Next.js App Router API Route: @ffmpeg-installer/ffmpeg throws "Module not found" and "server relative imports not implemented" error

After much struggle, I was able to get this to work. But I had to use ffmpeg static, and you have to use that as the path. There were some issues of its own with that. I hope this helps someone struggling with a similar issue.

Next.js wasn’t bundling ffmpeg-static, and fluent-ffmpeg didn’t know where the binary was located.
Two things fixed it:

✅ 1. Update your API Route to explicitly set the ffmpeg binary path

You must tell fluent-ffmpeg where the ffmpeg-static binary is, and you must run the route in a Node.js runtime (NOT Edge).

import { NextRequest, NextResponse } from "next/server";
import { getSession } from "@/lib/session";
import { S3Client, PutObjectCommand } from "@aws-sdk/client-s3";
import ffmpeg from "fluent-ffmpeg";
import ffmpegStatic from "ffmpeg-static"; // ✅ ADD THIS
import { writeFile, unlink, mkdir, readFile } from "fs/promises";
import { existsSync } from "fs";
import path from "path";
import { tmpdir } from "os";

export const runtime = "nodejs"; // ✅ Required for ffmpeg

// 🔥 Tell fluent-ffmpeg which binary to use
ffmpeg.setFfmpegPath(ffmpegStatic as string);

const s3Client = new S3Client({
  region: process.env.AWS_REGION!,
  credentials: {
    accessKeyId: process.env.AWS_ACCESS_KEY_ID!,
    secretAccessKey: process.env.AWS_SECRET_ACCESS_KEY!
  }
});

export async function POST(request: NextRequest) {
  try {
    const user = await getSession();
    if (!user) {
      return NextResponse.json(
        { success: false, message: "Unauthorized" },
        { status: 401 }
      );
    }

    const formData = await request.formData();
    const file = formData.get("file") as File;

    if (!file) {
      return NextResponse.json(
        { success: false, message: "No file found" },
        { status: 400 }
      );
    }

    const tempDir = path.join(tmpdir(), "video-conversion");
    if (!existsSync(tempDir)) {
      await mkdir(tempDir, { recursive: true });
    }

    const inputPath = path.join(tempDir, `${Date.now()}-input.webm`);
    const outputPath = path.join(tempDir, `${Date.now()}-output.mp4`);

    const buffer = Buffer.from(await file.arrayBuffer());
    await writeFile(inputPath, buffer);

    // Convert using ffmpeg
    await new Promise<void>((resolve, reject) => {
      ffmpeg(inputPath)
        .setFfmpegPath(ffmpegStatic as string)
        .outputOptions(["-c:v libx264", "-preset fast", "-crf 23", "-r 30"])
        .toFormat("mp4")
        .on("end", resolve)
        .on("error", reject)
        .save(outputPath);
    });

    const key = `${user.id}/${Date.now()}.mp4`;
    const convertedBuffer = await readFile(outputPath);

    await s3Client.send(
      new PutObjectCommand({
        Bucket: process.env.S3_BUCKET_NAME!,
        Key: key,
        Body: convertedBuffer,
        ContentType: "video/mp4"
      })
    );

    await Promise.all([
      unlink(inputPath).catch(() => {}),
      unlink(outputPath).catch(() => {})
    ]);

    return new NextResponse(convertedBuffer, {
      headers: {
        "Content-Type": "video/mp4",
        "Content-Length": convertedBuffer.length.toString()
      }
    });
  } catch (error) {
    console.error("Video conversion failed:", error);
    return NextResponse.json(
      { success: false, message: "Video conversion failed" },
      { status: 500 }
    );
  }
}

✅ 2. Add ffmpeg-static + fluent-ffmpeg to external server packages

You need to update your nextjs config file. Next.js must not bundle these — it must load them from node_modules.

/** @type {import('next').NextConfig} */
const nextConfig = {
  experimental: {
    serverComponentsExternalPackages: ["ffmpeg-static", "fluent-ffmpeg"]
  }
};

export default nextConfig;

🎉 After this, ffmpeg finally ran correctly inside the Next.js route.

Hopefully this saves someone hours of debugging!