Vaultbeat vs MetricBridge: Write-Back MCP and Encrypted Cloud Sync vs a Zero-Dependency Local Server
Vaultbeat names us directly, so we are answering. Two products, same question, opposite answers: do you want an agent that can write to your health data through an encrypted cloud relay, or one that can only read a snapshot that never left your phone.
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Quick answer. Vaultbeat syncs Apple Health to an end-to-end encrypted cloud relay, then hands your agent 26 MCP tools, 10 of which can write back to your data. MetricBridge exports a local JSON snapshot and a zero-dependency server reads it with 16 read-only tools, making zero network calls, on an on-device AI that never sends your health data anywhere. Both are legitimately private. They differ on where your data lives at rest, whether your agent can write, and what the agent needs to reach it.
What Vaultbeat ships, credit where it is due
Vaultbeat (formerly Tether) is well documented and the first credible push on the write axis in this category. The facts from its own pages:
- 26 MCP tools, 16 read and 10 write. Writes cover food, strength, weight, notes and symptoms, and weight can write back to Apple Health if you opt in. Ships on PyPI as
vaultbeat-apple-health. - End-to-end encrypted background sync. Curve25519 ECDH key agreement, HKDF-SHA256 derivation, AES-GCM for the payload. The relay stores ciphertext only, and the keys live on your devices.
- An account, and a subscription after the trial. Free download, 3 day trial, then $9.99 a month, $29.99 a year, or $74.99 lifetime for agent access.
That write toolset is genuinely useful if you want an agent to log meals or lifts directly into your data. It is also the fork in the road.
The part the encryption claim does not say
"End-to-end encrypted" is true for the health payload, and it is also doing quiet work at the front of the sentence. Three caveats follow, straight from Vaultbeat's own privacy page and App Store listing:
- Ciphertext still leaves the phone. "Data never leaves your device" is literally false for any cloud-relay design. What leaves is encrypted, which is better than plaintext, but a copy of your record sits on a relay you do not run.
- The server must dial out. To read your data, the server on your computer fetches ciphertext from the relay and decrypts it locally. That is a network dependency a truly local server does not have.
- The identity around the data is not anonymous. The App Store privacy label lists Health & Fitness, email, user ID, purchases and usage data as Data Linked to You, including analytics on purchases and usage. The records are protected from the operator; the account is not.
None of this makes Vaultbeat unsafe. It makes it a different privacy story than the headline implies: protected records attached to an identity, synced through a server.
What MetricBridge ships instead
Our answer is the opposite shape. The iOS app writes a small JSON snapshot to a destination you choose (local folder, or iCloud Drive if you want it on your Mac), and the open-source health-export-mcp server reads that folder:
{
"mcpServers": {
"health-export": {
"command": "node",
"args": ["/Users/you/.health-export-mcp/server.mjs"],
"env": {
"HEALTH_DATA_DIR": "/Users/you/Metrics/HealthExport"
}
}
}
}- Zero network calls. The server launches over stdio, no port, no socket, no remote fetch. It reads a folder on your disk and answers from it.
- 16 read-only tools, no write path.
get_health_metrics,get_trends,compare_periods,get_freshnessand the rest can only look. No path adds, edits or deletes a record, so no prompt can corrupt your history. - No account, no sign-in, and the answers stay on the phone. The app also answers questions about your data on device with a provenance card showing the number, the date range and the samples behind it. Your health data is never sent to a server we operate.
One purchase covers the whole thing: $24.99 one-time, no subscription, nothing metered. Export, MCP setup, on-device Ask and Pro Chat, and the Weekly Brief all included.
The write-back fork, stated plainly
This is the real decision, and it is about trust, not features. A write tool means a large language model, which already hallucinates confidently about the weather, can append a number to your symptom log and, if you opt in, your Apple Health weight history. Vaultbeat ships that proudly. We ship read-only on purpose: the person careful enough to want their health data offline is usually the same person who does not want a prompt to have write access to it. If you want the agent to log your food, Vaultbeat fits today. If you want an agent that can only observe, read-only is the point.
