fix(edit): preserve guard after transformed writes

packages/coding-agent/CHANGELOG.md

@@ -4,7 +4,7 @@
 
 ### Fixed
 
-- Fixed seen-line guard retries forcing agents to resend entire unchanged patches. Complete inline reveals now issue one-shot `RETRY <token>` continuations that rerun validation against live files, while numbered lines in successful edit output join the returned snapshot's seen-line provenance.
+- Fixed seen-line guard retries forcing agents to resend entire unchanged patches. Complete inline reveals now issue one-shot `RETRY <token>` continuations that rerun validation against live files, while numbered lines in successful edit output join the returned snapshot's seen-line provenance only when the written content exactly matches that output.
 
 ## [17.3.1] - 2026-08-13
 

packages/coding-agent/src/edit/hashline/execute.ts

@@ -18,11 +18,13 @@ import {
 	commitClipboard,
 	forkClipboard,
 	MismatchError as HashlineMismatchError,
+	normalizeToLF,
 	Patch,
 	Patcher,
 	type PatchSectionResult,
 	type PreparedSection,
 	startClipboardBatch,
+	stripBom,
 	UnseenLinesError,
 } from "@oh-my-pi/hashline";
 import type { AgentToolResult } from "@oh-my-pi/pi-agent-core";
@@ -227,7 +229,9 @@ function renderSection(
 	const moveBlock = result.moveDest ? `\nMoved to ${result.moveDest}` : "";
 	const firstChangedLine = result.firstChangedLine ?? diff.firstChangedLine;
 	const text = `${result.header}${blockBlock}${moveBlock}${previewBlock}${warningsBlock}`;
-	recordSeenLinesFromBody(session, canonicalSnapshotKey(result.canonicalPath), result.fileHash, text);
+	if (normalizeToLF(stripBom(result.written).text) === result.after) {
+		recordSeenLinesFromBody(session, canonicalSnapshotKey(result.canonicalPath), result.fileHash, text);
+	}
 	return {
 		toolResult: {
 			content: [

packages/coding-agent/test/edit/seen-line-guard.test.ts

@@ -374,6 +374,36 @@ describe("read → edit seen-line guard", () => {
 		).rejects.toThrow(/never displayed \(it showed/);
 	});
 
+	it("does not trust requested diff lines after an ACP client transforms the write", async () => {
+		const file = path.join(tmpDir, "notes.txt");
+		const content = "ONE\nTWO\nTHREE\nFOUR\nFIVE\n";
+		await Bun.write(file, content);
+		const bridge = {
+			capabilities: { writeTextFile: true },
+			writeTextFile: async ({ path: target, content: requested }: { path: string; content: string }) => {
+				await Bun.write(target, `CLIENT\n${requested}`);
+			},
+		};
+		const session = {
+			...createSession(tmpDir),
+			getClientBridge: () => bridge,
+		} as ToolSession;
+		const store = getFileSnapshotStore(session);
+		const originalTag = store.record(canonicalSnapshotKey(file), content, [2]);
+
+		const first = await executeHashlineSingle(
+			execOptions(`[notes.txt#${originalTag}]\nPUT 2.=2:\n+TWO EDITED`, session),
+		);
+		const persistedTag = tagFromOutput(resultText(first));
+		const drifted = "CLIENT\nONE\nTWO EDITED\nTHREE\nFOUR\nFIVE\n";
+		expect(await Bun.file(file).text()).toBe(drifted);
+
+		await expect(
+			executeHashlineSingle(execOptions(`[notes.txt#${persistedTag}]\nPUT 1.=1:\n+OVERWRITE`, session)),
+		).rejects.toThrow(/never displayed/);
+		expect(await Bun.file(file).text()).toBe(drifted);
+	});
+
 	it("keeps the re-read fallback when the anchor set exceeds the inline reveal cap", async () => {
 		const file = path.join(tmpDir, "long.txt");
 		const lines = Array.from({ length: 200 }, (_, i) => `line ${i + 1}`);

packages/hashline/CHANGELOG.md

@@ -6,6 +6,10 @@
 
 - Added structured `UnseenLinesError.retryable` metadata so hosts can offer compact retry continuations only when every unseen anchor was revealed in full.
 
