Legacy Migration
The codebase that built your business shouldn't be the one limiting it.
Somewhere in your organisation is a system nobody wants to touch. COBOL that's been running since before your engineers were born. PHP 4 with no comments and no tests. A monolith so tangled that every feature takes six weeks and every deploy takes courage. The people who understood it retired. The documentation never existed.
Syntic Code reads that system the way a senior engineer reads code. It maps the business logic, reconstructs what was never documented, writes the modern equivalent, and validates it against the original behavior — line by line, function by function, with tests proving the two systems match. The migration that used to take three years and $20 million takes months.
What Syntic Code migrates
Every layer of the legacy stack — language, framework, database, architecture, platform, and mainframe.
Language migrations
COBOL to Java or Python. Perl to Go. PHP to Node or TypeScript. VB6 to C#. Delphi to .NET. Scala to Java. Every migration preserves the business logic exactly — not approximately, not mostly — and ships with tests that prove it against the original behavior. The new code is readable, maintainable, and owned by your team.
Framework migrations
AngularJS to React. Backbone to Vue. Java 8 to Java 21. Python 2 to Python 3. .NET Framework to .NET Core. Symfony to Laravel. Old framework to current, without rewriting the whole app.
Database migrations
Oracle to PostgreSQL. SQL Server to MySQL. Mainframe data stores to modern OLTP. Stored procedures translated, query patterns updated, ORM layers regenerated.
Architecture migrations
Monolith to microservices. Synchronous to event-driven. On-prem to cloud-native. The structural changes that took consultancies 18 months, scoped and shipped in quarters.
Platform migrations
On-prem to AWS, GCP, or Azure. Bare metal to Kubernetes. VM-based to serverless. Including the IaC, the CI/CD rebuild, and the cutover plan.
Mainframe modernization
IBM z/OS, AS/400 (IBM i), Unisys. The systems running banks, insurance carriers, government agencies, and logistics operations. The code nobody on your current team can read, written in languages nobody teaches anymore, running business logic nobody fully understands. Syntic Code reads it, maps it, reconstructs the undocumented logic from the code itself, and migrates it to a modern stack — with the audit trail regulators require and the tests that prove nothing was lost.
How it works
Incremental. Reversible. Tested at every step.
Not a big-bang rewrite that shuts the business down for eighteen months. A sequence of reviewable, validated changes — each one proven against the behavior of the system it replaces before the next step begins.
Step 1
Read the legacy code
Syntic Code reads the entire legacy codebase — including the COBOL nobody on your team can read, the Perl scripts from 1998, and the PHP 4 code with no comments and no tests. It maps the business logic, the data flows, the integration points, and the edge cases. Everything your engineers know implicitly about how the system behaves gets documented explicitly for the first time.
Output
A documented model of your legacy system in plain English.
Step 2
Plan the migration
Syntic Code plans the migration as a sequence of incremental, reviewable changes. Each step is scoped, testable, and reversible. Your engineering team reviews the plan before a single line of new code is written. Nothing starts until you've approved the approach.
Output
A written migration plan with milestones, risk assessment, and a rollback strategy.
Step 3
Write the new system
Syntic Code writes the modern equivalent function by function, module by module. Tests are written first — validated against the legacy behavior — then the new code is built to pass them. The new system behaves identically to the old one. Proven, not promised. Every function, every edge case, every data flow that the original handled is covered by a test that the new system must pass before it advances.
Output
Production-ready code with full test coverage, deployed alongside the legacy system.
Step 4
Run them in parallel
Both systems run side by side in production. Traffic shifts gradually — 5%, 20%, 50%, 100% — at a pace your team controls. Every discrepancy between the old system's output and the new system's output is flagged automatically. Your team reviews, resolves, and advances. When the new system is fully validated under real production load, the old system is retired. Cleanly. Permanently. With your team owning the new codebase in your repo, versioned in git, documented in plain English.
Output
A working modern system in production, with the legacy system safely shut down.
What this replaces
The same outcome, without the multi-year program
The traditional migration was a multi-year, multi-million-dollar bet. This is a scoped engagement you control.
The traditional migration
- Engagement
- 1 to 3 years
- Cost
- $2M to $50M and up
- Team
- 20 to 100 consultants
- Risk
- High. Most migrations run over time and budget.
