diff --git a/pyrit/converter/token_smuggling/ascii_smuggler_converter.py b/pyrit/converter/token_smuggling/ascii_smuggler_converter.py
index c91059d3ea..be18271662 100644
--- a/pyrit/converter/token_smuggling/ascii_smuggler_converter.py
+++ b/pyrit/converter/token_smuggling/ascii_smuggler_converter.py
@@ -59,26 +59,33 @@ def encode_message(self, *, message: str) -> tuple[str, str]:
 
         Returns:
             tuple[str, str]: A tuple with a summary of code points and the encoded message.
+
+        Raises:
+            ValueError: If ``message`` contains a character outside the ASCII printable range (0x20-0x7E),
+                which has no Unicode Tag representation. Such characters were previously dropped while the
+                result still reported success; use ``SneakyBitsSmugglerConverter`` or
+                ``VariationSelectorSmugglerConverter`` to smuggle arbitrary Unicode losslessly.
         """
+        invalid_chars = "".join(char for char in message if not 0x20 <= ord(char) <= 0x7E)
+        if invalid_chars:
+            raise ValueError(
+                f"Cannot encode {len(invalid_chars)} character(s) outside the ASCII printable range "
+                f"(0x20-0x7E): {invalid_chars!r}. Use SneakyBitsSmugglerConverter or "
+                f"VariationSelectorSmugglerConverter for lossless Unicode smuggling."
+            )
+
         encoded = ""
         code_points = ""
-        invalid_chars = ""
         if self.unicode_tags:
             encoded += chr(0xE0001)
             code_points += "U+E0001 "
         for char in message:
-            if 0x20 <= ord(char) <= 0x7E:
-                code_point = 0xE0000 + ord(char)
-                encoded += chr(code_point)
-                code_points += f"U+{code_point:X} "
-            else:
-                invalid_chars += char
+            code_point = 0xE0000 + ord(char)
+            encoded += chr(code_point)
+            code_points += f"U+{code_point:X} "
         if self.unicode_tags:
             encoded += chr(0xE007F)
             code_points += "U+E007F"
-
-        if invalid_chars:
-            logger.error(f"Invalid characters detected: {invalid_chars}")
         return code_points, encoded
 
     def decode_message(self, *, message: str) -> str:
diff --git a/tests/unit/converter/test_ascii_smuggler_converter.py b/tests/unit/converter/test_ascii_smuggler_converter.py
index f5fe442862..30d48fcedb 100644
--- a/tests/unit/converter/test_ascii_smuggler_converter.py
+++ b/tests/unit/converter/test_ascii_smuggler_converter.py
@@ -46,3 +46,24 @@ async def test_ascii_smuggler_input_not_supported():
     converter = AsciiSmugglerConverter(action="encode")
     with pytest.raises(ValueError, match="Input type not supported"):
         await converter.convert_async(prompt="test", input_type="image_path")
+
+
+@pytest.mark.parametrize(
+    "prompt", ["caf\u00e9", "\u4f60\u597d\u4e16\u754c", "na\u00efve r\u00e9sum\u00e9", "\U0001f600", "line1\nline2"]
+)
+async def test_ascii_smuggler_encode_unrepresentable_chars_raise(prompt: str):
+    # Only the ASCII printable range (0x20-0x7E) maps to Unicode tags. Characters outside it used to be
+    # logged and dropped while convert_async still returned a success result, so a non-ASCII (or multi-line)
+    # objective was smuggled as a corrupted payload and an attack could be scored against a target that was
+    # never sent the objective. Encoding must fail loudly instead.
+    converter = AsciiSmugglerConverter(action="encode")
+    with pytest.raises(ValueError, match="ASCII printable range"):
+        await converter.convert_async(prompt=prompt, input_type="text")
+
+
+async def test_ascii_smuggler_encode_printable_boundaries_do_not_raise():
+    # 0x20 (space) and 0x7E (~) are the inclusive edges of the representable range and must still encode.
+    converter = AsciiSmugglerConverter(action="encode")
+    result = await converter.convert_async(prompt=" ~", input_type="text")
+    assert isinstance(result, ConverterResult)
+    assert result.output_type == "text"