Side by side
| Question | Vaultbeat | MetricBridge |
|---|---|---|
| Where does your data live at rest? | Encrypted in a cloud relay, decrypted on your machine | A local JSON snapshot you place |
| Does a copy leave the phone? | Yes, as ciphertext | No |
| MCP tools | 26 (16 read, 10 write) | 16 (all read-only) |
| Can the agent write? | Yes, food, strength, weight, notes, symptoms | No, by design |
| Server network calls | Must dial the relay to fetch and decrypt | Zero |
| Account required | Yes, sign-in required | No account |
| Pricing | Free download, then $9.99/mo, $29.99/yr or $74.99 lifetime | One $24.99 one-time unlock, everything included |
The one honest trade we admit
Vaultbeat has a real advantage: because the relay is in the cloud, its server can fetch your data from any machine with internet, including a VPS. Our zero-network server has to sit where the snapshot lands, so it is a Mac or a machine that can reach the synced folder. Run agents on a headless cloud server and that is a point for Vaultbeat. Run them on the machine you already trust and it is a wash, and the network call becomes pure downside.
Vaultbeat describes us as the "Mac-only setup with no developer server" option. We would put it differently: we are the local-first option that never asks your health data to cross a wire at all. Both descriptions are true. Pick your machine, pick your threat model.
What we are not claiming
Neither product diagnoses, detects, treats or monitors any condition. Where a metric moves away from your recent range, the language is deviation from your personal baseline, never abnormal. This is wellness and data plumbing, not medical advice, and no comparison table changes that.
A read-only server, and answers that stay on your phone
Ask your Apple Health data anything on the iPhone and see the numbers behind every answer, then export 190 metrics to a zero-dependency, read-only MCP server that makes zero network calls. One purchase covers it all: $24.99 one-time, no subscription.
Frequently asked questions
Does Vaultbeat's encrypted cloud sync actually keep my health data private?
The health records themselves are encrypted with Curve25519 ECDH, HKDF-SHA256 and AES-GCM, and the relay stores ciphertext only, so the sync provider cannot read them. But the app still needs an account and sign-in, and its App Store privacy label lists Health & Fitness data, email and user ID as Data Linked to You. The health data is protected; the account identity around it is not anonymous.
What does end-to-end encrypted background sync encrypt, and what does it not?
It encrypts the health records in transit and at rest in the relay. It does not remove the fact that a copy of your data leaves the phone for a cloud relay, that the MCP server on your computer must dial out to the internet to fetch and decrypt it, or that sign-in, email, user ID, purchases and usage analytics are linked to you. Ciphertext still leaves the device even when plaintext does not.
Can my AI agent write to Apple Health with Vaultbeat?
Yes. Vaultbeat's MCP server exposes 26 tools, 16 read and 10 write. The write tools cover food, strength, weight, notes and symptoms. Weight can also be written back to Apple Health if you opt in. Whether you want an agent that can modify your health record is a trust decision, not a feature checkbox.
Why is MetricBridge's MCP server read-only?
By design. The 16 tools only read a local snapshot, there is no write path, so no prompt can ever add, change or delete a health record. If you want the agent to log food or weight, this is the wrong product for that. If you want an agent that can only look and never touch your history, that is exactly what a read-only server means.
Do Vaultbeat and MetricBridge both require an account?
Vaultbeat requires an account and sign-in, because the relay is keyed to your identity. MetricBridge has no account and no sign-in at any stage: the app writes a JSON snapshot to a folder you choose, and the open-source server reads that folder with zero network calls.
What does each one cost?
Vaultbeat is a free download with a 3 day trial, then $9.99 a month, $29.99 a year, or a $74.99 lifetime unlock for agent access. MetricBridge is one $24.99 one-time unlock covering the whole product: export, MCP setup, on-device Ask and Pro Chat, and the Weekly Brief. The open-source server is free and MIT licensed.