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日本の給与・社会保険・労働法

日本の給与計算と労務判断。47都道府県の社会保険料率、源泉所得税、割増賃金、有給、標準報酬月額の改定、最低賃金。根拠の条文を必ず添える。

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Install 日本の給与・社会保険・労働法

Pick your client, then add it in one click or paste the config.

STDIONode.js
~/Library/Application Support/Claude/claude_desktop_config.json
claude_desktop_config.json
{
  "mcpServers": {
    "mcp-1788779857343": {
      "command": "npx",
      "args": [
        "-y",
        "jp-payroll-mcp"
      ]
    }
  }
}

Settings → Developer → Edit Config, merge this into mcpServers, then fully quit and reopen Claude Desktop.

Install not yet sandbox-tested— we haven't run this command through our automated sandbox yet. Not a red flag; we're working through the catalog.

Overview

Security & health

What it can reach and how reliably it runs — from our own checks, not the maintainer’s claims.

ActiveLast checked 2d ago

Recent health check succeeded.

6/6 checks healthy over the last 40d

Supply-chain signal

No high-severity advisories surfaced by our automated scan.

Critical 0High 0Medium 0Low 0

Scanned 1d ago via OSV.dev · jp-payroll-mcp (npm)

Adoption & maintenance

Factual signals from GitHub, npm, and our automated checks — not a rating.

GitHub stars2Stargazers on the source repository.
npm downloads217Package downloads in the last 30 days.
Last commit1mo agoMost recent push to the default branch.

From the project README

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:

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

Terminal
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。

以下は英語の詳細仕様です。

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):

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

Terminal
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:

"What does the law say?""So what do I pay?"
labor-law-mcp45 labour and social insurance laws, MHLW and JAISH notices—
tax-law-mcp24 tax laws, 17 NTA circulars, tribunal decisions—
hourei-mcp-serverAny Japanese law, via e-Gov—
jp-payroll-mcpThe 28 provisions it cites, in fullPremiums, withholding tax, grade revisions, exemptions

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

EndpointDescription
GET /API info and endpoint list
Payroll and insurance
GET /v1/prefecturesAll 47 prefectures with JIS codes
GET /v1/insurance-rates?prefecture=TokyoHealth, long-term care, pension, child-support rates
GET /v1/standard-remuneration?remuneration=350000Grade lookup for a monthly amount
GET /v1/standard-remuneration/tableFull 50-grade table
GET /v1/employment-insurance?business_type=generalEmployment insurance rates
GET /v1/payroll?prefecture=Tokyo&monthly_salary=350000&age=40Full deduction breakdown
Minimum wage
GET /v1/minimum-wage?prefecture=Tokyo&date=2020-01-01Rate in effect on a date
GET /v1/minimum-wage/history?prefecture=TokyoFull history since FY2002
Calendar
GET /v1/holidays?year=2026Public holidays (or from=/to= for a range)
GET /v1/holidays/check?date=2026-01-01Holiday / weekend / business-day flags
GET /v1/business-days?from=&to=Count business days in a range
GET /v1/business-days/shift?date=&days=1Move N business days forward or back
Tax
GET /v1/consumption-tax?date=&amount=&reduced=Rate in force, optionally applied to an amount
GET /v1/consumption-tax/historyEvery rate change since 1989
Identifiers
GET /v1/corporate-number/validate?number=8700110005901法人番号 check digit (Peppol ICD 0188)
GET /v1/corporate-number/check-digit?base=700110005901Check digit for a 12-digit base number
GET /v1/invoice-number/validate?number=T8700110005901Qualified invoice registration number
Withholding tax
GET /v1/withholding-tax?taxable_amount=300000&dependants=2Monthly withholding income tax (月額表)
GET /v1/withholding-tax/daily?taxable_amount=12000&column=heiDaily table (日額表), including the 丙 column
GET /v1/withholding-tax/computer?taxable_amount=400000Same, by the formula method (電算機計算の特例)
Bonuses
GET /v1/bonus-tax?bonus=500000&previous_month_pay=350000Withholding on a bonus (賞与の算出率表)
GET /v1/bonus-insurance?prefecture=Tokyo&bonus=800000&age=40Social insurance on a bonus, with both caps
Standard remuneration decisions
GET /v1/standard-remuneration/regular?months=350000:30,352000:31,349000:30定時決定 (算定基礎) from April–June
GET /v1/standard-remuneration/revision?current_remuneration=&months=&fixed_pay_change=Is a 随時改定 (月額変更) due?
GET /v1/standard-remuneration/leave-end?kind=childcare&current_remuneration=&months=Revision on returning from leave
POST /v1/standard-remuneration/annual-average年間平均による保険者算定, for seasonal work
Year-end adjustment
POST /v1/year-end-adjustment年末調整 (令和8年分): every box of the 源泉徴収簿 from ⑦ to ㉗, plus an "if filed" estimate for medical/donation/casualty deductions
Resident tax
POST /v1/resident-tax住民税の見込み額 from last year's income and the municipality: income and per-capita levies, adjustment credit, ふるさと納税, exemptions, every prefecture's surtax
Eligibility and leave
GET /v1/eligibility?month=2026-03&left_on=2026-03-30Is a premium due in a joining or leaving month?
GET /v1/leave-exemption?kind=childcare&start=&end=Which months a maternity or childcare leave exempts
GET /v1/age-milestones?birth_date=1986-04-01When 40, 65, 70 and 75 are reached, and what changes
Batch
POST /v1/payroll/batchUp to 500 payslips in one call, with run totals
Statutes
GET /v1/statute?ref=健康保険法第43条Full text of a provision this API cites
GET /v1/statute/indexEvery provision available, with its law
include=statute_textAdd to any endpoint to attach the text of whatever it cited
Meta
GET /v1/enumsEvery accepted enum value and error code
GET /v1/data-freshnessWhat each dataset covers and when it changes next

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:

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

Read the full README View source on GitHub

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Frequently Asked Questions about 日本の給与・社会保険・労働法

日本の給与・社会保険・労働法 is an MCP server that computes Japanese payroll, social insurance, and labor-law results rather than merely retrieving tables. It covers insurance rates for all 47 prefectures, withholding tax, standard remuneration revisions, leave exemptions, overtime pay, paid leave, minimum wages, business-day calculations, and estimated resident tax, with the supporting statute or notice.

MCPクライアントにリモートURLを登録するか、npxでローカル実行します。リモート接続先は https://japan-payroll-api.tsumugi.workers.dev/mcp で、Claudeでは `claude mcp add --transport http jp-payroll https://japan-payroll-api.tsumugi.workers.dev/mcp` を使えます。無料で、APIキーは不要です。

47都道府県の社会保険料率、源泉所得税、標準報酬月額の決定・随時改定、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、令和8年分の年末調整、住民税の見込み額に対応しています。

計算結果には、根拠となる条文または通知が付きます。公表された料額表・税額表との照合に加え、標準報酬月額の改定などで参照する通知も示されるため、数字だけでなく判断の根拠を確認できます。

MCPサーバーはAIアシスタントから日本語で質問するためのAPI薄層で、HTTP APIはソフトウェアへの組み込み向けです。MCPは30ツール、HTTP APIは45エンドポイントとOpenAPI 3.0・一括処理に対応し、両者は同じ答えを返します。

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