diff --git a/src/compaction.ts b/src/compaction.ts
new file mode 100644
index 0000000..6bf2ff4
--- /dev/null
+++ b/src/compaction.ts
@@ -0,0 +1,12 @@
+import type { CompactionSource } from "./state.js";
+
+/** Runtime compaction-source detection between input → before_compact → session_compact (SPEC §8). */
+export interface CompactionRuntime {
+	pendingCompactionSource: CompactionSource | null;
+}
+
+export function createCompactionRuntime(): CompactionRuntime {
+	return {
+		pendingCompactionSource: null,
+	};
+}