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Feather Flag exposes two separate APIs:
APIBase URLPurpose
Management APIhttps://nest.feather.mupeni.devCRUD for flags, environments, keys, proposals, analytics
Delivery APIhttps://fly.feather.mupeni.devFlag evaluation at the edge (used by SDKs)
This reference covers the Management API. See Evaluation for the Delivery API.

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Versioning

All Management API endpoints are prefixed with /v1.

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Authentication

Programmatic API requests require an Authorization header: 
Authorization: Bearer <api_key>
Dashboard requests may also be authenticated with a session cookie. Session auth is intended for browser-based dashboard usage, not external API clients. See Authentication for full details on API keys and session auth.

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Request format

Request bodies must be JSON with Content-Type: application/json.

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Response format

All responses are JSON. Successful responses return the resource directly (not wrapped in a data key).

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Error shape

{
  "error": {
    "code": "RESOURCE_NOT_FOUND",
    "message": "Flag 'my-flag' not found in project.",
    "details": {}
  }
}

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Common error codes

CodeStatusDescription
UNAUTHORIZED401Missing Authorization header
INVALID_API_KEY401API key is invalid or has been revoked
CSRF_MISSING_ORIGIN403CSRF header absent on session request
CSRF_ORIGIN_MISMATCH403CSRF origin header doesn’t match
UPGRADE_REQUIRED403Feature requires a higher plan
LIMIT_EXCEEDED403Monthly evaluation limit reached
PROTECTED_ENVIRONMENT403Environment requires a change proposal
SELF_APPROVAL_FORBIDDEN403Proposer cannot approve their own proposal
RESOURCE_NOT_FOUND404Resource does not exist or is not in your org
CONFLICT409Duplicate slug/key, or proposal state conflict
PROPOSAL_ALREADY_PENDING409A pending proposal already exists for this flag+env
PROPOSAL_NOT_PENDING409Proposal is not in pending state
IMMUTABLE_FIELD422Attempted to update an immutable field
VALIDATION_ERROR422Request body failed validation

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Pagination

Endpoints that return lists use cursor-based pagination:
  • Pass cursor (an opaque timestamp) to fetch the next page.
  • Pass limit (1–100, default 50) to control page size.
  • The response includes nextCursor (null when no more pages) and hasMore.
{
  "items": [...],
  "nextCursor": 1718000000000,
  "hasMore": true
}

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Partial success (207)

Some write endpoints return 207 Multi-Status when the primary operation succeeded but an edge cache sync failed. The response body includes a syncWarning field describing the failure. 
{
  "id": "flag_abc",
  "syncWarning": "Edge cache sync failed: timeout. Config saved but evaluations may serve stale data."
}
Always handle 207 as a success for the primary resource, but alert on syncWarning — evaluations may serve stale data until the next sync. Use Trigger Sync to force a retry.