From 4c751c48a815519efd471db43499e5e407e0e299 Mon Sep 17 00:00:00 2001
From: can1357 <me@can.ac>
Date: Wed, 1 Jul 2026 21:08:56 +0200
Subject: [PATCH] fix(hashline): validate unique stale anchor remaps

---
 packages/hashline/src/recovery.ts             | 21 ++++++++++++++++++-
 .../test/recovery-session-chain.test.ts       | 18 ++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/packages/hashline/src/recovery.ts b/packages/hashline/src/recovery.ts
index de0aafc75..e64b0091d 100644
--- a/packages/hashline/src/recovery.ts
+++ b/packages/hashline/src/recovery.ts
@@ -168,6 +168,20 @@ function validateDuplicateAnchorContext(
 	return checked;
 }
 
+function validateUniqueAnchorContext(
+	line: number,
+	mapped: number,
+	previousLines: readonly string[],
+	lineMap: ReadonlyMap<number, number>,
+	anchorLines: ReadonlySet<number>,
+): boolean {
+	const offset = mapped - line;
+	const after = nearestContextLine(line, 1, anchorLines, previousLines.length);
+	if (after !== undefined) return lineMap.get(after) === after + offset;
+	const before = nearestContextLine(line, -1, anchorLines, previousLines.length);
+	return before !== undefined && lineMap.get(before) === before + offset;
+}
+
 function validateRemappedAnchorContext(
 	previousText: string,
 	currentText: string,
@@ -181,7 +195,12 @@ function validateRemappedAnchorContext(
 	for (const line of anchorLines) {
 		const mapped = lineMap.get(line);
 		if (mapped === undefined) return false;
-		if (!lineIsDuplicated(previousLines, line) && !lineIsDuplicated(currentLines, mapped)) continue;
+		if (!lineIsDuplicated(previousLines, line) && !lineIsDuplicated(currentLines, mapped)) {
+			if (!validateUniqueAnchorContext(line, mapped, previousLines, lineMap, anchorLines)) {
+				return false;
+			}
+			continue;
+		}
 		if (!validateDuplicateAnchorContext(line, mapped, previousLines, lineMap, anchorLines)) {
 			return false;
 		}
diff --git a/packages/hashline/test/recovery-session-chain.test.ts b/packages/hashline/test/recovery-session-chain.test.ts
index 8af0314d4..9d789080c 100644
--- a/packages/hashline/test/recovery-session-chain.test.ts
+++ b/packages/hashline/test/recovery-session-chain.test.ts
@@ -139,4 +139,22 @@ describe("Recovery — session-chain replay anchor-content gate", () => {
 
 		expect(recovered).toBeNull();
 	});
+
+	it("refuses unique-line remaps when following context no longer matches", () => {
+		const store = new InMemorySnapshotStore();
+		const v0Text = lines("L1", "L2", "L3", "L4", "T", "L6");
+		const h0 = store.record(PATH, v0Text);
+		const v1Text = lines("X", "L1", "L2", "L3", "L4", "T", "T_CHANGED", "L6");
+		store.record(PATH, v1Text);
+		const { edits } = parsePatch("SWAP 5.=5:\n+MODEL");
+
+		const recovered = new Recovery(store).tryRecover({
+			path: PATH,
+			currentText: v1Text,
+			fileHash: h0,
+			edits,
+		});
+
+		expect(recovered).toBeNull();
+	});
 });