fix(engine): sanitize YAML date values in frontmatter to ISO strings

PyYAML safe_load converts date strings (e.g. '2026-03-06') into
datetime.date objects, which are not JSON-serializable. This caused
failures when frontmatter results were passed through the workflow
engine's JSON serialization pipeline.

- Add recursive _sanitize() in extract_markdown_frontmatter()
- Convert datetime.date and datetime.datetime to ISO strings
- Remove duplicate file_ext assignment in ReadFilesExecutor

Author: ibacalu

src/workflows_mcp/engine/executors_file.py

@@ -780,7 +780,6 @@ async def _execute_single_file(
                 file_path, mode
             )
             # Extract markdown intelligence for outline/summary mode
-            file_ext = file_path.suffix.lower()
             if file_ext in (".md", ".markdown"):
                 raw_content = file_path.read_text(encoding="utf-8", errors="replace")
                 frontmatter = extract_markdown_frontmatter(raw_content)

src/workflows_mcp/engine/file_outline.py

@@ -508,10 +508,28 @@ def extract_markdown_frontmatter(content: str) -> dict | None:
         return None
 
     try:
+        from datetime import date, datetime
+
         import yaml as _yaml
 
         result = _yaml.safe_load(yaml_text)
-        return result if isinstance(result, dict) else None
+        if not isinstance(result, dict):
+            return None
+
+        # PyYAML safe_load converts date strings (e.g. "2026-03-06") to
+        # datetime.date objects — sanitize to ISO strings for JSON safety.
+        def _sanitize(obj: object) -> object:
+            if isinstance(obj, datetime):
+                return obj.isoformat()
+            if isinstance(obj, date):
+                return obj.isoformat()
+            if isinstance(obj, dict):
+                return {k: _sanitize(v) for k, v in obj.items()}
+            if isinstance(obj, list):
+                return [_sanitize(v) for v in obj]
+            return obj
+
+        return _sanitize(result)  # type: ignore[return-value]
     except Exception:
         return None
 

tests/test_readfiles_sections.py

@@ -327,6 +327,19 @@ def test_invalid_yaml(self) -> None:
         """Invalid YAML returns None (no crash)."""
         assert extract_markdown_frontmatter("---\n[invalid: yaml:\n---\nContent\n") is None
 
+    def test_dates_converted_to_strings(self) -> None:
+        """YAML date values are converted to ISO strings for JSON safety."""
+        import json
+
+        content = "---\ncreated: 2026-03-06\ntitle: Test\n---\n# Title\n"
+        result = extract_markdown_frontmatter(content)
+
+        assert result is not None
+        assert isinstance(result["created"], str)
+        assert result["created"] == "2026-03-06"
+        # Must be JSON-serializable
+        json.dumps(result)  # should not raise
+
 
 # ── extract_markdown_references ────────────────────────────────────────