diff --git a/src/kept.ts b/src/kept.ts
index ad0a446..9461103 100644
--- a/src/kept.ts
+++ b/src/kept.ts
@@ -1,4 +1,5 @@
 import type { SessionEntry } from "@earendil-works/pi-coding-agent";
+import { parseSkillBlocksFromText } from "./detect.js";
 
 /** Branch slice from firstKeptEntryId through tail (SPEC §6.4). */
 export function getKeptEntries(branch: SessionEntry[], firstKeptEntryId: string): SessionEntry[] {
@@ -8,3 +9,48 @@ export function getKeptEntries(branch: SessionEntry[], firstKeptEntryId: string)
 	}
 	return branch.slice(startIndex);
 }
+
+function extractUserMessageText(content: unknown): string {
+	if (typeof content === "string") {
+		return content;
+	}
+	if (!Array.isArray(content)) {
+		return "";
+	}
+	const parts: string[] = [];
+	for (const part of content) {
+		if (!part || typeof part !== "object") {
+			continue;
+		}
+		const block = part as { type?: string; text?: string };
+		if (block.type === "text" && typeof block.text === "string") {
+			parts.push(block.text);
+		}
+	}
+	return parts.join("\n");
+}
+
+/** Skill names already present as blocks in kept user messages (SPEC §6.4). */
+export function skillsPresentInKeptWindow(
+	keptEntries: SessionEntry[],
+	skillNames: readonly string[],
+): Set<string> {
+	const present = new Set<string>();
+	const namesToCheck = new Set(skillNames);
+	if (namesToCheck.size === 0) {
+		return present;
+	}
+
+	for (const entry of keptEntries) {
+		if (entry.type !== "message" || entry.message.role !== "user") {
+			continue;
+		}
+		const text = extractUserMessageText(entry.message.content);
+		for (const block of parseSkillBlocksFromText(text)) {
+			if (namesToCheck.has(block.name)) {
+				present.add(block.name);
+			}
+		}
+	}
+	return present;
+}