diff --git a/pkg/tools/filesystem.go b/pkg/tools/filesystem.go
index d82100970..de9749121 100644
--- a/pkg/tools/filesystem.go
+++ b/pkg/tools/filesystem.go
@@ -569,7 +569,7 @@ func (t *ReadFileLinesTool) Execute(ctx context.Context, args map[string]any) *T
 	reader := bufio.NewReaderSize(io.MultiReader(bytes.NewReader(sample), file), 32*1024)
 
 	var content strings.Builder
-	var lineIndex int64
+	lineIndex := int64(1)
 	var linesRead int64
 	var bytesRead int64
 	var reachedEOF bool
@@ -589,7 +589,8 @@ func (t *ReadFileLinesTool) Execute(ctx context.Context, args map[string]any) *T
 	}
 
 	for !reachedEOF && (limit < 0 || linesRead < limit) {
-		remaining := t.maxSize - bytesRead
+		prefix := formatReadFileLinePrefix(lineIndex)
+		remaining := t.maxSize - bytesRead - int64(len(prefix))
 		if remaining <= 0 {
 			byteBudgetTruncated = true
 			break
@@ -604,8 +605,9 @@ func (t *ReadFileLinesTool) Execute(ctx context.Context, args map[string]any) *T
 			break
 		}
 
+		content.WriteString(prefix)
 		content.Write(line)
-		bytesRead += int64(len(line))
+		bytesRead += int64(len(prefix) + len(line))
 		linesRead++
 		lineIndex++
 
@@ -648,12 +650,12 @@ func (t *ReadFileLinesTool) Execute(ctx context.Context, args map[string]any) *T
 		)
 	case byteBudgetTruncated:
 		header += fmt.Sprintf(
-			"\n[TRUNCATED - byte budget reached. Call read_file again with offset=%d to continue at the next line.]",
+			"\n[TRUNCATED - byte budget reached. Call read_file again with start_line=%d to continue at the next line.]",
 			startLine+linesRead,
 		)
 	case !reachedEOF && limit > 0 && linesRead >= limit:
 		header += fmt.Sprintf(
-			"\n[PARTIAL - more content remains. Call read_file again with offset=%d to continue.]",
+			"\n[PARTIAL - more content remains. Call read_file again with start_line=%d to continue.]",
 			startLine+linesRead,
 		)
 	default:
@@ -672,6 +674,10 @@ func (t *ReadFileLinesTool) Execute(ctx context.Context, args map[string]any) *T
 	return NewToolResult(header + "\n\n" + content.String())
 }
 
+func formatReadFileLinePrefix(lineNumber int64) string {
+	return strconv.FormatInt(lineNumber, 10) + "|"
+}
+
 func isBinaryReadFileData(data []byte) bool {
 	if len(data) == 0 {
 		return false
diff --git a/pkg/tools/filesystem_test.go b/pkg/tools/filesystem_test.go
index 2507042ec..14b34b604 100644
--- a/pkg/tools/filesystem_test.go
+++ b/pkg/tools/filesystem_test.go
@@ -888,14 +888,14 @@ func TestReadFileLinesTool_ChunkedReading(t *testing.T) {
 	if result1.IsError {
 		t.Fatalf("Chunk 1 failed: %s", result1.ForLLM)
 	}
-	if !strings.Contains(result1.ForLLM, "line 2\nline 3\n") {
+	if !strings.Contains(result1.ForLLM, "1|line 1\n2|line 2\n") {
 		t.Fatalf("expected first two lines, got: %s", result1.ForLLM)
 	}
 	if !strings.Contains(result1.ForLLM, "lines 1-2") {
 		t.Fatalf("expected line range 1-2, got: %s", result1.ForLLM)
 	}
-	if !strings.Contains(result1.ForLLM, "offset=3") {
-		t.Fatalf("expected continuation offset=3, got: %s", result1.ForLLM)
+	if !strings.Contains(result1.ForLLM, "start_line=3") {
+		t.Fatalf("expected continuation start_line=3, got: %s", result1.ForLLM)
 	}
 
 	result2 := tool.Execute(context.Background(), map[string]any{
@@ -906,11 +906,11 @@ func TestReadFileLinesTool_ChunkedReading(t *testing.T) {
 	if result2.IsError {
 		t.Fatalf("Chunk 2 failed: %s", result2.ForLLM)
 	}
-	if !strings.Contains(result2.ForLLM, "line 4\nline 5\n") {
+	if !strings.Contains(result2.ForLLM, "3|line 3\n4|line 4\n") {
 		t.Fatalf("expected middle chunk, got: %s", result2.ForLLM)
 	}
-	if !strings.Contains(result2.ForLLM, "offset=5") {
-		t.Fatalf("expected continuation offset=5, got: %s", result2.ForLLM)
+	if !strings.Contains(result2.ForLLM, "start_line=5") {
+		t.Fatalf("expected continuation start_line=5, got: %s", result2.ForLLM)
 	}
 
 	result3 := tool.Execute(context.Background(), map[string]any{
@@ -921,7 +921,7 @@ func TestReadFileLinesTool_ChunkedReading(t *testing.T) {
 	if result3.IsError {
 		t.Fatalf("Chunk 3 failed: %s", result3.ForLLM)
 	}
-	if !strings.Contains(result3.ForLLM, "line 6\n") {
+	if !strings.Contains(result3.ForLLM, "5|line 5\n6|line 6\n") {
 		t.Fatalf("expected final chunk, got: %s", result3.ForLLM)
 	}
 	if !strings.Contains(result3.ForLLM, "[END OF FILE") {
@@ -946,11 +946,11 @@ func TestReadFileLinesTool_DefaultOffsetAndRemainingLines(t *testing.T) {
 	if result.IsError {
 		t.Fatalf("Execute() error = %s", result.ForLLM)
 	}
-	if !strings.Contains(result.ForLLM, "line 2\nline 3\n") {
+	if !strings.Contains(result.ForLLM, "1|line 1\n2|line 2\n3|line 3\n") {
 		t.Fatalf("expected remaining lines by default, got: %s", result.ForLLM)
 	}
-	if !strings.Contains(result.ForLLM, "lines 1-2") {
-		t.Fatalf("expected line range 1-2, got: %s", result.ForLLM)
+	if !strings.Contains(result.ForLLM, "lines 1-3") {
+		t.Fatalf("expected line range 1-3, got: %s", result.ForLLM)
 	}
 }
 
@@ -1052,7 +1052,10 @@ func TestReadFileLinesTool_TruncatesSingleLongLineAtByteBudget(t *testing.T) {
 	if !strings.Contains(result.ForLLM, "was cut mid-line") {
 		t.Fatalf("expected explicit mid-line truncation warning, got: %s", result.ForLLM)
 	}
-	if strings.Contains(result.ForLLM, "first line") {
-		t.Fatalf("did not expect the skipped first line in output, got: %s", result.ForLLM)
+	if !strings.Contains(result.ForLLM, "1|first line\n") {
+		t.Fatalf("expected the first line with line prefix, got: %s", result.ForLLM)
+	}
+	if !strings.Contains(result.ForLLM, "2|") {
+		t.Fatalf("expected line prefix for the truncated line, got: %s", result.ForLLM)
 	}
 }