diff --git a/cylc/flow/parsec/validate.py b/cylc/flow/parsec/validate.py
index 4ef66e19862..0f69e5d8ef1 100644
--- a/cylc/flow/parsec/validate.py
+++ b/cylc/flow/parsec/validate.py
@@ -26,7 +26,7 @@
 import shlex
 from collections import deque
 from textwrap import dedent
-from typing import List, Dict, Any, Tuple
+from typing import List, Dict, Any, Tuple, Union
 
 from metomi.isodatetime.data import Duration, TimePoint
 from metomi.isodatetime.dumpers import TimePointDumper
@@ -373,7 +373,7 @@ def coerce_range(cls, value, keys):
         return Range((int(min_), int(max_)))
 
     @classmethod
-    def coerce_str(cls, value, keys):
+    def coerce_str(cls, value: Union[str, List[str]], keys: List[str]) -> str:
         """Coerce value to a string.
 
         Examples:
@@ -512,16 +512,16 @@ def parse_int_range(cls, value):
             return None
 
     @classmethod
-    def strip_and_unquote(cls, keys, value):
+    def strip_and_unquote(cls, keys: List[str], value: str) -> str:
         """Remove leading and trailing spaces and unquote value.
 
         Args:
-            keys (list):
+            keys:
                 Keys in nested dict that represents the raw configuration.
-            value (str):
+            value:
                 String value in raw configuration.
 
-        Return (str):
+        Return:
             Processed value.
 
         Examples:
@@ -1136,23 +1136,25 @@ def _coerce_type(cls, value):
         return val
 
 
-# BACK COMPAT: BroadcastConfigValidator
-# The DB at 8.0.x stores Interval values as neither ISO8601 duration
-# string or DurationFloat. This has been fixed at 8.1.0, and
-# the following class acts as a bridge between fixed and broken.
-# url:
-#     https://github.com/cylc/cylc-flow/pull/5138
-# from:
-#    8.0.x
-# to:
-#    8.1.x
-# remove at:
-#    8.x
 class BroadcastConfigValidator(CylcConfigValidator):
     """Validate and Coerce DB loaded broadcast config to internal objects."""
     def __init__(self):
         CylcConfigValidator.__init__(self)
 
+    @classmethod
+    def coerce_str(cls, value: Union[str, List[str]], keys: List[str]) -> str:
+        """Coerce value to a string.
+
+        Examples:
+            >>> BroadcastConfigValidator.coerce_str('abc#def', None)
+            'abc#def'
+        """
+        # Prevent ParsecValidator from assuming '#' means comments;
+        # '#' has valid uses in shell script such as parameter substitution
+        if isinstance(value, str) and '#' in value:
+            value = f'"{value}"'
+        return ParsecValidator.coerce_str(value, keys)
+
     @classmethod
     def strip_and_unquote_list(cls, keys, value):
         """Remove leading and trailing spaces and unquote list value.
@@ -1177,6 +1179,18 @@ def strip_and_unquote_list(cls, keys, value):
             value = value.lstrip('[').rstrip(']')
         return ParsecValidator.strip_and_unquote_list(keys, value)
 
+    # BACK COMPAT: BroadcastConfigValidator.coerce_interval
+    # The DB at 8.0.x stores Interval values as neither ISO8601 duration
+    # string or DurationFloat. This has been fixed at 8.1.0, and
+    # the following class acts as a bridge between fixed and broken.
+    # url:
+    #     https://github.com/cylc/cylc-flow/pull/5138
+    # from:
+    #    8.0.x
+    # to:
+    #    8.1.x
+    # remove at:
+    #    8.x
     @classmethod
     def coerce_interval(cls, value, keys):
         """Coerce an ISO 8601 interval into seconds.
diff --git a/tests/functional/broadcast/10-file-1/broadcast.cylc b/tests/functional/broadcast/10-file-1/broadcast.cylc
index f429f8a2740..eaadb6b4579 100644
--- a/tests/functional/broadcast/10-file-1/broadcast.cylc
+++ b/tests/functional/broadcast/10-file-1/broadcast.cylc
@@ -1,6 +1,8 @@
 script="""
 printenv CYLC_FOOBAR
 
+# (This hash char should not cause the rest of the script to be stripped out)
+
 if (($CYLC_TASK_TRY_NUMBER < 2 )); then
     false
 fi
diff --git a/tests/unit/parsec/test_validate.py b/tests/unit/parsec/test_validate.py
index 8c73b840eb3..ff68e9e78d6 100644
--- a/tests/unit/parsec/test_validate.py
+++ b/tests/unit/parsec/test_validate.py
@@ -456,11 +456,9 @@ def test_coerce_int_list():
             validator.coerce_int_list(value, ['whatever'])
 
 
-def test_coerce_str():
-    """Test coerce_str."""
-    validator = ParsecValidator()
-    # The good
-    for value, result in [
+@pytest.mark.parametrize(
+    'value, expected',
+    [
         ('', ''),
         ('Hello World!', 'Hello World!'),
         ('"Hello World!"', 'Hello World!'),
@@ -474,9 +472,15 @@ def test_coerce_str():
          'Hello:\n    foo\nGreet\n    baz'),
         ('False', 'False'),
         ('None', 'None'),
-        (['a', 'b'], 'a\nb')
-    ]:
-        assert validator.coerce_str(value, ['whatever']) == result
+        (['a', 'b'], 'a\nb'),
+        ('abc#def', 'abc'),
+    ]
+)
+def test_coerce_str(value: str, expected: str):
+    """Test coerce_str."""
+    validator = ParsecValidator()
+    # The good
+    assert validator.coerce_str(value, ['whatever']) == expected
 
 
 def test_coerce_str_list():