Stellara MCP: Deterministic Swiss Ephemeris for AI Agents (Hermes, Claude, Grok)

Give your AI agent a real astronomical calculator. stellara-mcp is a tiny, open-source Model Context Protocol server that turns any MCP-capable agent — Claude.ai / Claude Desktop / Claude Code, ChatGPT, Codex, Cursor, Windsurf, Gemini CLI, Grok, Hermes — into an astrologer that never invents a planetary position. Connect by URL (no install) or run the package locally with an API key.

It exposes four tools backed by Swiss Ephemeris (the same engine professional astrologers use) running in the Stellara cloud:

Plus two MCP prompts: collect_birth_profile, a guided intake of the data every reading depends on (date → time with certainty → place → optional city, name, gender → confirmation card with age), and astrologer_system_prompt, which switches the agent into a disciplined professional-astrologer style for interpreting that JSON.

The server does not interpret and does not call any LLM. It is a proxy adapter: numbers in, numbers out. Your agent does the reading; Swiss Ephemeris does the math.

Why this exists: LLMs lie about natal charts

Ask a language model "where was my Moon on 17 May 1990 at 14:30 in Warsaw?" and it will answer confidently — and, more often than not, wrongly. Ecliptic longitudes, house cusps and aspect orbs are the output of numerical integration of planetary motion plus spherical trigonometry for the local horizon. That is not something a next-token predictor can do in its head. Typical failure modes:

• the Moon (13° per day) lands in the wrong sign;
• the Ascendant is guessed from the Sun sign, ignoring time and latitude;
• house cusps are fabricated or silently assume Equal houses;
• "Saturn square Sun" is asserted with no orb, or with an orb that does not exist;
• retrograde status is invented.

Every one of those errors poisons the whole interpretation that follows. A reading built on a wrong Ascendant is worthless no matter how eloquent it is.

stellara-mcp fixes this at the root. The agent calls a tool; the tool returns Swiss-Ephemeris-grade positions (sub-arcsecond precision, tropical zodiac, ten house systems, exact orbs, applying/separating derived from real planetary speeds); the agent interprets only that data. Deterministic senses, deterministic calculator — the language model is left to do what it is actually good at.

Quick start (1-click configs)

Two ways to connect:

• Remote, no install (Claude.ai, Claude Desktop, Claude Code, ChatGPT connectors, Grok, any client that takes an MCP URL): add a custom connector with the URL https://mcp.stellara.natlex.it/mcp and sign in with your Stellara account (the same account as the Stellara app; Free = 10 requests / day, Stellara Pro = 5 000). Claude: Settings → Connectors → Add custom connector → paste the URL → Connect → sign in. Claude Code: claude mcp add --transport http stellara https://mcp.stellara.natlex.it/mcp, then /mcp to sign in. Nothing to install, nothing to configure; tokens are short-lived and can be revoked from the app.
• Local package + API key (below): for servers, scripts, Cursor / Windsurf / Codex and anyone who prefers a key. Get one at stellara.natlex.it/#api — Free (5 requests / day) or Stellara API Pro (5 000 / day, $9 / month), delivered by e-mail within a minute. Then pick your client.

The package runs on Node.js ≥ 18.17 and is launched with npx, so nothing to install globally. All configs below are the same three lines: command, args, env.

Claude Desktop

Edit claude_desktop_config.json (macOS: ~/Library/Application Support/Claude/, Windows: %APPDATA%\Claude\):

{
  "mcpServers": {
    "stellara": {
      "command": "npx",
      "args": ["-y", "stellara-mcp"],
      "env": {
        "STELLARA_API_KEY": "sk_stellara_your_key_here"
      }
    }
  }
}

Restart Claude Desktop. The three tools appear under the 🔌 icon; the astrologer_system_prompt prompt appears in the prompts (+) menu.

