fix(edit): enforced seen-line guard on write tags

crates/pi-edit/src/modes/hashline/patcher.rs

@@ -346,6 +346,10 @@ fn rescue_shifted_anchors(
 	Ok(rescued)
 }
 
+/// Reject anchors the tagged snapshot never displayed, revealing their content
+/// inline. `None` provenance (tag issued without a display record) skips the
+/// guard; an empty set (a `write` tag) means no line was displayed, so every
+/// anchor is unseen.
 fn assert_seen_lines(
 	section: &PatchSection,
 	expected: &str,
@@ -359,9 +363,6 @@ fn assert_seen_lines(
 	let Some(seen) = snapshot.seen_lines else {
 		return Ok(());
 	};
-	if seen.is_empty() {
-		return Ok(());
-	}
 	let mut unseen = section
 		.collect_anchor_lines()?
 		.into_iter()

packages/coding-agent/CHANGELOG.md

@@ -13,6 +13,10 @@
 
 - Background task guidance now lives in the Task tool description (and in same-name extension replacements that delegate to Task) instead of being repeated in every spawn acknowledgment ([#15477](https://github.com/can1357/oh-my-pi/pull/15477) by [@iliaal](https://github.com/iliaal))
 
+### Fixed
+
+- Line-anchored edits against the tag a `write` returns are now rejected with the anchor lines revealed instead of applying at guessed line numbers ([#15666](https://github.com/can1357/oh-my-pi/issues/15666))
+
 ## [18.10.4] - 2026-10-13
 
 ### Added

packages/coding-agent/test/sdk-file-write-fallback-extension.test.ts

@@ -218,14 +218,18 @@ describe("registerFileWriteFallback end-to-end (real extension, real session)",
 				const headerLine = resultText(writeResult).split("\n")[0] ?? "";
 				expect(HASHLINE_HEADER_LINE.test(headerLine)).toBe(true);
 
-				// (iii) a subsequent hashline edit on the SAME real path works — this only
-				// holds if the write tool recorded its snapshot under `targetPath` itself
-				// (not a temp path the fallback happened to route through).
+				// (iii) a subsequent hashline edit on the SAME real path works after
+				// its anchor is revealed. The rejection requires the write snapshot
+				// to be recorded under `targetPath`, not a temporary broker path.
 				const editTool = session.getToolByName("edit") as AgentTool | undefined;
 				expect(editTool).toBeDefined();
 				const editInput = `${headerLine}\nPUT 1-1:\n+export const value = 43;\n`;
-				const editResult = await editTool!.execute("call-edit-1", { input: editInput });
+				const rejected = await editTool!.execute("call-edit-1", { input: editInput });
+				expect(rejected.isError).toBe(true);
+				expect(resultText(rejected)).toContain("1:export const value = 42;");
+				expect(fs.readFileSync(targetPath, "utf8")).toBe(content);
 
+				const editResult = await editTool!.execute("call-edit-2", { input: editInput });
 				expect(editResult.isError).not.toBe(true);
 				expect(fs.readFileSync(targetPath, "utf8")).toBe("export const value = 43;\n");
 			} finally {

packages/coding-agent/test/write-hashline-header.test.ts

@@ -70,7 +70,7 @@ describe("write tool hashline header", () => {
 		expect(snapshot).toBe(content);
 	});
 
-	it("makes the post-write tag usable by the hashline patcher", async () => {
+	it("guards line-anchored edits on the post-write tag until the anchor is revealed", async () => {
 		const filePath = path.join(tmpDir, "config.ts");
 		const session = createSession(tmpDir);
 		const tool = new WriteTool(session);
@@ -80,13 +80,18 @@ describe("write tool hashline header", () => {
 		const headerLine = resultText(writeResult).split("\n")[0] ?? "";
 		expect(HASHLINE_HEADER_LINE.test(headerLine)).toBe(true);
 
-		// Apply a hashline patch immediately, using only the tag the write tool
-		// returned — no intervening `read`.
+		// A write displays no numbered lines: anchoring on its tag without a
+		// `read` is rejected with the anchor revealed, and the file is untouched.
 		const patchInput = `${headerLine}\nPUT 1-1:\n+export const enabled = true;\n`;
-		await new EditTool(session, "hashline").execute("call-2", { input: patchInput });
-
-		const final = await fs.readFile(filePath, "utf8");
-		expect(final).toBe("export const enabled = true;\n");
+		const edit = new EditTool(session, "hashline");
+		const rejected = resultText(await edit.execute("call-2", { input: patchInput }));
+		expect(rejected).toContain("never displayed");
+		expect(rejected).toContain("1:export const enabled = false;");
+		expect(await fs.readFile(filePath, "utf8")).toBe(content);
+
+		// The reveal counts as having seen the line, so the same patch now applies.
+		await edit.execute("call-3", { input: patchInput });
+		expect(await fs.readFile(filePath, "utf8")).toBe("export const enabled = true;\n");
 	});
 
 	it("names a local:// write by its URL, and the header round-trips through edit and write", async () => {
@@ -100,9 +105,9 @@ describe("write tool hashline header", () => {
 		expect(writeLine).toBe(`Successfully wrote ${content.length} bytes to local://notes.ts`);
 
 		await new EditTool(session, "hashline").execute("call-2", {
-			input: `${headerLine}\nPUT 1-1:\n+export const enabled = true;\n`,
+			input: `${headerLine}\nPUT >$:\n+export const flag = true;\n`,
 		});
-		expect(await fs.readFile(backingPath, "utf8")).toBe("export const enabled = true;\n");
+		expect(await fs.readFile(backingPath, "utf8")).toBe(`${content}export const flag = true;\n`);
 
 		// The URL-form header also addresses the same file as a `write` path.
 		await new WriteTool(session).execute("call-3", { path: headerLine, content: "export const v = 2;\n" });