Portamp vs. GitHub Copilot
Copilot's "App Modernization" product genuinely does generate and run tests to catch breaking changes during a migration -- for Java and .NET. Point either Copilot's general chat/agent mode or Cursor at a legacy front-end port instead, and proving the old and new versions behave the same is left entirely to you. Portamp records what the old app actually does and writes that as a named, failing test the moment the port drops it.
GitHub Copilot's App Modernization feature, generally available since September 2025, is a real, specific answer to "how do I know this migration didn't break anything" -- it generates and migrates unit tests as part of a Java or .NET upgrade, runs a fix-and-test loop until they pass, and logs its work. That's genuine, purpose-built parity verification, and it's worth being clear that it exists rather than pretending Copilot has nothing like it.
Where the comparison actually sits
That feature is scoped to backend Java and .NET framework upgrades and Azure migration -- it isn't documented as covering front-end framework porting, which is the specific problem Portamp is built for: taking a legacy Angular, jQuery, or native Windows front end and porting it to React, Vue, Svelte, or a dependency-free custom element. For that job, general-purpose Copilot Chat or agent mode -- and Cursor, which has no comparable built-in feature either -- can write and update tests as part of normal AI-assisted coding if you ask, but neither has a dedicated mechanism that proves the before-and-after versions behave identically. That verification is left to the developer's own test-writing and prompting discipline in both tools.
Where Portamp differs
Portamp builds that verification into the pipeline rather than leaving it to whoever's driving the port. It walks the running legacy app and records what each action actually did, then writes that recording as a conformance test suite against the port -- a port that silently drops a validation rule doesn't just "look different," it fails a named test. The porting itself runs through one intermediate representation: every reader (Angular, jQuery, WinForms, a PDF spec, even a running app with no source at all, driven like a person would) turns its input into that one IR, and every emitter turns the IR back into its own target, so supporting N source dialects and M output frameworks is a reader plus a printer each, not N×M hand-written translators -- Portamp's own CI proves byte-identical output across four targets built from two different source dialects. The whole core is 729 lines across four files with zero runtime dependencies, and it recovers a design system rather than pixels -- a token file for density, type scale, spacing and color roles, fitted from the app's own sizes rather than imposed, then brought up to WCAG contrast without changing the brand color. It also enforces policy in the kernel rather than trusting the operator to remember: a credential found in legacy source stops the run before anything is written, live or billable calls are refused unless explicitly flagged, and reconstructing a system you don't own outright requires a signed authorization on disk first.
The honest read: Copilot's App Modernization proves the concept works and matters enough that a major platform shipped it for Java and .NET. Portamp is the same idea -- generate the proof, don't just hope the port is faithful -- applied to the front-end migration problem neither Copilot nor Cursor currently covers.