Open Ephemeris

Open Ephemeris MCP Server

Astrology that works inside ChatGPT and Claude: real computed charts from NASA JPL DE440, not AI guesses.

[npm version] [System Status] [License: MIT] [Ephemeris: JPL DE440]

[Open Ephemeris in Claude — ask in plain language, get a real computed chart]

Use it in Claude: add https://mcp.openephemeris.com/mcp as a custom connector (leave the OAuth Client ID and Secret blank) or install the Open Ephemeris plugin from the Claude directory. Sign-in happens in the browser and creates a free account with 150 credits.

Model Context Protocol server for Open Ephemeris — typed astrology tools powered by the NASA JPL DE440 ephemeris. Zero hallucination on planetary positions, dates, and degrees. Covers 1,100 years of astronomical data.

[The catalog: 90 bodies and 124 fixed stars — asteroids, trans-Neptunians, Uranian points, computed in one engine]

Hosted endpoint: https://mcp.openephemeris.com/mcp (Streamable HTTP, MCP 2025-11-25 spec)

Quick Start

Quick install

Claude Code, one command:

claude mcp add openephemeris -- npx -y @openephemeris/mcp-server

Any client that takes a hosted URL (Claude Web, ChatGPT, custom agents): use https://mcp.openephemeris.com/mcp and send your API key in the X-API-Key header. Other clients are below.

Connect from code (Streamable HTTP)

import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js"import { Client } from "@modelcontextprotocol/sdk/client/index.js"
const transport = new StreamableHTTPClientTransport(  new URL("https://mcp.openephemeris.com/mcp"),  { requestInit: { headers: { "X-API-Key": "your-openephemeris-api-key" } } })
const client = new Client({ name: "my-app", version: "1.0.0" }, { capabilities: {} })await client.connect(transport)

One-click install (Cursor)

[Install in Cursor]

Replace YOUR_API_KEY_HERE in Cursor MCP settings with your API key from https://openephemeris.com/dashboard.

Cursor deeplink payload:

{  "command": "npx",  "args": [    "-y",    "@openephemeris/mcp-server"  ],  "env": {    "OPENEPHEMERIS_PROFILE": "dev",    "OPENEPHEMERIS_BACKEND_URL": "https://api.openephemeris.com",    "OPENEPHEMERIS_API_KEY": "YOUR_API_KEY_HERE"  }}

Manual install (stdio MCP clients)

{  "mcpServers": {    "openephemeris": {      "command": "npx",      "args": ["-y", "@openephemeris/mcp-server"],      "env": {        "OPENEPHEMERIS_PROFILE": "dev",        "OPENEPHEMERIS_BACKEND_URL": "https://api.openephemeris.com",        "OPENEPHEMERIS_API_KEY": "YOUR_API_KEY_HERE"      }    }  }}

Platform guide

Detailed setup walkthroughs for each platform are in SETUP.md.

Client install walkthroughs

1. Cursor
   • Click the "Install in Cursor" button above, then replace YOUR_API_KEY_HERE in Cursor MCP settings.
   • If you prefer manual setup, paste the mcpServers.openephemeris block from "Manual install" into ~/.cursor/mcp.json.
2. Claude Desktop (macOS/Windows)
   • Open the platform config file from the table above.
   • Add the same mcpServers.openephemeris block from "Manual install".
   • Restart Claude Desktop.
3. Windsurf
   • Open ~/.codeium/windsurf/mcp_config.json (or the legacy ~/.codeium/mcp_config.json path).
   • Add the mcpServers.openephemeris block from "Manual install".
   • Restart Windsurf.

Remote-only clients (Claude Web, ChatGPT, etc.)

