Migrating a Legacy App to Next.js 15: A Realistic Timeline and Checklist

# How to Migrate to Next.js 15: A Realistic Timeline and Checklist

Summary: Migrating an enterprise legacy React single-page application to Next.js 15 requires an incremental strangler fig architecture rather than a high-risk complete rewrite. This engineering guide details a battle-tested 14-week migration roadmap covering reverse proxy routing, React 19 async API compatibility, server-side data fetching, and unified authentication. Technical leads and architects will find concrete Nginx configurations, TypeScript implementations, phased checklists, and realistic team velocity benchmarks to modernize production systems safely without stalling feature delivery.

The Flaw of Big-Bang Rewrites in Enterprise Systems

The Second-System Effect and Feature Velocity Freezes

Engineering organizations frequently fall into the trap of the total rewrite. When maintaining a sprawling single-page application (SPA) built on Create React App (CRA), legacy Webpack, or early React Router configurations, technical debt compounds. Bundle sizes swell past five megabytes, Time to Interactive (TTI) degrades on mobile devices, and search engine crawlers struggle with client-side JavaScript rendering.

The instinct of engineering leadership is often to halt feature development and declare a green-field rebuild. In production environments, this strategy fails consistently due to the Second-System Effect. While the engineering team spends six to nine months attempting to rebuild parity in a separate repository, business requirements evolve. The legacy application continues receiving critical bug fixes, compliance patches, and revenue-generating features.

The migration target becomes a moving boundary. When the projected launch date arrives, the new system lacks parity, the old system cannot be retired, and the team is forced to maintain two divergent codebases under immense executive pressure. A successful plan to migrate to Next.js 15 must decouple migration progress from feature freezes.

Architectural Divergence: Legacy React SPAs vs. Next.js 15

Modernizing a legacy React application is not a mere syntax upgrade; it is an architectural paradigm shift. Legacy SPAs operate on a client-centric model: the browser downloads an empty HTML shell, fetches massive JavaScript bundles, initializes client-side routers, renders component trees, and executes waterfalls of uncoordinated API calls via useEffect.

[Legacy SPA Request Lifecycle]
Client Request ──> HTML Shell (Empty <div>) ──> Fetch JS Bundle (3MB+)
                      │
                      ▼
               Parse & Execute JS ──> Client-Side Routing ──> useEffect Waterfall (Fetch API)
                                                                   │
                                                                   ▼
                                                            DOM Rendered & Interactive

Next.js 15, built on React 19, inverts this lifecycle through React Server Components (RSC) and the App Router. Components execute on the server by default, streaming prerendered HTML directly to the browser while keeping heavy dependencies (such as markdown parsers, date libraries, and data transformation utilities) off the client bundle.

[Next.js 15 App Router Request Lifecycle]
Client Request ──> Server Resolves Route & Async Request APIs (cookies, headers, params)
                      │
                      ▼
               Execute React Server Components ──> Stream Prerendered HTML + Flight Payload
                      │
                      ▼
               Browser Displays Content Immediately ──> Selective Hydration of Leaves ("use client")

Attempting to convert an entire legacy codebase to this server-driven model overnight introduces severe operational risk. The only viable path for mission-critical enterprise systems is an incremental migration using the Strangler Fig pattern.

Core Migration Pattern: The Strangler Fig Architecture

Ingress Traffic Partitioning

The Strangler Fig pattern places an ingress reverse proxy (such as Nginx, Cloudflare Workers, or AWS CloudFront) in front of both the legacy SPA and the new Next.js 15 application. To the outside world, both applications share the same origin, domain, and SSL certificates.

The proxy inspects the incoming request URI:

1. Migrated routes (for example, /login, /dashboard/analytics, or /settings) are routed directly to the Next.js 15 upstream cluster.

2. Unmigrated legacy routes (such as /legacy-reports or complex legacy workflows) fall back to the legacy SPA upstream.

3. Static assets and API traffic are routed deterministically based on path prefixes.

                            ┌───────────────────────────────────┐
                            │      Client (Browser / Bot)       │
                            └─────────────────┬─────────────────┘
                                              │
                                              ▼
                            ┌───────────────────────────────────┐
                            │    Ingress Proxy (Nginx / Edge)   │
                            └───────┬───────────────────┬───────┘
                                    │                   │
               Route: /dashboard/*  │                   │  Route: /* (Fallback)
                                    ▼                   ▼
    ┌───────────────────────────────────┐   ┌───────────────────────────────────┐
    │     Next.js 15 App Router         │   │        Legacy React SPA           │
    │     (React 19 / Node.js 20+)      │   │     (Webpack / CRA / Vite)        │
    └───────────────────────────────────┘   └───────────────────────────────────┘

This topology allows development teams to migrate one route cluster at a time, verify telemetry in production with real user traffic, and retain instantaneous rollback capabilities by tweaking proxy routing weights.

