diff --git a/src/compaction.ts b/src/compaction.ts
index e93152a..4645ced 100644
--- a/src/compaction.ts
+++ b/src/compaction.ts
@@ -6,12 +6,15 @@ export interface CompactionRuntime {
 	pendingCompactionSource: CompactionSource | null;
 	/** Manual compaction with default settings: clear stale pending on next user prompt (SPEC §16.5, §12.3). */
 	clearPendingReinjectOnNextUserInput: boolean;
+	/** Last compact firstKeptEntryId for debug kept-window snapshots (Phase 14). */
+	lastCompactionFirstKeptEntryId: string | null;
 }
 
 export function createCompactionRuntime(): CompactionRuntime {
 	return {
 		pendingCompactionSource: null,
 		clearPendingReinjectOnNextUserInput: false,
+		lastCompactionFirstKeptEntryId: null,
 	};
 }
 
diff --git a/src/diag.ts b/src/diag.ts
new file mode 100644
index 0000000..37189c6
--- /dev/null
+++ b/src/diag.ts
@@ -0,0 +1,42 @@
+import type { ExtensionContext, Skill } from "@earendil-works/pi-coding-agent";
+import type { SkillReinjectSettings } from "./settings.js";
+import type { ExtensionState } from "./state.js";
+
+export type ReinjectDiagPhase = "session_compact" | "before_agent_start";
+
+/** Filter snapshot for debug logging (Phase 14 / B-002). */
+export interface ReinjectDiagSnapshot {
+	tracked: string[];
+	kept: string[];
+	registered: string[];
+	planned: string[];
+	pending: string[];
+}
+
+export function buildReinjectDiagSnapshot(
+	state: ExtensionState,
+	registeredSkills: readonly Pick<Skill, "name">[],
+	keptPresent: ReadonlySet<string>,
+	planned: readonly string[],
+): ReinjectDiagSnapshot {
+	return {
+		tracked: state.skills.map((skill) => skill.name),
+		kept: [...keptPresent],
+		registered: registeredSkills.map((skill) => skill.name),
+		planned: [...planned],
+		pending: [...state.pendingReinject],
+	};
+}
+
+/** Log reinject filter state when settings.debug is on (Phase 14). */
+export function notifyReinjectDiag(
+	ctx: ExtensionContext | undefined,
+	settings: SkillReinjectSettings,
+	phase: ReinjectDiagPhase,
+	snapshot: ReinjectDiagSnapshot,
+): void {
+	if (!settings.debug || !ctx?.hasUI) {
+		return;
+	}
+	ctx.ui.notify(`skill-reinject [${phase}]: ${JSON.stringify(snapshot)}`, "info");
+}
diff --git a/src/index.ts b/src/index.ts
index e1661d5..7378ec5 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -14,7 +14,12 @@ import {
 	markAutoCompactionBeforeCompact,
 	markManualCompactionFromInput,
 } from "./compaction.js";
+import { buildReinjectDiagSnapshot, notifyReinjectDiag } from "./diag.js";
 import { detectSlashSkill, matchReadPathToSkillWhenEnabled, parseSkillBlocksFromText, userMessageText } from "./detect.js";
+import {
+	getKeptEntries,
+	skillsPresentInKeptWindow,
+} from "./kept.js";
 import {
 	applyPendingReinjectAfterCompact,
 	clearPendingReinjectOnUserPrompt,
@@ -92,6 +97,11 @@ export default function skillReinject(pi: ExtensionAPI): void {
 	function handleSessionCompact(event: SessionCompactEvent, ctx: ExtensionContext): void {
 		const settings = readSettings(ctx);
 		const skills = resolveRegisteredSkills(ctx.cwd, registeredSkills);
+		const branch = ctx.sessionManager.getBranch();
+		compactionRuntime.lastCompactionFirstKeptEntryId = event.compactionEntry.firstKeptEntryId;
+		const trackedNames = state.skills.map((skill) => skill.name);
+		const keptEntries = getKeptEntries(branch, event.compactionEntry.firstKeptEntryId);
+		const keptPresent = skillsPresentInKeptWindow(keptEntries, trackedNames);
 		const planned = planReinject(state, settings, ctx, event, skills);
 		const shouldReinject = consumeCompactionOnSessionCompact(
 			compactionRuntime,
@@ -101,13 +111,19 @@ export default function skillReinject(pi: ExtensionAPI): void {
 		);
 		const deliveryMode = resolveDeliveryMode(settings, runtime, state.sessionIntegrationOverride);
 
+		applyPendingReinjectAfterCompact(state, compactionRuntime, shouldReinject, planned);
+		notifyReinjectDiag(
+			ctx,
+			settings,
+			"session_compact",
+			buildReinjectDiagSnapshot(state, skills, keptPresent, planned),
+		);
+
 		if (deliveryMode === "defer") {
-			applyPendingReinjectAfterCompact(state, compactionRuntime, shouldReinject, planned);
 			persistState();
 			return;
 		}
 
