AI Agent Upgraded My Codebase to Nuxt 4

My personal site was running Nuxt 3 with a pile of outdated packages - the kind of cleanup you keep pushing to "next weekend."

I gave Claude Code one instruction:

"Upgrade everything to latest. Nuxt 4. Resolve any errors. Make sure the site works."

Two hours later: fully migrated, every dependency at latest, 45 passing tests, zero manual file edits.

Here's exactly what happened.

---

🔁 The Structural Migration

Nuxt 4 changes the entire source directory convention. Everything that lived at root moves into an app/ directory:

components/   →   app/components/
pages/        →   app/pages/
layouts/      →   app/layouts/
composables/  →   app/composables/
plugins/      →   app/plugins/

That's not just a rename - it cascades. The ~/ alias now resolves to app/, not the project root. Vitest alias mappings need updating too - ~~ for rootDir, @@ as a secondary alias. Auto-import behavior shifts in test environments.

The agent caught every cascading breakage without me pointing any of them out.

---

📦 The Package Upgrades

The kind that normally consumes an entire weekend:

| Package | Before | After | |---|---|---| | nuxt | 3.x | 4.3.1 | | vitest | 1.x | 4.0.18 | | @nuxt/image | 1.x | 2.0 | | jsdom | 24.x | 28.x | | @vitejs/plugin-vue | 5.x | 6.x |

The hidden landmines:

@nuxt/content v3 quietly dropped sqlite3 in favour of better-sqlite3. No loud warning - just a cryptic runtime error. The agent diagnosed it, swapped the dependency, moved on.

After the major esbuild jump, the native binary for Apple Silicon was missing. The agent ran:

npm rebuild esbuild

No panic. Just progression.

nuxt-security 2.5.x started injecting new default response headers - Cross-Origin-Resource-Policy, Cross-Origin-Opener-Policy, Permissions-Policy - that didn't exist in the previous version. The agent updated the security config to account for all of them.

---

🧪 The Test Fixes

This was the real test of the agent's reasoning ability.

Problem 1: Nuxt auto-imports don't exist in bare Vitest

ref, computed, inject and the rest of the Vue Composition API are auto-imported by Nuxt at runtime - but that runtime doesn't exist in a plain Vitest environment. Every test was throwing inject is not defined.

The agent exposed the entire Composition API globally in tests/setup.ts:

import { ref, computed, inject, /* ... */ } from 'vue'
Object.assign(globalThis, { ref, computed, inject /* ... */ })

Problem 2: <Teleport> breaks wrapper.find()

<Teleport to="body"> renders its slot content outside the component wrapper in jsdom. So wrapper.find('ul') was returning false on a perfectly working mobile nav component.

The agent diagnosed the root cause and stubbed Teleport globally:

config.global.stubs = {
  Teleport: { template: '<div><slot /></div>' }
}

Problem 3: Loading state race condition

The loading state test was passing a mock that resolved immediately - so isSubmitting was already false by the time the assertion ran. The agent switched to a never-resolving Promise for that specific test:

mockCreateContact.mockReturnValue(new Promise(() => {})) // never resolves
wrapper.find('form').trigger('submit.prevent') // intentionally no await
await wrapper.vm.$nextTick()
expect(wrapper.text().toLowerCase()).toContain('sending')

45 tests. All green ✅

---

🧠 What This Actually Means

I've been a developer for years. This migration would have taken a full weekend:

• Reading Nuxt 4 changelogs and migration guides
• Debugging cryptic errors one by one
• Cross-referencing GitHub issues for obscure package behaviour
• Fighting jsdom edge cases in test environments

It took 2 hours with agentic coding.

We're not at "AI writes your features" yet. But we're absolutely at:

AI handles the painful, cross-cutting, high-context engineering work - the kind that drains entire days.

The skill shift isn't learning to code less.

It's learning to direct well - knowing when to let the agent run, when to step in, and how to frame the problem so it doesn't go in circles. That's closer to being a good tech lead than it is to prompting.

---

📌 Takeaways

• Nuxt 4 migration is non-trivial - the app/ restructure has deep downstream effects that aren't obvious from the docs alone.
• Test environments need manual bridging - Nuxt's auto-import magic doesn't transfer to plain Vitest without explicit setup.
• Agentic coding shines on high-context work - the agent's ability to hold the full dependency graph in mind while fixing cascading failures is where it genuinely beats manual debugging.
• The real skill is direction - a clear initial instruction and sharp follow-up questions matter more than any prompting trick.

---

Thanks for reading.
Namaskaram 🙏