Desktop app: image attachments fail above ~100KB while a 17MB PDF attaches fine (macOS, arm64)

Summary

On the Claude desktop app, attaching a PNG larger than roughly 100KB fails, while PDF attachments of 17MB+ go through without any problem. Every Mac user on our team reproduces this.

Environment

| | |
|---|---|
| App | Claude desktop 2.2553.1 |
| Electron / Node | 44.2.0 / 24.20.0 |
| OS | macOS 26.6.2 (25G83) |
| Platform | darwin arm64 |
| Locale | ko-KR |

Symptom

One of two toasts appears when attaching an image (localized, translated here):

"image processing failed, you can try again"

"cannot read 1 attachment: <name>.png, attach it again or remove it before sending"

The attachment thumbnail sometimes renders as a black placeholder labelled "cannot read file".

Boundary (reproduced)

| Size | Result |
|---|---|
| 61KB | OK |
| 78KB | OK |
| 94KB | OK |
| 100KB+ | Fails consistently |
| 988KB | Fails |
| 17.3MB PDF | OK |

128KB succeeded once and failed on a later attempt, so behaviour right at the boundary is not fully deterministic.

Ruled out

| Factor | Evidence |
|---|---|
| Filename | Korean characters and spaces are not the cause; a plain test.png of the same size fails identically |
| File location | Same result from Desktop and from inside the project folder |
| Attachment method | Clipboard paste, Finder copy/paste, drag-and-drop and typed file path all behave the same |
| Network / gateway body-size limit | A 17.3MB PDF attaches successfully over the same connection |
| 2000px resize threshold | Failures occur well below that, at ~100KB regardless of pixel dimensions |

Hypothesis

The image-specific path (native resize module) fails, while PDFs bypass it entirely. Likely related to #85251 (image processing unavailable on macOS 26) since this machine is on macOS 26.6.2, and to #84251, but the trigger here is a ~100KB file-size threshold rather than pixel dimensions.

Workaround in use

Wrapping the screenshot in a PDF (sips -s format pdf shot.png --out shot.pdf), or compressing below ~90KB.