Batch Migration

Batch Migration

Recipes for large-scale code changes that touch tens or hundreds of files. Each recipe distributes work across agents, handles conflicts, and verifies changes after each batch.

How /batch Works

/batch <recipe> [options]        # Run a batch migration
/batch cjs-to-esm --dry-run     # Preview changes without writing
/batch status                    # Check running batch progress
/batch rollback                  # Revert last batch (via git)

Architecture

  /batch recipe
       |
       v
  Discovery Phase
  (find all files to change)
       |
       v
  Plan Phase
  (group into batches of 10-20 files)
       |
       v
  Execute Phase -----> Agent 1: Batch 1 (files 1-15)
       |          +--> Agent 2: Batch 2 (files 16-30)
       |          +--> Agent 3: Batch 3 (files 31-45)
       |
       v
  Verify Phase
  (typecheck, lint, test per batch)
       |
       v
  Commit Phase
  (one commit per batch or squash all)

Safety: The Batch Contract

Every batch migration follows these rules:

1. Discovery before execution -- always know the full scope first
2. Dry run first -- preview every change before writing
3. Batch size limit -- max 20 files per batch (prevents context overflow)
4. Verify after each batch -- typecheck + lint + test
5. Rollback ready -- each batch is a separate commit (easy to revert)
6. Conflict resolution -- stop on conflict, don't guess

Recipe 1: CJS to ESM

Convert CommonJS require() / module.exports to ES module import / export.

Command

/batch cjs-to-esm
/batch cjs-to-esm --dry-run           # Preview only
/batch cjs-to-esm --dir src/          # Limit scope

Discovery

Find all .js/.ts files containing:
  - require('...')
  - module.exports
  - exports.something

Transforms

Post-Migration

• Update package.json: add "type": "module"
• Update tsconfig.json: set "module": "ESNext", "moduleResolution": "bundler"
• Fix dynamic imports: require(var) becomes await import(var)

Verification

Per batch:
  1. npx tsc --noEmit (typecheck)
  2. npm run lint (lint)
  3. npm test (tests)

Recipe 2: React Class to Hooks

Convert React class components to function components with hooks.

Command

/batch class-to-hooks
/batch class-to-hooks --dir src/components/

Discovery

Find all .tsx/.jsx files containing:
  - extends React.Component
  - extends Component
  - class.*extends.*PureComponent

Transforms

Edge Cases (Human Gate)

These require manual review and are flagged, not auto-converted:

• Components using getSnapshotBeforeUpdate
• Components using componentDidCatch / error boundaries
• Components with complex this references beyond props/state
• Higher-order components wrapping class components

Verification

Per batch:
  1. npx tsc --noEmit
  2. npm run lint
  3. npm test (especially component tests)
  4. Visual verify (if available) for UI components

Recipe 3: Multi-File Rename

Rename a symbol (function, class, type, variable) across the entire codebase.

Command

/batch rename --from "UserProfile" --to "AccountProfile"
/batch rename --from "getUserData" --to "fetchUserProfile" --dir src/

Discovery

Find all files containing the symbol:
  - Import statements
  - Export statements
  - Usage in code
  - Type references
  - JSDoc comments
  - Test files
  - Documentation

Transforms

• Rename in import/export statements
• Rename in usage sites
• Rename in type annotations
• Rename in string literals (only if clearly a reference, e.g., route paths)
• Rename corresponding test files (e.g., UserProfile.test.tsx to AccountProfile.test.tsx)
• Rename file if it matches the symbol name

Conflict Resolution

If the new name already exists in a file's scope:

1. Stop the batch
2. Report the conflict with file path and line number
3. Wait for human decision

Recipe 4: Dependency Upgrade (Major Version)

Upgrade a dependency across a monorepo with breaking changes.

Command

/batch upgrade react@19
/batch upgrade next@15 --packages "apps/*"

Steps

1. Discovery: Find all package.json files referencing the dependency
2. Changelog review: Read the upgrade guide / migration docs
3. Update versions: Bump version in all package.json files
4. Install: Run install in each package
5. Apply codemods: Run official codemods if available (e.g., npx @next/codemod)
6. Fix breaking changes: Apply transforms from changelog
7. Verify: Typecheck, lint, test per package

Example: React 18 to 19

Batch 1: Update package.json files (all packages)
Batch 2: Run react-codemod (automated transforms)
Batch 3: Fix remaining type errors (manual transforms)
Batch 4: Update test utilities (testing-library)
Batch 5: Verify all packages build and test

Recipe 5: API Schema Change

Update all consumers when an API response shape changes.

Command

/batch api-schema --schema api/schema.ts --change "User.name -> User.displayName"

Steps

1. Discovery: Find all files that reference the changed field
2. Type update: Update the schema/type definition
3. Server update: Update API handlers that produce the field
4. Client update: Update all consumers that read the field
5. Test update: Update fixtures, mocks, and assertions
6. Migration: Generate database migration if field is persisted

Conflict Resolution

For ambiguous references (e.g., name is too generic):

• Use type-aware search (follow imports from the schema type)
• Flag ambiguous matches for human review
• Never rename unrelated name fields

Recipe 6: Linter Rule Enforcement

Apply a new lint rule across the entire codebase.

Command

/batch lint-enforce --rule "no-explicit-any"
/batch lint-enforce --rule "prefer-const" --autofix

Steps

1. Enable rule: Add rule to .eslintrc / eslint.config.js
2. Discovery: Run linter to find all violations
3. Autofix round: Apply --fix for auto-fixable violations
4. Manual round: Fix remaining violations in batches
5. Verify: Run linter (zero violations), then run tests

Common Enforcement Patterns

Batch Execution Strategy

Parallel Agents

For independent files (no cross-file dependencies):

Agent 1: files 1-15   → verify → commit
Agent 2: files 16-30  → verify → commit
Agent 3: files 31-45  → verify → commit

Sequential Batches

For dependent changes (types used by other files):

Batch 1: Update type definitions → verify → commit
Batch 2: Update implementations → verify → commit
Batch 3: Update tests → verify → commit
Batch 4: Update documentation → verify → commit

Conflict Resolution

If two agents modify the same file:

1. Both agents stop
2. Merge the changes manually (or let the second agent rebase)
3. Verify the merged result
4. Resume

Checkpoint Schema

{
  "task": "batch-migration",
  "recipe": "cjs-to-esm",
  "status": "in-progress",
  "discovery": {
    "totalFiles": 87,
    "batches": 6,
    "filesPerBatch": 15
  },
  "progress": {
    "completedBatches": 3,
    "completedFiles": 45,
    "failedFiles": 2,
    "skippedFiles": 1
  },
  "verification": {
    "typecheck": "passing",
    "lint": "3 warnings",
    "tests": "142/142 passing"
  },
  "errors": [
    { "file": "src/legacy/parser.js", "error": "Dynamic require with variable path" },
    { "file": "src/plugins/loader.js", "error": "Circular dependency detected" }
  ]
}

Anti-Patterns

• Never batch-migrate without a dry run first
• Never modify more than 20 files in a single agent context (context overflow)
• Never skip verification between batches (one bad batch cascades)
• Never force-push batch migration commits (makes rollback impossible)
• Never auto-resolve merge conflicts in batch mode (always stop and report)