S3FileSystem.isfile() returns True for a directory-like prefix (key/prefix collision), and the result changes after calling isdir()

[csniu]

I have these two objects:

• s3://my-bucket/some/prefix (an object key exactly equal to the “directory” name)
• s3://my-bucket/some/prefix/file.txt (an object under that prefix)

Then:

from s3fs import S3FileSystem

fs  = S3FileSystem()

print(fs.isfile('det-study-a/some/prefix/'))  # True
print(fs.isdir('det-study-a/some/prefix/'))  # True
print(fs.isfile('det-study-a/some/prefix/'))  # False
print(fs.isdir('det-study-a/some/prefix/'))  # True

Environment

• s3fs version: 2025.12.0
• Python: 3.12