What
A marketplace is a catalogue of plugins that Claude Code can install from. Some sessions run without being able to write files to disk. In those sessions, trying to add a claude.ai marketplace now stops straight away with the message "claude.ai marketplaces can't be added in this session, because it can't save them". Claude Code no longer tries to write its list of known claude.ai marketplaces to a file.
Why
You get a plain explanation of why the add did not work, instead of a save that fails or is quietly lost.
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
It is not clear which kind of session counts as unable to write to disk, or what happened in that situation before.