ASP.NET to Angular + .NET Web Modernization Playbook

Tell us what you're building

We use these answers to surface the prompts, skills, and MCP configs that fit your stack — and to substitute stack values like {{database}} into the prompts you copy. Content (PRDs, code, etc.) stays in your repo. Everything you enter here is stored in your browser — nothing is sent to a server.

Steps you'll go through

1. 01

   Audit the Legacy App

   2-3 weeks

   Reverse-engineer the existing app — tech stack, business logic locations, integrations, and risks — before deciding how to migrate.

2. 02

   Define the Target Architecture

   1-2 weeks

   Decide what you're building toward — Angular structure, .NET API structure, data layer, hosting, observability — and document it as ADRs.

3. 03

   Plan the Strangler Fig Sequence

   1 week

   Slice the migration into independently shippable chunks and decide the order. Slice 1 should be foundational, not the most valuable feature.

4. 04

   Lock Down Current Behavior

   2 weeks

   Capture the legacy system's behavior as a parity test suite before changing anything. The new system has to match the legacy — including its quirks.

5. 05

   Build the Coexistence Layer

   2-3 weeks

   Stand up the routing, auth, and observability infrastructure that lets old and new run side-by-side. Every later slice depends on this.

6. 06

   Migrate Vertical Slice 1

   4-6 weeks

   Ship one complete user-facing capability through the new stack, end-to-end, behind a feature flag. The first slice is your proof point.

7. 07

   Migrate Slice 2 (Apply Lessons from Slice 1)

   3-5 weeks (faster than slice 1)

   Take the next slice. Most infrastructure is built. The team knows the patterns. Slice 2 should ship faster than slice 1.

8. 08

   Continue Migrating Remaining Slices

   1-3 quarters depending on app size

   Repeat the slice migration pattern for everything else. By the end of this phase, all user-facing capabilities should run on the new system.

9. 09

   Cutover and Decommission

   2-3 weeks

   Turn off the legacy system. By this point traffic should already be zero — this phase is the formal teardown plus the post-migration write-up.