diff --git a/lib/types.d.ts b/lib/types.d.ts
index 043e8000..50dea24f 100644
--- a/lib/types.d.ts
+++ b/lib/types.d.ts
@@ -13,6 +13,7 @@ declare namespace beeline {
     serviceName?: string;
     sampleRate?: number;
     enabledInstrumentations?: string[];
+    impl?: "libhoney-event" | "mock";
 
     samplerHook?(event: unknown): SamplerResponse;
     presendHook?(event: unknown): void;