HealthLog/docs/integrations/withings.md

Withings integration

HealthLog connects to Withings via OAuth2 plus a push webhook for near-real-time sync. Once linked, every Body+ scale weighing, BPM blood-pressure reading, ScanWatch sleep stage, and activity sample lands in HealthLog within seconds — no polling, no third-party relay.

What gets synced

The Withings client (src/lib/withings/client.ts) maps the following metric types into Measurement rows tagged source = WITHINGS:

Withings meastype	HealthLog MeasurementType
1	WEIGHT
5	FAT_FREE_MASS
9	BLOOD_PRESSURE_DIA
10	BLOOD_PRESSURE_SYS
11	PULSE
54, 35	OXYGEN_SATURATION (ScanWatch / pulse-ox products)
Plus the body-composition family (fat ratio, muscle mass, bone mass, hydration) and sleep / heart-rate variability via the heart-list endpoint

Activity data (steps, active energy, distance, floors climbed) lands via the user.activity OAuth scope shipped in v1.4.25. Sleep stages arrive via the sleep series endpoint.

1. Register a Withings developer app

1. Sign in at https://developer.withings.com/ and create a new public-API application. The "Public" tier is free and supports the OAuth2 flow HealthLog uses; you do not need the partner tier.
2. Callback URL: https://your-instance.example.com/api/withings/callback. Withings allows multiple callbacks per app — add one entry per environment (production, staging, local development against a tunnel).
3. Scopes: request user.metrics AND user.activity. Both are needed — user.metrics covers weight / BP / heart-list / sleep / SpO₂ / body comp / temperature / VO₂ max; user.activity is the workout-equivalent stream for steps / energy / distance / floors. See WITHINGS_OAUTH_SCOPE in src/lib/withings/client.ts.
4. Save the application. Withings will issue a Client ID and a Client Secret — keep the secret out of any chat log; it grants read access to every linked account's full health history.

2. Wire the credentials in Settings → Integrations

Withings credentials are stored per user, not in .env. Sign in as the target user, open Settings → Integrations → Withings, and paste the client ID and secret. They are encrypted AES-256-GCM at rest (src/lib/withings/credentials.ts). On a shared instance every user brings their own Withings app credentials; there is no server-level client-id/secret fallback.

Two server-level env vars remain:

WITHINGS_WEBHOOK_SECRET="$(openssl rand -hex 32)"
WITHINGS_REDIRECT_URI="https://your-instance.example.com/api/withings/callback"

WITHINGS_WEBHOOK_SECRET is the path-segment token the webhook handler validates against. Generate a fresh random value — anything that fits in a URL path segment works, but a 64-character hex string keeps it indistinguishable from random. Restart the app container after saving.

WITHINGS_REDIRECT_URI is optional — when unset the callback URL defaults to ${NEXT_PUBLIC_APP_URL}/api/withings/callback. Set it only when the OAuth callback must differ from that default.

3. Set the webhook URL

When HealthLog calls Withings Notify.subscribe() for a newly-linked account, it hands Withings the callback URL constructed by getWithingsWebhookCallbackUrl() (src/lib/withings/sync.ts). The shape is:

https://your-instance.example.com/api/withings/webhook/<WITHINGS_WEBHOOK_SECRET>

Withings has no facility for adding HTTP headers or signing webhook bodies, so the callback URL itself is the only authenticity surface a subscriber controls. The v1.4.25 W17a migration moved the secret from a query parameter (?secret=…, captured by every reverse-proxy access log) to a path segment, which keeps it out of the query_string column most proxies log by default (src/app/api/withings/webhook/[token]/route.ts:13-23).

You do not need to register the webhook URL anywhere in the developer portal — HealthLog subscribes per-user during the OAuth callback. The portal only owns the OAuth callback URL.

