Summary

{:ok, snapshot} = PostHog.FeatureFlags.evaluate_flags("user-123")

if PostHog.FeatureFlags.Evaluations.enabled?(snapshot, "new-dashboard") do
  render_new_dashboard()
end

PostHog.FeatureFlags.set_in_context(snapshot)
PostHog.capture("page_viewed", %{distinct_id: "user-123"})

One /flags call, multiple flag checks, plus event enrichment from the same snapshot — replacing N check/3 round-trips.

• New PostHog.FeatureFlags.evaluate_flags/2 returns an Evaluations snapshot. Pass flag_keys: [...] to scope the underlying request body (forwarded as flag_keys_to_evaluate).
• Snapshot exposes enabled?/2, get_flag/2, get_flag_payload/2, only/2, keys/1, event_properties/1. enabled?/2 and get_flag/2 fire $feature_flag_called with full metadata; get_flag_payload/2 doesn't.
• set_in_context/2 copies the snapshot's $feature/<key> and $active_feature_flags into the per-process context so subsequent PostHog.capture/3 calls pick them up — no new capture arity needed.
• %PostHog.FeatureFlags.Result{} now carries :id, :version, :reason, :request_id, :evaluated_at, and :errors_while_computing. Both paths attach the matching $feature_flag_* properties (including $feature_flag_error: "errors_while_computing_flags") when the response provides them. Snapshot path also fires $feature_flag_error: "flag_missing" for unknown keys, combined comma-style with the response-level error when both apply.
• check/3, check!/3, get_feature_flag_result/4, and get_feature_flag_result!/4 are now @deprecated (compile-time warnings + admonitions in @doc). They continue to return the same values; removal is planned for the next major.

RFC: https://github.com/PostHog/requests-for-comments-internal/pull/1020. Reference implementations: posthog-python#539, posthog-js#3476.

Design decisions

• No capture(flags: snapshot) argument. Elixir's SDK already propagates $feature/* through the per-process context that check/3 writes to and capture/3 reads back. set_in_context/2 slots into that mechanism instead of fighting it.
• Snapshot is pure-functional, not Agent-backed. Mutable access tracking would require an Agent or threading a new snapshot through every query — both awkward in Elixir. only/2 (filter by explicit keys) is enough; only_accessed/1 from the cross-SDK API is dropped.
• Predicate is enabled?/2, not is_enabled/2. Idiomatic Elixir; is_* is reserved for guard-safe predicates and Credo flags it.
• Both paths share build_result/3 and log_feature_flag_usage/3. Single source of truth for $feature_flag_called properties — including the rich metadata and the new $feature_flag_error codes.
• Snapshot fires flag_missing events; existing single-flag path keeps its {:error, ...} / {:ok, nil} contract for missing flags. Mirrors posthog-python's per-call telemetry from a snapshot while not changing existing check/3 behavior.
• No dedup cache. The Elixir SDK has no per-distinct_id dedup for $feature_flag_called; both paths fire on every access. Adding the cache is a separate behavior change to existing call sites and lands in its own PR.
• No new config option. only/2 silently drops unknown keys (matching Map.take/2), so there's no warning to silence.

Out of scope (future PRs)

• Per-distinct_id dedup cache for $feature_flag_called events, applied to both paths.
• Local-evaluation poller, with locally_evaluated, $feature_flag_reason: "Evaluated locally", and $feature_flag_definitions_loaded_at plumbed through the snapshot.
• only_accessed/1 if users ask for it.
• Phase 3: removal of the deprecated single-flag methods in the next major.

227/227 tests pass, format + Credo strict clean. Compile warnings on the four deprecated functions are expected — they mark call sites that need migrating.

Created with PostHog Code