feat: add EvaluateFlags() API for single-call flag evaluation

CHANGELOG.md

@@ -1,3 +1,15 @@
+## Unreleased
+
+### New Features
+
+* **`EvaluateFlags`**: New method on `Client` that returns a `FeatureFlagEvaluations` snapshot for a user using a single `/flags` request. The snapshot powers any number of `IsEnabled` / `GetFlag` / `GetFlagPayload` checks, fires deduped `$feature_flag_called` events with full v4 metadata (id, version, reason, request_id), and can be attached to a `Capture` event via the new `Capture.Flags` field to populate `$feature/<key>` and `$active_feature_flags` without another network call.
+* **`Capture.Flags`**: New optional field on `Capture` that accepts a `*FeatureFlagEvaluations` snapshot. Takes precedence over `SendFeatureFlags`, avoids a hidden `/flags` request per event, and lets caller-supplied `Properties` override the auto-generated `$feature/<key>` values on conflict.
+
+### Internal
+
+* Refactored the `$feature_flag_called` dedup logic into a shared helper so the existing single-flag path and the new snapshot path use identical semantics against the same per-distinct_id LRU cache.
+* `$feature_flag_called` events from the snapshot path combine response-level errors (`errors_while_computing_flags`, `quota_limited`) with per-flag errors (`flag_missing`) comma-joined in `$feature_flag_error`, matching the granularity of the legacy single-flag path.
+
 ## 1.12.4 - 2026-04-30
 
 * [Full Changelog](https://github.com/PostHog/posthog-go/compare/v1.12.3...v1.12.4)

capture.go

@@ -83,6 +83,13 @@ type Capture struct {
 	Properties       Properties
 	Groups           Groups
 	SendFeatureFlags SendFeatureFlagsValue
+	// Flags, when set, attaches $feature/<key> and $active_feature_flags
+	// properties from a snapshot returned by Client.EvaluateFlags. It is
+	// preferred over SendFeatureFlags: the snapshot guarantees the event
+	// carries the exact values the application branched on and avoids a
+	// hidden /flags request on every capture. Flags takes precedence when
+	// both are set.
+	Flags *FeatureFlagEvaluations
 }
 
