diff --git a/test/kept-window.test.ts b/test/kept-window.test.ts
new file mode 100644
index 0000000..72633dd
--- /dev/null
+++ b/test/kept-window.test.ts
@@ -0,0 +1,117 @@
+import type { SessionEntry } from "@earendil-works/pi-coding-agent";
+import { describe, expect, it } from "vitest";
+import {
+	filterSkillsNeedingReinject,
+	getKeptEntries,
+	skillsPresentInKeptWindow,
+} from "../src/kept";
+import type { TrackedSkill } from "../src/state";
+
+const ts = "2024-12-03T14:00:00.000Z";
+
+function userMessage(id: string, content: string): SessionEntry {
+	return {
+		type: "message",
+		id,
+		parentId: null,
+		timestamp: ts,
+		message: { role: "user", content },
+	} as SessionEntry;
+}
+
+function assistantMessage(id: string, content: string): SessionEntry {
+	return {
+		type: "message",
+		id,
+		parentId: null,
+		timestamp: ts,
+		message: { role: "assistant", content: [{ type: "text", text: content }] },
+	} as SessionEntry;
+}
+
+function compactionEntry(id: string, firstKeptEntryId: string): SessionEntry {
+	return {
+		type: "compaction",
+		id,
+		parentId: null,
+		timestamp: ts,
+		summary: "summary",
+		firstKeptEntryId,
+		tokensBefore: 1000,
+	} as SessionEntry;
+}
+
+const skillBlock = (name: string) =>
+	`<skill name="${name}" location="/skills/${name}/SKILL.md">\nbody\n</skill>`;
+
+const trackedSkills: TrackedSkill[] = [
+	{
+		name: "alpha",
+		filePath: "/skills/alpha/SKILL.md",
+		baseDir: "/skills/alpha",
+		firstSeenAt: 1,
+		lastSeenAt: 1,
+		sources: ["slash"],
+	},
+	{
+		name: "beta",
+		filePath: "/skills/beta/SKILL.md",
+		baseDir: "/skills/beta",
+		firstSeenAt: 2,
+		lastSeenAt: 2,
+		sources: ["skill-block"],
+	},
+];
+
+describe("getKeptEntries", () => {
+	const branch: SessionEntry[] = [
+		userMessage("e1", "old"),
+		userMessage("e2", "kept start"),
+		assistantMessage("e3", "reply"),
+		compactionEntry("e4", "e2"),
+		userMessage("e5", "after compact"),
+	];
+
+	it("slices from firstKeptEntryId through tail", () => {
+		expect(getKeptEntries(branch, "e2").map((entry) => entry.id)).toEqual(["e2", "e3", "e4", "e5"]);
+	});
+
+	it("returns empty when firstKeptEntryId is missing", () => {
+		expect(getKeptEntries(branch, "missing")).toEqual([]);
+		expect(getKeptEntries([], "e1")).toEqual([]);
+	});
+});
+
+describe("skillsPresentInKeptWindow", () => {
+	it("detects skill blocks in kept user messages", () => {
+		const kept = [userMessage("k1", skillBlock("alpha")), assistantMessage("k2", "ignored")];
+		expect(skillsPresentInKeptWindow(kept, ["alpha", "beta"])).toEqual(new Set(["alpha"]));
+	});
+
+	it("returns empty when no tracked skills or no blocks", () => {
+		expect(skillsPresentInKeptWindow([userMessage("k1", "plain")], ["alpha"])).toEqual(new Set());
+		expect(skillsPresentInKeptWindow([userMessage("k1", skillBlock("alpha"))], [])).toEqual(new Set());
+		expect(skillsPresentInKeptWindow([], ["alpha", "beta"])).toEqual(new Set());
+	});
+});
+
+describe("filterSkillsNeedingReinject", () => {
+	it("returns registered tracked skills absent from kept window", () => {
+		const keptPresent = new Set(["alpha"]);
+		const registered = new Set(["alpha", "beta"]);
+		expect(filterSkillsNeedingReinject(trackedSkills, keptPresent, registered)).toEqual(["beta"]);
+	});
+
+	it("excludes unregistered skills and preserves tracked order", () => {
+		const keptPresent = new Set<string>();
+		const registered = new Set(["alpha", "beta"]);
+		expect(filterSkillsNeedingReinject(trackedSkills, keptPresent, registered)).toEqual(["alpha", "beta"]);
+		expect(filterSkillsNeedingReinject(trackedSkills, keptPresent, new Set(["beta"]))).toEqual(["beta"]);
+	});
+
+	it("returns empty when all tracked skills are in kept window", () => {
+		const keptPresent = new Set(["alpha", "beta"]);
+		const registered = new Set(["alpha", "beta"]);
+		expect(filterSkillsNeedingReinject(trackedSkills, keptPresent, registered)).toEqual([]);
+	});
+});