diff --git a/checker/types.go b/checker/types.go
index ed4a9fc..ee67132 100644
--- a/checker/types.go
+++ b/checker/types.go
@@ -220,6 +220,24 @@ type ObservationProvider interface {
 	Collect(ctx context.Context, opts CheckerOptions) (any, error)
 }
 
+// ObservationSharer is an optional interface an ObservationProvider can
+// implement to declare that its observation depends only on a subset of the
+// inputs and may therefore be shared across check targets.
+//
+// ShareKey returns a stable string derived purely from the inputs that affect
+// the result (e.g. the sorted IP set + ping count). Two collections with the
+// same ShareKey produce identical data; the host may serve one from the other.
+// Return "" to opt out (the host falls back to per-target caching).
+//
+// ShareKey MUST be a pure function of opts: no network, no I/O. The host calls
+// it before deciding whether to collect, possibly on the locally registered
+// provider even when actual collection is delegated to a remote endpoint.
+//
+// Detect support with a type assertion: _, ok := provider.(ObservationSharer)
+type ObservationSharer interface {
+	ShareKey(opts CheckerOptions) (string, error)
+}
+
 // CheckRuleInfo is the JSON-serializable description of a rule, for API/UI listing.
 type CheckRuleInfo struct {
 	Name        string                       `json:"name"`