Tether MCP Server

Let your own AI agent read your health data — without the cloud ever seeing it.

This is the official local MCP server for Tether — AI Health Sync, the iOS app that syncs Apple Health data (sleep, heart rate, menstrual cycle, weight, water, symptoms) between partners and to your own AI — end-to-end encrypted.

Tether embeds no AI and runs no model on your phone. Intelligence lives where you control it: Claude Code, Claude Desktop, or any MCP-capable agent running on your own machine. This server is the bridge — it holds a private key that never leaves your computer, pulls ciphertext from the cloud, and decrypts only locally.

iPhone (Apple Health) ──E2EE──▶ cloud (ciphertext only) ──E2EE──▶ this server (your machine) ──▶ your AI agent

Requirements

• Tether — AI Health Sync on iOS, with a Pro subscription (the AI-agent interface is the Pro tier)

• Python 3.11+ on the machine where your agent runs (macOS / Linux / Windows)

Related MCP server: bsp-mcp

Quick start

1. Install

With uv (recommended — no clone needed):

uvx tether-mcp status

Or with pip:

pip install 'tether-mcp[qr]'

2. Bind your phone

tether-mcp bind

This generates a keypair on your machine and prints a QR code. In the Tether iOS app, open Settings → Data & AI → MCP Server and scan it (or import a QR screenshot from Photos). The app authorizes this machine and starts sealing your health envelopes to its public key. The private key stays in ~/.tether/mcp-local/ (owner-only 0600 permissions, OS keychain where available) — it is never uploaded anywhere.

3. Connect your agent

Claude Code (one line):

claude mcp add tether-health -- uvx tether-mcp serve --transport stdio

Claude Desktop (claude_desktop_config.json):

{
  "mcpServers": {
    "tether-health": {
      "command": "uvx",
      "args": ["tether-mcp", "serve", "--transport", "stdio"]
    }
  }
}

Any other MCP client: run tether-mcp serve --transport stdio, or serve --transport http for a loopback streamable-HTTP endpoint with bearer-token auth.

Claude Desktop note: it does not inherit your shell PATH. If uvx isn't found, use the absolute path (which uvx) as command.

Debugging: npx @modelcontextprotocol/inspector uvx tether-mcp serve --transport stdio

Then just ask your agent: “How did we sleep last night?”

MCP tools (16)

Every data tool accepts owner (a user-ID prefix) to filter to one person — the server may hold both your and your partner's shared records, and omitting owner mixes them into one pool, so per-person questions should always pass it. (Earlier releases named some tools get_partner_*; they were renamed in the 16-tool release since they return whichever owners' records this server holds, not specifically the partner's.)

Reads are cache-first: decrypted records are cached locally (owner-only files, 600 s TTL, TETHER_MCP_CACHE_TTL to override) so repeat queries answer in ~0.2 s with zero network; pass fresh=true to force a cloud round trip. The same service layer backs a full CLI (tether-mcp sleep / water / weight / … — every data subcommand takes --owner too) if you prefer scripts over MCP.

Privacy & security model

• End-to-end encryption: Curve25519 ECDH + HKDF-SHA256 + AES-GCM. Every health record is sealed on-device to each authorized recipient's public key (your partner, and this server once bound).

• The cloud only ever holds ciphertext. Tether's backend cannot read your health data — architecturally, not just by policy.

• Decryption happens here, on hardware you own. The private key and server token are never exposed through any tool result.

• Sensitive kinds (menstrual cycle, symptoms, notes) reach this server only if explicitly opted in inside the iOS app, and are never re-exported by the server.

• HTTP transport binds to loopback by default and requires a bearer token; binding a non-loopback address fails closed unless explicitly allowed — front it with TLS if you must expose it.

You can audit all of the above in this repository — that is why it is open source.

Development

pip install -e '.[dev,qr]'
pytest

License

MIT. The Tether iOS app and cloud service are separate proprietary components; this repository covers the local MCP server only.

Website: tetherme.app · App Store: Tether — AI Health Sync · Bugs & feedback: tether-community