The server is hosted at https://mcp.openephemeris.com/mcp with full Streamable HTTP support (MCP 2025-11-25 spec). Remote-only clients can connect directly — no bridge/proxy required:

• Claude Web: Add https://mcp.openephemeris.com/mcp as a custom connector URL — leave OAuth Client ID and Secret blank. The server uses OAuth 2.1 + PKCE (Dynamic Client Registration), so Claude handles authentication via a browser popup automatically.
• ChatGPT: Open Ephemeris is an approved app in the ChatGPT app directory. Open the listing, click Install plugin, approve the sign-in (that also creates your free Open Ephemeris account), then type @Open Ephemeris in any chat. Charts render inline, exactly as they do in Claude. Prefer to wire it yourself? Settings → Plugins → Advanced → Developer mode → + Create app accepts https://mcp.openephemeris.com/mcp with Authentication on OAuth — no client ID or secret to enter.
• Legacy SSE: retired in 3.20.0 — use Streamable HTTP at /mcp

Auth and upgrade behavior in MCP clients

• Missing/invalid credentials (401): tool call fails with a message that points users to sign up/sign in at https://openephemeris.com/login?signup=true&redirect=%2Fdashboard%3Ftab%3Daccount, then create/manage keys in https://openephemeris.com/dashboard?tab=account.
• Tier-gated endpoint (403): tool call returns an upgrade-required message with https://openephemeris.com/pay and dashboard billing/key management link.
• Out of credits (402): tool call returns a one-tap top-up link (Explorer's 150 free credits are one-time and do not reset; plan allowances renew each billing period) plus the dashboard usage link. Failed calls (any 4xx/5xx) are refunded.
• Burst/rate limit (429): tool call returns retry guidance and links to dashboard usage monitoring.

What You Can Ask

"Calculate a natal chart for 1990-04-15 at 2:30 PM in Chicago.""Find all Saturn transits to my natal Sun in the next 6 months.""Get the current moon phase and void-of-course status.""Find the next solar eclipse visible from Tokyo.""Find the best time to sign a contract in March — electional window.""Generate a Human Design chart for my birth data.""What is my Vedic (sidereal) chart?""Calculate my Chinese BaZi (Four Pillars) chart.""Show me my Astrocartography power lines — where is my Venus line on the map?""Find all ACG lines within 3° of Paris for my chart.""Calculate a synastry chart between two people.""Find the next Venus Star Point and my relationship to it.""What are the active planetary stations in the next 3 months?""Calculate primary directions for the next 5 years.""Find my Firdaria time lord period.""What is the sidereal time and delta-T right now?"

Interactive Charts

Nine of the tools don't answer with JSON. They open a chart in the conversation — a real one, drawn from the same calculation, that you can click around in.

This matters more than it sounds. A natal chart returned as JSON is a list of numbers you have to already understand to read. The same chart rendered as a wheel is something you can point at. Click a planet and you get that placement explained; click a house and you get what's in it. The chart stays on screen while you keep talking, and it doesn't cost another credit to keep looking at it.

These need a host that supports MCP Apps. Claude and ChatGPT both do, and they render the same widget — there is no separate ChatGPT build. MCP Apps (SEP-1865) was co-authored by Anthropic and OpenAI and became the first official MCP extension in January 2026, so one ui:// resource serves both. In ChatGPT, install it from the app directory; in Claude, add the connector (see SETUP.md).

In a client without app support the same tools still work; you get the underlying data instead of the picture, so nothing breaks, you just don't get the wheel.

Ask for these the way you'd ask a person: "show me my chart", "put today's transits over my Human Design", "what's the moon doing right now". The model picks the app.

Two things worth knowing. The chart wheel and bi-wheel accept a click on an aspect line, not just on the two planets it joins — so "why does this line matter" is one click rather than a paragraph of setup. And the bodygraph's mandala toggle rearranges the whole chart into concentric rings without another API call, so switching views is free.

Tools at a Glance

Tooling Model

• Typed tools are preferred for common workflows (natal, transits, moon phase, eclipse, synastry, relocation, electional, Human Design).
• Generic tools: find_calculation searches the allowlisted operations by plain words and returns the path, inputs, an example and the credit cost (dev_list_allowed lists them all, outside core); dev_read_api invokes allowlisted GET (read) operations and dev_write_api invokes allowlisted POST/PUT/PATCH/DELETE (write/compute) operations, each by method + path. Read and write are kept as separate tools so a safe read surface never shares a tool with state-changing writes.
• Security model: default-deny with explicit allowlist in config/dev-allowlist.json.
• Deny prefixes block sensitive route families (/auth, /billing, /admin, etc.).

dev_read_api / dev_write_api input

Environment Variables

Legacy aliases (ASTROMCP_*, MERIDIAN_*) remain supported.

