ANSWER
How do you retire a legacy system with no documentation?
Reviewed · Reviewed by Paulo Pierrondi, founder.
You can retire a legacy system with no documentation, as long as the documentation is rebuilt from real behavior before cutover. Instead of trusting outdated manuals or the memory of people who have left, you observe what the system receives and delivers, write that down as an equivalence contract and only switch it off when the replacement proves, in parallel, that it delivers the same result within the agreed criterion.
Step by step
- List who uses the system, for what, and which reports, integrations and files depend on it.
- Rebuild behavior from real inputs and outputs, logs and historical data, not from old manuals.
- Turn what you observed into an equivalence contract with rules, tolerances and a cutover criterion.
- Run the replacement in parallel with the legacy system and classify every variance before looking at the aggregate rate.
- Cut over in stages, with a written rollback plan and the new documentation kept as evidence of the process.
How an agent does this
In AgenticosCore, the equivalence contract becomes the missing documentation. Parallel compares the agent with the current system, Trail keeps every run and decision, and the Evidence Pack gathers what was proven before cutover, which remains your decision.
Frequently asked questions
Do we need to document the whole system before starting?
No. Start with the process you will retire first and document its behavior. The rest of the system can stay as it is until its turn comes.
What if nobody from the original team is still around?
That is common. Behavior observed in inputs, outputs and historical data replaces team memory, and the people running the process today validate what was found.
Primary sources
- Managing legacy technology — GOV.UK
- Retiring service components securely — UK Government Security