For engineering organizations evaluating team bandwidth, engaging specialized legacy modernization services accelerates this initial infrastructure audit while establishing deterministic proxy routing before internal teams touch core business logic.

Production Nginx Reverse Proxy Configuration

Below is a concrete, production-grade Nginx configuration demonstrating path-based routing between a legacy SPA container and a Next.js 15 server. It handles header preservation, streaming buffers for React Server Components, and WebSocket connections for real-time channels:

# /etc/nginx/conf.d/application_proxy.conf

upstream legacy_spa_upstream {
    server 10.0.1.50:80;
    keepalive 32;
}

upstream nextjs15_upstream {
    server 10.0.2.100:3000;
    keepalive 64;
}

server {
    listen 443 ssl http2;
    server_name app.enterprise.com;

    ssl_certificate /etc/ssl/certs/app.enterprise.com.crt;
    ssl_certificate_key /etc/ssl/private/app.enterprise.com.key;
    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers HIGH:!aNULL:!MD5;

    # Shared Cookie and Header Propagation
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_http_version 1.1;

    # 1. Next.js 15 Static Assets & Build Artifacts
    location /_next/ {
        proxy_pass http://nextjs15_upstream;
        proxy_set_header Connection "";
        proxy_cache_valid 200 365d;
        add_header Cache-Control "public, max-age=31536000, immutable";
    }

    # 2. Migrated Route Domain: Modern Dashboard & Reporting
    location ~ ^/(dashboard|analytics|settings|auth)(/.*)?$ {
        proxy_pass http://nextjs15_upstream;
        proxy_set_header Connection "";
        
        # Disable buffering to allow React 15/19 Server Component streaming
        proxy_buffering off;
        proxy_read_timeout 60s;
        
        # Custom header to track upstream routing in observability tools
        add_header X-App-Engine "NextJS-15-AppRouter" always;
    }

    # 3. Next.js 15 Server Actions Ingress
    location /api/actions/ {
        proxy_pass http://nextjs15_upstream;
        proxy_set_header Connection "";
        proxy_buffering off;
    }

    # 4. Fallback: Legacy React Single Page Application
    location / {
        proxy_pass http://legacy_spa_upstream;
        proxy_set_header Connection "";
        
        # Standard SPA routing: unmigrated pages hit index.html on legacy origin
        proxy_intercept_errors on;
        error_page 404 = /index.html;
        
        add_header X-App-Engine "Legacy-React-SPA" always;
    }
}

State and Authentication Continuity Across Boundaries

The hardest challenge in a strangler migration is state continuity. If a user logs into the legacy SPA, navigates to a migrated Next.js 15 route, and encounters a redirected login screen, the architecture has failed.

To guarantee continuous user context:

1. Move Authentication to Secure HTTP-Only Cookies: Eliminate all dependencies on localStorage or sessionStorage for bearer tokens. Both upstreams must access the session via domain-scoped HttpOnly, SameSite=Lax, Secure cookies.

2. Standardize JWT Signature Validation: The Next.js 15 Edge or Node runtime must share verification secrets or public JWKS endpoints with the legacy backend.

3. Decouple Global Client Stores: Avoid attempting to synchronize Redux or Zustand state across the boundary via iframe messaging or local storage listeners. Instead, treat the URL and server-side session stores as the authoritative source of truth.

Architectural Comparison: Legacy React SPA vs. Next.js 15 App Router

The following matrix contrasts the operational characteristics of a traditional client-side React architecture against Next.js 15:

The Realistic 14-Week Migration Timeline

Engineering estimates published in vendor tutorials often suggest migrating an enterprise app in two weeks. In reality, unexpected cross-cutting concerns—such as legacy CSS conflicts, obscure authentication flows, and tightly coupled Redux state—quickly derail oversimplified timelines.

