diff --git a/src/core/diff/strategies/__tests__/multi-search-replace.test.ts b/src/core/diff/strategies/__tests__/multi-search-replace.test.ts
index dc971d3a10..7d19f37cba 100644
--- a/src/core/diff/strategies/__tests__/multi-search-replace.test.ts
+++ b/src/core/diff/strategies/__tests__/multi-search-replace.test.ts
@@ -2407,4 +2407,168 @@ function two() {
 			expect(description).toContain("</apply_diff>")
 		})
 	})
+
+	describe("line marker validation in REPLACE sections", () => {
+		let strategy: MultiSearchReplaceDiffStrategy
+
+		beforeEach(() => {
+			strategy = new MultiSearchReplaceDiffStrategy()
+		})
+
+		it("should reject start_line marker in REPLACE section", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				":start_line:5\n" +
+				"replacement content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("Invalid line marker ':start_line:' found in REPLACE section")
+			expect(result.error).toContain(
+				"Line markers (:start_line: and :end_line:) are only allowed in SEARCH sections",
+			)
+		})
+
+		it("should reject end_line marker in REPLACE section", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				":end_line:10\n" +
+				"replacement content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("Invalid line marker ':end_line:' found in REPLACE section")
+			expect(result.error).toContain(
+				"Line markers (:start_line: and :end_line:) are only allowed in SEARCH sections",
+			)
+		})
+
+		it("should reject both line markers in REPLACE section", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				":start_line:5\n" +
+				":end_line:10\n" +
+				"replacement content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("Invalid line marker ':start_line:' found in REPLACE section")
+		})
+
+		it("should reject line markers in multiple diff blocks where one has invalid markers", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				":start_line:1\n" +
+				"content1\n" +
+				"=======\n" +
+				"replacement1\n" +
+				">>>>>>> REPLACE\n\n" +
+				"<<<<<<< SEARCH\n" +
+				"content2\n" +
+				"=======\n" +
+				":start_line:5\n" +
+				"replacement2\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("Invalid line marker ':start_line:' found in REPLACE section")
+		})
+
+		it("should allow valid markers in SEARCH section with content in REPLACE", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				":start_line:5\n" +
+				":end_line:10\n" +
+				"-------\n" +
+				"content to find\n" +
+				"=======\n" +
+				"replacement content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(true)
+		})
+
+		it("should allow escaped line markers in REPLACE content", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				"replacement content\n" +
+				"\\:start_line:5\n" +
+				"more content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(true)
+		})
+
+		it("should allow escaped end_line markers in REPLACE content", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				"replacement content\n" +
+				"\\:end_line:10\n" +
+				"more content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(true)
+		})
+
+		it("should allow both escaped line markers in REPLACE content", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				"replacement content\n" +
+				"\\:start_line:5\n" +
+				"\\:end_line:10\n" +
+				"more content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(true)
+		})
+
+		it("should reject line markers with whitespace in REPLACE section", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				"  :start_line:5  \n" +
+				"replacement content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("Invalid line marker ':start_line:' found in REPLACE section")
+		})
+
+		it("should reject line markers in middle of REPLACE content", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				"some replacement\n" +
+				":end_line:15\n" +
+				"more replacement\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("Invalid line marker ':end_line:' found in REPLACE section")
+		})
+
+		it("should provide helpful error message format", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" + "content\n" + "=======\n" + ":start_line:5\n" + "replacement\n" + ">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("CORRECT FORMAT:")
+			expect(result.error).toContain("INCORRECT FORMAT:")
+			expect(result.error).toContain(":start_line:5    <-- Invalid location")
+		})
+	})
 })
diff --git a/src/core/diff/strategies/multi-search-replace.ts b/src/core/diff/strategies/multi-search-replace.ts
index af5a03e468..9e740a6571 100644
--- a/src/core/diff/strategies/multi-search-replace.ts
+++ b/src/core/diff/strategies/multi-search-replace.ts
@@ -243,6 +243,30 @@ Only use a single line of '=======' between search and replacement content, beca
 				">>>>>>> REPLACE\n",
 		})
 
+		const reportLineMarkerInReplaceError = (marker: string) => ({
+			success: false,
+			error:
+				`ERROR: Invalid line marker '${marker}' found in REPLACE section at line ${state.line}\n` +
+				"\n" +
+				"Line markers (:start_line: and :end_line:) are only allowed in SEARCH sections.\n" +
+				"\n" +
+				"CORRECT FORMAT:\n" +
+				"<<<<<<< SEARCH\n" +
+				":start_line:5\n" +
+				"content to find\n" +
+				"=======\n" +
+				"replacement content\n" +
+				">>>>>>> REPLACE\n" +
+				"\n" +
+				"INCORRECT FORMAT:\n" +
+				"<<<<<<< SEARCH\n" +
+				"content to find\n" +
+				"=======\n" +
+				":start_line:5    <-- Invalid location\n" +
+				"replacement content\n" +
+				">>>>>>> REPLACE\n",
+		})
+
 		const lines = diffContent.split("\n")
 		const searchCount = lines.filter((l) => l.trim() === SEARCH).length
 		const sepCount = lines.filter((l) => l.trim() === SEP).length
@@ -254,6 +278,16 @@ Only use a single line of '=======' between search and replacement content, beca
 			state.line++
 			const marker = line.trim()
 
+			// Check for line markers in REPLACE sections (but allow escaped ones)
+			if (state.current === State.AFTER_SEPARATOR) {
+				if (marker.startsWith(":start_line:") && !line.trim().startsWith("\\:start_line:")) {
+					return reportLineMarkerInReplaceError(":start_line:")
+				}
+				if (marker.startsWith(":end_line:") && !line.trim().startsWith("\\:end_line:")) {
+					return reportLineMarkerInReplaceError(":end_line:")
+				}
+			}
+
 			switch (state.current) {
 				case State.START:
 					if (marker === SEP)