[iamnbutler]

Part of #26. Related: #105, #269.

A non-image file in the composer (dropped, picked, or pasted) no longer fails with "Only images can be attached". If it decodes as UTF-8, it is sent with the message, and every model in the chat reads it.

• Protocol: TextFile = { name, text }, carried as files? on say, ask, the message event, and QueuedMessage.
• Storage: no new store and no migration. Each file is its own text block after the message in pi's user message, <attachment name="...">\n…\n</attachment>. The first text block is still author: text, so input()/human() read existing transcripts unchanged and parse files back from later blocks. The size budget is shared: checkAttachments replaces checkImages, and images plus file text (UTF-8 bytes) must fit under the existing 1.5 MB limit (the Durable Object row cap), with at most 10 files. Names must be one line.
• Prompt: one added line in the channel section says attachments are inline, not on disk. Without it, Haiku searched the filesystem for the file before answering.
• App: images.ts became attachments.ts. It reads text files first, then shrinks images to fit the remaining budget. A binary file is refused with "X isn't a text file; only images and text files can be attached". Sent files appear in the timeline as the existing file chunk/FileView block, and by name in the queue. A withdrawn message brings its files back into the composer as chips.
• UI package: Attachment gets an optional file, which useAttachments sets, so contents without a preview can be read. The chip UI is unchanged.

Product calls:

• Files go to the agent as text in the transcript, not into the workspace. This matches how images already work, and it works the same for hosted and compute channels and for teammates on other machines.
• Binary files (PDF, zip, …) are refused for now. They would need workspace upload or provider document blocks.
• Older clients and hosts reading a new ask message see the file text joined onto the message text. Nothing is lost.

Remaining for #26: binary/non-text files (into the lane or workspace, or as documents for providers that accept them), and opening an attached file's contents from the timeline.

Smoke-tested with a source host on isolated ACE_HOME/ACE_CONFIG_HOME and real anthropic/claude-haiku-4-5 runs:

• say and ask with files: the model answered from the attachment, a name with quotes and contents containing </attachment> round-tripped, and replayed message events carried files.
• The channel refused a multi-line name, 11 files, and 1.6 MB of text.
• In the served app, file blocks render in user messages.
• The app's attachments() in Bun accepted a .md file and refused random bytes.
• I didn't drive the native file picker end to end; macOS's open panel wasn't observable to the desktop tools.

Built in Ace

Built with Claude Code outside Ace, in a session already running there; no Ace gap involved.