@codeagentjp/egov-law-mcp

Local stdio MCP server for searching Japanese laws and retrieving article text from e-Gov Law Search through the e-Gov Law API Version 2.

This server does not call an LLM. It only returns source-backed law data and e-Gov URLs so your MCP client (Claude Desktop, Claude Code, Cursor, or any other agent) can cite the original source.

Design choices were informed by reading Digital Agency's open-sourced Lawsy-Custom-BQ (released 2026-04-24 as part of the Gennai government AI OSS release). See the design notes at codeagent.jp.

Why another e-Gov MCP

There is an existing egov-law-mcp on npm by another author. This package differs in four ways:

1. find_related_laws — looks up enforcement orders (施行令) and regulations (施行規則) for a given base law name or abbreviation. Lawsy-Custom-BQ has the same step server-side; useful because definitions and delegated rules often live outside the parent act.
2. Source attribution baked into every tool result — every response includes the law name, law ID, article number, the law revision ID (版), the retrieval time, and the canonical e-Gov URL, so the calling LLM cannot drop the citation.
3. Point-in-time retrieval — every tool accepts asof (YYYY-MM-DD), so you can pin the same version of a law that a benchmark or an earlier answer used.
4. Single-file .mjs, no build step — bin/egov-law-mcp.mjs runs directly under Node 20+. Easier to audit, smaller install.

Status

MVP. The API surface is intentionally small:

• search_laws — search current Japanese laws by title, abbreviation, law number, or law ID.
• get_article — retrieve a specific article (or paragraph) from a law by law ID, law number, or law revision ID.
• get_law — retrieve basic metadata and a text preview for a law.
• find_related_laws — find likely related enforcement orders and regulations.

All four accept an optional asof. Since v0.2.0 every request goes to Law API v2 (/api/2/laws and /api/2/law_data/{id}); nothing calls the v1 endpoints any more. See Compatibility with v0.1.x.

Requirements

• Node.js 20 or later
• Network access to https://laws.e-gov.go.jp

Install

From npm:

{
  "mcpServers": {
    "egov-law": {
      "command": "npx",
      "args": ["-y", "@codeagentjp/egov-law-mcp"]
    }
  }
}

From source for development:

git clone https://github.com/SHAYOUWORLD/egov-law-mcp.git
cd egov-law-mcp
node bin/egov-law-mcp.mjs

{
  "mcpServers": {
    "egov-law": {
      "command": "node",
      "args": ["/absolute/path/to/egov-law-mcp/bin/egov-law-mcp.mjs"]
    }
  }
}

Tools

search_laws

Searches laws that are not repealed (repeal_status=None) with one laws request. Keywords that look like a law ID (322AC0000000049) go to law_id, keywords that look like a law number (平成十五年法律第五十七号, …第五十七号) go to law_num, and everything else goes to law_title, which e-Gov matches against both the title and its abbreviations (個人情報保護法 finds 個人情報の保護に関する法律). With several space-separated words, the longest goes to e-Gov and the rest are checked locally. Up to 500 matches are ranked locally; truncated: true means e-Gov had more and the keyword should be narrowed.

{
  "keyword": "個人情報保護法",
  "category": "all",
  "limit": 10,
  "asof": "2026-10-29"
}

category maps to law_type: acts = Constitution, Act; orders = CabinetOrder, ImperialOrder; rules = MinisterialOrdinance, Rule; all = no filter.

get_article

Retrieves an article from the main provision (本則) with law_data?elm=MainProvision-Article_N[-Paragraph_M]. Provide either lawId or lawNum. article accepts 32, 32_2, 32の2, 第32条の2, or 第三十二条の二; paragraph accepts 2 or 第二項.

{
  "lawId": "322AC0000000049",
  "article": "32",
  "paragraph": "2",
  "asof": "2026-10-29"
}

To pin an exact version, pass a law revision ID as lawId (for example 322AC0000000049_20260717_508AC0000000060); e-Gov then ignores asof. e-Gov accepts asof from 2017-04-01 onward.

get_law

Retrieves basic metadata and a plain text preview of a law.

{
  "lawId": "503AC0000000035",
  "previewChars": 3000
}

find_related_laws

Searches for laws whose names or abbreviations look related to a base law name, including enforcement orders and regulations. It sends one laws?law_title= request with the name minus a trailing 法律 / 法 (and minus 施行令 / 施行規則, so a regulation also finds its parent act).

{
  "lawName": "個人情報の保護に関する法律",
  "limit": 10
}

What the results contain

get_article and get_law return lawId, lawNum, lawTitle, lawRevisionId, and asof, plus a source object:

{
  "name": "e-Gov法令検索",
  "api": "法令API Version 2",
  "url": "https://laws.e-gov.go.jp/law/322AC0000000049",
  "apiUrl": "https://laws.e-gov.go.jp/api/2/law_data/322AC0000000049?elm=MainProvision-Article_32-Paragraph_2&asof=2026-10-29&response_format=json&law_full_text_format=xml",
  "lawId": "322AC0000000049",
  "lawRevisionId": "322AC0000000049_20260717_508AC0000000060",
  "amendmentEnforcementDate": "2026-07-17",
  "asof": "2026-10-29",
  "attribution": "出典: e-Gov法令検索（https://laws.e-gov.go.jp/）",
  "processing": "法令API Version 2 の law_data で取得した法令XMLからタグを除き、平文にした。",
  "retrievedAt": "2026-10-29T05:57:09.260Z"
}

url opens the current version on e-Gov Law Search. apiUrl reproduces the exact request, and lawRevisionId identifies the version that was returned. search_laws and find_related_laws return source with apiUrl, asof, and retrievedAt at the top level, and each result carries lawRevisionId, abbreviations, and the e-Gov URL.

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