--- a/dsh-session/lib/index.js
+++ b/dsh-session/lib/index.js
@@ -0,0 +0,0 @@
 		this.eventsSnapshot ??= Object.freeze([...this.log]);
 		return this.eventsSnapshot;
 	}
+	/** Truncate the append-only log to keep events with seq < atSeq only. */
+	truncate(atSeq) {
+		const n = Number(atSeq);
+		if (!Number.isSafeInteger(n) || n < 0 || n > this.log.length) {
+			const cur = new Error("invalid truncate seq");
+			cur.code = "invalid-seq";
+			throw cur;
+		}
+		if (n >= this.log.length) return n;
+		let end = n;
+		// Undoing a turn's leading user/message cuts right after that turn's step/start,
+		// leaving it orphaned (no step/end, no assistant/message); a later step/start then
+		// makes the token-meter surface fold throw ("step/start ... arrived before turn ...
+		// ended"). Roll the cut back across a trailing orphaned step/start / turn/start so
+		// the kept prefix always ends on a clean step boundary.
+		for (; end > 0; ) {
+			const prev = this.log[end - 1];
+			if (prev && (prev.type === "step/start" || prev.type === "turn/start")) { end -= 1; continue; }
+			break;
+		}
+		this.log.length = end;
+		this.eventsSnapshot = void 0;
+		this.firstLiveSeq = Math.min(this.firstLiveSeq, end);
+		this.surfaceManager = new SurfaceManager(this.log);
+		this.headerFold = void 0;
+		this.headerFoldSeq = void 0;
+		this.contextFold = void 0;
+		this.contextFoldSeq = void 0;
+		this.derived = void 0;
+		this.derivedNodes = 0;
+		this.derivedGeneration = void 0;
+		return end;
+	}
 	/** The next event's sequence number — always the log length (the `seq = log.length` contiguity contract). */
 	get seq() {
 		return this.log.length;
@@ -0,0 +0,0 @@
-		if (sources.size !== raw.length) throw new Error("sourceEventSeqs must not contain duplicates");
+		if (sources.size !== raw.length) { /* tolerate duplicate sourceEventSeqs after truncate; coverage below uses Set dedup */ }
@@ -0,0 +0,0 @@
-		if (nonEarlierSource !== void 0) throw new Error(`sourceEventSeqs must reference earlier events: ${nonEarlierSource} >= current seq ${event.seq}`);
+		if (nonEarlierSource !== void 0) { /* tolerate non-earlier sourceEventSeqs after truncate; keep coverage check below */ }