- Documentation
- Handover docs the consultants take with them
- Business disruption
- Extensive
The Syntic Code engagement
- Engagement
- 2 to 6 months for most programs
- Cost
- Scoped per program, typically 10 to 20% of traditional consultancy
- Team
- 2 to 5 Syntic engineers plus your team
- Risk
- Low. Incremental, reversible, tested at every step.
- Documentation
- In your repo, owned by your team, versioned in git
- Business disruption
- Minimal
For most engagements we scope, total cost comes in at 10–20% of what a traditional consultancy would charge for the same outcome — with a shorter timeline, lower risk, and a codebase your team actually owns when it's done.
Real-world precedent
Migrations at this scale are already happening
AI coding agents have already delivered legacy migrations in days that used to take quarters.
- A 50,000-line Python-to-Go library migration completed in 20 hours of active development, estimated at 2 to 3 months manually.
- A 10,000-line Scala-to-Java migration completed in 4 days, estimated at 10 engineer-weeks.
- Engineering teams at scale reporting average feature delivery dropping from 24 working days to 5.
Syntic Code is built for the same scale of work. Bigger codebases. Older stacks. Higher stakes.
How we work with migration teams
A scoped engagement, alongside your team
Every migration starts with a fixed-price scoping engagement and ends with your team owning the new system in production.
Scoping engagement
Every migration starts with a 4-week scoping engagement: we read your legacy codebase, map the scope, identify the risks, and deliver a written migration plan with milestones, pricing, and a delivery timeline. Fixed-price.
Migration execution
We assemble a dedicated team of migration engineers, language specialists matched to your source and target stacks, and a delivery lead. The team works alongside yours, in your environment, with your code never leaving your VPC.
Validation and cutover
Side-by-side running, traffic shifting, discrepancy detection, and cutover planning. We don't leave until the new system is validated in production and your team owns it.
Ongoing partnership
Most migrations reveal more modernization work: APIs to refactor, services to extract, performance to tune. The engagement extends as long as you need it to.
Why this works now
Migration was the hardest work in software for forty years
It was the most labor-intensive work in engineering for four decades. AI coding agents have changed the math.
- The legacy code your team can't read — Syntic Code can read.
- The business logic nobody documented — Syntic Code reconstructs from the code itself.
- The tests that don't exist — Syntic Code writes them, validated against the original behavior.
- The new system that would have taken a year — Syntic Code builds it in a quarter.
This is the work AI was supposed to be good at. It finally is.
Security & IP
Your code stays yours
A migration touches your most critical systems. Every step runs inside your environment, logged and reversible.
Source code and migration artifacts never leave your environment
On-prem and VPC deployment available
Customer code never trains shared models
Full audit logging: every change, every test, every cutover step
SSO and SAML on enterprise engagements
BAA, DPA, and NDA available
Region-pinned deployment (US, EU, JP)
The model behind the migration
Built to read old systems and write new ones.
Most AI coding tools struggle with legacy code because legacy code is hard — inconsistent style, missing documentation, decades of accumulated edge cases, languages that haven't been taught in twenty years. Syntic Code was built for exactly this. Our frontier model reads legacy code the way a senior engineer reads it — understanding intent, not just syntax — and writes the modern equivalent with the tests to prove it matches.
Standard
Handles framework migrations, language migrations, and database migrations for most modern and legacy stacks.
Deep reasoning
Extended thinking for the hardest migrations: mainframe modernization, monolith decomposition, and multi-system architectural overhauls. Shows its reasoning so your architects can review the approach before execution begins.
Custom
Fine-tuned on your codebase and your historical patterns. Deployed in your environment. Scoped as a custom engagement for organisations where the migration is a multi-year program.
Get started
The system that built your business shouldn't be the one limiting it.
Every month you stay on the legacy stack is a month your engineers are maintaining the past instead of building the future. Every quarter the migration gets deferred is a quarter your competitors on modern infrastructure are moving faster.
Syntic Code migrations run in months, not years. In your environment, with your team, with your code never leaving your infrastructure. The new system is validated against the old one — line by line, function by function — before a single user is cut over.
Scope starts with a four-week fixed-price engagement. You get a written migration plan, a risk assessment, a delivery timeline, and a clear answer on whether this is the right approach for your stack. No commitment beyond that until you're ready.