diff --git a/tests/cpydiff/types_str_formatsep_float.py b/tests/cpydiff/types_str_formatsep_float.py
new file mode 100644
index 0000000000..b487cd3758
--- /dev/null
+++ b/tests/cpydiff/types_str_formatsep_float.py
@@ -0,0 +1,11 @@
+"""
+categories: Types,str
+description: MicroPython accepts but does not properly implement the "," or "_" grouping character for float values
+cause: To reduce code size, MicroPython does not implement this combination. Grouping characters will not appear in the number's significant digits and will appear at incorrect locations in leading zeros.
+workaround: Do not use a format string like ``{:,f}`` if exact CPython compatibility is required.
+"""
+
+print("{:,f}".format(3141.159))
+print("{:_f}".format(3141.159))
+print("{:011,.2f}".format(3141.159))
+print("{:011_.2f}".format(3141.159))