Below is an honest, 14-week enterprise migration roadmap for a mid-to-large React codebase (~100,000 to 250,000 lines of code) staffed by a dedicated squad of four senior software engineers.

Engineering Effort and Velocity Reality Check

Week  1 -  2: [Phase 0] Audits, Dependency Upgrades, React 19 Validation
Week  3 -  4: [Phase 1] Reverse Proxy Ingress, Shell Setup, Token Harmonization
Week  5 -  7: [Phase 2] Route Mapping, Nested Layouts, Static Pages
Week  8 - 10: [Phase 3] Data Layer Decoupling, RSC Implementation, Server Actions
Week 11 - 12: [Phase 4] Auth Hardening, Middleware Context, Session Sync
Week 13 - 14: [Phase 5] CI/CD Pipeline, Observability, Load Testing, Canary Cutover

Phase 0: Discovery, Dependency Auditing, and React 19 Baseline (Weeks 1–2)

Before writing Next.js code, assess your package manifest. Next.js 15 requires React 19. If your legacy application relies on abandoned libraries that hook into obsolete React lifecycles (UNSAFE_componentWillReceiveProps, obsolete context APIs, or legacy React 16/17 reconciler internals), builds will fail.

Execute an audit checklist:

• Run npm outdated and flag packages that lack React 19 support.
• Identify legacy CSS-in-JS libraries. Older versions of Emotion and Styled-Components rely on runtime style injection that fails inside React Server Components. Plan to isolate them inside "use client" wrappers or transition to Tailwind CSS / Vanilla Extract.
• Run Next.js codemods on experimental branches:

  npx @next/codemod@canary upgrade latest

Phase 1: Ingress Infrastructure and Shared Design System Shell (Weeks 3–4)

Establish the hybrid deployment environment:

• Provision containerized environments in your staging cloud (AWS ECS, Google Cloud Run, or Kubernetes).
• Deploy the Nginx or Cloudflare Worker ingress proxy. Verify that requests route properly to the legacy SPA by default.
• Create a shared design token repository or export CSS custom properties from the legacy app into the Next.js project. Ensure font definitions, colors, and layout widths render identically across both systems to prevent jarring visual layout shifts during page transitions.

Phase 2: Routing Architecture and App Router Layouts (Weeks 5–7)

Begin strangling low-risk, high-impact pages. Excellent candidates include:

• Authentication screens (/login, /signup, /forgot-password).
• Static marketing and documentation pages.
• Standalone reporting views with few interactive forms.

Construct the root layout hierarchy (app/layout.tsx and app/(authenticated)/layout.tsx). Replace legacy client-side route guards with Next.js middleware and server-side layout redirects.

Phase 3: Data Layer Refactoring and Server Component Adoption (Weeks 8–10)

Migrate core interactive dashboard pages. This phase represents the most substantial refactor:

• Deconstruct monolithic Redux slices. If data is only read by a dashboard view, delete the Redux action creators, reducers, and selectors. Fetch data directly in async Server Components.
• Convert client-side form submissions from imperative axios.post() calls to React Server Actions with validation via libraries such as Zod.
• Audit component trees to enforce strict separation: keep data fetching at the server root, passing clean serialized data down to interactive interactive leaves marked with "use client".

Phase 4: Unified Authentication and Session Propagation (Weeks 11–12)

Solidify the security boundary:

• Implement Next.js 15 middleware.ts to intercept protected route groups.
• Validate incoming session cookies. Decorate downstream request headers with user identity, tenant IDs, and role permissions.
• Implement comprehensive fallback handling: if an authenticated user moves from Next.js back to a legacy route, ensure session refresh tokens renew silently without forcing re-authentication.

Phase 5: CI/CD Hardening, Observability, and Final Cutover (Weeks 13–14)

Prepare the production deployment pipeline:

• Configure OpenTelemetry exporters in instrumentation.ts to capture distributed traces across Nginx, Next.js Server Components, and downstream microservices.
• Execute synthetic load testing using k6 or Locust to model expected traffic distribution across both runtimes.
• Shift production traffic incrementally: route 10% of users to Next.js via proxy weighting, monitor error rates for 48 hours, scale to 50%, and finally reach 100%. Once stable, decommission the legacy routes.

