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betterCode(), GenAI Summit · Mannheim · June 10, 2026

AI in Practice:
Strategies for Modernizing Complex Legacy Systems

Deutsche Telekom Legacy Modernization Agentic Engineering

Who We Are

Two perspectives on the same modernization loop.

Sigrid Braun

Sigrid Braun

Enterprise Architect at Deutsche Telekom with a focus on AI. She established the use of AI for modernizing legacy systems at Telekom IT GmbH and is currently shaping a group-wide architecture framework for multi-agent systems.

Architecture & ODA
Laura Fuhlbrügge

Laura Fuhlbrügge

Product Owner at Deutsche Telekom IT GmbH in the field of artificial intelligence. With her team, she works on AI-supported modernization of critical enterprise applications and operates a chatbot platform for intelligent knowledge management.

Product & Delivery

Agenda

From legacy challenge to scalable AI practice.

01Problem Statement
02Deutsche Telekom: Legacy Modernization Approach
03Live Demo Migration Plugin
04Success Stories & Success Factors
05First Steps & Wrap-up
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Problem Statement

Legacy Systems: A Barrier to Speed, Scale, and Efficiency.

With the Migration Plugin, we want to tackle the challenges of legacy modernization to increase the innovation cycle.

Legacy Systems

A barrier to speed, scale, and efficiency.

Complexity accumulates Legacy Target Higher Maintenance Costs Technical Limitations Loss of Expertise Frequent Upgrades needed Cost Savings Better Scalability Knowledge Transfer Faster Time to Market

Deutsche Telekom Approach

Migration Plugin: Driving Measurable Business Impact Through Structured AI Modernization.

Input
Codebase
Documentation
ConfluenceDocumentation flowJira
Legacy Code AI Automation butterflyMigration Pluginfor AI assistants
AI

Coding Assistance

Claude CodeClaude CodeGitHub CopilotGitHub CopilotCursorCursor
Frameworks & Tools

Validation Stack

RAGPipelineDeepEvalDeepEvalSpec KitSpec KitCucumberCucumber
Phase 01

Reverse Engineering

Legacy flowsDependenciesBusiness logic
Phase 02

Architecting

PlanInterfacesTechnology Stack
Phase 03

Forward Engineering

TasksTestsSource CodeEvaluationValidation

Migration Plugin · Multi-Phase Workflow

AI agents run an intelligent workflow across three modernization phases.

Phase 01

Reverse Engineering

Deep Explorer AgentAnalysis AgentDiagram Generator Agent
Phase 02

Architecting

Architect AgentPlan Migration AgentSpecification Agent
Phase 03

Forward Engineering

Code Generator AgentValidation Agent... and more Agents
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Live Demo

Migration Plugin in Practice

From legacy artifact to validated target increment: reverse engineering, specification, forward engineering and evaluation in one structured loop.

Success Stories

Three examples show where the leverage sits: Reverse + Forward Engineering.

Fixed-Line System

“Your plugin developed a running feature that is now ready for testing.”

65,000 lines of legacy code were refactored into 4,400 lines. A new feature powered by a modern API merges data from two databases.

5xReverse Engineering
5xForward Engineering

Fiber System

“We delivered 8 to 12 months of work in just one month.”

35+ repositories and 4 million lines of code were analyzed. The team delivered end-to-end modernization of a component across repositories.

Reverse Engineering Enabled
4-6xForward Engineering

Login System

“Without your plugin we would not have met our timeline for migration.”

Business logic was extracted from 1.7 million lines of code and new software was implemented for three business processes.

12xReverse Engineering
16xForward Engineering

Success Factors

Modernization only scales when four layers are managed together.

01

Principles

Human in/on the loop, AI coding assistants, lossless work and quality shifted left.

02

Organization

AI buddies, clear priorities, small business processes and sequenced technical/business transformation.

03

Cost Control

Models, context, retrieval, reuse and monitoring are managed deliberately as an operating loop.

04

Security

Scope, model access, secrets, least privilege, human gates and auditability as default controls.

Strategic Principles

Following these principles, migration gets faster and quality stays under control.

01Human in and on the Loop
02AI Coding Assistants
03Nothing is lost
04Shift left Quality
05Structured Evaluations
06Spec-driven development

Enterprise Scale

Making agentic modernization work at enterprise scale.

