Unclear Which characters count as disallowed is not shown.
What
When you attach a file, Claude Code used to check only that the file exists, is inside the working folder and is a normal file. It now also refuses file names and paths that contain disallowed characters. The message asks you to give a file name without those characters. If the file really has that name, copy or rename it to a name without them and send that, or correct the path.
Why
Some attachments that used to send will now fail with a message until the name is changed.
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Which characters count as disallowed is not shown.