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diff --git a/packages/frontend/src/lib/snapshot-sequencer.ts b/packages/frontend/src/lib/snapshot-sequencer.ts
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+/**
+ * Tiny race guard for "the most-recent request wins" semantics.
+ *
+ * When a frontend component fans out multiple HTTP calls that each return a
+ * full snapshot of shared state — and applying an older snapshot would clobber
+ * a newer one — wrap each call with `seq = sequencer.begin()` before send and
+ * `sequencer.accept(seq)` before applying the response. Older sequences are
+ * rejected.
+ *
+ * Why: the Claude Wake Schedule's POST /toggle and GET /wake-schedule both
+ * return the *whole* schedule. If a user toggles hour 9 (request A) and then
+ * hour 10 (request B), and B's response arrives before A's, the older A
+ * response — which doesn't know about hour 10 yet — would otherwise overwrite
+ * hour 10 right out of the UI. A per-hour counter is NOT enough because the
+ * race spans different hours (and also covers the initial-load vs first-click
+ * race).
+ *
+ * `>=` on accept is intentional: if seq equals the latest applied seq, the
+ * response is a redundant arrival of the most-recent winner — accepting it
+ * (idempotently) is fine. The discriminator is *strictly less than*.
+ */
+export class SnapshotSequencer {
+	private nextSeq = 0;
+	private latestApplied = 0;
+
+	/** Tag a new request. Call before sending; pass the returned seq to accept(). */
+	begin(): number {
+		this.nextSeq += 1;
+		return this.nextSeq;
+	}
+
+	/**
+	 * Decide whether to apply a response. Returns true if this seq is the
+	 * newest seen so far (and updates the watermark); false if a newer
+	 * response has already won.
+	 */
+	accept(seq: number): boolean {
+		if (seq < this.latestApplied) return false;
+		this.latestApplied = seq;
+		return true;
+	}
+
+	/** Inspect (for tests / debugging). */
+	get state(): { nextSeq: number; latestApplied: number } {
+		return { nextSeq: this.nextSeq, latestApplied: this.latestApplied };
+	}
+}