Critical Next.js 15 Breaking Changes and Engineering Pitfalls

Migrating to Next.js 15 introduces distinct architectural changes from earlier versions (such as Next.js 13 and 14). Architects must address these breaking modifications explicitly during implementation.

Asynchronous Request APIs (Headers, Cookies, Params)

In Next.js 15, runtime request data—specifically cookies(), headers(), params, and searchParams—has transitioned from synchronous access to asynchronous Promises. This change enables the underlying React runtime to optimize rendering pipelines and prepare for future streaming compiler improvements.

Failure to await these functions will throw runtime warnings or fatal execution errors during build time.

Legacy / Next.js 14 Pattern (Synchronous)

// app/dashboard/analytics/[metricId]/page.tsx (OBSOLETE)
import { cookies } from 'next/headers';

interface PageProps {
  params: { metricId: string };
  searchParams: { timeframe?: string };
}

export default function AnalyticsPage({ params, searchParams }: PageProps) {
  // Synchronous access in Next.js 14 causes breaking runtime errors in Next.js 15
  const cookieStore = cookies();
  const token = cookieStore.get('session_token');
  const metricId = params.metricId;
  const timeframe = searchParams.timeframe || '7d';

  return <div>Metric: {metricId} - Timeframe: {timeframe}</div>;
}

Modern Next.js 15 Implementation (Fully Asynchronous)

// app/dashboard/analytics/[metricId]/page.tsx (CORRECT NEXT.JS 15)
import { cookies, headers } from 'next/headers';
import { Suspense } from 'react';
import { notFound } from 'next/navigation';

interface PageProps {
  params: Promise<{ metricId: string }>;
  searchParams: Promise<{ timeframe?: string }>;
}

async function MetricDataViewer({ 
  metricIdPromise, 
  timeframePromise 
}: { 
  metricIdPromise: Promise<{ metricId: string }>;
  timeframePromise: Promise<{ timeframe?: string }>;
}) {
  const { metricId } = await metricIdPromise;
  const { timeframe = '7d' } = await timeframePromise;
  
  const cookieStore = await cookies();
  const sessionToken = cookieStore.get('session_token')?.value;

  if (!sessionToken) {
    notFound();
  }

  // Fetch telemetry data directly in the Server Component
  const response = await fetch(`https://api.enterprise.internal/v1/metrics/${metricId}?range=${timeframe}`, {
    headers: {
      Authorization: `Bearer ${sessionToken}`,
      'X-Request-Source': 'NextJS15-RSC',
    },
    // Explicit caching semantics required in Next.js 15
    cache: 'no-store',
  });

  if (!response.ok) {
    throw new Error(`Failed to load metric data: ${response.statusText}`);
  }

  const data = await response.json();

  return (
    <div className="rounded-lg border border-slate-800 p-6 bg-slate-900 text-slate-100">
      <h2 className="text-xl font-semibold mb-2">Metric: {metricId}</h2>
      <p className="text-sm text-slate-400 mb-4">Timeframe Range: {timeframe}</p>
      <pre className="text-xs bg-slate-950 p-4 rounded overflow-x-auto">
        {JSON.stringify(data, null, 2)}
      </pre>
    </div>
  );
}

export default async function AnalyticsPage(props: PageProps) {
  const headerStore = await headers();
  const correlationId = headerStore.get('x-correlation-id') || 'unknown';

  return (
    <main className="container mx-auto py-8">
      <div className="mb-6 flex justify-between items-center">
        <h1 className="text-2xl font-bold tracking-tight">Enterprise Telemetry</h1>
        <span className="text-xs font-mono text-slate-500">Trace: {correlationId}</span>
      </div>
      
      <Suspense fallback={<div className="h-48 w-full animate-pulse bg-slate-800 rounded-lg" />}>
        <MetricDataViewer 
          metricIdPromise={props.params} 
          timeframePromise={props.searchParams} 
        />
      </Suspense>
    </main>
  );
}

Uncached Fetch Defaults and Caching Semantics

In Next.js 14, standard fetch calls defaulted to force-cache (aggressive caching), causing unexpected stale data issues for engineers unfamiliar with the App Router.

Next.js 15 changes the default behavior: fetch requests now default to no-store.

