日本の給与・社会保険・労働法

jp-payroll-mcp

日本の給与計算・社会保険・労務を、表を引くのではなく計算して返すAPIとMCPサーバーです。 47都道府県の保険料率、源泉所得税、標準報酬月額の決定と改定、休業中の保険料免除、 割増賃金、年次有給休暇、最低賃金、営業日計算、令和8年分の年末調整、住民税の見込み額まで。答えには根拠の条文または通知が付きます。

公表されている料額表・税額表と1セルずつ突き合わせ、変更のたびに 4,689件 の検証を実行します。

使いかたは2通り

MCPサーバー — AIアシスタントに日本語で聞く。30ツール、無料、APIキー不要。 URL を貼るだけ(remote MCP)か、npx で入れるか:

claude mcp add --transport http jp-payroll https://japan-payroll-api.tsumugi.workers.dev/mcp

claude mcp add jp-payroll -- npx -y jp-payroll-mcp

Claude.ai のカスタムコネクタ、ChatGPT、Cursor には https://japan-payroll-api.tsumugi.workers.dev/mcp を貼ります。

HTTP API — ソフトウェアに組み込む。45エンドポイント、OpenAPI 3.0、一括処理:

curl "https://japan-payroll-api.tsumugi.workers.dev/v1/payroll?prefecture=Tokyo&monthly_salary=350000&birth_date=1986-04-01"

MCPサーバーはAPIの薄い層なので、どちらも同じ答えを返します。 人が聞くのか、プログラムが聞くのかの違いです。

なぜ要るのか

自前で書くと、もっともらしく見えて間違っている数字が出ます。3月31日退職と3月30日退職では 1か月分の社会保険料が動きます(東京・40歳・月給30万円で労使合計95,130円)。4月1日生まれの人は 3月31日に40歳になるので介護保険料が1か月早く始まります。随時改定の「2等級以上」は健康保険法に 書いておらず、昭和36年保発第4号にあります。

• 解説記事: [日本の給与計算・社会保険をAPIとMCPにしました](https://zenn.dev/kishida_devil/articles/9d5a645a105c0b)
• MCPサーバーの詳細: [mcp/README.md](mcp/README.md)
• 大量処理の有料プラン: RapidAPI

以下は英語の詳細仕様です。

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Japanese payroll, social insurance and labour law, computed rather than looked up — premiums for all 47 prefectures, withholding tax, standard remuneration decisions and revisions, leave exemptions, minimum wage — with the statute or ministerial notice each answer rests on.

Verified against the published tables cell by cell: 4,689 assertions on every change.

Two ways in

As an MCP server, for asking questions through an AI assistant. 30 tools, free, no key. Remote (paste a URL) or local (npx):

claude mcp add --transport http jp-payroll https://japan-payroll-api.tsumugi.workers.dev/mcp

claude mcp add jp-payroll -- npx -y jp-payroll-mcp

For Claude.ai custom connectors, ChatGPT and Cursor, the URL is https://japan-payroll-api.tsumugi.workers.dev/mcp.

As an HTTP API, for building it into software. 45 endpoints, OpenAPI 3.0, batch:

curl "https://japan-payroll-api.tsumugi.workers.dev/v1/payroll?prefecture=Tokyo&monthly_salary=350000&birth_date=1986-04-01"

The MCP server is a thin layer over the API, so both give the same answers. Which you want depends on whether a person or a program is asking.

• MCP server source and its own README: mcp/ · English
• Live API: https://japan-payroll-api.tsumugi.workers.dev
• OpenAPI spec: /openapi.json

Related tools

Japanese statutory MCP servers mostly *retrieve* — they hand you the text of a law and leave the reasoning to you. This one *computes*, and returns the provision it relied on. They fit together rather than compete:

If you already run one of those, add this alongside it. An assistant with both picks the right one per question.

Why this exists

• No consolidated API exists. Developers assemble this from 協会けんぽ, 厚生労働省 and each prefectural labour bureau separately.
• The rules are fiddly. Premiums are computed on the *standard monthly remuneration* (a 50-grade step function), not on actual salary — except employment insurance, which uses actual salary. Pension caps at grade 32. Long-term care applies only to ages 40–64. The employee share rounds *half down* (≤ 0.50 yen truncates). Getting one of these wrong produces numbers that look plausible and are wrong.

MCP server

Source in mcp/, with its own README (日本語) · English. Test it with npm run mcp:test — it drives a real stdio transport with the real MCP client, because a tool with a broken handler still lists perfectly and only fails when something calls it.

The MCP server is free and always will be. It is a distribution channel rather than a revenue one: npm pays nothing and MCP has no billing of its own. That is deliberate — the problem was never billing, which RapidAPI already handles, but discovery, and MCP is where the traffic for Japanese statutory data measurably is.

Endpoints

prefecture accepts an English name (Tokyo, case-insensitive), Japanese (東京 or 東京都), or a JIS code (13).

The one call that matters

Running payroll for one employee is a single request:

curl "https://japan-payroll-api.tsumugi.workers.dev/v1/payroll?prefecture=Tokyo&monthly_salary=350000&age=40&dependants=2"

gross                    350,000

social insurance -55,750

---

after social insurance 294,250 <- the base withholding tax is charged on withholding income tax -4,480

---

net pay 289,770

That middle line is the point. Income tax is charged on pay after social insurance, not on gross pay, and deriving it by hand is the mistake this endpoint exists to stop. The response also carries the grade that was resolved, every premium split into employee and employer shares, and which bracket produced the tax — so the arithmetic can be audited rather than trusted.

Resident tax (住民税) is assessed by the municipality and notified to the employer; no API can compute it. Pass resident_tax= and it will be subtracted from net pay.

Pass income_tax=false to get social insurance only.

Before you integrate

• GET /v1/enums lists every accepted value — business_type, column, calendar — and every error code, so they can be read at build time instead of discovered from a 400.
• Errors carry a stable code. invalid_request and missing_parameter mean fix the call; out_of_coverage means the input was valid but falls outside what is published, which needs a different branch. Do not match on the English prose — it will change.
• GET /v1/data-freshness tells you how current each dataset is.

Data

All figures are extracted programmatically from the official spreadsheets — not transcribed by hand. See `scrip