fix: Clipboard image paste fails with AI_DownloadError due to binary corruption and encoding mismatch #3816

1. What's the bug/issue?

When pasting images from clipboard on Linux, the application fails with an AI_DownloadError when attempting to process the image data URL. This is caused by two bugs in the clipboard handling code:

Bug #1: Binary Data Corruption
The Linux clipboard code (Wayland and X11) uses .text() to read binary PNG data, which attempts to decode the binary image bytes as UTF-8 text. This completely corrupts the image data.

Bug #2: Encoding Mismatch
The clipboard code uses base64url encoding (URL-safe variant with - and _ characters) instead of standard base64 encoding (with + and / characters) required by the data URL specification (RFC 2397).

Location: packages/opencode/src/cli/cmd/tui/util/clipboard.ts lines 33-50

2. How to reproduce it

1. Environment: Opencode v1.0.15, Ubuntu 25.04 (Omabub), in an Alacritty/Zellij terminal session
2. Take a screenshot using the system shortcut, saving the image (approx. 800KB - 1.5MB PNG) to the clipboard
3. In the opencode editor (e.g., in a ~sticky session), paste the image using the terminal's paste command/shortcut (e.g., Ctrl+V)

Resulting Error:

AI_DownloadError: Failed to download data:image/png;base64,77-9UE5HDQoaCgAAAA1JSERSAAAB77-9AAABDggGAAAAUe-_ve-_vWcAAAAEc0JJVAgICAh8CGTvv70AAAAZdEVYdFNvZnR3YXJlAGdub21lLXNjcmVlbnNob3Tvv70D77-9PgAAAC10RVh0Q3JlYXRpb24gVGltZQBNb24gMDMgTm92IDIwMjUgMTE6MzE6MTIgQU0gQ0VU77-9SC8eAAAgAElFTkTvv71CYO-_vQ: Error: failed to fetch the data URL

3. What's the expected vs actual behavior?

Expected Behavior:
When an image is pasted from clipboard, it should be:

1. Read as binary data (not decoded as text)
2. Encoded using standard base64 (not base64url)
3. Embedded in a proper data URL format: data:image/png;base64,<standard-base64-data>
4. Successfully processed by the AI SDK

Actual Behavior:
The application corrupts the binary image data by decoding it as UTF-8 text, then encodes it with base64url instead of standard base64, resulting in an invalid data URL that the AI SDK cannot process.

4. What's the proposed fix?

In packages/opencode/src/cli/cmd/tui/util/clipboard.ts:

For Linux (Wayland, lines 33-35):

• Change .text() to .arrayBuffer() to preserve binary data
• Change toString('base64url') to toString('base64')
• Add validation: if (wayland && wayland.byteLength > 0)

For Linux (X11, lines 37-39):

• Change .text() to .arrayBuffer() to preserve binary data
• Change toString('base64url') to toString('base64')
• Add validation: if (x11 && x11.byteLength > 0)

For Windows (line 50):

• Change toString('base64url') to toString('base64')

This aligns with how macOS correctly handles clipboard images (line 25) and ensures data URLs comply with RFC 2397.