What this means for migration:

• Dynamic API calls will always hit your upstream backend unless explicitly configured otherwise.
• To prevent overwhelming internal microservices with duplicate requests, implement explicit static revalidation or request deduplication:

// Explicitly opt into Incremental Static Regeneration (ISR) when data changes infrequently
const productCatalog = await fetch('https://api.enterprise.internal/products', {
  next: { revalidate: 3600 }, // Cache on server for 1 hour
});

// Explicitly opt into persistent static caching
const appConfig = await fetch('https://api.enterprise.internal/config', {
  cache: 'force-cache',
});

Client Component Hydration Boundaries

A common failure mode during a legacy React migration is placing "use client" at the very top of a migrated page to bypass Server Component rules. While this temporarily silences build errors, it forces the entire sub-tree into the client bundle, defeating the primary architectural benefits of Next.js 15.

Enforce the leaf-node rule:

• Keep data fetching, auth verification, and heavy business transformations in Server Components.
• Pass plain JavaScript primitives (strings, numbers, objects) as props across the boundary.
• Reserve "use client" exclusively for interactive leaf components that manage DOM events, local interactive animations, or browser hooks (useState, useCallback, onClick).

// app/components/interactive-counter.tsx (LEAF COMPONENT)
'use client';

import { useState } from 'react';

interface CounterProps {
  initialCount: number;
  metricLabel: string;
}

export function InteractiveCounter({ initialCount, metricLabel }: CounterProps) {
  const [count, setCount] = useState(initialCount);

  return (
    <div className="flex items-center gap-3">
      <span className="text-sm font-medium">{metricLabel}: {count}</span>
      <button 
        onClick={() => setCount((c) => c + 1)}
        className="px-3 py-1 bg-blue-600 hover:bg-blue-500 text-white text-xs rounded transition"
      >
        Increment
      </button>
    </div>
  );
}

Production-Ready Migration Checklist

Use this checklist during weekly architectural syncs to ensure no foundational steps are bypassed:

1. Build & Runtime Environment

• [ ] Upgrade Node.js runtime across all CI/CD runners to Node.js 20.x or 22.x LTS.
• [ ] Initialize Next.js 15 with Turbopack enabled for local development (next dev --turbo).
• [ ] Migrate tsconfig.json to enforce "strict": true, "moduleResolution": "bundler", and "isolatedModules": true.
• [ ] Audit dependencies against React 19 peer requirements using npm ls react.
• [ ] Isolate legacy CSS-in-JS libraries inside client boundary wrappers or migrate styles to Tailwind CSS.

2. Ingress & Routing Topology

• [ ] Deploy Nginx, Cloudflare Workers, or AWS CloudFront with path-based upstream routing.
• [ ] Implement asset isolation: ensure /_next/* routes to Next.js and /static/* routes to the legacy SPA.
• [ ] Verify SSL certificate termination and enforce HTTP/2 or HTTP/3 at the proxy layer.
• [ ] Implement an automated fallback rule returning legacy index.html on unmigrated paths.
• [ ] Configure proxy buffer sizes and disable buffering for Server-Sent Events (SSE) and RSC streaming.

3. Routing & Layout Architecture

• [ ] Establish root layout hierarchy (app/layout.tsx) containing essential meta tags and global fonts.
• [ ] Convert React Router route definitions into nested file-system folders under app/.
• [ ] Ensure all dynamic parameters (params and searchParams) in layouts and pages use await promises.
• [ ] Implement global error boundaries (app/error.tsx) and 404 handlers (app/not-found.tsx).
• [ ] Replace imperative navigation (history.push()) with next/navigation (useRouter, <Link>).

4. Data Layer & Caching

• [ ] Remove useEffect data fetching loops on migrated routes; transition to async Server Components.
• [ ] Explicitly declare caching policies (cache: 'no-store' or next: { revalidate: N }) on every fetch.
• [ ] Replace client-side mutation endpoints with React Server Actions and Zod schema validation.
• [ ] Wrap streaming Server Components in <Suspense> boundaries with skeleton placeholders.
• [ ] Audit backend API access logs to ensure Server Components do not cause N+1 query storms.

5. Identity, Session & Security Boundaries

• [ ] Standardize session storage on HttpOnly, SameSite=Lax, Secure cookies.
• [ ] Implement Next.js middleware.ts for route-level authentication and RBAC checks.
• [ ] Inject user identity and correlation IDs into downstream request headers.
• [ ] Implement CSRF protection on all Server Action endpoints.
• [ ] Verify that navigating across legacy and modern routes does not trigger unexpected session termination.

