--- a/js/public/GCAPI.h
+++ b/js/public/GCAPI.h
@@ -327,19 +327,20 @@ enum GCProgress {
     GC_SLICE_BEGIN,
     GC_SLICE_END,
     GC_CYCLE_END
 };
 
 struct JS_PUBLIC_API(GCDescription) {
     bool isCompartment_;
     JSGCInvocationKind invocationKind_;
+    gcreason::Reason reason_;
 
-    GCDescription(bool isCompartment, JSGCInvocationKind kind)
-      : isCompartment_(isCompartment), invocationKind_(kind) {}
+    GCDescription(bool isCompartment, JSGCInvocationKind kind, gcreason::Reason reason)
+      : isCompartment_(isCompartment), invocationKind_(kind), reason_(reason) {}
 
     char16_t* formatSliceMessage(JSRuntime* rt) const;
     char16_t* formatSummaryMessage(JSRuntime* rt) const;
     char16_t* formatJSON(JSRuntime* rt, uint64_t timestamp) const;
 
     JS::dbg::GarbageCollectionEvent::Ptr toGCEvent(JSRuntime* rt) const;
 };
 

--- a/js/src/gc/Statistics.cpp
+++ b/js/src/gc/Statistics.cpp
@@ -940,17 +940,17 @@ Statistics::beginSlice(const ZoneGCStats
 
     runtime->addTelemetry(JS_TELEMETRY_GC_REASON, reason);
 
     // Slice callbacks should only fire for the outermost level
     if (++gcDepth == 1) {
         bool wasFullGC = zoneStats.isCollectingAllZones();
         if (sliceCallback)
             (*sliceCallback)(runtime, first ? JS::GC_CYCLE_BEGIN : JS::GC_SLICE_BEGIN,
-                             JS::GCDescription(!wasFullGC, gckind));
+                             JS::GCDescription(!wasFullGC, gckind, reason));
     }
 }
 
 void
 Statistics::endSlice()
 {
     if (!aborted) {
         slices.back().end = PRMJ_Now();
@@ -972,17 +972,17 @@ Statistics::endSlice()
     if (last)
         endGC();
 
     // Slice callbacks should only fire for the outermost level
     if (--gcDepth == 0) {
         bool wasFullGC = zoneStats.isCollectingAllZones();
         if (sliceCallback)
             (*sliceCallback)(runtime, last ? JS::GC_CYCLE_END : JS::GC_SLICE_END,
-                             JS::GCDescription(!wasFullGC, gckind));
+                             JS::GCDescription(!wasFullGC, gckind, slices.back().reason));
     }
 
     /* Do this after the slice callback since it uses these values. */
     if (last)
         PodArrayZero(counts);
 }
 
 void