Resize web image attachments before forwarding them to model providers

[ashwin-pc]

Summary

Browser-uploaded image attachments are forwarded to session.prompt() at their original dimensions. This bypasses pi's normal image processing path, which automatically resizes images used through read and @file to a 2000×2000 maximum.

This can leave a session in a permanently failing state once it accumulates enough images for a provider's stricter many-image limits to apply. At that point, even a text-only follow-up fails because the oversized historical attachment is resent as part of the conversation context.

Reproduction

1. Start a session with an image-capable Anthropic model.
2. Upload one or more screenshots with a dimension greater than 2000px through the web composer.
3. Continue until the request contains enough historical images to trigger the provider's many-image validation rules.
4. Send another message, including a text-only message.

The provider rejects the request before generation with an error similar to:

Validation error: messages.N.content.M.image.source.base64.data:
At least one of the image dimensions exceed max allowed size for
many-image requests: 2000 pixels

Expected behavior

Web attachments should be normalized before their first model request so that no embedded image exceeds the provider-compatible maximum dimension. A previously accepted attachment should not make all future turns fail when the session later crosses a provider image-count threshold.

Current behavior

The browser reads the original file as a data URL, and the server persists and forwards the same base64 payload directly. The automatic resizing used by pi's other image ingestion paths is therefore not applied to web-composer attachments.

Suggested fix

• Process uploaded images server-side before adding them to session.prompt().
• Apply the same default maximum dimension as pi's image processor (2000×2000), preserving aspect ratio.
• Optionally keep the full-resolution upload on disk while embedding only the normalized copy in model history.
• Validate the decoded MIME type and payload size at the same boundary.
• Add a regression test with a synthetic image larger than 2000px and assert that the image passed to the session is resized.
• Consider a recovery mechanism for existing sessions that already contain oversized embedded images.