diff --git a/src/compaction.ts b/src/compaction.ts
index 715f2a9..e2a20b2 100644
--- a/src/compaction.ts
+++ b/src/compaction.ts
@@ -1,4 +1,5 @@
-import type { CompactionSource } from "./state.js";
+import { effectiveEnabled, type SkillReinjectSettings } from "./settings.js";
+import type { CompactionSource, ExtensionState } from "./state.js";
 
 /** Runtime compaction-source detection between input → before_compact → session_compact (SPEC §8). */
 export interface CompactionRuntime {
@@ -24,3 +25,34 @@ export function markAutoCompactionBeforeCompact(runtime: CompactionRuntime): voi
 		runtime.pendingCompactionSource = "auto";
 	}
 }
+
+/** Gate re-inject on session_compact from enabled layer and compaction source (SPEC §8). */
+export function shouldReinjectAfterCompaction(
+	sessionOverride: boolean | null,
+	settings: SkillReinjectSettings,
+	runtime: CompactionRuntime,
+): boolean {
+	if (!effectiveEnabled(sessionOverride, settings)) {
+		return false;
+	}
+	return (
+		runtime.pendingCompactionSource === "auto" ||
+		settings.reinjectOnManualCompaction
+	);
+}
+
+/**
+ * session_compact: evaluate gate, persist lastCompactionSource, clear pending (SPEC §8).
+ * Call before enqueueing deferred/immediate re-inject.
+ */
+export function consumeCompactionOnSessionCompact(
+	runtime: CompactionRuntime,
+	state: ExtensionState,
+	sessionOverride: boolean | null,
+	settings: SkillReinjectSettings,
+): boolean {
+	const shouldReinject = shouldReinjectAfterCompaction(sessionOverride, settings, runtime);
+	state.lastCompactionSource = runtime.pendingCompactionSource;
+	runtime.pendingCompactionSource = null;
+	return shouldReinject;
+}