6. Observability & Deployment Verification

• [ ] Configure OpenTelemetry tracing in instrumentation.ts targeting Datadog, New Relic, or Grafana.
• [ ] Set up Core Web Vitals monitoring capturing real-user INP, LCP, and CLS.
• [ ] Establish automated canary deployment pipelines capable of weighted traffic shifting.
• [ ] Validate automated rollback triggers: 5xx error spikes or latency degradations automatically revert traffic to legacy upstreams.

Falsifiability and Verification Protocol

An architectural plan without verifiable pass/fail metrics is an assumption. To confirm each phase has succeeded without silent degradation, enforce these falsifiability benchmarks before transitioning subsequent routes:

1. Hydration Parity: In staging and production environments, console hydration errors must equal zero (0). Any mismatch between server-rendered HTML and client hydration must fail the pull request build.

2. Core Web Vitals Thresholds:

- Largest Contentful Paint (LCP): Must stay below 1.8 seconds on 75th percentile mobile devices.

- Interaction to Next Paint (INP): Must stay below 150 milliseconds under heavy interaction tests.

- Time to First Byte (TTFB): Must remain below 250 milliseconds from the edge proxy.

3. Upstream Error Rates: During the strangler transition, the Nginx reverse proxy 5xx error rate must not exceed 0.01% over a trailing 24-hour evaluation window.

4. Backend Database Concurrency: Upstream internal API response latencies must remain stable (variance < 5%) before and after Server Component adoption, confirming that server-side data fetching does not cause database connection pool exhaustion.

If any of these verification gates fail, halt the migration cadence, inspect trace payloads in OpenTelemetry, and resolve the architectural bottleneck before migrating additional route segments.

Frequently Asked Questions

How should we manage shared client state between the legacy SPA and Next.js 15?

Do not attempt to synchronize global client-side state stores (such as Redux or Zustand) across the two applications. Shared client state across separate application runtimes introduces race conditions, memory leaks, and complex serialization logic. Instead, rely on the server and the URL as the single source of truth. Persist critical application state in URL search parameters or server-side session databases backed by Redis. When a user transitions from a legacy route to a Next.js 15 route, the Next.js Server Component reads the state directly from the request cookie or URL query string, completely bypassing the need for client store bridges.

Can we run React 18 alongside Next.js 15 in the same application bundle?

No. You cannot run two different versions of React within the same Next.js application bundle. Next.js 15 requires React 19. However, under the Strangler Fig architecture outlined in this guide, your legacy application can continue running React 16, 17, or 18 in its own container, while the Next.js 15 application runs React 19 independently. The two applications run in completely isolated container runtimes and communicate solely through standard HTTP cookies, headers, and the reverse proxy.

How does Next.js 15's default uncached fetch affect our internal API infrastructure?

Because Next.js 15 defaults to cache: 'no-store', every request to a Server Component that triggers a fetch will execute against your backend service unless configured otherwise. If your legacy SPA previously relied on client-side libraries that aggressively cached responses in memory, migrating to Server Components can lead to a tenfold increase in backend query volume. To mitigate this risk, profile all backend endpoints before migrating a route. For data that does not change every second, explicitly configure Incremental Static Regeneration using { next: { revalidate: 60 } } or implement a Redis caching layer in front of high-traffic database queries.

What is the exact rollback procedure if a migrated route fails in production?

Because the ingress reverse proxy (such as Nginx or Cloudflare Workers) controls route resolution, rollbacks do not require rolling back the Next.js container deployment. If a critical bug is detected in a migrated route (for example, /dashboard/analytics), an engineer updates the reverse proxy configuration to comment out the Next.js route mapping, forcing traffic to fall back to the legacy SPA upstream. With automated edge proxies, this configuration change takes effect globally in under five seconds, isolating the incident while the engineering team investigates error traces in a non-production environment.

Sources

• Next.js 15 Official Release Documentation - Complete overview of Next.js 15 features, Turbopack stability, and async request APIs.
• React 19 Upgrade Guide - Official React documentation outlining breaking changes, hook deprecations, and Server Component specifications.
• Martin Fowler on the Strangler Fig Application Pattern - Foundational architectural reference for incrementally modernizing monolithic and legacy applications.
• Web Vitals Specification - Technical thresholds and diagnostic methodology for LCP, INP, and CLS performance metrics.