What
Publishing a data file with an artifact by path now fails up front if the file's extension has no known content type, and the error tells you to rename it to something recognised or list it under files in map form with an explicit contentType.
Details
- The error text suggests
.jsonor.txtas known extensions. - Path resolution was reworked to try both the working directory and its real (symlink-resolved) path before computing the key the file is published under.
Evidence
has no known content type for its extension \u2014 rename it to a known one (e.g. .json or .txt)
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
Names in the bundlecontentTypefiles
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed since
Anthropic's documentation has since written up contentType, on skills-guide.
.contentType("text/markdown")build-with-claude/skills-guide see the edit
Confirmed since
Anthropic's documentation has since written up files, on Give Claude access to your tools.
Anthropic's **Security Guidance** [plugin](https://code.claude.com/docs/en/plugins) has Claude review the code it writes. With the plugin on in a channel, Claude is warned about risky patterns as it edits files, and the plugin reviews the …claude-tag/admins/add-connections see the edit
Added since
A small documentation edit on Changelog touched a line naming files after this was published.
A Windows update released September 8, 2026 (including KB5124008) stops Cowork from reaching your files when it runs on your Windows PC, so tasks fail or the workspace does not start. This affects Cowork on third-party inference deployment…cowork/changelog see the edit
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
A small documentation edit on Changelog touched a line naming files after this was published.