1

Set up the transformation

  • Board-level priority: sponsorship, funding, KPIs and transparent reporting
  • Start small and grow: prove value in one concrete business process
  • Sequence transformation deliberately: stabilize technical modernization first
  • Technology scouting: track new LLMs, agent skills and vendors with telco-specific value in mind
2

Scale the adoption

  • Run the plugin, skills and hooks as an internal product
  • Human adoption engine: AI ambassadors, AI buddies and SME-engineer tandems
  • Talent Initiative: identify early adopters and create multiplier effects
  • Train teams to delegate tasks to agents and validate reusable workflows
Success=Executive Mandate+Product Mindset+Staged Scaling+Adoption Engine

MARA Guardrails

Organisational Enablement: scaling legacy modernization through MARA guardrails, governance and adoption.

MARAMagenta AI-Centric Reference Architecture

The Magenta AI-Centric Reference Architecture provides comprehensive patterns, guidelines and best practices for building enterprise-grade AI solutions at Deutsche Telekom.

01Guardrails & governance

Steer decisions and owner accountability.

02AI-assisted legacy modernization

Reverse engineering, architecting and forward engineering.

03Adoption metrics

Score each app L0-L4, identify modernization gaps, prioritize waves and track coverage.

04Modernization roadmap

Capability-by-capability plan, funding, ownership and controlled cutover.

05Result

Governed transition from legacy complexity to AI-first solutions.

AI Adoption Metrics

Score adoption from manual modernization to AI-assisted cutover.

Key question: Are we industrializing AI-driven modernization and making modernization AI-first by default?

Level Adoption scoring
L0 Manual modernization only
L1 AI-assisted discovery / reverse engineering
L2 AI-assisted architecture & specification
L3 AI-supported forward engineering
L4 AI-assisted cutover

Cost Control

Cost-Efficiency: Technology Levers

Goal: lower model usage, integration effort and repeated context work

Problem statement

  • Model licenses and usage costs keep rising
  • Users have low awareness of cost impact

Cost thesis

Reduce spend by separating planning from execution, reusing tools and context, and routing each task to the smallest sufficient model.

Primary cost drivers to control

Cost control starts by making the main drivers visible, then managing them deliberately inside the workflow.

01 DriverToken volume
02 DriverContext window
03 DriverModel choice
04 DriverExternal data via MCP / RAG

Cost Control

Technology-to-cost levers

Four moves translate platform choices into lower run cost and less repeated context work.

01 Route compute

Keep strong models scarce

Use the expensive model only where it changes the outcome.
  • Agent pipeline design: keep reasoning scarce; use tool execution for deterministic, cheaper work.
  • Model selection: route simple work to cheaper models; reserve stronger models for high-context tasks.
  • Personal AI setup: reusable local AI setup using local LLMs and specific CLI.
02 Reuse capabilities

Avoid rework

Package repeatable work once, then make it callable.
  • Agent skills: reuse instructions for recurring tasks to reduce prompting effort and variance.
  • Plug-ins: package integrations once so agents call consistent, well-described capabilities.
  • Community marketplace: share components to avoid duplicated engineering and speed adoption.
03 Retrieve context

Load only relevant external data

Bring in the smallest useful context, not the whole estate.
  • Session hooks: load context automatically; reduce manual preparation and token waste.
  • MCP: standardize tool access; avoid rebuilding connector logic for every workflow.
  • RAG + vector DBs for smarter context: retrieve relevant legacy documentation on demand. Efficient access to external data, e.g. via RAG.
04 Accelerate delivery

Reduce engineering effort

Spend less human time on preparation and repeated execution.
  • AI coding assistants: accelerate reverse engineering, refactoring and forward-engineering tasks.
  • Agentic workflow: fewer manual preparation steps and faster adoption across teams.

Design principle: keep reasoning scarce, retrieval targeted, tool execution deterministic and integration reusable.

Scaling Patterns

Cost-Efficiency: Patterns for Scaling

Goal: prevent wasted runs, rework and uncontrolled usage

1. Reuse and scope control

Reduce repeated analysis, wasted tokens, unnecessary modernization work.

  1. 01
    Agent workflows

    Structured workflows reduce failed runs, rework and unnecessary model calls.

  2. 02
    Store intermediate results

    Persist plans, analyses and interview outputs so the same information is not recomputed.

  3. 03
    Business feature extraction

    Focus on valuable business logic; avoid modernizing dead or unused legacy code.

