Saju MCP — Korean Four Pillars & BaZi Astrology

An MCP (Model Context Protocol) server that wraps the Saju API — Korean Four Pillars of Destiny (사주팔자 / BaZi / 八字) — so any MCP-capable AI client (Claude Desktop, Cursor, VS Code, Windsurf, and custom agents) can compute, interpret, and compare Korean Saju charts directly in a conversation.

$SAJU_API_KEY="sajuapi_free_xxx" npx saju-mcp

30-second path: get a free key → add the config → ask your AI client "calculate the saju for someone born 1990-05-15 14:00, male."

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Why this MCP?

• The only production-grade Korean Saju engine available as an MCP server.
• KASI-validated lunar conversion (47,000+ days cross-checked, zero failures).
• Ten Gods (十神) + Yongshin (用神) + Daeun (大運) — interpretive features absent from generic Western astrology APIs that only return sun/moon signs.
• 10 output languages: Korean, English, Japanese, Chinese, Spanish, Portuguese, Vietnamese, Indonesian, Hindi, Thai.
• Free tier: 100 requests/day, no credit card. Freemium — start building today and upgrade only when your app needs production volume.

Backed by the live API at https://saju-api.pages.dev.

What it looks like in practice

Ask your AI client a natural-language question; it calls saju_calculate and gets back structured data it can reason over. This is a real, unedited response from the live API for { year: 1990, month: 5, day: 15, hour: 14, gender: "M", lang: "en" }:

{
  "pillars": {
    "year":  { "stem": "경", "branch": "오", "stem_hanja": "庚", "branch_hanja": "午" },
    "month": { "stem": "신", "branch": "사", "stem_hanja": "辛", "branch_hanja": "巳" },
    "day":   { "stem": "경", "branch": "진", "stem_hanja": "庚", "branch_hanja": "辰" },
    "hour":  { "stem": "계", "branch": "미", "stem_hanja": "癸", "branch_hanja": "未" }
  },
  "elements": { "wood": 0, "fire": 2, "earth": 2, "metal": 3, "water": 1 },
  "day_master": { "stem": "경", "element": "metal", "polarity": "yang" },
  "zodiac": "horse",
  "tier": "free",
  "remaining": 99
}

Every response is returned to the model as both human-readable text and structuredContent, so agents can branch on day_master.element, elements, a compatibility score, etc. without re-parsing prose.

Tools

Tool	Upstream endpoint	What it does
saju_calculate	POST /api/v1/calculate	Four Pillars (stem+branch+hanja), five-element distribution, Day Master, zodiac, from a solar birthdate.
saju_interpret	POST /api/v1/interpret	Full reading: Ten Gods (십신), hidden stems, Yongshin (용신), Daeun (대운), localized summaries.
saju_compatibility	POST /api/v1/compatibility	Two-person 궁합 score (0–100) with breakdown (element balance, Day Master relation, branch harmony/clash).
saju_daily	GET /api/v1/daily	Daily fortune snapshot (score + advice) for a Day Master and date.

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Quickstart

1. Get a free API key (no card)

The free tier is 100 requests/day, no credit card:

curl -X POST https://saju-api.pages.dev/api/v1/keys/create \
  -H "Content-Type: application/json" \
  -d '{"email":"dev@yourcompany.com"}'

The response contains an api_key of the form sajuapi_free_...:

{
  "api_key": "sajuapi_free_XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX",
  "tier": "free",
  "daily_limit": 100,
  "rps": 1,
  "monthly_price_usd": 0,
  "note": "Store this key safely — it is shown only once. Send with header `X-API-Key: <key>`."
}

The key is shown only once — store it now. It is passed to the server via the SAJU_API_KEY environment variable, never hardcoded. (Disposable / example.com email domains are rejected — use a real address.)

2. (Optional) Smoke-test without an MCP client

npx runs the server straight from npm — no clone, no local build:

$SAJU_API_KEY="sajuapi_free_xxx" npx -y saju-mcp

It speaks MCP over stdio and exposes the four saju_* tools. Press Ctrl-C to exit.

3. Register in your MCP client

The server is stdio-based, so every MCP client uses the same three pieces: command: npx, args: ["-y", "saju-mcp"], and an env with your SAJU_API_KEY.

Restart / reload your client. The four saju_* tools appear in its tool list.

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Example tool inputs

saju_calculate / saju_interpret:

{ "year": 1990, "month": 5, "day": 15, "hour": 14, "gender": "M", "lang": "en" }

(hour: -1 if the birth hour is unknown.)

saju_compatibility:

{
  "person_a": { "year": 1990, "month": 5, "day": 15, "hour": 14, "gender": "M" },
  "person_b": { "year": 1992, "month": 8, "day": 3,  "hour": 9,  "gender": "F" },
  "lang": "en"
}

saju_daily (Day Master from a prior calculate/interpret call):

{ "day_master": "갑", "date": "2026-06-17", "lang": "en" }

Input bounds (validated server-side, mirrors the API): year 1920–2050, month 1–12, day 1–31, hour -1–23, gender "M"|"F", lang one of the 10 supported codes (default ko).

Environment variables

Variable	Required	Default	Notes
SAJU_API_KEY	yes (for real calls)	(empty)	Your sajuapi_* key, sent as the X-API-Key header. Without it, every call returns 401 invalid_api_key.
SAJU_API_BASE	no	https://saju-api.pages.dev	Override the upstream base URL (e.g. a staging deploy).

Errors & troubleshooting

When an upstream call fails, the tool returns an MCP error result (isError: true) whose text is Saju API error <status>: <body> plus a hint. Common cases:

Symptom	HTTP status	Cause	Fix
401 invalid_api_key	401	SAJU_API_KEY is missing, mistyped, or revoked.	Set the env var to a valid sajuapi_* key. Get a free one.
429 (daily quota exceeded)	429	Free tier is 100 req/day, 1 rps.	Wait for the daily reset, or upgrade to a paid tier for production volume.
invalid_input	400	A field is out of bounds (e.g. month: 13) or missing.	Check the input bounds above; the reason field names the offending field.
Tools don't appear in the client	—	Client not restarted, or npx can't fetch the package.	Restart the client; run npx -y saju-mcp once in a terminal to confirm it starts.
non_json_response	any	Upstream returned non-JSON (rare; network/proxy).	Retry; if persistent, check SAJU_API_BASE is correct.

Keys never appear in tool output or logs. If a key leaks, mint a new one — the old one keeps its own quota and can be abandoned.

Develop / build from source

git clone https://github.com/ghdejr11-beep/saju-mcp.git
cd saju-mcp
npm install
npm run build      # compiles src/index.ts -> dist/index.js
npm run typecheck  # tsc --noEmit

Run the local build directly:

{
  "mcpServers": {
    "saju": {
      "command": "node",
      "args": ["/absolute/path/to/saju-mcp/dist/index.js"],
      "env": { "SAJU_API_KEY": "sajuapi_free_your_key_here" }
    }
  }
}

Requires Node.js 18+ (uses the built-in global fetch).

Upgrading to production

The free tier (100 req/day, 1 rps) is for building and evaluation. When your app ships, higher-volume tiers are available on the same API — see https://saju-api.pages.dev for current plans and the key endpoint. Your code and config don't change; only the key does.

Related

• Korea Calendar API — Korean public holidays, lunar↔solar conversion, the gapja (간지) pillars and the 24 solar terms over REST. Pairs naturally with this server when you need the raw calendar facts behind a saju reading: https://korea-calendar-api.kunstudio.workers.dev

License

Proprietary — KunStudio. Wraps the Saju API; subject to that API's terms.