Decide API

/decide evaluates every feature flag of a project for one distinct id, and returns the project’s session-recording config. It’s what the SDK calls at init and after identify()/reset(); you can call it directly from anywhere that can’t use the SDK.

POST https://ingest.kilden.io/decide
Content-Type: application/json

Same host as capture — SDKs need a single apiHost for events and flags.

Request

{
  "write_key": "YOUR_PUBLIC_WRITE_KEY",
  "distinct_id": "user_4821",
  "person_properties": { "plan": "enterprise" }
}

Field	Required	Notes
`write_key`	yes	Public keys only. A secret key is rejected with `403` — it must never leave your backend, and `/decide` never needs it
`distinct_id`	yes	The id to evaluate for — the same one your events carry
`person_properties`	no	Overrides the person’s stored traits for this evaluation only. Nothing is ever written; the event pipeline remains the only way to mutate persons

Body limit: 64 KiB. Cohort-based targeting is resolved server-side from precomputed cohort membership — there’s nothing to send.

Response

{
  "flags": {
    "new-checkout": true,
    "pricing-test": "variant-b",
    "sunset-legacy-ui": false
  },
  "sessionRecording": { "enabled": true, "sampleRate": 0.1 }
}

flags — every flag in the project, evaluated for distinct_id. Values: true, false, or a variant key string for multivariate flags. Inactive flags come back as false, so a consumer can distinguish “off” from “unknown flag”.
sessionRecording — the project’s session replay remote config: the kill switch and sample rate the SDK obeys. {"enabled": false, "sampleRate": 0} when recording is off.

Evaluation is deterministic: the same flag key and distinct id always produce the same value, across requests, replicas and deploys. See deterministic bucketing.

Errors

Errors are plain text, not JSON:

Status	Meaning
`400`	`invalid JSON body: …` (including bodies over 64 KiB) or `write_key and distinct_id are required`
`401`	`unknown write_key`
`403`	`secret keys must never leave your backend; call /decide with the project's public write key` — or `origin not allowed` (same allowed-origins check as capture)
`405`	Method other than `POST`/`OPTIONS`

Example

curl -X POST https://ingest.kilden.io/decide \
  -H 'Content-Type: application/json' \
  -d '{"write_key": "pk_your_public_key", "distinct_id": "user_4821"}'

CORS

Identical to capture: Access-Control-Allow-Origin: * on every response; preflight OPTIONS → 204 with Access-Control-Allow-Methods: POST and Access-Control-Max-Age: 86400.

Freshness

Flag config changes are visible within ~10 seconds; person traits and cohort membership are cached for ~30 seconds in the evaluation path; cohort membership itself is materialized about every 10 minutes. If you need a property override to apply immediately, pass it in person_properties.