UNKNOWN_UPSTREAM_ERROR when attaching any image; chat freezes until a new chat is started

Environment
- Product: Antigravity / Gemini CLI
- Version: @google/gemini-cli@0.53.1
- Model: gemini-3.6-flash-high
- OS: Windows 11 (64-bit)
- Reproduced BOTH directly in the terminal (Antigravity CLI)
AND when invoked through the AionUi wrapper → not wrapper-specific.

Steps to reproduce
1. Start a chat with gemini-3.6-flash-high.
2. Attach ANY image (screenshot or JPG) to a message.
3. Send the message.

Evidence
- Screenshot 1: UNKNOWN_UPSTREAM_ERROR while using Antigravity CLI
through the AionUi wrapper.
- Screenshot 2: the same UNKNOWN_UPSTREAM_ERROR when running the CLI
directly in the terminal (no wrapper). Occurs identically in [both.]

Actual result
- Fails with error code UNKNOWN_UPSTREAM_ERROR
("Agent execution terminated due to error").
- The image is shown in the message as a path (e.g.
C:\Users\nokia\Desktop\CLS STYLING\360a_1056813108.jpg).
- After the error the chat becomes completely unresponsive: further
messages are ignored (no reaction at all). Starting a brand-new chat
is the only way to recover, and the new chat works fine until the
next image is attached.

Expected result
- The image is attached and processed normally, without freezing the
conversation.

Ruled out
- Non-ASCII / Cyrillic path theory: I first suspected the failure was
caused by Cyrillic characters in the file path. I re-tested with a
purely ASCII path (see example above) and the error occurs identically.
So it is not a path-encoding issue.

Environment variables / tokens
- Using Antigravity's own token (no custom token configured).

Additional notes
- The failure is deterministic: any image attachment triggers it every
time in the same session.
- The chat is hard-frozen after the error (not just a failed step) —
this is the most disruptive part.

Please let me know if you need a full session log or a trace of the
upstream request to investigate further.

Reproduce the failure directly in the Gemini CLI on Windows by attaching an image to a chat, then inspect the full session log and upstream request trace mentioned in the issue. Done means image attachments are processed without UNKNOWN_UPSTREAM_ERROR and the existing chat remains responsive afterward.

Written by the indexing model from the issue text.