselves
```

Behind a reverse proxy, accept the public host name as well; otherwise the server's DNS
rebinding protection rejects requests whose `Host` is not localhost:

```bash
japan-law-util-mcp-server --host 127.0.0.1 --port 8000 --allowed-host demo.mcp.gotoai.com
```

When bound to a non-loopback address (for example `--host 0.0.0.0` on a LAN), that
protection is off; restrict access with a firewall or a proxy.

### In Kari

Add a custom extension of type **Streamable HTTP** with the URL `http://<host>:<port>/mcp`.
Give it an ASCII name such as `japanlaw` — the name becomes part of every tool name the
agent sees.

## Conditions of use

These are in the guideline the agent reads; they apply to anyone presenting the results:

- Credit the source as 「出典：e-Gov法令検索（デジタル庁）」 with the law's URL.
- Say so when the text has been summarised, excerpted or rearranged, and do not present
  the result as if the government had written it.
- Where e-Gov法令検索 differs from the 官報, the 官報 prevails; confirm anything that
  matters against the law's own page.
- Avoid excessive repeated access to the API.

e-Gov法令検索's content is published by the Digital Agency under the
[公共データ利用規約（第1.0版）](https://www.digital.go.jp/resources/open_data/public_data_license_v1.0);
its own terms are at <https://laws.e-gov.go.jp/terms/> and apply to whoever runs this
server.

## Coverage

e-Gov法令検索 holds national law only — 憲法・法律・政令・勅令・府令・省令・規則. It does
not hold 告示・通達, treaties, local ordinances or case law. Its data starts from the text
in force on 2017-04-01, covering acts and cabinet orders in force since 2015-08-01 and
ministerial ordinances and rules since 2016-10-01, so amendment history before then is
incomplete.

## Development

```bash
pip install -e ".[dev]"
pytest                      # offline tests
JAPAN_LAW_LIVE=1 pytest     # also the live tests against the real API
ruff check src tests
```

## License

MIT — see [LICENSE](LICENSE).