diff --git a/src/compaction.ts b/src/compaction.ts
index a49cf72..715f2a9 100644
--- a/src/compaction.ts
+++ b/src/compaction.ts
@@ -17,3 +17,10 @@ export function markManualCompactionFromInput(text: string, runtime: CompactionR
 		runtime.pendingCompactionSource = "manual";
 	}
 }
+
+/** session_before_compact: default to auto unless input already marked manual (SPEC §8). */
+export function markAutoCompactionBeforeCompact(runtime: CompactionRuntime): void {
+	if (runtime.pendingCompactionSource !== "manual") {
+		runtime.pendingCompactionSource = "auto";
+	}
+}