io.github.SHAYOUWORLD/egov-law-mcp

@codeagentjp/egov-law-mcp

Local stdio MCP server for searching Japanese laws and retrieving article text from e-Gov Law Search.

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 three ways:

1. find_related_laws — looks up enforcement orders (施行令) and regulations (施行規則) for a given base law name. 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, and the canonical e-Gov URL, so the calling LLM cannot drop the citation.
3. 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 keyword.
• get_article — retrieve a specific article from a law by law ID or law number.
• get_law — retrieve basic metadata and a text preview for a law.
• find_related_laws — find likely related enforcement orders and regulations.

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 the e-Gov law list.

{
  "keyword": "個人情報",
  "limit": 10
}

get_article

Retrieves article text. Provide either lawId or lawNum.

{
  "lawId": "503AC0000000035",
  "article": "2"
}

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 look related to a base law name, including enforcement orders and regulations.

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

Data Source and Attribution

This package uses the e-Gov Law Search API:

• e-Gov Law Search: https://laws.e-gov.go.jp/
• Law API documentation: https://laws.e-gov.go.jp/docs/law-data-basic/8529371-law-api-v1/
• e-Gov terms: https://developer.e-gov.go.jp/contents/terms
• MCP stdio transport: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports

Tool results include source attribution. When you publish or redistribute output based on this package, include an appropriate e-Gov source attribution.

Suggested attribution:

出典: e-Gov法令検索（https://laws.e-gov.go.jp/）

Safety Notes

• This package is a law reference tool, not legal advice. Verify important legal conclusions against the official e-Gov page and, where necessary, consult a qualified professional.
• It does not execute shell commands.
• It writes JSON-RPC messages only to stdout and logs only to stderr.
• It fetches only e-Gov Law Search endpoints.

Related

• Design notes: 源内OSSのLawsy実装をMCP化するなら、どこを残してどこを捨てるか (codeagent.jp)
• Background on Gennai OSS release: 源内OSS公開の背景 (codeagent.jp)
• Setup guides (Japanese): Claude Code / Codex CLI / Troubleshooting
• e-Gov Law API v2 guide (Japanese): codeagent.jp/guides/egov-law-api/
• Other Japanese public-data APIs and MCP servers: codeagent.jp/guides/public-data-api/
• Reference implementation we learned from: digital-go-jp/genai-ai-api/google-cloud/lawsy-custom-bq

License

MIT © codeagent.jp