fix(workers-ai): fix broken multipart FormData examples for Flux models

[mattrothenberg]

Summary

• Fix broken code examples for Flux 2 multipart API in docs
• Add explanatory comment about why the Response constructor workaround is needed

Problem

The existing code examples for Flux 2 models (dev, klein-4b, klein-9b) don't work. They either:

1. Reference undefined variables (formStream, formContentType)
2. Pass FormData directly with a hardcoded "multipart/form-data" content-type

Both approaches fail because the AI API requires the multipart boundary parameter in the Content-Type header to parse the form fields.

Solution

FormData doesn't expose its serialized body or boundary. Passing it to a Response (or Request) constructor serializes it and generates the Content-Type header with the boundary, which is required for the server to parse the multipart fields.

const formResponse = new Response(form);
const formStream = formResponse.body;
const formContentType = formResponse.headers.get('content-type');

Files Changed

• src/components/models/code/Flux-2.astro - shared code example component
• src/content/changelog/workers-ai/2026-01-15-flux-2-klein-4b-workers-ai.mdx
• src/content/changelog/workers-ai/2026-01-28-flux-2-klein-9b-workers-ai.mdx

Testing

Verified all 3 models work with the fixed code:

• @cf/black-forest-labs/flux-2-dev ✓
• @cf/black-forest-labs/flux-2-klein-4b ✓
• @cf/black-forest-labs/flux-2-klein-9b ✓