fix: normalize 'ascii' encoding to 'utf-8' (ASCII is valid UTF-8)

Author: chrisdpurcell

src/services/file_service.py

@@ -80,13 +80,17 @@ def _detect_encoding(raw: bytes) -> str:
 
     Module-level so it can be tested without instantiating FileService.
     Falls back to utf-8 if chardet is unavailable or confidence is too low.
+    Normalizes 'ascii' to 'utf-8' because ASCII is a proper subset of UTF-8
+    and saving as utf-8 is always safe for ASCII content.
     """
     try:
         import chardet  # optional dependency
 
         result = chardet.detect(raw)
         if result["encoding"] and result["confidence"] >= _ENCODING_MIN_CONFIDENCE:
-            return result["encoding"]
+            detected = result["encoding"].lower()
+            # ASCII is a valid subset of UTF-8; normalize to avoid confusing the UI.
+            return "utf-8" if detected == "ascii" else result["encoding"]
     except ImportError:
         logger.debug("chardet not installed; defaulting to utf-8")
     return _ENCODING_FALLBACK

tests/unit/test_file_service.py

@@ -97,6 +97,25 @@ def test_falls_back_to_utf8_for_undetectable_bytes(self):
         assert encoding == "utf-8"
 
 
+class TestDetectEncodingNormalization:
+    def test_ascii_content_returns_utf8(self):
+        """Pure-ASCII bytes must be labelled utf-8, not ascii.
+
+        chardet correctly identifies ASCII bytes as 'ascii'. We normalize to
+        'utf-8' because ASCII is a proper subset of UTF-8 and saving as utf-8
+        is always safe for ASCII content.
+        """
+        from src.services.file_service import _detect_encoding
+        raw = b"hello world"  # pure ASCII
+        assert _detect_encoding(raw) == "utf-8"
+
+    def test_non_ascii_utf8_content_returns_utf8(self):
+        """Non-ASCII UTF-8 bytes (é, ñ) must return utf-8."""
+        from src.services.file_service import _detect_encoding
+        raw = "café résumé naïve".encode("utf-8")
+        assert _detect_encoding(raw) == "utf-8"
+
+
 class TestAtomicSaveCleanup:
     """Verify temp file is cleaned up when os.replace fails (lines 70-75)."""