4. Link from the Settings page

With the credentials saved in Settings, open /settings/integrations/withings and click Connect Withings. The flow:

1. The Connect button redirects to Withings' oauth2/authorize.
2. The user signs in on Withings and grants the requested scopes.
3. Withings redirects back to /api/withings/callback with an auth code.
4. HealthLog exchanges the code for an access + refresh token, stores them AES-256-GCM-encrypted at rest, subscribes the webhook URL, and triggers an initial sync of the past 30 days.
5. The settings page now shows Connected with the timestamp of the last successful sync.

Subsequent syncs run automatically on every webhook delivery plus an hourly safety-net cron pull (measurements at :00, sleep at :15, ECG at :41).

Source-priority interaction

HealthLog's source ladder ranks WITHINGS ≻ APPLE_HEALTH ≻ MANUAL for point measurements (weight, BP, pulse, body fat, body temperature, SpO₂, VO₂ max) and APPLE_HEALTH ≻ WITHINGS ≻ MANUAL for cumulative metrics (steps, active energy, distance, flights, sleep, HRV, resting HR). The defaults live in DEFAULT_SOURCE_PRIORITY in src/lib/validations/source-priority.ts.

Concrete consequences if you run Withings alongside an Apple Health import:

• Weight readings from a Withings scale win for display. Apple Health rows for the same metric (received second-hand from Health Mate) stay in the DB as audit trail but drop out of the active display.
• Steps from a ScanWatch go through HealthKit's daily aggregation when Apple Health is also linked, so the Apple Health stream wins for the cumulative axis. The Withings rows stay in the DB.

Override per-user via Settings → Sources (/settings/sources).

Troubleshooting

The Connect button errors out

• Withings credentials not configured — the signed-in user has not saved a client ID + secret under Settings → Integrations → Withings. Credentials live per user in the database, not in .env — paste the pair from the developer portal and retry.
• Callback URI mismatch — the URL HealthLog sends to Withings must match an entry in the developer-portal app's callback list. Add the public URL exactly, including the https:// scheme and the /api/withings/callback path.

The webhook stops firing

Webhook subscriptions can lapse after a token revoke or a portal- side cleanup. Re-subscribe by opening /settings/integrations/withings and clicking Disconnect then Connect Withings again — the callback exchange re-runs Notify.subscribe() with a fresh callback URL.

Test the inbound path with a HEAD request from outside the host:

curl -I https://your-instance.example.com/api/withings/webhook/<WITHINGS_WEBHOOK_SECRET>

A 200 OK means the path-segment secret matches. A 401 means the env var on the running container does not match the secret embedded in the URL Withings was handed at subscribe time. Re-subscribing fixes the mismatch by updating the URL Withings holds.

Sync is stuck on "Reauthorisation required"

Withings revokes refresh tokens on password change, app un-authorise, or after extended inactivity. The WithingsConnection row flips to error_reauth status and the scheduled sync no-ops until the user re-runs the OAuth flow (src/lib/withings/sync.ts). Click Connect Withings again from the settings page; the callback exchanges fresh tokens and the next sync runs through the last 30 days of history to catch up.

Disconnect cleanly

The Settings page Disconnect button:

1. Calls POST /api/withings/disconnect.
2. Server-side: unsubscribes the webhook categories (best-effort Withings Notify revoke per category), deletes the WithingsConnection row with its stored encrypted tokens, and audits the action. The Withings-side grant itself persists until the user un-authorises HealthLog in their Withings account.
3. Existing Measurement rows tagged source = WITHINGS stay in the database — disconnect does not delete data. Re-linking the same Withings account merges back onto the same metric timeline; the upsert path matches on (userId, type, measuredAt, source) so previously-synced rows update in place.

Connection test

POST /api/integrations/withings/test runs a read-only round-trip against the calling user's own stored credentials and returns the latency in milliseconds (rate-limited to 5/min per user). The Withings settings card surfaces it as the connection test. Use it when data looks stale — a green test plus a stale lastSyncedAt usually means the webhook URL Withings holds is out of sync (re-subscribe per the steps above).