Biome migration formatter drift during zero-downtime cutover

Formatter output divergence during incremental adoption

Biome's built-in migration command converts ESLint and Prettier configurations, but the formatter produces different output than Prettier for certain syntax patterns. When migrating a small application without downtime, both toolchains must coexist during the transition window.

Constraint: simultaneous toolchain execution

The application serves live traffic while developers run Biome on new changes and Prettier on legacy paths. CI pipelines execute both formatters against overlapping file sets. Any formatting discrepancy creates merge conflicts, review noise, or silent style regressions that accumulate until the final cutover.

Uncertainty: deterministic parity scope

Documentation notes formatter differences for "certain syntax patterns" without enumerating them. The migration guide suggests validating with biome check --apply but does not define a parity threshold for zero-downtime safety.

• Which syntax categories exhibit non-deterministic divergence between Biome and Prettier 3.x?
• Can a pre-migration audit quantify the drift surface area without full test-suite execution?
• Does Biome's LSP incremental mode guarantee identical formatting for unchanged files across toolchain boundaries?