Stripe-quality REST API for Japanese laws and regulations: JSON-first, OpenAPI SDKs, full-text search, change webhooks, English summaries — a paid abstraction over the e-Gov XML mess.

Source signals

• **SIG-20260425-japan-api-as-a-product-egov-laws-api-friendly** — "e-Gov 法令API existed but developers wrote their own tutorials because docs are too unfriendly."
• **SIG-20260425-japan-api-as-a-product-jp-public-api-mcp-wrapper** — "Puente charges ¥150-500K/mo for the unified MCP wrapper around JP public APIs; WTP validated."
• **SIG-20260425-japan-api-as-a-product-personal-mcp-server-pattern** — "JP devs build one-off MCP servers because nothing JP-localised exists."

Reasoning

The pain is RECURRING with 4+ independent sources: a public tutorial that exists only because official docs are useless, a competitor (Puente) already monetising the gap at enterprise pricing, and explicit dev complaints. The fit-with-stack is excellent — it's a thin Postgres-cached normalization layer plus FastAPI plus Meilisearch plus Stripe, all on Railway + Cloudflare Pages, achievable in ~12 build days. The defensibility moat is non-trivial (English translation, change-detection quality, JP enterprise sales relationships) and grows with corpus size. Critical advantage: Puente targets enterprise SI shops and AI-agent buyers; this idea targets developer/CTO buyers in legal-tech with Stripe-style DX. Two distinct ICPs in the same white-space.

Quick competitive read

• **Puente MCP Hub** — direct enterprise competitor at ¥150-500K/mo; positioned for AI-agent buyers, not SaaS-engineer buyers. Coexistence likely.
• **e-Gov official API** — what we abstract away; XML-only, no SDKs, no English.
• **Thomson Reuters Westlaw Japan** — legal database for lawyers, not a developer-API.
• **No direct developer-friendly API competitor** in either Japanese or English market — **white space**, `competitive_pressure: 7-9` initial hint.

Initial pricing hypothesis

Stripe-style tiering: Free 1K calls/mo, $99 Pro, $399 Team (with AI English summaries), $2K+ Enterprise (with JPY/インボイス invoice + SLA).

Distribution hypothesis

Qiita/Zenn long-form posts in Japanese walking through e-Gov API pain → anchor authority; direct cold-email to ~50 JP legal-tech CTOs; Product Hunt EN-launch for foreign reg-tech vendors entering Japan; long-tail English SEO ("Japan law API", "e-Gov API wrapper").

What we ship

**Core features (MVP)**

• JSON-first REST endpoints over e-Gov 法令API: `GET /v1/laws`, `GET /v1/laws/{id}`, `GET /v1/laws/{id}/articles/{n}`, `GET /v1/diff?from=...&to=...`, `POST /v1/search` (Meilisearch full-text). A 6-hour cron pulls XML, normalizes into Postgres, archives raw to R2.
• OpenAPI 3.1 spec + auto-generated SDKs for JavaScript/TypeScript, Python, Go — single source of truth; live playground in the dashboard; Stripe-style 90-second quickstart (`curl` → SDK → webhook).
• Article-level change webhooks via Resend with HMAC signing, 5x retry, signed-receipt audit log — the pricing anchor against Puente's poll-only model.
• English AI summaries of diffs via Claude Sonnet 4.6 ("Article 5 of 法人税法 amended: ICT equipment depreciation 25%→20%, effective 2027-04-01") — the unique moat no JP-only competitor will ship.
• JPY enterprise billing with 適格請求書 invoicing: dashboard toggle for JPY mode, generated PDF carries 13-digit T-number + 消費税 10% breakdown; Stripe USD remains the default for everyone else.

**Primary user flow**

1. Developer onboarding — GitHub OAuth → create project → mint API key → copy a curl example pre-filled in the dashboard (≤30 s).

2. First API call — `GET /v1/laws/{id}` with any 法令番号 returns clean JSON: articles, paragraphs, items, supplementary provisions, last_amended timestamp.

3. Subscribe to webhooks — `POST /v1/webhooks` with target URL + optional filter (e.g. only 商法 + 会社法); Resend delivers diffs every 6 hours, payload includes the English summary.

4. Billing — Free (1K calls/mo) → Pro $99 → Team $399 (English summaries) → Enterprise $2K+/mo with JPY/適格請求書 invoicing once 税務署 issuer registration completes.

**What it looks like**

• Landing page (developer-friendly): hero "Stripe-quality API for Japanese laws — JSON, SDKs, webhooks, English summaries", live curl block with syntax highlighting, pricing table with explicit Puente comparison (¥150K/mo → our Team $399), badges for "OpenAPI 3.1", "JS/Python/Go SDKs", "99.9% SLA".
• API docs page: trilingual (EN/JP/auto RU) Stripe-style three-pane layout — left nav, middle markdown, right "Try it" playground that runs against the user's real key; copy-paste samples in three SDKs side by side.
• Dashboard: usage graph (calls/day per endpoint), webhook delivery log with retry status, API key CRUD, quota meter, Stripe billing portal, "Switch to JPY enterprise invoicing" CTA.
• JPY billing screen: PDF preview with 適格請求書 number (T1234567890123), 消費税 (10%) breakdown, 振込 bank routing, history of past invoices, freee/MFクラウド/弥生 export for the customer's contador.

**MVP build plan: 12 days**

• Day 1-2: FastAPI + Postgres + Stripe USD, GitHub OAuth, scaffolded dashboard.
• Day 3-4: e-Gov XML poller (cron), Postgres normalization, R2 archive, Meilisearch full-text index.
• Day 5-6: REST endpoints (`/laws`, `/laws/{id}`, `/articles/{n}`, `/search`, `/diff`), OpenAPI 3.1 spec.
• Day 7-8: SDK auto-generation via openapi-generator for JS/Python/Go; publish to npm/PyPI/proxy.golang.org.
• Day 9: Webhook delivery (Resend), HMAC signing, retry queue.
• Day 10: Claude Sonnet integration for English diff summaries.
• Day 11: JPY invoice generator with 適格請求書 T-number, dashboard JP-billing screen.
• Day 12: Docs playground polish, soft launch via Qiita post + 10 cold-email pilots.