-		applyPendingReinjectAfterCompact(state, compactionRuntime, shouldReinject, planned);
 		if (!shouldReinject) {
 			persistState();
 			return;
@@ -149,6 +165,24 @@ export default function skillReinject(pi: ExtensionAPI): void {
 		registeredSkills = event.systemPromptOptions.skills ?? registeredSkills;
 
 		const settings = readSettings(ctx);
+		const skills = resolveRegisteredSkills(ctx.cwd, registeredSkills);
+		const trackedNames = state.skills.map((skill) => skill.name);
+		const keptPresent =
+			compactionRuntime.lastCompactionFirstKeptEntryId === null
+				? new Set<string>()
+				: skillsPresentInKeptWindow(
+						getKeptEntries(
+							ctx.sessionManager.getBranch(),
+							compactionRuntime.lastCompactionFirstKeptEntryId,
+						),
+						trackedNames,
+					);
+		notifyReinjectDiag(
+			ctx,
+			settings,
+			"before_agent_start",
+			buildReinjectDiagSnapshot(state, skills, keptPresent, []),
+		);
 		const pendingBefore = state.pendingReinject.length;
 		const deferred = tryConsumeDeferredReinject(
 			state,
@@ -157,6 +191,9 @@ export default function skillReinject(pi: ExtensionAPI): void {
 			ctx,
 			compactionRuntime,
 		);
+		if (deferred) {
+			compactionRuntime.lastCompactionFirstKeptEntryId = null;
+		}
 		if (pendingBefore > 0) {
 			persistState();
 		}
diff --git a/src/settings.ts b/src/settings.ts
index cb16220..4c8947b 100644
--- a/src/settings.ts
+++ b/src/settings.ts
@@ -26,6 +26,8 @@ export interface SkillReinjectSettings {
 	suffix: string;
 	/** Soft warn threshold; omit for unlimited with default warn above 3 (SPEC §15). */
 	maxSkills?: number;
+	/** Verbose reinject filter logging via ui.notify (Phase 14 / B-002). */
+	debug: boolean;
 }
 
 /** Defaults from SPEC §7.3 — extension off until explicitly enabled. */
@@ -36,6 +38,7 @@ export const DEFAULT_SKILL_REINJECT_SETTINGS: Readonly<SkillReinjectSettings> =
 	reinjectOnManualCompaction: false,
 	autoCompactIntegration: "auto",
 	suffix: "[skill-reinject] Re-applied after compaction.",
+	debug: false,
 };
 
 export function createDefaultSettings(): SkillReinjectSettings {
@@ -77,6 +80,9 @@ export function parseSkillReinjectPartial(raw: unknown): PartialSkillReinjectSet
 	if (typeof obj.maxSkills === "number" && Number.isInteger(obj.maxSkills) && obj.maxSkills > 0) {
 		result.maxSkills = obj.maxSkills;
 	}
+	if (typeof obj.debug === "boolean") {
+		result.debug = obj.debug;
+	}
 	return result;
 }
 
diff --git a/test/diag.test.ts b/test/diag.test.ts
new file mode 100644
index 0000000..186e773
--- /dev/null
+++ b/test/diag.test.ts
@@ -0,0 +1,81 @@
+import { describe, expect, it, vi } from "vitest";
+import { buildReinjectDiagSnapshot, notifyReinjectDiag } from "../src/diag.js";
+import { createDefaultSettings } from "../src/settings.js";
+import { createInitialState } from "../src/state.js";
+
+describe("buildReinjectDiagSnapshot", () => {
+	it("collects tracked, kept, registered, planned, and pending", () => {
+		const state = createInitialState();
+		state.skills.push({
+			name: "alpha",
+			filePath: "/skills/alpha/SKILL.md",
+			baseDir: "/skills/alpha",
+			firstSeenAt: 1,
+			lastSeenAt: 1,
+			sources: ["slash"],
+		});
+		state.pendingReinject = ["alpha"];
+
+		expect(
+			buildReinjectDiagSnapshot(
+				state,
+				[{ name: "beta" }],
+				new Set(["gamma"]),
+				["alpha"],
+			),
+		).toEqual({
+			tracked: ["alpha"],
+			kept: ["gamma"],
+			registered: ["beta"],
+			planned: ["alpha"],
+			pending: ["alpha"],
+		});
+	});
+});
+
+describe("notifyReinjectDiag", () => {
+	it("no-ops when debug is off", () => {
+		const notify = vi.fn();
+		notifyReinjectDiag(
+			{
+				hasUI: true,
+				ui: { notify },
+			} as never,
+			createDefaultSettings(),
+			"session_compact",
+			{
+				tracked: [],
+				kept: [],
+				registered: [],
+				planned: [],
+				pending: [],
+			},
+		);
+		expect(notify).not.toHaveBeenCalled();
+	});
+
+	it("notifies with JSON snapshot when debug is on", () => {
+		const notify = vi.fn();
+		const settings = { ...createDefaultSettings(), debug: true };
+		const snapshot = {
+			tracked: ["a"],
+			kept: [],
+			registered: [],
+			planned: ["a"],
+			pending: ["a"],
+		};
+		notifyReinjectDiag(
+			{
+				hasUI: true,
+				ui: { notify },
+			} as never,
+			settings,
+			"before_agent_start",
+			snapshot,
+		);
+		expect(notify).toHaveBeenCalledWith(
+			`skill-reinject [before_agent_start]: ${JSON.stringify(snapshot)}`,
+			"info",
+		);
+	});
+});
diff --git a/test/settings.test.ts b/test/settings.test.ts
index fe505d9..5e079c9 100644
--- a/test/settings.test.ts
+++ b/test/settings.test.ts
@@ -54,6 +54,10 @@ describe("parseSkillReinjectPartial", () => {
 			}),
 		).toEqual({ enabled: true, suffix: "custom" });
 	});
+
+	it("parses debug flag", () => {
+		expect(parseSkillReinjectPartial({ debug: true })).toEqual({ debug: true });
+	});
 });
 
 describe("mergeSkillReinjectSettings", () => {