Article: Jarred Sumner drove a Claude-powered, adversarial-review workflow to rewrite the Bun runtime from Zig to Rust. He pushed more than a million lines of code through 6,778 commits and squashed 16,000 compiler errors. The effort ran 50 Claude Code workflows, peaked at 64 concurrent Claude agents, and left a reproducible playbook for massive language migrations.

Why the rewrite matters

For any project wrestling with legacy code or a strategic language move, the “Sumner Method” offers a concrete template instead of a vague promise.

The workflow that turned AI into a partner, not a replacement

Sumner’s process loops tightly:

  1. Task assignment – an agent receives a concrete migration task.
  2. Implementation – a second agent writes the Rust code.
  3. Adversarial review – a third agent assumes the code is wrong, combs the diff, and tries to prove every claim false using source files and tests.
  4. Fixes – the implementation agent incorporates the reviewer’s findings.
  5. Automated gates – compiler, test suite, and static-analysis checks verify the changes.

The crucial rule is separation. The writer never sees the reviewer’s reasoning, and the reviewer never sees the writer’s intent. By stripping bias, the system forces the reviewer to hunt for hidden bugs instead of giving a perfunctory “looks good” sign-off.

Rust’s machine-checkable feedback turns thousands of potential human-readable errors into a manageable queue. Each compiler error, borrow-checker failure, or Clippy warning becomes a work item that the adversarial reviewer can target directly.

The eight-phase playbook

Sumner distilled the workflow into eight sequential phases, each with its own task queue, definition of done, review prompts, and automated gates:

  • Phase A – Fact extraction & guide authoring – Gather architectural facts and produce a migration guide.
  • Phase B – Mechanical file translation – Convert Zig files to Rust skeletons.
  • Phase C – Compile-error remediation – Resolve the 16,000 compiler errors logged during translation.
  • Phase D – Runtime-behavior matching – Verify that Rust output mirrors Zig’s behavior.
  • Phase E – Test suite completion – Pass every existing test.
  • Phase F – Performance recovery – Eliminate any slowdown introduced by the rewrite.
  • Phase G – Code-quality polishing – Apply idiomatic Rust patterns and refactor for readability.
  • Phase H – Security hardening – Run unsafe audits and address any discovered vulnerabilities.

Each phase feeds into the next, ensuring no step is skipped and that regressions are caught early.

Where to find the playbook

Sumner has open-sourced the full set of templates, prompts, and phase definitions at https://github.com/Lumafy/sumner-method. A companion article walks through the Bun migration in detail: https://dev.to/lumafy/the-sumner-method-what-buns-ai-assisted-zig-rust-rewrite-teaches-about-large-migrations-2gpo. A Telegram community for ongoing discussion lives at https://t.me/GyaanSetuAi.