@getbirthchart/mcp

Official MCP server `0.2.0` for GetBirthChart astrology calculations. It gives

MCP-compatible AI clients access to structured calculations through the public

GetBirthChart API; it does not contain or reimplement the astrology engine.

Requirements

• Node.js 20 or newer
• A GetBirthChart developer API key

Create a key at getbirthchart.com/developers. Keep it private and do not commit MCP host configuration containing the real key.

Quick start

The package runs over MCP stdio and can be launched with `npx`:

{
  "mcpServers": {
    "getbirthchart": {
      "command": "npx",
      "args": ["-y", "@getbirthchart/mcp@0.2.0"],
      "env": {
        "GETBIRTHCHART_API_KEY": "gbc_live_your_key_here"
      }
    }
  }
}

This is the standard command-based configuration for hosts that support MCP stdio servers. Use your client’s current documentation for the exact configuration file or UI location; this repository has been protocol-tested with the official MCP TypeScript client, not vendor-specific clients.

Environment variables

The key is read at startup, never accepted as a tool argument, and never written to stdout, logs, resources, or tool results.

Available tools

All tools are read-only and return structured calculation facts. Inputs use strict fields: `date`, optional `time` and `place`, required `latitude`, `longitude`, and `timezone`, plus optional `unknown_time` and calculation settings documented below.

The current public API does not geocode `place`; provide latitude, longitude,

and an IANA timezone even when a place label is included. The MCP server never

invents a birth time. The backend may use local midnight as the labeled

calculation anchor for an unknown-time assessment, but it never presents that

anchor as the person's birth time and never guesses houses, the Ascendant, or

an ambiguous Moon sign.

Example input:

{
  "date": "1990-01-15",
  "time": "12:00",
  "place": "New York, NY",
  "latitude": 40.7128,
  "longitude": -74.006,
  "timezone": "America/New_York"
}

Unknown time:

{
  "date": "1990-01-15",
  "unknown_time": true,
  "latitude": 40.7128,
  "longitude": -74.006,
  "timezone": "America/New_York"
}

Calculation options

The natal tools accept the exposed core `gbc-astro 1.13.0` options `house_system`,

`node_type`, `aspect_preset`, `custom_aspect_rules`, `additional_points`,

`fold`, `zodiac`, and `ayanamsa`. Omit them for the legacy-compatible defaults:

Tropical, Placidus, True Node, Standard aspects, Chiron on, and Lilith off.

Sidereal calculations require a named `ayanamsa`; Lahiri is the recommended

product choice. Custom aspects require `custom_aspect_rules` and each rule uses

`type`, `exact_angle`, and `orb`.

The implementation exposes the core's product-relevant house systems

(`placidus`, `whole_sign`, `equal`) plus the registered engine systems, Mean

Node, Standard/Extended/Custom natal aspects, Mean/True Lilith, Vertex and Part

of Fortune requests, and named sidereal ayanamsas. The MCP contract does not

add calculation logic or interpretation. Composite and Davison are core/SDK

operations but are not MCP tools in this release.

Synastry accepts a relationship-level `node_type` so both charts use one node

convention, plus optional `relationship_type`, `topic`, and `target_instant`.

The server preserves response metadata and additive schema 1.x fields. A natal

HTTP response may omit `calculationHash`; that is valid and is never required.

The server preserves a returned `v2:` calculation hash without changing or

truncating it. Core compatibility is natal schema `1.9.0` and synastry schema

`1.5.0`.

Resources

• `getbirthchart://methodology` — calculation conventions and unknown-time boundaries.
• `getbirthchart://data-sources` — ephemeris, timezone, and location-input provenance.
• `getbirthchart://engine-info` — provider and public API metadata.

Authoritative web references: Methodology and Data Sources.

Errors

Tool failures use structured `error` data with a safe machine-readable `code`, message, `retryable`, and optional `retry_after`. Common codes include `validation_error`, `authentication_required`, `birth_time_required`, `location_not_found`, `ambiguous_location`, `rate_limit_exceeded`, `timeout`, and `internal_error`.

Troubleshooting

• Missing API key: set `GETBIRTHCHART_API_KEY` in the MCP host's server

environment; it is never a tool argument.

• Authentication or rate-limit errors: use the structured error `code` and

`retry_after` when present. Do not put the key in a tool call or client log.

• Unknown-time validation: omit `time` when `unknown_time` is `true`; the

server will not substitute noon or invent a Rising sign.

• Location validation: include numeric WGS84 `latitude`, `longitude`, and

an IANA `timezone`; `place` is only a label and is not geocoded here.

• Local development: use `GETBIRTHCHART_API_BASE_URL` only with HTTPS, or

HTTP on localhost. Never send a production key to an untrusted host.

Privacy and security

Birth data is passed only to the configured public API for the requested calculation. This package does not persist, cache, or log birth inputs, and it has no analytics or telemetry. The optional base URL override changes the trust boundary; do not send a production key to an untrusted host.

Report security issues privately through the process in SECURITY.md. Do not include API keys in bug reports.

Development

npm install
npm run lint
npm run typecheck
npm test
npm run build

Tests use mocked clients and do not call the production API. To run the built server locally, set `GETBIRTHCHART_API_KEY` and execute `node dist/cli.js`; normal protocol traffic stays on stdout, while startup failures are written to stderr.

Links

• Developer hub
• GitHub repository
• Core 1.13.0 GitHub release
• Core 1.13.0 on PyPI
• Core OpenAPI source
• Concept DOI · Version DOI
• npm package

MCP Registry metadata

Registry metadata is prepared in `server.json` with server name `io.github.getbirthchart-com/getbirthchart-mcp`. Publish the npm package first, then authenticate with the official `mcp-publisher` tool and publish the metadata. Registry submission is intentionally not part of the package build or CI workflow.