Hermes Agent Desktop

Add the same block to ~/.hermes/config.json:

{
  "mcpServers": {
    "stellara": {
      "command": "npx",
      "args": ["-y", "stellara-mcp"],
      "env": {
        "STELLARA_API_KEY": "sk_stellara_your_key_here"
      }
    }
  }
}

Hermes discovers the tools on next launch. If your Hermes build uses a different top-level key for MCP servers, keep the command / args / env triple as is — it is the standard stdio launch spec.

Cursor / Windsurf

Cursor: Settings → MCP → Add new global MCP server (or .cursor/mcp.json in a project). Windsurf: Settings → Cascade → MCP servers (or ~/.codeium/windsurf/mcp_config.json). Both take the identical JSON:

{
  "mcpServers": {
    "stellara": {
      "command": "npx",
      "args": ["-y", "stellara-mcp"],
      "env": {
        "STELLARA_API_KEY": "sk_stellara_your_key_here"
      }
    }
  }
}

Any other MCP client (Grok bridges, Continue, Zed, custom agents)

Launch npx -y stellara-mcp as a stdio server with the environment variable STELLARA_API_KEY set. That is the whole integration surface.

From a local clone (for development)

git clone https://github.com/mihnin/stellara-mcp.git
cd stellara-mcp
npm install && npm run build

then use "command": "node", "args": ["/absolute/path/to/stellara-mcp/dist/index.js"] in any of the configs above.

Example conversation

User: Build my natal chart — I was born on 17 May 1990 at 14:30 in Warsaw — and tell me why work feels so heavy today.

Agent (loads astrologer_system_prompt, asks whether 14:30 is exact or approximate, calls resolve_birth_place("Warsaw, Poland", "1990-05-17T14:30") → 52.23 / 21.01, Europe/Warsaw, 1990-05-17T14:30:00+02:00, then calculate_natal_chart with those values, then get_transits_and_aspects for today)

Your chart is built from Swiss Ephemeris positions, so let's start with what's actually there. Sun at 26°14' Taurus in the 9th house, Moon at 3°52' Capricorn in the 4th, Ascendant 8°41' Virgo. The tightest natal aspect is Saturn conjunct Neptune (orb 0.6°) sitting in your 5th house — a lifelong tension between structure and drift in creative work.

Why today is heavy: transiting Saturn is squaring your natal Sun with an orb of 1.2° and it is applying, so the pressure is still building toward exact in about two weeks. Saturn squares to the Sun feel like carrying a backpack of obligations — output is fine, but everything costs more effort. At the same time transiting Mars is passing through your natal 6th house of daily work (Aquarius), conjunct your natal Mercury with a 0.4° orb: fast, irritable, prone to arguing with tools and colleagues.

Practical read: this is a "finish, don't start" window. Close the two tasks with deadlines, defer the brainstorming to after the Saturn square separates, and don't read the Mars–Mercury irritability as a sign that the job is wrong. If you can, I'd like your birth time to the minute — the Ascendant at 8° Virgo is close to a sign boundary, and the house placements above depend on it.

Every degree, orb and "applying" in that answer came from a tool result; the interpretation is the agent's. That separation is the entire point.

Tool reference

Chart tools take datetimes as ISO-8601 with a timezone (1990-05-17T14:30:00+02:00 or ...Z); naive values are rejected before any network call. Coordinates are decimal degrees (north / east positive). The intended flow is resolve_birth_place first — it produces exactly those values from what the user actually says.

resolve_birth_place

Output:

{
  "query": "Warsaw", "display_name": "Warszawa, województwo mazowieckie, Polska",
  "latitude": 52.2333742, "longitude": 21.0711489, "timezone": "Europe/Warsaw",
  "local_datetime": "1990-05-17T14:30:00", "utc_offset": "+02:00",
  "birth_datetime": "1990-05-17T14:30:00+02:00", "birth_datetime_utc": "1990-05-17T12:30:00Z",
  "time_assumed_noon": false, "local_time_ambiguity": "none", "age_years": 36,
  "source": "nominatim"
}