Codex use case
Run code migrations
Migrate legacy stacks in controlled checkpoints.
Use Codex to map a legacy system to a new stack, land the move in milestones, and validate parity before each transition.
Best for
- Legacy-to-modern stack moves where frameworks, runtimes, build systems, or platform conventions need to change.
- Teams that need compatibility layers, phased transitions, and explicit validation at each migration checkpoint.
Contents
Run code migrations
Migrate legacy stacks in controlled checkpoints.
Use Codex to map a legacy system to a new stack, land the move in milestones, and validate parity before each transition.
Use Codex to map a legacy system to a new stack, land the move in milestones, and validate parity before each transition.
Best for
- Legacy-to-modern stack moves where frameworks, runtimes, build systems, or platform conventions need to change.
- Teams that need compatibility layers, phased transitions, and explicit validation at each migration checkpoint.
Skills & Plugins
- Check risky migrations, dependency changes, and exposed surfaces before you merge.
- Work through failing CI after each migration milestone instead of leaving cleanup until the end.
- Use framework-specific guidance when a migration touches ASP.NET Core app models, `Program.cs`, middleware, testing, performance, or version upgrades.
| Skill | Why use it |
|---|---|
| Security Best Practices | Check risky migrations, dependency changes, and exposed surfaces before you merge. |
| Gh Fix Ci | Work through failing CI after each migration milestone instead of leaving cleanup until the end. |
| Aspnet Core | Use framework-specific guidance when a migration touches ASP.NET Core app models, `Program.cs`, middleware, testing, performance, or version upgrades. |
Starter prompt
Introduction
When you are moving from one stack to another, you can leverage Codex to map and execute a controlled migration: routing, data models, configuration, auth, background jobs, build tooling, deployment, tests, or even the language and framework conventions themselves.
Codex is useful here because it can inventory the legacy system, map old concepts to new ones, and land the change in checkpoints instead of one giant rewrite. That matters when you are moving off a legacy framework, porting to a new runtime, or incrementally replacing one stack with another while the product still has to keep working.
How to use
- Start by inventorying the migration surface: legacy packages, framework conventions, routing, data access, auth, configuration, build tooling, tests, deployment assumptions, and any external contracts that must survive the move.
- Ask Codex to map the legacy concepts to the target stack and call out what has no direct match.
- Choose an incremental strategy: compatibility layer, module-by-module port, branch-by-abstraction, or a strangler-style replacement around one boundary at a time.
- Keep behavior stable until the migration itself forces a visible change, and name those exceptions explicitly.
- After each milestone, run the smallest validation that proves parity: lint, type-check, focused tests, contract tests, smoke tests, or a side-by-side check against the legacy path.
- Review the diff and the remaining transition risk after each checkpoint instead of waiting for the full rewrite.
Leverage ExecPlans
In our code modernization cookbook, we introduce ExecPlans: documents that let Codex keep an overview of the cleanup, spell out the intended end state, and log validation after each pass. When you ask Codex to run a complex migration, ask it to create an ExecPlan for each part of the system to make sure every decision and tech stack choice is recorded and can be reviewed later.
Combine with a goal
For long-running migration slices, use a goal to guide Codex through the work. Set the goal with a clear end state, parity checks, rollback expectations, and a stopping condition.
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