diff --git a/src/reinject.ts b/src/reinject.ts
new file mode 100644
index 0000000..b99f54a
--- /dev/null
+++ b/src/reinject.ts
@@ -0,0 +1,39 @@
+import type {
+	ExtensionContext,
+	SessionCompactEvent,
+	Skill,
+} from "@earendil-works/pi-coding-agent";
+import {
+	filterSkillsNeedingReinject,
+	getKeptEntries,
+	skillsPresentInKeptWindow,
+} from "./kept.js";
+import type { SkillReinjectSettings } from "./settings.js";
+import type { ExtensionState } from "./state.js";
+
+/** Names still registered in resourceLoader (SPEC §5.2). */
+export function registeredSkillNames(skills: readonly Pick<Skill, "name">[]): ReadonlySet<string> {
+	return new Set(skills.map((skill) => skill.name));
+}
+
+/**
+ * Skill names to re-inject after compaction: tracked, absent from kept window, still registered (SPEC §5.2).
+ * `registeredSkills` comes from resourceLoader — ExtensionContext has no getSkills(); wired in index.ts.
+ */
+export function planReinject(
+	state: ExtensionState,
+	_settings: SkillReinjectSettings,
+	ctx: ExtensionContext,
+	compactionEvent: SessionCompactEvent,
+	registeredSkills: readonly Pick<Skill, "name">[],
+): string[] {
+	const branch = ctx.sessionManager.getBranch();
+	const keptEntries = getKeptEntries(branch, compactionEvent.compactionEntry.firstKeptEntryId);
+	const trackedNames = state.skills.map((skill) => skill.name);
+	const keptPresent = skillsPresentInKeptWindow(keptEntries, trackedNames);
+	return filterSkillsNeedingReinject(
+		state.skills,
+		keptPresent,
+		registeredSkillNames(registeredSkills),
+	);
+}