[BUG] Desktop app (Windows): pasted long text becomes an attachment the model receives as empty

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Scope / labels requested: this is a Claude Desktop chat app bug on Windows, not Claude Code CLI. Please triage under area:desktop + platform:windows rather than closing as invalid.

Related: #77607 ("Claude Desktop: can't load attachments or read pastes-as-attachments after forced update to 1.21459.0") describes the same underlying failure, but was closed as invalid / doesn't seem to be related to Claude Code, so it never reached the desktop team. Desktop-app issues that were triaged correctly under area:desktop: #71536, #72179.

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In the Claude for Windows desktop app, pasting a long block of plain text (roughly 100+ lines, e.g. docker logs output) into the message box causes the app to convert the paste into an attachment instead of keeping it as inline text.

The message sends and the attachment renders in the UI on my side, but the model receives that attachment as empty — it reports that the document has no readable content and asks me to provide the data again.

Important distinction: explicitly uploading a .txt file works fine — the model reads that without any problem. The failure is specific to pastes that the app auto-converts into attachments; those arrive empty on the model's side.

This fails silently from the user's point of view: the attachment looks present and correct in the UI, so there is no indication that anything was lost.

What Should Happen?

Pasted text must reach the model in full — either kept as inline text, or converted to an attachment whose content is actually transmitted.

If the client chooses to convert a paste into an attachment, that attachment's content must be delivered to the model. If for some reason it cannot be, the app should surface a visible error instead of silently sending an empty attachment.

Error Messages/Logs

Steps to Reproduce

1. Open the Claude for Windows desktop app and start (or open) any chat
2. Copy roughly 100+ lines of plain text to the clipboard — e.g. the output of docker logs <container>, a stack trace, or any long console log
3. Paste it into the chat input box. The app converts the paste into an attachment instead of keeping it as inline text
4. Send the message
5. Ask the model to quote or summarise the pasted content

Result: the model reports that the attached document is empty / has no readable content.

Control test (works correctly): save the exact same text as a .txt file and attach it via the normal file-upload button — the model reads it without any problem. Only the auto-converted paste arrives empty.

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

N/A — this is the Claude Desktop chat app, not Claude Code CLI (see Additional Information for the desktop app version)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Claude Desktop app version: Claude 1.24012.9 (03c61d) 2026-07-24T04:59:17.000Z

Impact: blocks any workflow that involves sharing longer logs, stack traces or command output by pasting. Because the attachment looks present in the UI, the user gets no indication that the content was lost, and ends up being asked for the same data repeatedly. Current workarounds are saving to a .txt file and uploading it, or splitting the text into many small messages.

Reproducibility: consistent, every time a paste is large enough to be auto-converted into an attachment.

Note on #77607: that report ties the same symptom to the forced auto-update to 1.21459.0 on Windows. If my version is at or above that build, it is likely the same regression.