Block migrates go/console from Base Web to Fluent UI without a flag day using an agentic migration system
Block's internal platform go/console had 11k tracked files mostly tied to Base Web, which was scaling back maintenance. With 40 to 60 engineers landing changes daily and no acceptable freeze window or downtime, a standard migration cutover was impossible.
How it works
Common implementation structure
How this type of workflow is generally built, generalized across documented cases — not tied to any one vendor's stack. Click any stage to read what happens there. Specific products that implement these stages appear in “Tools commonly seen” below.
Stage 1 · File selected for migration
The TypeScript language server answers which file to migrate next and what references and related files matter.
Tools used
TypeScript language serviceGooseANTLRJest
Outcome
The migration succeeded: more than 80 distinct targets were migrated to Fluent, Base Web was fully removed in February 2026, and the entire effort across 451 days was driven primarily by one IC, producing 1,004 commits and 14,713 file changes.
What failed first
Single-shot prompting produced code that was syntactically valid but semantically wrong — the model would generate React-incorrect patterns like creating makeStyles hook factories inside the component body. Attempting to fix output quality with global hint libraries consumed 25% of the 256k context window before the model even saw the target file.