Remote Control can download attachments ahead of time, and SDK hosts get new control requests

What

Claude Code can now start downloading files attached to a Remote Control message before the message itself is processed. Remote Control is the feature that lets you drive a Claude Code session from claude.ai or the mobile app. A host program sends a control request (an instruction passed to Claude Code outside the conversation) called prefetch_attachments, listing up to 16 file UUIDs. When a later message names one of those files, Claude Code uses the copy it already downloaded. Before this release, attachments were fetched only when the message was handled.

• prefetch_attachments: takes up to 16 file UUIDs and is passed through the same way mcp_call and rename_session are.
• Limits on prefetching: a cap on how much downloaded data can be held in memory, a limit on how many downloads may start in a given time window, a cutoff that discards stale requests, and an expiry for files nobody ends up using.
• claim_session: a second new control request, sent to a parked "spare" Claude Code process started with --await-claim. It is refused in the same state where set_cwd and add_directory are refused. When it fails, it returns a reason such as not_a_spare, already_claimed, busy, turn_started or cwd_not_found. It is accepted only by a process started with --await-claim that has not already been claimed.

The prefetching code sits behind a server-side flag, tengu_dapper_brooks. Nothing has been read about that flag's state.

Why

Prefetching could make images and files sent from claude.ai or mobile appear sooner, because the download can already be finished when the message arrives. For anyone building on the SDK, these are two new request types a host can send: one to warm up attachments and one to take over a spare process that is already running.