 func (msg Capture) internal() {

feature_flag_evaluations.go

@@ -0,0 +1,325 @@
+package posthog
+
+import (
+	"sort"
+	"strings"
+	"sync"
+)
+
+// FeatureFlagEvaluations is a snapshot of feature-flag evaluations for a
+// single distinct_id, returned by Client.EvaluateFlags. It is the recommended
+// way to evaluate many flags for a single user with a single network request:
+// the snapshot can power any number of IsEnabled / GetFlag checks and can be
+// attached to a Capture event via Capture.Flags so the event carries the
+// exact values the application branched on, with no additional /flags call.
+//
+// Calls to IsEnabled and GetFlag fire $feature_flag_called (deduped per
+// (distinct_id, key, device_id)). GetFlagPayload does not fire an event.
+type FeatureFlagEvaluations struct {
+	host                    featureFlagEvaluationsHost
+	distinctId              string
+	deviceId                *string
+	groups                  Groups
+	flags                   map[string]evaluatedFlagRecord
+	requestId               string
+	evaluatedAt             *int64
+	flagDefinitionsLoadedAt *int64
+	// errorsWhileComputing and quotaLimited capture response-level error
+	// signals from the /flags request. They are combined with per-flag
+	// errors in the $feature_flag_called event so consumers see the same
+	// granularity the legacy single-flag path emits.
+	errorsWhileComputing bool
+	quotaLimited         bool
+
+	mu       sync.Mutex
+	accessed map[string]struct{}
+}
+
+// evaluatedFlagRecord is the per-flag entry stored in a snapshot. All fields
+// are optional except Key and Enabled, mirroring the v4 /flags response shape
+// with an additional LocallyEvaluated marker for poller-resolved flags.
+type evaluatedFlagRecord struct {
+	Key              string
+	Enabled          bool
+	Variant          *string
+	Payload          *string
+	ID               *int
+	Version          *int
+	Reason           *string
+	LocallyEvaluated bool
+	Error            *string
+}
+
+// featureFlagEvaluationsHost is the small callback surface a snapshot uses to
+// talk back to the client. It is intentionally narrow so tests can construct
+// snapshots without spinning up a full client. Warnings are routed through
+// the SDK's Logger; users who want them silenced should pass a Logger that
+// drops Warnf calls.
+type featureFlagEvaluationsHost struct {
+	captureFlagCalledIfNeeded func(distinctId, key string, deviceId *string, properties Properties, groups Groups)
+	logger                    Logger
+}
+
+// IsEnabled reports whether the flag is enabled for this snapshot's user.
+// Unknown flags return false. The first call for a given key fires
+// $feature_flag_called with full evaluation metadata; subsequent calls with
+// the same response are deduped against the client's per-distinct_id cache.
+func (e *FeatureFlagEvaluations) IsEnabled(key string) bool {
+	if e == nil {
+		return false
+	}
+	flag, ok := e.flags[key]
+	e.recordAccess(key)
+	if !ok {
+		return false
+	}
+	return flag.Enabled
+}
+
+// GetFlag returns the value of the flag: a string for multivariate flags,
+// true or false for boolean flags, or nil for unknown flags. The first call
+// for a given key fires $feature_flag_called with full evaluation metadata;
+// subsequent calls with the same response are deduped.
+func (e *FeatureFlagEvaluations) GetFlag(key string) interface{} {
+	if e == nil {
+		return nil
+	}
+	flag, ok := e.flags[key]
+	e.recordAccess(key)
+	if !ok {
+		return nil
+	}
+	if !flag.Enabled {
+		return false
+	}
+	if flag.Variant != nil {
+		return *flag.Variant
+	}
+	return true
+}
+
+// GetFlagPayload returns the raw JSON payload string for the flag, or "" if
+// no payload is configured or the flag is unknown. Unlike IsEnabled and
+// GetFlag, it does not record an access and does not fire an event.
+func (e *FeatureFlagEvaluations) GetFlagPayload(key string) string {
+	if e == nil {
+		return ""
+	}
+	flag, ok := e.flags[key]
+	if !ok || flag.Payload == nil {
+		return ""
+	}
+	return *flag.Payload
+}
+
+// Keys returns the sorted list of evaluated flag keys.
+func (e *FeatureFlagEvaluations) Keys() []string {
+	if e == nil {
+		return nil
+	}
+	keys := make([]string, 0, len(e.flags))
+	for k := range e.flags {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	return keys
+}
+
+// OnlyAccessed returns a filtered copy of this snapshot containing only the
+// flags that were accessed via IsEnabled or GetFlag before this call.
+//
+// The method honors its name: if nothing has been accessed yet, the returned
+// snapshot is empty. Pre-access the flags you want to attach before calling.
+//
+// The returned snapshot tracks access independently from the receiver;
+// further accesses on the child do not influence what the parent sees.
+func (e *FeatureFlagEvaluations) OnlyAccessed() *FeatureFlagEvaluations {
+	if e == nil {
+		return nil
+	}
+	e.mu.Lock()
+	filtered := make(map[string]evaluatedFlagRecord, len(e.accessed))
+	for k := range e.accessed {
+		if flag, ok := e.flags[k]; ok {
+			filtered[k] = flag
+		}
+	}
+	e.mu.Unlock()
+	return e.cloneWith(filtered)
+}
+
+// Only returns a filtered copy of this snapshot keeping only the named flags
+// that were actually evaluated. Unknown keys are dropped with a warning.
