JP Invoice Check

An Agent Skill, CLI and MCP server that checks a Japanese qualified invoice (適格請求書): the six required items, reduced-rate marks, totals per rate, and the rule that consumption tax is rounded once per invoice per rate.

Japan's invoice system requires six items on every qualified invoice, and the tax must be rounded once per invoice for each rate, not line by line (NTA pamphlet: 「端数処理は一請求書当たり、税率ごとに1回ずつ」). A language model that writes an invoice tends to compute tax per line and add it up. On the example below that gives 597 yen instead of 599 for the 10% items and 285 instead of 286 for the 8% items.

This package computes instead of guessing:

• checks the six items and returns each as true/false (required_items)
• recomputes tax per rate for floor, half-up and ceil rounding (the method is the issuer's choice) and tells you which one the invoice used
• names the per-line rounding mistake (E_TAX_LINE_ROUNDING) separately from other wrong tax
• flags 8% items without a reduced-rate mark, and reduced marks on 10% items
• normalizes the registration number (full-width, hyphens) and checks T + 13 digits and the corporate-number check digit
• reads ISO dates, 2026年10月28日, 令和8年10月28日, and periods

Every message comes in English and Japanese. Python standard library only. No network. MIT.

Use as an Agent Skill

Copy this folder into your skills folder (for Claude: ~/.claude/skills/jp-invoice-check/). SKILL.md tells the agent to put the invoice into JSON and run the checker instead of computing tax itself.

CLI

python -m jpinvoice examples/ok.json                    # exit 0
python -m jpinvoice examples/line_rounding_mistake.json # exit 1, lists the errors
python -m jpinvoice invoice.json --rounding floor       # require one rounding method

from jpinvoice import check_invoice, expected_tax
expected_tax(5997, 10)                      # 599  (exclusive, floor)
expected_tax(1000, 10, "inclusive")         # 90
check_invoice(invoice)["errors"]            # [{level, code, field, message, message_ja}, ...]

MCP server (for AI agents)

{"mcpServers": {"jp-invoice-check": {"command": "python", "args": ["-m", "jpinvoice.server"], "cwd": "/path/to/jp-invoice-check"}}}

All tools are read-only.

Error codes

E_REGNO_MISSING E_REGNO_FORMAT E_ISSUER_NAME_MISSING E_DATE_MISSING E_DATE_INVALID E_ITEMS_MISSING E_ITEM_DESCRIPTION_MISSING E_RATE_INVALID E_REDUCED_NOT_MARKED E_REDUCED_RATE_MISMATCH E_TOTAL_BY_RATE_MISSING E_TOTAL_MISMATCH E_TAX_MISSING E_TAX_LINE_ROUNDING E_TAX_MISMATCH E_BUYER_MISSING; warnings W_REGNO_NORMALIZED W_REGNO_CHECK_DIGIT W_TOTAL_WITHOUT_ITEMS.

Limits

• Offline. It does not check whether a registration number is registered today; use the NTA publication site for that.
• The check digit rule used is the corporate-number rule. For a sole proprietor's number a mismatch is reported only as a warning.
• Simplified invoices (適格簡易請求書), return invoices and other special cases are not covered.
• This is a checking tool, not tax advice.
• Tests: python -m unittest discover -s tests (18 tests)

日本語

適格請求書(インボイス)を検査する Agent Skill・コマンド・MCPサーバーです。

• 記載事項6つ(発行者名と登録番号・取引年月日・取引内容と軽減税率の印・税率ごとの合計と税率・税率ごとの消費税額・買い手の名称)を1つずつ判定
• 消費税は「1つの請求書につき税率ごとに1回」端数処理。品目ごとに端数処理して足す誤り(AIがよくする)を E_TAX_LINE_ROUNDING で知らせます。切捨て・四捨五入・切上げのどれを使ったかも返します
• 8%の品目に※などの印がない、10%なのに印がある、を検出
• 登録番号を整え(全角・ハイフン)、「T+13桁」と法人番号の検査用数字を確認
• 西暦・和暦(令和・平成)・期間の日付に対応

外部通信なし(登録が今も有効かは国税庁の公表サイトで確認してください)・Python標準ライブラリのみ・MIT。税務の助言ではありません。