From 5066aad92282068dfd3ce5e1bf87164264618cfe Mon Sep 17 00:00:00 2001
From: Adam Malczewski <github@tradam.dev>
Date: Mon, 22 Jun 2026 12:24:43 +0900
Subject: fix(metrics): tail-align when stepId matching fails — prevent
 misaligned turns
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When stepIds are absent on persisted chunks (or don't match), the
sequential fallback was HEAD-aligning — matching the OLDEST entries
(trimmed turns) to the NEWEST segments. This showed 'turn 1' on turn 20's
content and placed the wrong metrics on the wrong segments.

Fix: when stepId matching produces ZERO matches, use TAIL-ALIGNMENT
instead — match the LAST T entries to the T segments (the loaded chunks
are always the newest). The oldest entries (trimmed turns) are unmatched
and emit standalone turn-metrics rows at the top.

686 tests green.
---
 src/core/metrics/place.ts | 45 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 34 insertions(+), 11 deletions(-)

diff --git a/src/core/metrics/place.ts b/src/core/metrics/place.ts
index b60c675..47d3c5f 100644
--- a/src/core/metrics/place.ts
+++ b/src/core/metrics/place.ts
@@ -120,18 +120,41 @@ export function interleaveTurnMetrics(
 	}
 
 	// Pass 2: sequential fallback for unmatched segments.
-	let nextUnused = 0;
-	for (let seg = 0; seg < T; seg++) {
-		if (segmentEntry.has(seg)) continue;
-		while (nextUnused < K && usedEntries.has(nextUnused)) nextUnused++;
-		if (nextUnused < K) {
-			usedEntries.add(nextUnused);
-			const e = entries[nextUnused];
-			if (e !== undefined) {
-				segmentEntry.set(seg, e);
-				segmentEntryIndex.set(seg, nextUnused);
+	// If NO segments were matched by stepId (pass 1), use TAIL-ALIGNMENT:
+	// the loaded chunks are always the NEWEST (chat-limit/windowing keeps the
+	// newest and trims the oldest), so match the LAST T entries to the T
+	// segments. This prevents misaligning oldest (trimmed) entries to newest
+	// segments — which would show "turn 1" on turn 20's content.
+	const pass1Matches = segmentEntry.size;
+	if (pass1Matches === 0 && K >= T) {
+		// Tail-align: skip the first K-T entries (trimmed turns).
+		for (let seg = 0; seg < T; seg++) {
+			if (segmentEntry.has(seg)) continue;
+			const entryIdx = K - T + seg;
+			if (entryIdx < K && !usedEntries.has(entryIdx)) {
+				usedEntries.add(entryIdx);
+				const e = entries[entryIdx];
+				if (e !== undefined) {
+					segmentEntry.set(seg, e);
+					segmentEntryIndex.set(seg, entryIdx);
+				}
+			}
+		}
+	} else {
+		// Head-align fallback for remaining unmatched segments.
+		let nextUnused = 0;
+		for (let seg = 0; seg < T; seg++) {
+			if (segmentEntry.has(seg)) continue;
+			while (nextUnused < K && usedEntries.has(nextUnused)) nextUnused++;
+			if (nextUnused < K) {
+				usedEntries.add(nextUnused);
+				const e = entries[nextUnused];
+				if (e !== undefined) {
+					segmentEntry.set(seg, e);
+					segmentEntryIndex.set(seg, nextUnused);
+				}
+				nextUnused++;
 			}
-			nextUnused++;
 		}
 	}
 
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