Download the PHP package tomvondracek/bolt-ai-image-alt without Composer

AI Image Alt Texts for Bolt CMS

https://github.com/user-attachments/assets/bad597c5-481c-49e3-b9bd-765218955408

Generates image ALT texts in the Bolt 6 admin, in the editor's browser:

• captions come from Florence-2 running through Transformers.js (WebGPU, or WebAssembly as a fallback);
• Chrome's built-in Translator API translates them into the language of the content.

Image data never leaves the browser, and there are no API costs. Only the model weights are downloaded once from Hugging Face (a few hundred MB) and cached by the browser.

Features

• "Generate ALT" button under every image alt input (image fields without alt: false, and every imagelist item, including images inside collections and sets). The main click uses the configured task; its dropdown lets the editor pick a level of detail (short / detailed / very detailed) for that image.
• Automatic generation right after an image is uploaded, picked from the library or uploaded from a URL. An alt is filled only if it is empty, and never replaced if the editor typed something in the meantime.
• Correct language: localized fields use the edit locale, others use the ContentType's first locale or the site default (%locale%).
• Batch page (/bolt/ai-alt/batch, sidebar "AI ALT") to fill missing alts on existing content, one image at a time, with "review before saving" (on by default), pause/resume and an ETA.
• The generated text is marked in the editor ("AI – please review") until the editor edits it.
• UI in English, Czech, Slovak, German, French, Dutch, Italian, Spanish, Romanian, Ukrainian and Russian, following the backend user's language. Strings live in assets/langs/<language>.json; add a file to add a language.

Installation

extensions:configure copies the prebuilt browser files to public/extensions/ai-alt/. Node.js is not needed on the server. Run it again after every update of the extension.

Configuration

config/extensions/tomvondracek-aialt.yaml (created on first use; see config/config.yaml for all options):

Saving from the batch page also requires the edit permission on each record.

Browser support

Without WebGPU the model runs on WebAssembly single-threaded: the Bolt admin isn't cross-origin isolated (no COOP/COEP headers), so ONNX Runtime can't use worker threads. Expect several seconds per image there. GPUs without shader-f16 get fp32 weights; if WebGPU fails anyway, the worker falls back to WebAssembly. A caption that takes longer than 3 minutes is aborted and the model reloaded, so a hung GPU can't block the queue.

When no translator is available, the alt stays empty by default and the chip says so. With fallback_without_translator: english the English caption is written and marked "EN – please translate". English sites need no translator.

Content Security Policy

Bolt sets no CSP. If your hosting adds one, the admin needs:

(connect-src for Hugging Face is not needed with self-hosted weights, see model_host.)

Self-hosted model weights

Download the model repository (config.json, preprocessor_config.json, tokenizer*.json, generation_config.json, onnx/*.onnx) into e.g. public/ai-alt-models/onnx-community/Florence-2-base-ft/ and set model_host: /ai-alt-models. Don't use public/extensions/ai-alt/, because extensions:configure replaces it.

model_host is read like this: a bare origin (https://huggingface.co, or a mirror like https://hf-mirror.com) uses the Hub layout {model}/resolve/main/…; a URL with a path (/ai-alt-models, https://cdn.example.com/models) is a plain copy, {model}/…. If Bolt runs in a subdirectory, include it (/cms/ai-alt-models). The extension's own assets and thumbnails follow the subdirectory automatically.

Limitations

• Captions come from a small model: they are generic, and sometimes wrong (a kangaroo may become a deer). Review them.
• The batch page covers top-level image / imagelist fields; images inside collections and sets are only handled in the editor.
• The batch page runs only while its tab is open. Open it again later to continue: finished images drop out of the list.
• Bolt's Media entity has no alt, so only content fields are filled.

Development

To try it in a Bolt project from a local checkout:

Layout

License

MIT