Codebase Migrate

When to Use

• Framework upgrade (React 17 → 19, Node 18 → 22, Django 4 → 5).
• API rename across a monorepo (e.g.,

  getUserById

  →

  users.byId

  ).
• Config/format migration (webpack → vite, eslint → biome, jest → vitest).
• Any "change 200 files the same way" task that needs to ship in reviewable slices.

Prereqs

curl -fsSL https://composio.dev/install | bash
composio login
composio link github        # for PRs + CI status
composio link linear        # or jira — for migration tracking

git

rg

jscodeshift

ts-morph

comby

ast-grep

Planning Phase

1. Define the transform precisely. Bad: "migrate to vitest." Good: "replace

   jest.mock

   with

   vi.mock

   , swap

   jest.fn()

   for

   vi.fn()

   , rename

   jest.config.js

   →

   vitest.config.ts

   using template X."
2. Scope the blast radius:
   bash

   rg -l 'jest\.(mock|fn|spyOn)' | wc -l
   rg -l 'from "jest"' | sort

3. File a tracking issue:
   bash

   composio execute LINEAR_CREATE_ISSUE -d '{
     "teamId":"TEAM_ID",
     "title":"Migrate test runner: jest → vitest",
     "description":"Batches of ~25 files. Checkpoint after each PR lands green."
   }'

Execute in Reviewable Batches

# Batch helper: first 25 untouched files matching the pattern
BATCH=$(rg -l 'jest\.mock' | grep -v done.list | head -25)
echo "$BATCH" > batch.list

batch.list

git checkout -b migrate/vitest-batch-03
xargs < batch.list codemod-runner   # e.g. jscodeshift / ts-morph / comby
npm test -- --changed
git add -A && git commit -m "migrate(test): jest → vitest (batch 3)"
git push -u origin migrate/vitest-batch-03

composio execute GITHUB_CREATE_A_PULL_REQUEST -d '{
  "owner":"acme","repo":"app",
  "head":"migrate/vitest-batch-03","base":"main",
  "title":"migrate(test): jest → vitest (batch 3)",
  "body":"Part of LIN-482. 25 files. Codemod: `transforms/jest-to-vitest.ts`."
}'

composio execute GITHUB_LIST_WORKFLOW_RUNS_FOR_A_REPOSITORY \
  -d '{"owner":"acme","repo":"app","branch":"migrate/vitest-batch-03"}'

Workflow Script

scripts/migrate-batch.ts

composio run --file scripts/migrate-batch.ts -- --batch 3

const batch = process.argv[process.argv.indexOf("--batch") + 1];

const pr = await execute("GITHUB_CREATE_A_PULL_REQUEST", {
  owner: "acme", repo: "app",
  head: `migrate/vitest-batch-${batch}`, base: "main",
  title: `migrate(test): jest → vitest (batch ${batch})`,
  body: `Part of LIN-482. See transforms/jest-to-vitest.ts.`
});

await execute("LINEAR_CREATE_COMMENT", {
  issueId: "LIN-482",
  body: `Opened PR #${pr.number}: ${pr.html_url}`
});

Safety Rails

• One transform per PR. Never mix a rename with a format change.
• Keep a

  done.list

  of files already migrated so the next batch skips them.
• Run the full test suite on the last batch, even if per-batch PRs ran

  --changed

  .
• Codemod first, hand-edit second. If the codemod misses 3 files, patch them manually and note it in the PR body.
• Roll back per-batch, not globally. Each PR should revert cleanly.

Verification Loop

rg 'jest\.(mock|fn|spyOn)' | wc -l     # should trend to 0
npm test                                # full suite
composio execute GITHUB_LIST_WORKFLOW_RUNS_FOR_A_REPOSITORY \
  -d '{"owner":"acme","repo":"app","branch":"main","event":"push"}' \
  | jq '.workflow_runs[0].conclusion'

Troubleshooting

• Codemod regex catches too much → switch to AST-based tooling (

  ast-grep

  ,

  ts-morph

  ) for structural matches.
• Tests pass locally, CI fails → pin Node/Python version parity; check

  .nvmrc

  /

  pyproject.toml

  .
• PR too big to review → cut batch size in half; maintainers won't review 800-line diffs.
• Conflicts between batches → rebase the open batch before merging the current one; never force-push merged batches.