Next.js 15 API Route TypeScript Error: Fixing the Invalid GET Export Type for Dynamic Route Parameters

This build error happens because Next.js 15 now validates the exact type signature of App Router route handlers. If your dynamic API route exports a GET function with a manually typed params object that does not match what Next expects, the project may run in development but fail during npm run build.

Understanding the Root Cause

In a dynamic route such as app/api/case/[id]/route.ts, Next.js passes route parameters as the second argument to the handler. In older examples, many developers typed the handler like this:

export async function GET(req: Request, { params }: { params: { id: string } }) {
  // ...
}

That pattern was commonly accepted, but in Next.js 15 the route handler types became stricter. The framework checks whether exported HTTP methods such as GET, POST, and DELETE match its expected internal contract. For dynamic segments, the second argument is no longer something you should freely redefine in a way that conflicts with framework expectations.

The reported error usually appears during build with wording similar to an invalid GET export type. The reason is not your business logic. The real problem is the function signature.

There are two important technical details behind this:

• App Router handlers are framework-controlled exports, so the signature must match what Next can infer and validate.
• For dynamic route params in newer versions, params may need to be awaited depending on the generated route context typing, especially in examples aligned with current Next.js behavior.

If your route currently looks like the one referenced in the issue at the repository file, the fix is to update the handler signature and access pattern for params.

Step-by-Step Solution

The safest fix is to type the second argument in a way compatible with Next.js 15 route handlers and then extract the dynamic parameter correctly.

1. Locate the dynamic API route

Open:

app/api/case/[id]/route.ts

2. Replace the old handler signature

If your code looks similar to this:

export async function GET(
  request: Request,
  { params }: { params: { id: string } }
) {
  const id = params.id
  // existing logic
}

Update it to a Next.js 15-compatible form:

import { NextRequest, NextResponse } from 'next/server'

export async function GET(
  request: NextRequest,
  context: { params: Promise<{ id: string }> }
) {
  const { id } = await context.params

  try {
    return NextResponse.json({ id })
  } catch (error) {
    return NextResponse.json(
      { error: 'Failed to fetch case' },
      { status: 500 }
    )
  }
}

If you want to keep destructuring, use:

import { NextRequest, NextResponse } from 'next/server'

export async function GET(
  request: NextRequest,
  { params }: { params: Promise<{ id: string }> }
) {
  const { id } = await params

  return NextResponse.json({ id })
}

3. Why this works

This aligns the route handler with the stricter typing model used by Next.js 15. Instead of forcing a custom synchronous params object, you accept the route context in the form the framework expects and resolve the dynamic segment via await.

4. Full practical example for a database-backed route

Here is a more realistic example for a route that fetches a case by ID:

import { NextRequest, NextResponse } from 'next/server'

export async function GET(
  request: NextRequest,
  context: { params: Promise<{ id: string }> }
) {
  const { id } = await context.params

  if (!id) {
    return NextResponse.json(
      { error: 'Missing case id' },
      { status: 400 }
    )
  }

  try {
    const caseData = await getCaseById(id)

    if (!caseData) {
      return NextResponse.json(
        { error: 'Case not found' },
        { status: 404 }
      )
    }

    return NextResponse.json(caseData)
  } catch (error) {
    console.error('GET /api/case/[id] failed:', error)
    return NextResponse.json(
      { error: 'Internal server error' },
      { status: 500 }
    )
  }
}

async function getCaseById(id: string) {
  return { id, title: 'Example Case' }
}

5. Run a production build again

npm run build

If the issue was caused by the route export type, the build should now pass.

6. Optional cleanup: use NextRequest and NextResponse consistently

Although Request can work in some cases, using NextRequest and NextResponse makes your handler clearer and more aligned with Next.js conventions, especially when you later need cookies, headers helpers, or URL parsing.

Common Edge Cases

Using { params: { id: string } } still compiles in one file but fails elsewhere

This usually happens when local editor inference differs from the stricter production type validation. Always trust the result of next build over editor hints.

Destructuring params without awaiting it

const { id } = params

In Next.js 15-compatible patterns, this can be wrong if params is typed as a Promise. Use:

const { id } = await params

Using the same outdated pattern in POST, PUT, PATCH, or DELETE

The same issue can affect any exported route handler. If one method is fixed but another still uses the old context type, the build can continue failing.

export async function DELETE(
  request: NextRequest,
  context: { params: Promise<{ id: string }> }
) {
  const { id } = await context.params
  return NextResponse.json({ deleted: id })
}

Mismatch between folder name and param name

If your route folder is [id], your code must read id. If the folder is [caseId], then the property must be caseId. A mismatch will not be fixed by changing types alone.

Confusing API route handlers with page components

In page and layout files, dynamic params have their own typing patterns. Do not assume the exact same examples apply to route.ts files. Route handlers are validated differently from server components.

Mixing Pages Router APIs with App Router APIs

If you import NextApiRequest or NextApiResponse, you are using the wrong API surface. In app/api, use App Router route handlers with NextRequest and NextResponse.

FAQ

Why does the route work in development but fail during build?

Development mode is often more permissive, while the production build performs stricter validation of exported route handler types. That is why this bug commonly appears only when running npm run build.

Do I have to use NextRequest instead of the standard Request?

Not always, but it is recommended. The core issue is usually the context params typing, not the first argument alone. Still, NextRequest is the better choice for consistency and future framework features.

Is awaiting params really necessary in Next.js 15?

For this class of route typing issue, yes—when your route context is defined as { params: Promise<...> }. This matches the newer framework-compatible pattern and resolves the invalid export type error.

Can I define a reusable type for dynamic route context?

Yes. If multiple routes use the same pattern, create a shared type:

type IdRouteContext = {
  params: Promise<{ id: string }>
}

export async function GET(request: NextRequest, context: IdRouteContext) {
  const { id } = await context.params
  return NextResponse.json({ id })
}

That keeps your handlers clean while staying compatible with Next.js 15.

The key fix is simple: in dynamic App Router API routes, stop using the older manually shaped synchronous params signature and switch to the Next.js 15-compatible route context. Once you update the handler in app/api/case/[id]/route.ts, the invalid GET export type error should disappear during build.