A chat attached via the "+" menu or dropped from the sidebar references an
existing chat by id and carries no url. add_file_context() filtered on
`file.get('url')`, so the reference was dropped from <attached_files>
entirely and the model was never told it existed.
When the RAG file-context path is enabled the chat content still reaches
the model as <source> context, which masked this. With file_context
disabled that path is skipped, and get_attached_knowledge() only promotes
collection/note items into <attached_knowledge> - so an attached chat was
visible in the UI but invisible to the model, which then reported having
no chat attachments despite having a view_chat tool available.
Keep chat references and emit their id so the model can resolve them with
view_chat. The url attribute is now conditional, since a chat has none;
the id guard it replaces was dead once the filter guarantees a url or a
chat id.
Co-authored-by: Claude <noreply@anthropic.com>
