@codeagentjp/law-diff-mcp

Local stdio MCP server for finding Japanese e-Gov law revisions and diffing law text by law history ID.

This server does not call an LLM. It only returns source-backed e-Gov data and a mechanical text diff so your MCP client can cite the original source.

Status

MVP. The API surface is intentionally small:

• list_updates — list laws updated on a specific date via the e-Gov updated-law list API.
• find_revisions — scan updated-law lists across a short date range and find revision IDs for a law or keyword.
• get_revision_text — retrieve current law text or historical revision text by law ID / law history ID.
• diff_revisions — compare two current law IDs or law history IDs and return a normalized article/paragraph text diff.

What this is for

egov-law-mcp answers "what does the current law say?"

law-diff-mcp answers "what changed between these two e-Gov law revisions?"

The tool is useful for:

• triaging legal amendments before reading the official text manually;
• checking whether a later revision changed a specific law area;
• giving Claude Code / Claude Desktop / Cursor source-backed amendment context.

Requirements

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

Install

From npm, after publish:

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

From source for development:

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

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

Tools

list_updates

Lists e-Gov updated-law records for one date.

{
  "date": "2023-02-01",
  "keyword": "液化石油ガス",
  "limit": 10
}

Returns amendment metadata such as amendName, amendNo, enforcementDate, lawId, revisionId, and lawUrl.

find_revisions

Scans updated-law lists for up to 31 days.

{
  "from": "2023-02-01",
  "until": "2023-02-28",
  "keyword": "液化石油ガス",
  "limit": 20
}

Use this to find candidate revision IDs before calling diff_revisions.

get_revision_text

Retrieves current or historical e-Gov law text.

{
  "revisionId": "343CO0000000014_20230401_505CO0000000007",
  "previewChars": 8000
}

You can also pass a lawUrl returned by list_updates or find_revisions.

diff_revisions

Compares two e-Gov law IDs or law history IDs.

{
  "oldRevisionId": "343CO0000000014_20230401_505CO0000000007",
  "newRevisionId": "343CO0000000014",
  "maxChanges": 120
}

The diff is line-level and generated from normalized article/paragraph text. It is meant for research triage. Always verify important conclusions against official e-Gov pages and, where necessary, official new-old comparison tables.

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/
• Law revisions documentation: https://laws.e-gov.go.jp/docs/law-data-basic/da91fe9-law-revisions/
• 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 and diff triage tool, not legal advice.
• The generated diff is mechanical. It does not interpret transitional provisions, delayed enforcement, "なお効力を有する" clauses, or amendment intent.
• 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

• @codeagentjp/egov-law-mcp
• @codeagentjp/houan-mcp

License

MIT © codeagent.jp