Telemetry

This server reports anonymous usage so we know which tools are worth maintaining and which are broken. Three events: session start, tool call, tool error.

What is sent: the tool name, how long it took, error status, which MCP client connected (e.g. Claude Desktop, Cursor) and its version, the server version, and a stable id for the session: a one-way SHA-256 prefix of your API key, or your account ID when you sign in with OAuth.

What is never sent: your API key or token, birth data, dates, names, coordinates, tool arguments, or tool results. No request or response bodies, ever.

To turn it off — either works, checked before anything is sent:

OPENEPHEMERIS_TELEMETRY=0# or the cross-tool standardDO_NOT_TRACK=1

Tool surface

By default the server advertises a focused core set of everyday tools rather than the entire catalog. Large tool lists cost context and make model tool-selection worse, so the default is tuned for real conversations: one interactive app per tradition, the primary data tool per domain, geocoding, and the allowlist-gated generic proxy.

Nothing is removed. The surface is a filter on tools/list only — every tool stays registered and stays callable by name. If you know the tool you want, call it and it works, listed or not.

To advertise the full catalog:

OPENEPHEMERIS_TOOLS=full npx -y @openephemeris/mcp-server

On the remote HTTP server, append ?profile=full to the connector URL (or send X-OE-Tool-Surface: full):

https://mcp.openephemeris.com/mcp?profile=full

Toolsets by tradition

If you work in one tradition, ask for it by name instead of taking the general-purpose default. You get that tradition in full — including the long-tail tools the core set leaves out — for a fraction of the context.

OPENEPHEMERIS_TOOLS=hd npx -y @openephemeris/mcp-server        # Human DesignOPENEPHEMERIS_TOOLS=astrology,moon npx -y @openephemeris/mcp-server

https://mcp.openephemeris.com/mcp?profile=hd,bazi

Every selection also includes geocoding (location_search, timezone_resolve), account_usage, and the allowlist-gated proxy — so a birthplace is always resolvable and nothing is stranded.

Combine with commas; unknown names are ignored rather than rejected, so a typo degrades to a smaller surface instead of a dead connector. As with core/full, this only filters tools/list — every tool remains callable by name.

Why it matters: tool definitions are re-sent to the model on every pass. astrology,moon advertises a few more tools than the default but costs ~700 fewer tokens per message and covers more of the tradition.

The surface is fixed when the session initializes — this server does not advertise tools.listChanged, so switching requires reconnecting. On every surface, find_calculation searches every operation reachable through the generic proxy.

Contributing & Support

• Something wrong with a result? Open an issue — include the tool, your inputs, and what you expected.
• Want to contribute? See CONTRIBUTING.md. Integration examples and new skills are the most useful things you can add.
• Found a security problem? Please report it privately — see SECURITY.md.
• Tools timing out? Check status.openephemeris.com first.

If this saved you from an LLM confidently inventing a Saturn position, a ⭐ helps other people find it.

Legal

This package is licensed under the MIT License. However, use of this package to access the Open Ephemeris API constitutes use of the Service and is governed by the Open Ephemeris Terms of Service. By using this package, you agree to those terms. See also the Privacy Policy and Acceptable Use Policy.

Development

The server source is maintained in a private monorepo; this repository holds the public docs, plugin, skills and examples. Bug reports and contributions are welcome — see CONTRIBUTING.md.

Architecture

┌─────────────────────────────────────────────────────────┐│                    MCP Clients                          ││  Codex · Gemini   · Claude Web · ChatGPT · Remote apps  │└──────────────────┬──────────────────────────────────────┘                   │ Streamable HTTP (MCP 2025-11-25)                   │ https://mcp.openephemeris.com/mcp                   │┌─────────────────────────────────────────────────────────┐│          Cursor · Claude Desktop · Windsurf             │└──────────────────┬──────────────────────────────────────┘                   │ stdio JSON-RPC                   │ npx @openephemeris/mcp-server                   │              ┌────▼────────────────────┐              │  openephemeris-mcp      │              │  Node.js MCP Server     │              │  typed tools            │              │  auth: Key > JWT        │              └────────────┬────────────┘                           │ HTTPS                           ▼              ┌────────────────────────┐              │  Open Ephemeris API    │              │  api.openephemeris.com │              │  NASA JPL DE440        │              │  1,100 years of data   │              └────────────────────────┘

Runtime Snapshot (Generated)

Generated by npm run sync:readme from config/dev-allowlist.json and the live tool registry.

