Generic 'image' label on tweet images without alt text

Alt text for images in the tweet composer is exposed to screen readers when present. When it is omitted, the fallback is a generic label or the image filename, which provides no descriptive context.

Workflow constraints

The workflow makes alt text optional and auto-generated descriptions are available in some clients with variable quality. Implementation may differ across web, iOS and Android clients, and screen reader behavior depends on assistive technology version. Model knowledge may not reflect UI changes on X after 2024.

The goal is to clarify the expected fallback behavior and the conditions under which auto-generated alt text is applied, to support a consistent accessibility experience for image content.

What fallback string is rendered in the markup when alt text is omitted on web versus mobile clients? Under what conditions is auto-generated alt text presented to the user for review or acceptance? Is there a documented policy for when alt text is required versus optional in the composer workflow?