  4. 04
    Spec-driven development

    Clear specifications reduce ambiguity, retries and costly implementation corrections.

  5. 05
    Knowledge graph

    Compactly represent code and dependencies. Faster navigation, duplicate-analysis reduction and hidden relationship discovery.

Cost-efficiency effect

Less recomputation • Smaller context windows • Fewer abandoned implementation paths

Use this area to prioritize which code and evidence the AI should spend tokens on.

Scaling Patterns

Cost-Efficiency: Patterns for Scaling

Goal: prevent wasted runs, rework and uncontrolled usage

2. Automated Validation

Shift review effort into executable checks and targeted model judging.

  1. 06
    Validation

    Automated validation catches modernization errors earlier and reduces manual review.

  2. 07
    LLM as a judge

    Reserve expensive reasoning models for scoring rather than every generation step.

  3. 08
    Reduced human annotation

    AI-assisted labelling and review lowers the volume of costly manual annotation.

  4. 09
    TDD Test driven development

    Write tests before implementation. Use test suites as automated gates for AI/code changes.

    Impact: fewer regressions, faster debugging and less rework.

  5. 10
    BDD Behavior-driven development

    Translate requirements into Given/When/Then scenarios and executable acceptance checks.

    Impact: shared language, less handoff ambiguity and reduced manual review.

Cost Control

Cost-controlled Modernization with AI

Make every modernization workflow cheaper by design: measure spend, route intelligently, reuse context, automate repeatable work, then package what proves reusable.

Cost-control operating loop

1Baseline spend

Track tokens, model choice, context size and workflow execution.

2Route work

Use the smallest sufficient model; reserve reasoning for complex decisions.

3Retrieve context

Pull only relevant legacy facts; reuse stored outputs.

4Execute deterministically

Automate repeated steps with tools and scripts.

5Package reuse

Turn proven workflows into skills, plug-ins and marketplace assets.

Cost Control

Cost optimization guardrails

Use the operating loop with explicit guardrails so teams reduce model spend without losing quality or reusable acceleration assets.

Cost optimization guardrails

Measure first

Baseline cost per workflow before changing prompts or models.

Use reasoning sparingly

Route low-complexity tasks to cheaper models and deterministic tools.

Keep context targeted

Retrieve only the facts needed for the next decision.

Reuse before build

Promote proven patterns into skills, plug-ins and reusable assets.

Outcome: lower model spend, shorter modernization cycles, consistent quality and reusable acceleration assets.

Security Requirements

Guardrails for safe, code-only agentic modernization.

1Clearly bounded scope

Code migration only: no runtime operations, production data or system administration.

2Sensitive data protection

Repositories are scanned and cleaned before migration. Agents never access secrets or credentials.

3Controlled model access

Route interactions through approved model hubs with governed providers and safeguards.

4Least privilege

Restrict agents to approved repositories, branches, files, tools and actions.

5Human gates

Engineers approve generated changes before merge, deployment or irreversible action.

6Auditability

Trace prompts, model calls, tool actions, code changes, approvals and versions.

7Secure SDLC

Treat generated migration changes like production code with review, tests and rollback paths.

How to Start

Start with one bounded business process and prove the AI modernization loop end-to-end.

Minimum Setup
  • Owned repository + documentation
  • Legacy SME available for validation
  • Known feature / business process
  • Initial target architecture guardrails
  • Approved LLM hosting, security & privacy path
01Select pilot

Choose a valuable feature, not a whole application. Define success criteria and scope.

02Prepare evidence

Load code, docs, APIs, UI traces and existing tests. Keep context targeted.

03Reverse engineer

Use agents / coding assistants to extract flows, dependencies and business logic.

04Specify & build

Create specs as guardrails, design target architecture, generate tasks and code.

05Validate & scale

SME review, tests, security checks and LLM judge. Package reusable skills.

Output: validated modernization backlog + first generated target increment + reusable workflow assets.

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Wrap-up

AI modernization is ready enough to start — but it must be engineered, governed and measured.

Start now

Do not wait for the perfect market solution. Use current coding assistants and agents on a pilot.

Engineer the loop

Reverse engineering, specs, forward engineering, validation and human approval belong together.

Scale what works

Turn proven workflows into skills, plug-ins and guardrails to lower cost and improve quality.

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Thank you

Thank you for your attention.

Time for questions.