• Allowlisted operations: 141
• Methods: GET=45, POST=96, PUT=0, PATCH=0, DELETE=0
• Registered tools (OPENEPHEMERIS_PROFILE=dev): 104
• Typed tools: account_usage, acg_hits, acg_power_lines, auth_login, auth_logout, auth_status, bazi_annual_pillar, bazi_chart, bazi_compatibility, bazi_element_balance, bazi_luck_pillars, bazi_recalculate, bazi_ten_gods, bi_wheel_on_cross_aspect_click, bi_wheel_on_house_click, bi_wheel_on_planet_click, bi_wheel_recalculate, bi_wheel_synopsis, bodygraph_recalculate, chart_wheel_on_aspect_click, chart_wheel_on_house_click, chart_wheel_on_planet_click, chart_wheel_recalculate, chinese_bazi, electional_angle_crossings, electional_aspect_search, electional_moment_analysis, electional_station_tracker, ephemeris_angles_points, ephemeris_aspect_check, ephemeris_bi_wheel, ephemeris_chart_wheel, ephemeris_composite, ephemeris_composite_midpoint, ephemeris_dignities, ephemeris_electional, ephemeris_fixed_stars, ephemeris_hermetic_lots, ephemeris_house_cusps, ephemeris_ingresses, ephemeris_lunar_return, ephemeris_midpoints, ephemeris_moon_phase, ephemeris_natal_batch, ephemeris_natal_chart, ephemeris_natal_transits, ephemeris_next_eclipse, ephemeris_next_lunar_phase, ephemeris_overlay, ephemeris_planet_position, ephemeris_planetary_return, ephemeris_progressed_chart, ephemeris_relocation, ephemeris_retrograde_status, ephemeris_solar_return, ephemeris_synastry, ephemeris_time_lords, ephemeris_time_sensitivity, ephemeris_transits, explore_bazi_chart, explore_bi_wheel, explore_human_design, explore_human_design_connection, explore_human_design_transit, explore_moon_phase, explore_natal_chart, explore_time_lords, explore_transit_timeline, explore_vedic_chart, geodetic_chart, hd_connection_matrix, hd_on_center_click, hd_on_channel_click, hd_on_connection_channel_click, hd_on_gate_click, hd_on_planet_click, hd_on_transit_channel_click, hd_on_variable_click, hd_opposition, hd_planetary_return, human_design_bodygraph, human_design_chart, human_design_composite, human_design_extended, human_design_penta, human_design_transit_timeline, location_search, moon_phase_recalculate, mundane_chart, timezone_resolve, vedic_chart, vedic_chart_recalculate, vedic_matching, venus_eight_year_star, venus_elongations, venus_phase, venus_star_points, venus_star_points_conjunctions, venus_stations, ziwei_chart
• Generic tools: dev_list_allowed, dev_read_api, dev_write_api, find_calculation

Allowlist Families

Why Open Ephemeris for AI Agents?

Most LLMs (like Claude and ChatGPT) struggle heavily with astronomical calculations (trigonometry, Julian date conversions, and planetary lookups). Open Ephemeris serves as a secure, remote math engine.

By pairing LLMs with the Open Ephemeris MCP server, your agents can instantly access:

• Zero-hallucination coordinates: Direct, NASA JPL DE440 calculations covering 1550–2650 CE, within about a tenth of an arcsecond of JPL Horizons for modern dates.
• LLM-optimized tokens (format=llm): We compress standard 25,000 token JSON chart responses into minimal text blocks, cutting your inference costs by 50–73% depending on endpoint.
• Ready-to-use astrology layers: Built-in support for Astrocartography geoJSON lines, Hermetic Lots, Fixed Stars, and complex Human Design matrix generation.