diff --git a/src/kept.ts b/src/kept.ts
index 9461103..6855872 100644
--- a/src/kept.ts
+++ b/src/kept.ts
@@ -1,5 +1,6 @@
 import type { SessionEntry } from "@earendil-works/pi-coding-agent";
 import { parseSkillBlocksFromText } from "./detect.js";
+import type { TrackedSkill } from "./state.js";
 
 /** Branch slice from firstKeptEntryId through tail (SPEC §6.4). */
 export function getKeptEntries(branch: SessionEntry[], firstKeptEntryId: string): SessionEntry[] {
@@ -54,3 +55,18 @@ export function skillsPresentInKeptWindow(
 	}
 	return present;
 }
+
+/** Tracked skills missing from kept window but still registered (SPEC §5.2, §6.4). */
+export function filterSkillsNeedingReinject(
+	tracked: readonly TrackedSkill[],
+	keptPresent: ReadonlySet<string>,
+	registeredNames: ReadonlySet<string>,
+): string[] {
+	const needing: string[] = [];
+	for (const skill of tracked) {
+		if (registeredNames.has(skill.name) && !keptPresent.has(skill.name)) {
+			needing.push(skill.name);
+		}
+	}
+	return needing;
+}