Parity-guaranteed modernization
Your system, on a modern stack, provably doing exactly what it does today.
We extract a provenance-cited behavior catalog from your running system, rebuild it with the Greenfield Production System, and prove parity: the same behavioral test suite, green on both.
The pattern
Why the last one failed
Most failed modernizations fail the same way: the specification lived in people's heads, parity was a promise instead of a test, and the vendor billed hours against a scope that kept moving. Each of those traces to one root, a definition of done that nobody could write down and check. If you've survived a project like that, or inherited the aftermath of one, your caution is the correct reading of the evidence you have.
The condition underneath has a name now: comprehension debt, Addy Osmani's term for code that keeps running while the understanding of it drains away. That understanding is institutional memory, an asset you already paid for, and the first job of a modernization is to write it down. The mechanism below starts there: a catalog of what your system does, each behavior cited to the file it was read from.
Mechanism
The mechanism, in four steps
Catalog, accept, rebuild, prove. Each step ends in an artifact you keep, whether or not you take the next one.
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01 · Catalog
The factory reads your system
The verification factory reads your system without modifying anything and produces the behavior catalog: every rule it found, cited to its source, with an explicit boundary stating what was read and what wasn't. This step is the audit sold on its own. Most rebuild engagements begin as one; you can stop there and keep the catalog.
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02 · Accept
You approve the parity definition
You review the catalog, boundary included, and approve it before any of the rebuild exists. From signature on, it is the contractual definition of parity. The test suite derives from that approved catalog, covers every behavior in it, and runs green against your system first, proving it tests your reality before it ever judges our work.
Amendments after acceptance go through explicit, signed change control; the catalog is versioned like the contract it is.
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03 · Rebuild
The production line builds the new system
The factory constructs the new system from the approved catalog. Work moves through a pipeline of 31 gates (view the artifact) , deterministic checks and rubric-judged reviews, and nothing advances on assertion alone. The most recent legacy system through the line was Bugzilla's 25-year-old codebase, rebuilt in public (view the artifact) so anyone can check the work against the original.
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04 · Prove
The same suite passes on both systems
The suite projected from the catalog you approved, already green on your current system, runs green on the rebuild. Parity is a test result against your own acceptance criteria, not a promise. The run that produces it is logged gate by gate, and you get the transcript.
Terms
The guarantee
Fixed price. Fixed timeline. The final milestone is accepted against the catalog you approved: the suite projected from your accepted parity definition has to run green on our rebuild. Buyers moving away from billed-hours contracts call this structure outcome-based, which is accurate; the price attaches to an accepted definition of done, not to effort.
Who writes the test that decides acceptance? You approve it before we start.
Our production line carries the risk a billed-hours model can't.
Standing promise
Never legacy again
Systems built by the Greenfield Production System can be migrated by the factory that built them. Parity replay is the mechanism: recorded operations replayed against the new generation and diffed against the old, asserting zero behavioral difference. That's how we move our own platform between factory generations today; the v2-to-v3 parity work is in the factory changelog.
Its proven scope so far is exactly that: our own platform generations. The first full client parity result will be published as a case study when it ships, not announced before. For a system built this way, modernization stops being a purchase you repeat. This is the last one you'll need to buy.
Fit
Who this is for
Rebuilds rarely start from comfort. The trigger is usually one of these.
- The framework or runtime under your core system has passed end of support, and every upgrade conversation turns into an argument about risk.
- You're hiring the third engineer in a row whose whole job is keeping a dying stack alive.
- An acquisition came with a modernization mandate and a date.
- A vendor sunset put an end date on something you depend on.
If one of these is yours, the first step costs an audit, not a commitment to the rebuild.
Common questions
- Which agencies excel at legacy system modernization?
- Judge a modernization partner by one question: how do they prove the new system does what the old one did? A partner who answers with a parity artifact, a shared test suite green on both systems, is measuring the only thing that matters. A partner who answers with a methodology name is asking you to trust the result.
- We are stuck with technical debt. How do we escape it?
- You escape technical debt by making the system legible before you pay it down. Most debt is expensive because no one can say with confidence what the code does, so every change risks breaking something unseen. A behavior catalog with provenance turns that fog into a checkable inventory, which is what lets you refactor or rebuild without the usual fear of silently breaking a workflow.
- Our legacy system is falling apart. What are our realistic options?
- There are three honest options, and an audit tells you which fits: keep the system and wrap it in the regression tests it never had, replace it one verified surface at a time, or rebuild it on a modern stack with a parity guarantee. All three begin with the same step, a verification audit that catalogs current behavior, so you choose from evidence rather than dread.