Prerequisite Flags

Prerequisites let you create flag dependencies — a flag will only evaluate to its configured value if all its prerequisite flags are also enabled and evaluating to true.

Use Case

You're building a multi-step feature rollout:

1. new-api-v2 — The new API must be enabled first
2. new-dashboard — Depends on the new API
3. new-dashboard-analytics — Depends on the new dashboard

If new-api-v2 is disabled, both new-dashboard and new-dashboard-analytics automatically return their default values.

Setting Prerequisites

Via API

curl -X POST http://localhost:8080/v1/projects/$PROJECT_ID/flags \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "key": "new-dashboard",
    "name": "New Dashboard",
    "flag_type": "boolean",
    "default_value": false,
    "prerequisites": ["new-api-v2"]
  }'

Via Dashboard

1. Open the flag detail page
2. In the Prerequisites section, select flags to depend on
3. Save

Evaluation Behavior

During evaluation, each prerequisite is recursively evaluated:

1. For each prerequisite key in the flag's prerequisites array:
   • Evaluate the prerequisite flag with the same context
   • If the result is NOT_FOUND, DISABLED, or boolean false → fail
2. If any prerequisite fails → return PREREQUISITE_FAILED with the flag's default value
3. If all pass → continue normal evaluation

Recursive Dependencies

Prerequisites are evaluated recursively. If flag A depends on B, and B depends on C, then A is only active when both B and C are enabled and evaluating to true.

Circular Dependencies

Avoid circular dependencies (A → B → A). The evaluation engine has no explicit cycle detection and will hit the Go stack limit, returning an error.

Example

Flag: "premium-feature"
  prerequisites: ["billing-enabled", "premium-plan"]

Evaluation for user "alice":
  1. Evaluate "billing-enabled" → true ✓
  2. Evaluate "premium-plan"   → false ✗
  3. Result: PREREQUISITE_FAILED, value: false (default)

API Response

{
  "flag_key": "premium-feature",
  "value": false,
  "reason": "PREREQUISITE_FAILED"
}