fix(tools) diff preview for files without trailing newline

pkg/tools/shared/diff_result.go

@@ -1,6 +1,7 @@
 package toolshared
 
 import (
+	"bytes"
 	"fmt"
 	"path/filepath"
 	"strings"
@@ -8,11 +9,15 @@ import (
 	"github.com/pmezard/go-difflib/difflib"
 )
 
-const noContentChangeDiffMessage = "(no content change)"
+const (
+	noContentChangeDiffMessage = "(no content change)"
+	noNewlineAtEOFMarker       = `\ No newline at end of file`
+)
 
 // DiffResult creates a user-visible tool result containing a unified diff for
 // a successful file edit. The diff is included for both the LLM and the user so
-// the follow-up assistant response can reason about the exact change set.
+// the follow-up assistant response can reason about the resulting change set,
+// including EOF newline transitions.
 func DiffResult(path string, before, after []byte) *ToolResult {
 	diff, err := buildUnifiedDiff(path, before, after)
 	if err != nil {
@@ -25,8 +30,8 @@ func DiffResult(path string, before, after []byte) *ToolResult {
 
 func buildUnifiedDiff(path string, before, after []byte) (string, error) {
 	diff, err := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
-		A:        difflib.SplitLines(string(before)),
-		B:        difflib.SplitLines(string(after)),
+		A:        splitDiffLinesPreservingEOF(before),
+		B:        splitDiffLinesPreservingEOF(after),
 		FromFile: "a/" + diffDisplayPath(path),
 		ToFile:   "b/" + diffDisplayPath(path),
 		Context:  3,
@@ -43,6 +48,36 @@ func buildUnifiedDiff(path string, before, after []byte) (string, error) {
 	return diff, nil
 }
 
+func splitDiffLinesPreservingEOF(content []byte) []string {
+	if len(content) == 0 {
+		return nil
+	}
+
+	lines := make([]string, 0, bytes.Count(content, []byte{'\n'})+1)
+	lineStart := 0
+	for i, b := range content {
+		if b != '\n' {
+			continue
+		}
+		lines = append(lines, string(content[lineStart:i+1]))
+		lineStart = i + 1
+	}
+	if lineStart < len(content) {
+		lines = append(lines, string(content[lineStart:]))
+	}
+
+	if lacksTrailingNewline(content) {
+		lines[len(lines)-1] += "\n"
+		lines = append(lines, noNewlineAtEOFMarker+"\n")
+	}
+
+	return lines
+}
+
+func lacksTrailingNewline(content []byte) bool {
+	return len(content) > 0 && !bytes.HasSuffix(content, []byte("\n"))
+}
+
 func diffDisplayPath(path string) string {
 	displayPath := strings.TrimLeft(filepath.ToSlash(path), "/")
 	if displayPath == "" {

pkg/tools/shared/diff_result_test.go

@@ -26,7 +26,7 @@ func TestDiffResult_UserVisibleUnifiedDiff(t *testing.T) {
 		"```diff",
 		"--- a/tmp/example.txt",
 		"+++ b/tmp/example.txt",
-		"@@ -1,4 +1,4 @@",
+		"@@ -1,3 +1,3 @@",
 		" alpha",
 		"-beta",
 		"+beta 2",
@@ -48,6 +48,42 @@ func TestBuildUnifiedDiff_NoContentChange(t *testing.T) {
 	}
 }
 
+func TestBuildUnifiedDiff_PreservesTrailingNewlineRemoval(t *testing.T) {
+	diff, err := buildUnifiedDiff("test.txt", []byte("same\n"), []byte("same"))
+	if err != nil {
+		t.Fatalf("buildUnifiedDiff() error = %v", err)
+	}
+
+	for _, want := range []string{
+		"--- a/test.txt",
+		"+++ b/test.txt",
+		" same",
+		"+" + noNewlineAtEOFMarker,
+	} {
+		if !strings.Contains(diff, want) {
+			t.Fatalf("buildUnifiedDiff() missing %q:\n%s", want, diff)
+		}
+	}
+}
+
+func TestBuildUnifiedDiff_PreservesTrailingNewlineAddition(t *testing.T) {
+	diff, err := buildUnifiedDiff("test.txt", []byte("same"), []byte("same\n"))
+	if err != nil {
+		t.Fatalf("buildUnifiedDiff() error = %v", err)
+	}
+
+	for _, want := range []string{
+		"--- a/test.txt",
+		"+++ b/test.txt",
+		" same",
+		"-" + noNewlineAtEOFMarker,
+	} {
+		if !strings.Contains(diff, want) {
+			t.Fatalf("buildUnifiedDiff() missing %q:\n%s", want, diff)
+		}
+	}
+}
+
 func TestBuildUnifiedDiff_UsesNormalizedDisplayPaths(t *testing.T) {
 	diff, err := buildUnifiedDiff("/tmp/nested/example.txt", []byte("before\n"), []byte("after\n"))
 	if err != nil {