The full upstream README, mirrored here for reference. Install config, tool schemas, adoption signals, and an original overview live on the Stellara MCP listing page.
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:
| 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.
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:
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.
Two ways to connect:
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.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.
Edit claude_desktop_config.json
(macOS: ~/Library/Application Support/Claude/, Windows: %APPDATA%\Claude\):
Restart Claude Desktop. The three tools appear under the 🔌 icon; the
astrologer_system_prompt prompt appears in the prompts (+) menu.
Add the same block to ~/.hermes/config.json:
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: 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:
Launch npx -y stellara-mcp as a stdio server with the environment variable
STELLARA_API_KEY set. That is the whole integration surface.
then use "command": "node", "args": ["/absolute/path/to/stellara-mcp/dist/index.js"]
in any of the configs above.
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, callsresolve_birth_place("Warsaw, Poland", "1990-05-17T14:30")→ 52.23 / 21.01, Europe/Warsaw,1990-05-17T14:30:00+02:00, thencalculate_natal_chartwith those values, thenget_transits_and_aspectsfor 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.
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:
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 larger nearby city / add the country) and
GEOCODER_UNAVAILABLE (transient — retry). Geocoding uses OpenStreetMap
Nominatim; nothing about the request is stored.
calculate_natal_chart| Input | Type | Notes |
|---|---|---|
birth_datetime | string | ISO-8601 with tz |
latitude | number | −90 … 90 |
longitude | number | −180 … 180 |
house_system | enum | placidus (default), koch, porphyry, regiomontanus, campanus, equal, whole_sign, topocentric, morinus, alcabitius |
include_minor_aspects | boolean | default false — majors only (conjunction 10°, opposition 10°, trine 8°, square 8°, sextile 6°) |
Output (abridged):
get_transits_and_aspects| Input | Type |
|---|---|
natal | object — same fields as calculate_natal_chart |
target_datetime | string — ISO-8601 with tz |
include_minor_aspects | boolean |
Returns natal (the full chart above), target_datetime_utc, transiting_planets
(same point shape plus natal_house), and aspects_to_natal:
movement is computed with the natal point held fixed — only the transiting body
moves — using the real planetary speed for that instant.
calculate_synastry| Input | Type |
|---|---|
person_a, person_b | object — same fields as calculate_natal_chart |
include_minor_aspects | boolean |
Returns both full charts, inter_aspects (point_a belongs to A, point_b to B,
with orb), and house_overlays (a_in_b_houses, b_in_a_houses).
collect_birth_profileArgument (optional): language. A fixed-order intake the agent walks through
before any chart: 1 date of birth → 2 time of birth with its certainty
(exact / approximate / unknown → 12:00 and no house interpretation) → 3 place
of birth, resolved through resolve_birth_place and confirmed with the user →
4 current city (optional, only to interpret "today" in local time) → 5
name (optional) → 6 gender (optional, asked only for synastry) → 7 a
confirmation card with the resolved coordinates, timezone, birth_datetime and
age. Mirrors the profile the Stellara app collects, minus anything the
calculation does not need.
astrologer_system_promptArguments (both optional): language — the language to answer in; focus — the
user's current concern. The prompt carries the same intake rules, mandates
resolve_birth_place for coordinates and offsets, and instructs the agent to use
tool JSON as the only source of positions, cite orbs, respect applying/separating,
flag unreliable houses when the birth time is unknown, and keep interpretation
separate from calculation.
| Variable | Required | Default | Purpose |
|---|---|---|---|
STELLARA_API_KEY | yes | — | Access key, sent as X-API-Key. Request one at stellara.natlex.it/#api. Without it every tool returns a clear error that says where to get one. |
STELLARA_API_BASE_URL | no | https://api.stellara.natlex.it/api/v1/astrology | Point at a self-hosted Stellara backend (the raw Cloud Run URL https://stellara-api-923668916124.us-central1.run.app/api/v1/astrology also works) |
STELLARA_TIMEOUT_MS | no | 15000 | Per-request timeout |
Errors are returned as MCP tool errors with an actionable message — invalid or
revoked key (401), place not found (404), rate limit (429, 60 requests / minute /
IP; 20 for resolve_birth_place), daily quota used up (429 with
QUOTA_EXCEEDED — the message tells the agent when the quota resets at 00:00 UTC
and where to upgrade the key), backend validation detail (422), geocoder or
backend outage (5xx), network failure, timeout. The server never
crashes at launch because of configuration; it explains the problem when a tool is
called, which is what desktop agents handle best.
Keys come in tiers: free (5 requests / day — enough to try every tool) and Stellara API Pro (5 000 / day, $9 / month) — see stellara.natlex.it/#api.
The same package also runs as an HTTP service — node dist/remote.js — which is what
https://mcp.stellara.natlex.it/mcp is (Cloud Run, stateless, one McpServer per
request). It is an OAuth 2.1 resource server: GET /.well-known/oauth-protected-resource/mcp
points clients at the authorization server (the Stellara backend, https://api.stellara.natlex.it,
DCR + CIMD + PKCE S256), an unauthenticated POST /mcp answers 401 with
WWW-Authenticate: Bearer … resource_metadata="…", and a valid token — an OAuth access
token or an sk_stellara_ key sent as Authorization: Bearer — is forwarded to the API,
which enforces the daily quota. GET /health for monitoring; GET /mcp is 405 (no SSE
stream, no sessions).
| Variable | Required | Default | Purpose |
|---|---|---|---|
JWT_SECRET | yes | — | HS256 secret shared with the authorization server (the service refuses to start without it) |
MCP_PUBLIC_URL | no | https://mcp.stellara.natlex.it | Public origin; <origin>/mcp is the token audience (RFC 8707 resource) |
OAUTH_ISSUER | no | https://api.stellara.natlex.it | Authorization server (RFC 8414 issuer) |
PORT | no | 8080 | Listen port (Cloud Run sets it) |
GLAMA_CLAIM_TOKEN | no | — | When set, GET /.well-known/glama.json publishes the Glama connector ownership claim (public by design) |
STELLARA_API_BASE_URL, STELLARA_TIMEOUT_MS | no | as above | Same as stdio mode |
The calculator behind the tools is the Stellara API (FastAPI + kerykeion / pyswisseph),
hosted at https://api.stellara.natlex.it. The MCP server itself is a thin, stateless
adapter: point STELLARA_API_BASE_URL at another instance of the API and everything
else stays the same. The backend is not published as open source; for a private
deployment write to info@natlex.it.
docs/PUBLISHING.md there) and is not published.This repository is the public source of the npm package and the remote service. Day-to-day
development happens in the private Stellara monorepo; every release is synced here as a
single commit (sync: stellara-mcp <version>). Issues are welcome here; pull requests are
merged into the monorepo and land here with the next sync.
Tests connect an MCP Client to the server over an in-memory transport, stub
fetch, and assert exactly what a desktop agent would observe: tool discovery,
schema rejection before any network call, request shape (URL, headers, body), the
JSON passthrough, every error mapping, timeouts and the prompt text.
MIT © Natlex. Swiss Ephemeris is © Astrodienst AG and is used on the server side under its license via kerykeion.