> ## Documentation Index
> Fetch the complete documentation index at: https://docs.bluegenai.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Path B: Legacy System Modernization (CAB)

> Use CAB to reverse-engineer an existing legacy system and develop requirements for its replacement.

This path is for government offices, Business Analysts, and contracted teams modernizing a legacy system. It is the primary entry point for the modernization scenario.

## When to use this path

* A government office has an existing legacy system that needs to be modernized.
* The team needs to understand what the current system does before defining what the replacement should do.
* Resources are already under contract — this is post-award or in-house work.

## Prepare your legacy system artifacts for upload

1. Gather source code — package it as a ZIP archive if it's a full codebase.
2. Include configuration files: `package.json`, `requirements.txt`, `docker-compose.yml`, `.env.example`.
3. Include database DDL scripts and schema exports.
4. Gather any existing documentation: user guides, system manuals, process flows, ATO artifacts.
5. Take screenshots of the existing application UI if available.
6. Gather interface specifications for any connected systems.

<Warning>
  Do not clean or filter the code before uploading. CAB handles messy, inconsistent code — removing files you think are unimportant risks losing context the analyzer needs.
</Warning>

## Set up CAB and upload your inputs

<Steps>
  <Step title="Open your project">
    Navigate to the CAB viewer within your project.
  </Step>

  <Step title="Upload your prepared files">
    Add your source code ZIP, documentation, and screenshots.
  </Step>
</Steps>

<Tip>
  Source code + documentation + screenshots together produce the richest blueprint. Any single input type alone will be more limited.
</Tip>

## Select the right model tier

Always use the **High** tier for legacy system analysis. Legacy systems tend to be complex with deeply intertwined logic — High tier gives CAB the room it needs to surface nuance. Low and Medium tiers are only suitable for quick explorations.

## Generate the architectural blueprint

Click Generate and monitor progress through CAB's seven pipeline phases:

1. Parse and extract content from each uploaded file.
2. Analyze codebase structure — languages, frameworks, entry points, dependencies.
3. Identify architectural patterns and map them to a normalized structure.
4. Simplify and consolidate patterns.
5. Reassemble into a coherent system description.
6. Create a semantic search index for cross-referencing.
7. Generate the final blueprint sections.

Credit usage is tracked per phase — review the breakdown after generation completes.

## Read the architectural blueprint

The completed blueprint (`CAB.md`) covers:

* **System overview** — what the legacy system does and who it serves
* **Architecture and components** — frontend, backend, data, and integration layers
* **Data models** — entity definitions, relationships, and key data structures
* **Business logic and workflows** — the rules the system enforces and the processes it supports
* **Integration points** — external systems, APIs, and services the application connects to
* **UI patterns** — page layouts and form structures (present only if screenshots or frontend code were uploaded)

## Validate the blueprint with SMEs

<Steps>
  <Step title="Share the read-only link">
    Send it to government SMEs for review.
  </Step>

  <Step title="Walk through the blueprint together">
    Confirm the analysis accurately reflects the real system.
  </Step>

  <Step title="Note gaps or inaccuracies">
    These inform the requirements you write for the replacement system.
  </Step>
</Steps>

## Share the blueprint

CAB generates a shareable read-only link automatically. Copy and share it — recipients must be logged in to BlueGenAI. Disable the share link from Project Settings once reviews are complete.

## Review and edit generated user stories (CAB Phase 2)

<Warning>
  This section requires a Platform Owner walkthrough before it can be fully documented. The workflow below reflects what's confirmed so far — treat it as provisional.
</Warning>

* Review AI-generated user stories for the future-state modernized system.
* Edit and update user stories to reflect what the replacement should do.
* Whether edits propagate automatically to dependent artifacts is still being confirmed with the Platform Owner.

## Prepare Phase 2 outputs for handoff to Phase 3

<Steps>
  <Step title="Save the architectural blueprint">
    It becomes a reference artifact for System Design.
  </Step>

  <Step title="Save approved user stories">
    Include any supporting documentation.
  </Step>

  <Step title="Document what should change">
    Record decisions about what the replacement system must do differently from the legacy system.
  </Step>

  <Step title="Proceed to Phase 3">
    Continue to System Design once your requirements and blueprint are finalized.
  </Step>
</Steps>

<Card title="Phase 3: System Design" icon="arrow-right" href="/building-web-apps/phase-3-system-design">
  Continue to the next phase.
</Card>