+//
+// The returned snapshot tracks access independently from the receiver.
+func (e *FeatureFlagEvaluations) Only(keys []string) *FeatureFlagEvaluations {
+	if e == nil {
+		return nil
+	}
+	filtered := make(map[string]evaluatedFlagRecord, len(keys))
+	var missing []string
+	for _, k := range keys {
+		if flag, ok := e.flags[k]; ok {
+			filtered[k] = flag
+		} else {
+			missing = append(missing, k)
+		}
+	}
+	if len(missing) > 0 && e.host.logger != nil {
+		e.host.logger.Warnf("FeatureFlagEvaluations.Only() was called with flag keys that are not in the evaluation set and will be dropped: %s", strings.Join(missing, ", "))
+	}
+	return e.cloneWith(filtered)
+}
+
+// eventProperties returns the $feature/<key> and $active_feature_flags
+// properties for the snapshot, suitable for merging into a Capture event.
+func (e *FeatureFlagEvaluations) eventProperties() Properties {
+	props := NewProperties()
+	if e == nil || len(e.flags) == 0 {
+		return props
+	}
+	active := make([]string, 0, len(e.flags))
+	for key, flag := range e.flags {
+		var value interface{}
+		switch {
+		case !flag.Enabled:
+			value = false
+		case flag.Variant != nil:
+			value = *flag.Variant
+		default:
+			value = true
+		}
+		props.Set("$feature/"+key, value)
+		if flag.Enabled {
+			active = append(active, key)
+		}
+	}
+	sort.Strings(active)
+	if len(active) > 0 {
+		props.Set("$active_feature_flags", active)
+	}
+	return props
+}
+
+// recordAccess marks key as accessed, builds the $feature_flag_called event
+// properties, and forwards them to the client's dedup helper. Access from a
+// snapshot bound to an empty distinct_id is silently dropped — those events
+// would be useless and would pollute analytics with empty distinct_ids.
+func (e *FeatureFlagEvaluations) recordAccess(key string) {
+	e.mu.Lock()
+	if e.accessed == nil {
+		e.accessed = make(map[string]struct{})
+	}
+	_, alreadyAccessed := e.accessed[key]
+	e.accessed[key] = struct{}{}
+	e.mu.Unlock()
+
+	if e.distinctId == "" {
+		return
+	}
+	if e.host.captureFlagCalledIfNeeded == nil {
+		return
+	}
+
+	flag, found := e.flags[key]
+
+	var response interface{}
+	switch {
+	case !found:
+		response = nil
+	case !flag.Enabled:
+		response = false
+	case flag.Variant != nil:
+		response = *flag.Variant
+	default:
+		response = true
+	}
+
+	properties := NewProperties().
+		Set("$feature_flag", key).
+		Set("$feature_flag_response", response).
+		Set("$feature/"+key, response).
+		Set("locally_evaluated", found && flag.LocallyEvaluated)
+
+	if e.deviceId != nil {
+		properties.Set("$device_id", *e.deviceId)
+	}
+	if e.requestId != "" {
+		properties.Set("$feature_flag_request_id", e.requestId)
+	}
+	if found {
+		if flag.Payload != nil {
+			properties.Set("$feature_flag_payload", *flag.Payload)
+		}
+		if flag.ID != nil {
+			properties.Set("$feature_flag_id", *flag.ID)
+		}
+		if flag.Version != nil {
+			properties.Set("$feature_flag_version", *flag.Version)
+		}
+		if flag.Reason != nil {
+			properties.Set("$feature_flag_reason", *flag.Reason)
+		}
+		if flag.LocallyEvaluated {
+			if e.flagDefinitionsLoadedAt != nil {
+				properties.Set("$feature_flag_definitions_loaded_at", *e.flagDefinitionsLoadedAt)
+			}
+		} else if e.evaluatedAt != nil {
+			properties.Set("$feature_flag_evaluated_at", *e.evaluatedAt)
+		}
+	}
+
+	// Build the comma-joined $feature_flag_error matching the single-flag
+	// path's granularity: response-level errors (errors-while-computing,
+	// quota-limited) are combined with per-flag errors so consumers can
+	// filter by type.
+	var errs []string
+	if e.errorsWhileComputing {
+		errs = append(errs, FeatureFlagErrorErrorsWhileComputing)
+	}
+	if e.quotaLimited {
+		errs = append(errs, FeatureFlagErrorQuotaLimited)
+	}
+	if !found {
+		errs = append(errs, FeatureFlagErrorFlagMissing)
+	} else if flag.Error != nil {
+		errs = append(errs, *flag.Error)
+	}
+	if len(errs) > 0 {
+		properties.Set("$feature_flag_error", strings.Join(errs, ","))
+	}
+
+	// Dedup is owned by the client cache; alreadyAccessed is a per-snapshot
+	// hint that lets us skip the client call when we know we have already
+	// fired for this key from this snapshot.
+	if alreadyAccessed {
+		return
+	}
+	e.host.captureFlagCalledIfNeeded(e.distinctId, key, e.deviceId, properties, e.groups)
+}
+
+// cloneWith builds a child snapshot with the given flag set. The accessed set
+// is copied so further access on the child does not leak back into the
+// parent's accessed set.
+func (e *FeatureFlagEvaluations) cloneWith(flags map[string]evaluatedFlagRecord) *FeatureFlagEvaluations {
+	e.mu.Lock()
+	accessedCopy := make(map[string]struct{}, len(e.accessed))
+	for k := range e.accessed {
+		accessedCopy[k] = struct{}{}
+	}
+	e.mu.Unlock()
+
+	return &FeatureFlagEvaluations{
+		host:                    e.host,
+		distinctId:              e.distinctId,
+		deviceId:                e.deviceId,
+		groups:                  e.groups,
+		flags:                   flags,
+		requestId:               e.requestId,
+		evaluatedAt:             e.evaluatedAt,
+		flagDefinitionsLoadedAt: e.flagDefinitionsLoadedAt,
+		errorsWhileComputing:    e.errorsWhileComputing,
+		quotaLimited:            e.quotaLimited,
+		accessed:                accessedCopy,
+	}
+}