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Use case: code migration

AI Code Migration and Refactoring at Scale

Framework upgrades, dependency bumps, and deprecated-API removals are mechanical, repetitive, and risky to do by hand across a large codebase. CodeCourier does the mechanical part in isolated sandboxes and proves nothing broke with your test suite before the pull request opens.

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Migrations no one wants to own

Every codebase carries a backlog of migrations someone keeps deferring: a framework a major version behind, a dependency with a security advisory, a deprecated API that will break on the next upgrade. The work is mechanical but huge - hundreds of near-identical edits across files - and risky, because one missed call site fails in production. It is too tedious for a senior engineer and too dangerous to rush, so it sits there accruing risk until it becomes an emergency.

How it works

How autonomous code migration works

Step 1

Map the surface

CodeCourier loads your repo in an isolated sandbox and finds every call site, import, and pattern the migration touches - so the change is scoped to reality, not a guess at where the old API lives.

Step 2

Migrate consistently

It applies the migration uniformly across the codebase, following the new API and your team's conventions through its persona, so hundreds of edits are consistent rather than hand-varied.

Step 3

Prove nothing broke

It runs your full test suite in the sandbox after the migration and iterates until it is green, catching the call site a manual sweep would have missed before any of it reaches a PR.

Step 4

Open a reviewable PR

CodeCourier opens a pull request with the full migration diff, a summary of what changed, and proof the suite still passes - reviewable in one place instead of a scary all-at-once merge.

Why the sandbox matters

A migration that touches hundreds of files is exactly where you want a contained dry run. CodeCourier applies the whole change in a disposable, isolated sandbox and runs your test suite there, so a broken call site surfaces in the sandbox - not in production after merge. You review a migration that has already been proven to build and pass, instead of crossing your fingers on a sweeping diff.

More on sandbox isolation

What it does well

  • Framework and major-version upgrades with mechanical, repeatable changes
  • Dependency bumps and security-advisory remediations across the codebase
  • Retiring deprecated APIs and replacing them with their supported successors
  • Large, consistent refactors verified by your existing test suite

What it will not do

  • Migrations with thin test coverage to verify against - it flags the gap first
  • Re-architecture or redesign disguised as a migration
  • Judgement calls about whether to migrate at all - that stays your decision
  • Sweeping changes still merge only through your review and policy

Representative of how CodeCourier runs as of June 2026. Results depend on your codebase, test coverage, and the scope of the job. CodeCourier escalates to a human when it cannot reproduce or verify a change rather than guessing.

Proof

Run a migration end to end

Take a deferred dependency bump or deprecated-API removal and hand it over. You will get a tested migration PR you can actually review - see the existing migration guide for the playbook.

Read the issue-to-PR walkthrough
FAQ
What kinds of migrations can CodeCourier handle?
The mechanical, repeatable kind: framework and major-version upgrades, dependency bumps including security-advisory remediations, retiring deprecated APIs, and large consistent refactors. It maps every affected call site in a sandbox, applies the change uniformly, and proves your suite still passes. The honest limit is verification - if there is too little test coverage to confirm correctness, it flags that gap before sweeping the codebase.
How do I know the migration did not break anything?
Because it runs your full test suite in the isolated sandbox after the migration and iterates until it is green, catching the missed call site a manual sweep tends to leave behind. What you review is a migration already proven to build and pass, not a hopeful diff. If the suite cannot be made green, it reports the blocker instead of opening a PR you would have to debug.
Can it migrate a large codebase consistently?
Consistency is the point. A human doing hundreds of near-identical edits drifts; CodeCourier applies the new pattern uniformly across every call site, following your conventions through its persona. Because the whole change happens in a sandbox and is verified there, you get one reviewable PR for a sweeping migration instead of a risky, hand-varied all-at-once merge.
Do I still review the changes before they merge?
Yes. Even a fully tested migration opens as a reviewable pull request, and merge policy stays yours - sweeping changes are exactly the case where most teams keep a human in the loop. CodeCourier gives you the diff, a summary of what changed, and the green test run so the review is fast, but the decision to merge is still yours.

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