+### Fixed
+
+- Distinguished absent seen-line provenance from an explicitly observed empty set, so transformed writes can require a fresh read without weakening the guard.
+
 ## [17.3.0] - 2026-08-13
 
 ### Fixed

packages/hashline/src/patcher.ts

@@ -569,7 +569,7 @@ export class Patcher {
 		// false "drift" purely from BOM/line-ending restoration asymmetry.
 		const recorded = normalizeToLF(stripBom(write.text).text);
 		const driftedOnWrite = recorded !== after;
-		const fileHash = this.#recordFullSnapshot(canonicalPath, recorded);
+		const fileHash = this.#recordFullSnapshot(canonicalPath, recorded, driftedOnWrite ? [] : undefined);
 		const allWarnings = driftedOnWrite ? [...warnings, writeDriftWarning(section.path)] : warnings;
 
 		return {
@@ -604,18 +604,19 @@ export class Patcher {
 		}
 	}
 
-	#recordFullSnapshot(canonicalPath: string, normalized: string): string {
-		return this.snapshots.record(canonicalPath, normalized);
+	#recordFullSnapshot(canonicalPath: string, normalized: string, seenLines?: Iterable<number>): string {
+		return this.snapshots.record(canonicalPath, normalized, seenLines);
 	}
 
 	/**
 	 * Reject an anchored edit that references a line the read which minted
 	 * `expected` never displayed. `matchedSnapshot` is the store version whose
 	 * text equals the live normalized content — the exact snapshot the model
-	 * anchored against. Absent means no provenance was recorded (the tag was
-	 * externally minted or aged out), so the edit applies as before. Only runs
-	 * on the no-drift path, where anchor line numbers index the tagged content
-	 * 1:1.
+	 * anchored against. A missing snapshot or undefined `seenLines` means no
+	 * provenance was recorded (the tag was externally minted or aged out), so
+	 * the edit applies as before. An empty set means provenance is active but no
+	 * exact lines were displayed, so every anchor remains guarded. Only runs on
+	 * the no-drift path, where anchor line numbers index the tagged content 1:1.
 	 *
 	 * The rejection inlines the actual file content at the unseen anchor lines
 	 * (from `matchedSnapshot.text`, which by definition equals the live
@@ -635,7 +636,7 @@ export class Patcher {
 	 */
 	#assertSeenLines(section: PatchSection, expected: string, matchedSnapshot: Snapshot | null): void {
 		const seen = matchedSnapshot?.seenLines;
-		if (!seen || seen.size === 0) return;
+		if (seen === undefined) return;
 		const unseen = section.collectAnchorLines().filter(line => !seen.has(line));
 		if (unseen.length === 0) return;
 		const sourceLines = matchedSnapshot?.text.split("\n") ?? [];

packages/hashline/src/snapshots.ts

@@ -41,7 +41,8 @@ export interface Snapshot {
 	 * bodies) leaves this sparse; a whole-file read fills every line. Multiple
 	 * reads of the same content union into one set. `undefined` means "no
 	 * provenance recorded" — the patcher then skips the seen-line check and
-	 * applies as before. Mutated in place as more of the same content is read.
+	 * applies as before. An empty set means provenance is active but no exact
+	 * lines were displayed, so every anchored line remains guarded.
 	 */
 	seenLines?: Set<number>;
 }

packages/hashline/test/patcher.test.ts

@@ -193,9 +193,13 @@ describe("Patcher snapshot tag stays honest across a write-time content transfor
 		// what turned a one-line edit into unexplained whole-file corruption.
 		expect(section.warnings.some(w => w.includes(PATH) && /reformatted it on save/.test(w))).toBe(true);
 
-		// A follow-up edit anchored on the returned tag must succeed against
-		// the real (drifted) file instead of failing a stale-tag mismatch.
-		await patcher.apply(Patch.parse(`[${PATH}#${section.fileHash}]\nPUT 1-1:\n+function g() {`));
+		// The returned tag must still resolve against the real drifted file.
+		// Because no exact persisted lines were displayed after the transform,
+		// the seen-line guard first reveals the anchor, then the same-tag retry
+		// succeeds instead of failing a stale-tag mismatch.
+		const followUp = `[${PATH}#${section.fileHash}]\nPUT 1-1:\n+function g() {`;
+		await expect(patcher.apply(Patch.parse(followUp))).rejects.toThrow(/never displayed/);
+		await patcher.apply(Patch.parse(followUp));
 		expect(fs.get(PATH)).toBe("function g() {\n\treturn 2;\n}\n");
 	});
 });
@@ -417,6 +421,18 @@ describe("Patcher seen-line provenance", () => {
 		expect(fs.get(PATH)).toBe(wideContent);
 	});
 
+	it("guards every anchor when provenance recorded no displayed lines", async () => {
+		const fs = new InMemoryFilesystem([[PATH, CONTENT]]);
+		const snapshots = new InMemorySnapshotStore();
+		const tag = snapshots.record(PATH, CONTENT, []);
+		const patcher = new Patcher({ fs, snapshots });
+
+		await expect(patcher.apply(Patch.parse(`[${PATH}#${tag}]\nPUT 4-4:\n+L4`))).rejects.toThrow(
+			/never displayed \(it showed/,
+		);
+		expect(fs.get(PATH)).toBe(CONTENT);
+	});
+
 	it("skips the check when no seen lines were recorded (absent → allow)", async () => {
 		const fs = new InMemoryFilesystem([[PATH, CONTENT]]);
 		const snapshots = new InMemorySnapshotStore();