Context

Context is the set of key-value pairs the client passes with each eval request. Rules match against these keys.

What is context?

Context describes the environment of an evaluation: who is asking, where they are, what version they run, what plan they're on. Every key-value pair you include is available to rules.

There is no fixed schema — you define your own context keys and reference them in rule field definitions. Common keys:

Example key	Example value	Typical use
country	DE	Geo-targeting, GDPR jurisdiction
plan	pro	Feature gating by subscription tier
user_id	usr_01HXYZ	Per-user flag overrides
app_version	2.5.0	Semver-based rollouts
email	alice@acme.com	Internal / beta user targeting

Setting context at runtime

Use setContext / removeContext after the client is initialized — e.g. once the user logs in.

// After the user logs in:
flags.setContext('user_id', user.id);
flags.setContext('plan', user.plan);

// Remove a key from context:
flags.removeContext('country');

Per-call override

Pass a second argument to eval() to add or override context for a single call without mutating the client instance.

// Override or add context for a single call — does not mutate the client:
const result = await flags.eval('beta_features', { user_id: 'usr_01' });

Raw HTTP

When calling the API directly, every key in the JSON body (other than api_key) is treated as context.

# Every key in the JSON body (except api_key) is treated as context:
curl -X POST https://your-ftflg-instance.com/eval/dark_mode \
  -H 'Content-Type: application/json' \
  -d '{
    "api_key": "ff_your_api_key",
    "country": "DE",
    "plan": "pro",
    "app_version": "2.5.0"
  }'

Missing keys

If a rule references country but the eval request does not include country, that rule fails. This means the flag returns enabled: false, value: null. Always send the full context your rules depend on, or handle null gracefully on the client.