fix: sanitize changes diagram input

pr_agent/tools/pr_description.py

@@ -461,11 +461,9 @@ def _prepare_data(self):
         if 'description' in self.data:
             self.data['description'] = self.data.pop('description')
         if 'changes_diagram' in self.data:
-            changes_diagram = self.data.pop('changes_diagram').strip()
-            if changes_diagram.startswith('```'):
-                if not changes_diagram.endswith('```'):  # fallback for missing closing
-                    changes_diagram += '\n```'
-                self.data['changes_diagram'] = '\n'+ changes_diagram
+            sanitized = sanitize_diagram(self.data.pop('changes_diagram'))
+            if sanitized:
+                self.data['changes_diagram'] = sanitized
         if 'pr_files' in self.data:
             self.data['pr_files'] = self.data.pop('pr_files')
 
@@ -771,6 +769,30 @@ def add_file_data(self, delta_nbsp, diff_plus_minus, file_change_description_br,
 """
         return pr_body
 
+def sanitize_diagram(diagram_raw: str) -> str:
+    """Sanitize a diagram string: fix missing closing fence and remove backticks."""
+    if not isinstance(diagram_raw, str):
+        return ''
+    diagram = diagram_raw.strip()
+    if not diagram.startswith('```mermaid'):
+        return ''
+
+    # fallback missing closing
+    if not diagram.endswith('```'):
+        diagram += '\n```'  
+
+    # remove backticks inside node labels: ["`label`"] -> ["label"]
+    result = []
+    for line in diagram.split('\n'):
+        line = re.sub(
+            r'\["([^"]*?)"\]',
+            lambda m: '["' + m.group(1).replace('`', '') + '"]',
+            line,
+        )
+        result.append(line)
+    return '\n' + '\n'.join(result)
+
+
 def count_chars_without_html(string):
     if '<' not in string:
         return len(string)

tests/unittest/test_pr_description.py

@@ -0,0 +1,78 @@
+import pytest
+import yaml
+from unittest.mock import MagicMock, patch
+from pr_agent.tools.pr_description import PRDescription, sanitize_diagram
+
+KEYS_FIX = ["filename:", "language:", "changes_summary:", "changes_title:", "description:", "title:"]
+
+def _make_instance(prediction_yaml: str):
+    """Create a PRDescription instance, bypassing __init__."""
+    with patch.object(PRDescription, '__init__', lambda self, *a, **kw: None):
+        obj = PRDescription.__new__(PRDescription)
+    obj.prediction = prediction_yaml
+    obj.keys_fix = KEYS_FIX
+    obj.user_description = ""
+    return obj
+
+
+def _mock_settings():
+    """Mock get_settings used by _prepare_data."""
+    settings = MagicMock()
+    settings.pr_description.add_original_user_description = False
+    return settings
+
+
+def _prediction_with_diagram(diagram_value: str) -> str:
+    """Build a minimal YAML prediction string that includes changes_diagram."""
+    return yaml.dump({
+        'title': 'test',
+        'description': 'test',
+        'changes_diagram': diagram_value,
+    })
+
+
+class TestPRDescriptionDiagram:
+
+    @patch('pr_agent.tools.pr_description.get_settings')
+    def test_diagram_not_starting_with_fence_is_removed(self, mock_get_settings):
+        mock_get_settings.return_value = _mock_settings()
+        obj = _make_instance(_prediction_with_diagram('graph LR\nA --> B'))
+        obj._prepare_data()
+        assert 'changes_diagram' not in obj.data
+
+    @patch('pr_agent.tools.pr_description.get_settings')
+    def test_diagram_missing_closing_fence_is_appended(self, mock_get_settings):
+        mock_get_settings.return_value = _mock_settings()
+        obj = _make_instance(_prediction_with_diagram('```mermaid\ngraph LR\nA --> B'))
+        obj._prepare_data()
+        assert obj.data['changes_diagram'] == '\n```mermaid\ngraph LR\nA --> B\n```'
+
+    @patch('pr_agent.tools.pr_description.get_settings')
+    def test_backticks_inside_label_are_removed(self, mock_get_settings):
+        mock_get_settings.return_value = _mock_settings()
+        obj = _make_instance(_prediction_with_diagram('```mermaid\ngraph LR\nA["`file`"] --> B\n```'))
+        obj._prepare_data()
+        assert obj.data['changes_diagram'] == '\n```mermaid\ngraph LR\nA["file"] --> B\n```'
+
+    @patch('pr_agent.tools.pr_description.get_settings')
+    def test_backticks_outside_label_are_kept(self, mock_get_settings):
+        mock_get_settings.return_value = _mock_settings()
+        obj = _make_instance(_prediction_with_diagram('```mermaid\ngraph LR\nA["`file`"] -->|`edge`| B\n```'))
+        obj._prepare_data()
+        assert obj.data['changes_diagram'] == '\n```mermaid\ngraph LR\nA["file"] -->|`edge`| B\n```'
+
+    @patch('pr_agent.tools.pr_description.get_settings')
+    def test_normal_diagram_only_adds_newline(self, mock_get_settings):
+        mock_get_settings.return_value = _mock_settings()
+        obj = _make_instance(_prediction_with_diagram('```mermaid\ngraph LR\nA["file.py"] --> B["output"]\n```'))
+        obj._prepare_data()
+        assert obj.data['changes_diagram'] == '\n```mermaid\ngraph LR\nA["file.py"] --> B["output"]\n```'
+
+    def test_none_input_returns_empty(self):
+        assert sanitize_diagram(None) == ''
+
+    def test_non_string_input_returns_empty(self):
+        assert sanitize_diagram(123) == ''
+
+    def test_non_mermaid_fence_returns_empty(self):
+        assert sanitize_diagram('```python\nprint("hello")\n```') == ''