stellara-mcp

# 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](https://modelcontextprotocol.io) 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:

| Tool | What you get back (exact JSON) |
| --- | --- |
| `resolve_birth_place` | "Warsaw, 14:30" → latitude / longitude, IANA timezone and a tz-aware `birth_datetime` with the **historically correct UTC offset** (old DST rules, decree time), plus age and DST-ambiguity flags — so the agent never guesses coordinates or offsets |
| `calculate_natal_chart` | 14 points (Sun → Pluto, Chiron, Lilith, North/South Node) with sign, degree, absolute longitude, house, retrograde, speed, declination · 12 house cusps · Ascendant / MC / Descendant / IC · every major aspect with its **exact orb** and applying / separating |
| `get_transits_and_aspects` | Transiting positions at any moment · every transit-to-natal aspect (planets **and** angles) with orb and applying / separating · which natal house each transiting planet occupies |
| `calculate_synastry` | All inter-chart aspects between two people with exact orbs · house overlays (A's planets in B's houses and vice versa) |

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](https://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\`):

```json
{
  "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`:

```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:

```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)

```bash
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`

| Input | Type | Notes |
| --- | --- | --- |
| `place` | string | Free text, any language: `Warsaw`, `Варшава`, `Springfield, Illinois, USA` |
| `local_datetime` | string (optional) | **Local wall-clock time without an offset**: `1990-05-17T14:30`, or `1990-05-17` if the time is unknown (12:00 assumed and flagged) |

Output:

```json
{
  "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"
}
```

`birth_datetime`, `latitude` and `longitude` go straight into the chart tools.
`local_time_ambiguity` is `ambiguous_dst_fallback` when the wall time happened
twice (autumn clock change) and `nonexistent_dst_gap` when the clocks skipped it —
the agent should ask which one the user means. Offsets come from the IANA tz
database, so 1985 Moscow resolves to +04:00 and 2015 Moscow to +03:00. Errors:
`PLACE_NOT_FOUND` (try a large