Create a feature flag
Creates a flag, disabled at a 0% rollout.
A string, number or json flag takes variants whose values match its type; a boolean flag has none.
Guide: Feature flags and A/B tests
A project secret key: vh_sk_... for production, vh_sk_test_... for test. Keep it on your servers.
In: header
Path parameters
The project's id, proj_.... A secret key works only in the project it belongs to.
Request body
application/json
TypeScript definitions
Use the request body type in TypeScript.
The flag's key, unique in the project; SDKs ask for the flag by it.
1 <= lengthWhat the flag controls.
What the flag evaluates to; defaults to boolean. It cannot change afterwards.
The values a string, number or json flag can evaluate to, at least one; traffic splits evenly between them. A boolean flag takes none.
Examples
curl -X POST "https://api.voidhash.com/api/v1/projects/proj_tz4a98xxat96iws9zmbrgj3a/feature-flags" \ -H "Authorization: Bearer vh_sk_..." \ -H "Content-Type: application/json" \ -d '{ "key": "checkout-headline", "description": "The headline of the checkout screen.", "type": "string", "variants": [ { "label": "Plain", "value": "Go Pro" }, { "label": "Benefit", "value": "Unlock every workout" } ] }'{ "id": "ff_d2k9m4x1c6v8b3n5q0w2e7h7", "projectId": "proj_tz4a98xxat96iws9zmbrgj3a", "key": "checkout-headline", "description": "The headline of the checkout screen.", "type": "string", "isEnabled": false, "rolloutPercentage": 0, "isArchived": false, "experimentId": null, "createdAt": "2026-03-12T10:00:00.000Z", "updatedAt": "2026-03-12T10:00:00.000Z"}{ "error": { "type": "invalid_request", "code": "invalid_variants", "message": "Boolean feature flags cannot have variants", "param": "variants" }}{ "error": { "type": "unauthenticated", "code": "invalid_api_key", "message": "This secret key is not valid." }}{ "error": { "type": "permission_denied", "code": "permission_denied", "message": "Insufficient permissions." }}{ "error": { "type": "not_found", "code": "project_not_found", "message": "There is no project proj_tz4a98xxat96iws9zmbrgj3a, or this credential cannot use it.", "param": "projectId" }}{ "error": { "type": "conflict", "code": "feature_flag_key_taken", "message": "Another feature flag in this project, archived or not, already uses the key \"new-onboarding\".", "param": "key" }}{ "error": { "type": "internal", "code": "internal_error", "message": "Something went wrong on our side. Try again, and contact support with the Request-Id header if it keeps failing." }}Response body
application/json
application/json
application/json
application/json
application/json
application/json
application/json
Errors
| Status | type | code | When |
|---|---|---|---|
| 400 | invalid_request | invalid_variants | When they do not fit. |
| 409 | conflict | feature_flag_key_taken | When another flag, archived or not, uses the key. |
It can also fail with the errors every operation shares, such as unauthenticated; see Errors.
More in Feature flags
- List feature flags
GET - Evaluate flags for an end user
POST - Retrieve a feature flag
GET - Update a feature flag
PATCH - Archive a feature flag
POST - Unarchive a feature flag
POST - List a flag's variants
GET - Replace a flag's variants
PUT - List a flag's overrides
GET - Set an override
PUT - Remove an override
DELETE - List an end user's overrides
GET - List a flag's targets
GET - Add a target
PUT - Remove a target
DELETE
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