[BUG] Cowork: images attached to a chat message are silently dropped — 2 of 3 never reach the agent, no failure signal either side

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)

### What's Wrong?

In Cowork on the desktop app, images attached directly to a chat message are silently dropped on the way to the agent.

I attached three screenshots to a single message. Only one of them reached the agent. The other two were never received, and the agent had no knowledge that any further images had been sent.

Nothing surfaced the failure. The attachments appeared to send normally on my side, and the agent received no indication that an attachment had been sent and had failed to arrive. The agent therefore carried on reasoning from partial input, believing it had everything.

### What Should Happen?

Every image attached to a message reaches the agent. If an attachment cannot be delivered, both the sender and the agent are told, so the agent can ask for it again instead of silently working with partial input.

### Error Messages/Logs

```shell
No error was produced. The failure is silent on both sides: the send appears to
succeed in the UI, and the agent receives no signal that an attachment was sent
and did not arrive.
```

### Steps to Reproduce

1. Open a Cowork session in the desktop app.
2. Attach three images directly to a single chat message and send it.
3. Ask the agent to describe each image it received.

Observed: the agent reports only one image. The other two never arrived, and no delivery failure is reported anywhere.

Workaround that does work reliably: put the same images into a zip archive and attach the archive instead. Every image survives that path, which suggests the problem is specific to the direct multi-image attach path rather than to file transfer in general.

### Claude Model

Not sure / Multiple models

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

Claude Desktop 1.34493.1 (the build currently installed on the affected machine).
The exact build in use on the day of the incident was not recorded, so this is the current
version rather than a confirmed-affected one.

### Platform

Claude Desktop app

### Operating System

macOS 26.5.2 (build 25F84), Apple silicon

### Terminal/Shell

Not applicable (desktop app chat, not a terminal session)

### Additional Information

Two asks, in priority order:

1. Reliable delivery of images attached directly to a message.
2. An agent-side signal when an attachment was sent but did not arrive. The silent case is the damaging one, because the agent cannot know to ask for the missing input.

Possibly related but not the same report: #87162 covers files dropped into the desktop Code tab on Windows being inaccessible to the model. That one is a total failure on a different surface and OS; this report is a partial, silent drop in Cowork chat on macOS with a working zip workaround.

No files, tests, or entry points are named. Start by reproducing the Cowork desktop flow with three direct image attachments, then compare it with the reliable zip-archive path. Done means all images reach the agent, and both sender and agent receive a failure signal when delivery does not complete.

索引モデルが issue の本文から書いたものです。