Handle diffs missing context prefixes

gptdiff/applydiff.py

@@ -96,8 +96,19 @@ def apply_patch_to_file(file_path, patch):
                         # Addition line: add to new_lines.
                         new_lines.append(pline[1:] + "\n")
                     else:
-                        print("Unexpected line in hunk:", pline)
-                        return False
+                        # Some diffs (especially LLM-generated) omit the leading space on
+                        # context lines. Treat bare lines as context when they match the
+                        # original file to make patch application more forgiving.
+                        expected = pline
+                        if current_index >= len(original_lines):
+                            print("Context line expected but file ended")
+                            return False
+                        orig_line = original_lines[current_index].rstrip("\n")
+                        if orig_line != expected:
+                            print("Context line mismatch. Expected:", expected, "Got:", orig_line)
+                            return False
+                        new_lines.append(original_lines[current_index])
+                        current_index += 1
                     i += 1
             else:
                 # Skip non-hunk header lines.

tests/test_applydiff_edgecases.py

@@ -141,6 +141,30 @@ def test_diff_with_whitespace_changes(tmp_project_dir):
     assert "line2  " in content, "Whitespace change should be reflected in the file"
 
 
+def test_diff_without_context_prefixes(tmp_project_dir):
+    """
+    Diffs generated by LLMs sometimes omit the leading space on context lines.
+    Ensure we still treat those lines as context when they match the file.
+    """
+    file = tmp_project_dir / "example.txt"
+    file.write_text("line1\nline2\nline3\n")
+    diff_text = (
+        "diff --git a/example.txt b/example.txt\n"
+        "--- a/example.txt\n"
+        "+++ a/example.txt\n"
+        "@@ -1,3 +1,3 @@\n"
+        "line1\n"
+        "-line2\n"
+        "+line2 updated\n"
+        "line3\n"
+    )
+
+    result = apply_diff(str(tmp_project_dir), diff_text)
+
+    assert result is True, "Diff without context prefixes should still apply"
+    assert file.read_text() == "line1\nline2 updated\nline3\n"
+
+
 def test_diff_file_deletion_edge(tmp_project_dir):
     """
     Test deletion diff for a file with minimal content.