feat(coding-agent): added automatic stripping of hashline prefixes from write tool output

- Added automatic stripping of hashline display prefixes (LINE#ID:) from write tool content when hashline edit mode is enabled, preventing models from accidentally copying display markers into files.
- Implemented stripHashlinePrefixes() utility to remove hashline-only prefixes without affecting diff markers.
- Added stripWriteContent() helper to conditionally clean write content based on file display mode configuration.
- Updated write tool result message to notify users when hashline prefixes were auto-stripped.

packages/coding-agent/CHANGELOG.md

@@ -1,8 +1,10 @@
 # Changelog
 
 ## [Unreleased]
+
 ### Added
 
+- Added automatic stripping of hashline display prefixes (LINE#ID:) from write tool content when hashline edit mode is enabled, preventing the model from accidentally copying display markers into files
 - Added `mcpServerName` and `mcpToolName` optional properties to custom tools for MCP server discovery and search metadata
 
 ## [13.13.1] - 2026-03-18

packages/coding-agent/src/patch/index.ts

@@ -136,6 +136,27 @@ export function stripNewLinePrefixes(lines: string[]): string[] {
 	});
 }
 
+/**
+ * Strip hashline display prefixes only (no diff markers).
+ *
+ * Unlike {@link stripNewLinePrefixes} which also handles `+` diff markers,
+ * this only strips `LINE#ID:` / `#ID:` prefixes. Used by the write tool
+ * where diff markers are not applicable.
+ *
+ * Returns the original array reference when no stripping is needed.
+ */
+export function stripHashlinePrefixes(lines: string[]): string[] {
+	let hashPrefixCount = 0;
+	let nonEmpty = 0;
+	for (const l of lines) {
+		if (l.length === 0) continue;
+		nonEmpty++;
+		if (HASHLINE_PREFIX_RE.test(l)) hashPrefixCount++;
+	}
+	if (nonEmpty === 0 || hashPrefixCount !== nonEmpty) return lines;
+	return lines.map(l => l.replace(HASHLINE_PREFIX_RE, ""));
+}
+
 export function hashlineParseText(edit: string[] | string | null): string[] {
 	if (edit === null) return [];
 	if (typeof edit === "string") {

packages/coding-agent/src/tools/write.ts

@@ -14,9 +14,11 @@ import { renderPromptTemplate } from "../config/prompt-templates";
 import type { RenderResultOptions } from "../extensibility/custom-tools/types";
 import { createLspWritethrough, type FileDiagnosticsResult, type WritethroughCallback, writethroughNoop } from "../lsp";
 import { getLanguageFromPath, type Theme } from "../modes/theme/theme";
+import { stripHashlinePrefixes } from "../patch";
 import writeDescription from "../prompts/tools/write.md" with { type: "text" };
 import type { ToolSession } from "../sdk";
 import { Ellipsis, Hasher, type RenderCache, renderStatusLine, truncateToWidth } from "../tui";
+import { resolveFileDisplayMode } from "../utils/file-display-mode";
 import { checkAutoGeneratedFile } from "./auto-generated-guard";
 import { invalidateFsScanAfterWrite } from "./fs-cache-invalidation";
 import { type OutputMeta, outputMeta } from "./output-meta";
@@ -60,6 +62,22 @@ function getLspBatchRequest(toolCall: ToolCallContext | undefined): { id: string
 	return { id: toolCall.batchId, flush: !hasLaterWrites };
 }
 
+/**
+ * Strip hashline display prefixes from write content.
+ *
+ * Only active when hashline edit mode is enabled — the model sees `LINE#ID:`
+ * prefixes in read output and sometimes copies them into write content.
+ */
+function stripWriteContent(session: ToolSession, content: string): { text: string; stripped: boolean } {
+	if (!resolveFileDisplayMode(session).hashLines) {
+		return { text: content, stripped: false };
+	}
+	const lines = content.split("\n");
+	const cleaned = stripHashlinePrefixes(lines);
+	if (cleaned === lines) return { text: content, stripped: false };
+	return { text: cleaned.join("\n"), stripped: true };
+}
+
 // ═══════════════════════════════════════════════════════════════════════════
 // Tool Class
 // ═══════════════════════════════════════════════════════════════════════════
@@ -109,10 +127,16 @@ export class WriteTool implements AgentTool<typeof writeSchema, WriteToolDetails
 				await checkAutoGeneratedFile(absolutePath, path);
 			}
 
-			const diagnostics = await this.#writethrough(absolutePath, content, signal, undefined, batchRequest);
+			// Strip hashline display prefixes (LINE#ID:) if the model copied them from read output
+			const { text: cleanContent, stripped } = stripWriteContent(this.session, content);
+
+			const diagnostics = await this.#writethrough(absolutePath, cleanContent, signal, undefined, batchRequest);
 			invalidateFsScanAfterWrite(absolutePath);
 
-			const resultText = `Successfully wrote ${content.length} bytes to ${path}`;
+			let resultText = `Successfully wrote ${cleanContent.length} bytes to ${path}`;
+			if (stripped) {
+				resultText += `\nNote: auto-stripped hashline display prefixes from content before writing.`;
+			}
 			if (!diagnostics) {
 				return {
 					content: [{ type: "text", text: resultText }],