fix: avoid replaying superseded compaction summaries

src/core/compaction.ts

@@ -272,9 +272,9 @@ export function newCompactionMarker(summary: string, details: CompactionDetails)
 	};
 }
 
-/** The ACTIVE compaction marker: the newest one in the record (chained
- * compactions — the last one rules; older markers fall inside the
- * dropped span and never render). */
+/** The ACTIVE compaction marker: the newest one with a retained-message
+ * id. Its summary supersedes older markers even when their stored
+ * positions remain inside its retained tail. */
 export function activeCompactionOf(messages: readonly LhMessage[]): (LhMarkerMessage & { details: CompactionDetails }) | null {
 	for (let i = messages.length - 1; i >= 0; i--) {
 		const m = messages[i]!;

src/core/native-messages.ts

@@ -562,8 +562,9 @@ export function toLlmMessages(messages: readonly AgentMessage[]): Message[] {
 	// THE COMPACTION CUT (owner round 14, 2026-10-05): the ACTIVE marker
 	// (the newest — chained compactions rule last) names the first kept
 	// message; everything before it is replaced by the marker's content
-	// (header + summary) rendered ONCE, up top. The marker itself is then
-	// skipped in place — it must never render twice. A cut naming a
+	// (header + summary) rendered ONCE, up top. All compaction markers
+	// in the retained tail are then skipped, including older summaries
+	// whose stored positions still lie after the cut. A cut naming a
 	// missing id degrades honestly (everything renders — never throw).
 	const lhMessages = messages as readonly LhMessage[];
 	const active = activeCompactionOf(lhMessages);
@@ -604,11 +605,10 @@ export function toLlmMessages(messages: readonly AgentMessage[]): Message[] {
 			// UI-only markers: plan blocks, and problem dismissals (the
 			// synthesized conclude pair carries the model's face).
 			if (marker.customType === "plan-block" || marker.customType === "problem-dismissed") continue;
-			// The ACTIVE compaction marker already rendered as the head —
-			// never twice. (Stale markers before the cut are unreachable; a
-			// marker whose cut could not resolve has no head and renders as
-			// its user text — honest degradation.)
-			if (marker.customType === "compaction" && head !== null && marker.id === active!.id) continue;
+			// The head supersedes every compaction marker, not only its own
+			// stored position. Without a resolved head, preserve the existing
+			// full-history fallback, including the summary text.
+			if (marker.customType === "compaction" && head !== null) continue;
 			out.push({
 				role: "user",
 				content: [{ type: "text", text: marker.content }],

src/server/compaction.ts

@@ -69,7 +69,7 @@ async function subagentSummarizer(prompt: string, engine: PiEngine): Promise<{ s
 		totalTokens: 0,
 		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
 	};
-	const { text } = await runSubagent([], {
+	const { text, newMessages } = await runSubagent([], {
 		// The engine override is always present (the chat's own conversation
 		// engine — owner ruling), so the role label never resolves an
 		// engine; "researcher" is the least-wrong existing ModelRole label
@@ -83,6 +83,12 @@ async function subagentSummarizer(prompt: string, engine: PiEngine): Promise<{ s
 			usage = u;
 		},
 	});
+	// A provider-declared incomplete response is not a usable checkpoint,
+	// even when it contains nonempty text. Match Pi's compaction failure
+	// semantics: leave the existing history and prior summary untouched.
+	if (newMessages.some((message) => message.role === "assistant" && message.stopReason === "length")) {
+		throw new Error("Compaction summary hit the output limit and is incomplete");
+	}
 	return { summary: text, usage };
 }
 
@@ -201,7 +207,14 @@ async function compact(deps: CompactionDeps, chatId: string): Promise<void> {
 		spanStart = prevKept;
 		priorSummary = previous.content;
 	}
-	const spanText = serializeForSummary(messages.slice(spanStart, cut));
+	const span = messages.slice(spanStart, cut);
+	// The separately supplied prior summary replaces all summary markers
+	// in this span. Without a resolved prior boundary, retain the existing
+	// legacy fallback rather than silently discarding summary-only context.
+	const summaryMessages = priorSummary
+		? span.filter((m) => m.role !== "custom" || m.customType !== "compaction")
+		: span;
+	const spanText = serializeForSummary(summaryMessages);
 	const fullSpan = priorSummary ? `[SYSTEM NOTE — prior compaction summary, still authoritative where not superseded]\n${priorSummary}\n\n${spanText}` : spanText;
 
 	const prompt = buildCompactionPrompt({