diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 775437f92..d6c0a5e49 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -3,7 +3,7 @@
 ## [Unreleased]
 
 ### Fixed
-
+- Fixed hashline edit payloads that use a readability space after `~` by warning on separator-padding-shaped payload blocks and tightening the model prompt. ([#1166](https://github.com/can1357/oh-my-pi/issues/1166))
 - Fixed ACP command and custom tool-call notifications to carry the original tool arguments in replayed and final updates, so command text is preserved and raw input is no longer wrapped
 - Fixed ACP async-job draining to be scoped by session owner so `getAsyncJobSnapshot` and `drainAsyncJobDeliveriesForAcp` no longer consume or expose jobs from other sessions
 - Fixed async job status reporting to include in-flight completions so queued/delivering indicators remain accurate while callbacks are still running
diff --git a/packages/coding-agent/src/hashline/parser.ts b/packages/coding-agent/src/hashline/parser.ts
index e0ed8fc33..0bb9ca1fc 100644
--- a/packages/coding-agent/src/hashline/parser.ts
+++ b/packages/coding-agent/src/hashline/parser.ts
@@ -74,19 +74,29 @@ export function cloneCursor(cursor: HashlineCursor): HashlineCursor {
 	if (cursor.kind === "after_anchor") return { kind: "after_anchor", anchor: { ...cursor.anchor } };
 	return cursor;
 }
+/** Returns true when every non-empty payload line starts with `${sep} ` (sep + one space). */
+function hasUniformSeparatorPadding(payload: string[]): boolean {
+	let any = false;
+	for (const text of payload) {
+		if (text.length === 0) continue;
+		if (!text.startsWith(" ")) return false;
+		any = true;
+	}
+	return any;
+}
 
 function collectPayload(
 	lines: string[],
 	startIndex: number,
 	opLineNum: number,
 	requirePayload: boolean,
-): { payload: string[]; nextIndex: number } {
+): { payload: string[]; nextIndex: number; paddingWarning?: string } {
 	const payload: string[] = [];
 	let index = startIndex;
 	while (index < lines.length) {
 		const line = lines[index];
 		if (line.startsWith(HL_EDIT_SEP)) {
-			payload.push(line.slice(1).trimEnd());
+			payload.push(line.slice(HL_EDIT_SEP.length).trimEnd());
 			index++;
 			continue;
 		}
@@ -115,7 +125,11 @@ function collectPayload(
 	if (payload.length === 0 && requirePayload) {
 		throw new Error(`line ${opLineNum}: + and < operations require at least one ${HL_EDIT_SEP}TEXT payload line.`);
 	}
-	return { payload, nextIndex: index };
+	const paddingWarning = hasUniformSeparatorPadding(payload)
+		? `line ${opLineNum}: all payload lines start with "${HL_EDIT_SEP} " (separator + space). ` +
+			`The space becomes file content. Remove it unless the target file requires leading spaces.`
+		: undefined;
+	return { payload, nextIndex: index, paddingWarning };
 }
 
 export function parseHashline(diff: string): HashlineEdit[] {
@@ -158,7 +172,8 @@ export function parseHashlineWithWarnings(diff: string): { edits: HashlineEdit[]
 		const insertBeforeMatch = INSERT_BEFORE_OP_RE.exec(line);
 		if (insertBeforeMatch) {
 			const cursor = parseInsertTarget(insertBeforeMatch[1], lineNum, "before");
-			const { payload, nextIndex } = collectPayload(lines, i + 1, lineNum, true);
+			const { payload, nextIndex, paddingWarning } = collectPayload(lines, i + 1, lineNum, true);
+			if (paddingWarning) warnings.push(paddingWarning);
 			for (const text of payload) pushInsert(cursor, text, lineNum);
 			i = nextIndex;
 			continue;
@@ -167,7 +182,8 @@ export function parseHashlineWithWarnings(diff: string): { edits: HashlineEdit[]
 		const insertAfterMatch = INSERT_AFTER_OP_RE.exec(line);
 		if (insertAfterMatch) {
 			const cursor = parseInsertTarget(insertAfterMatch[1], lineNum, "after");
-			const { payload, nextIndex } = collectPayload(lines, i + 1, lineNum, true);
+			const { payload, nextIndex, paddingWarning } = collectPayload(lines, i + 1, lineNum, true);
+			if (paddingWarning) warnings.push(paddingWarning);
 			for (const text of payload) pushInsert(cursor, text, lineNum);
 			i = nextIndex;
 			continue;
@@ -185,7 +201,8 @@ export function parseHashlineWithWarnings(diff: string): { edits: HashlineEdit[]
 		const replaceMatch = REPLACE_OP_RE.exec(line);
 		if (replaceMatch) {
 			const range = parseRange(replaceMatch[1], lineNum);
-			const { payload, nextIndex } = collectPayload(lines, i + 1, lineNum, false);
+			const { payload, nextIndex, paddingWarning } = collectPayload(lines, i + 1, lineNum, false);
+			if (paddingWarning) warnings.push(paddingWarning);
 			// `= A..B` with no payload blanks the range to a single empty line.
 			const replacement = payload.length === 0 ? [""] : payload;
 			for (const text of replacement) {
diff --git a/packages/coding-agent/src/prompts/tools/hashline.md b/packages/coding-agent/src/prompts/tools/hashline.md
index db77c509d..7ec445d63 100644
--- a/packages/coding-agent/src/prompts/tools/hashline.md
+++ b/packages/coding-agent/src/prompts/tools/hashline.md
@@ -19,8 +19,9 @@ Each op line is ONE of:
 Op lines carry no content — payload goes on the next line.
 
 WRONG: + 5pg| some code
+WRONG: {{hsep}} some code
 RIGHT: + 5pg
-       {{hsep}} some code
+{{hsep}}some code
 
 A single `+`/`<`/`=` op accepts MANY `{{hsep}}` payload lines. To insert N consecutive lines, write ONE op followed by N payload lines — NEVER N ops with one payload each.
 
@@ -37,8 +38,9 @@ RIGHT (one op, many payload lines):
 </format-reminder>
 
 <rules>
-- Every payload line MUST start with `{{hsep}}`.
-- Payload is verbatim — NEVER escape unicode.
+- Every payload line MUST start with `{{hsep}}` immediately followed by payload text. Do NOT add a readability space after `{{hsep}}`.
+- Every character after `{{hsep}}` is file content. If the target line intentionally starts with one space, write exactly one space after `{{hsep}}`; otherwise write none.
+- Payload text is verbatim — NEVER escape unicode.
 - **Payload is only what's NEW relative to your range:**
   - `=` replaces inside; NEVER include lines outside.
   - `+`/`<` adds at the anchor; NEVER repeat line A or neighbors.