diff --git a/.gitignore b/.gitignore
index 5402c8e..6719e04 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ test/fixture/nextoutput/
 test/fixture/next.config.js
 test/fixture2/.next/BUILD_ID
 test/fixture-next5/build/
+test/fixture-next6/.next/
diff --git a/README.md b/README.md
index 98bd996..d4c00dd 100644
--- a/README.md
+++ b/README.md
@@ -9,12 +9,29 @@
 
 Simple CLI and module that lets you define your own build id when using Next.js.
 
-This is necessary if you're running multiple instances of your Next.js app on different servers sitting behind a load balancer without session affinity. Otherwise, if your Next.js builds end up with different build ids, a client loading content from different servers can result in [this Next.js error](https://github.com/zeit/next.js/blob/52ccc14059673508803f96ef1c74eecdf27fe096/server/index.js#L444), which causes the app to blow up for that client.
+## New in version 2!
+
+When using Next.js 6+ (which introduced the [generateBuildId](https://github.com/zeit/next.js#configuring-the-build-id) config prop), you can use `next-build-id` as a module within your next.config.js logic to set the BUILD_ID to the most recent git commit hash. This approach means you don't need to use the `next-build-id` CLI - just use `next build` as normal and you'll get the build id you want!
+
+```js
+// next.config.js
+const nextBuildId = require('next-build-id')
+module.exports = {
+  generateBuildId: async () => {
+    const fromGit = await nextBuildId({ dir: __dirname })
+    return fromGit.id
+  }
+}
+```
+
+## Intro
+
+This tool is necessary if you're running multiple instances of your Next.js app on different servers sitting behind a load balancer without session affinity. Otherwise, if your Next.js builds end up with different build ids, a client loading content from different servers can result in [this Next.js error](https://github.com/zeit/next.js/blob/52ccc14059673508803f96ef1c74eecdf27fe096/server/index.js#L444), which causes the app to blow up for that client.
 
 This module updates/overrides the following:
 
 - uuid defined in `.next/BUILD_ID`
-- hashes for all chunks defined in `.next/build-stats.json`
+- hashes for all chunks defined in `.next/build-stats.json` (Next.js 4 or below)
 
 By default, this CLI/module will overwrite those values with the hash of the latest git commit (`git rev-parse HEAD`), but it will also allow you to define your own id.
 
@@ -77,6 +94,7 @@ This module exports a single function that accepts an options object and returns
 The options supported are:
 
 - `dir` (string): the directory built by `next build`
+- `write` (boolean): whether to overwrite the BUILD_ID in the dist dir (not needed when using `generateBuildId` in next.config.js)
 - `id` (string): define a custom id instead of deferring to `git rev-parse HEAD`
 
 The returned `Promise` resolves to a result object containing:
@@ -93,6 +111,7 @@ const nextBuildId = require('next-build-id')
 
 const opts = {}
 // opts.dir = '/path/to/input/dir'
+// opts.write = true
 // opts.id = 'my_custom_id'
 
 nextBuildId(opts).then(result => {
@@ -108,6 +127,7 @@ nextBuildId(opts).then(result => {
 
 ## Reference
 
+- [zeit/next.js#786](https://github.com/zeit/next.js/issues/786)
 - [zeit/next.js#2978 (comment)](https://github.com/zeit/next.js/issues/2978#issuecomment-334849384)
 - [zeit/next.js#3299 (comment)](https://github.com/zeit/next.js/issues/3299#issuecomment-344973091)
 - ["Handle BUILD_ID Mismatch Error" on Next.js wiki](https://github.com/zeit/next.js/wiki/Handle-BUILD_ID-Mismatch-Error)
diff --git a/cli.js b/cli.js
index 4e58da9..6db05f9 100755
--- a/cli.js
+++ b/cli.js
@@ -13,6 +13,7 @@ require('sywac')
   .outputSettings({ maxWidth: 60 })
   .parseAndExit()
   .then(argv => {
+    argv.write = true
     argv.dir = path.resolve(process.cwd(), argv.dir || '.')
     const nextBuildId = require('./index')
     return nextBuildId(argv)
diff --git a/index.js b/index.js
index 784d4e3..ec51a29 100644
--- a/index.js
+++ b/index.js
@@ -14,7 +14,7 @@ const defaultConfig = {
   assetPrefix: '',
   configOrigin: 'default',
   useFileSystemPublicRoutes: true,
-  // generateBuildId: () => '9f2a37be-4545-445e-91bd-' + String(new Date().getTime()).slice(1, 13),
+  generateBuildId: () => '9f2a37be-4545-445e-91bd-' + String(new Date().getTime()).slice(1, 13),
   generateEtags: true,
   pageExtensions: ['jsx', 'js']
 }
@@ -26,19 +26,19 @@ module.exports = function nextBuildId (opts) {
   return resolveInputDir(opts.dir)
     .then(inputDir => {
       result.inputDir = inputDir
-      return resolveOutputDir(inputDir)
+      return opts.write && resolveOutputDir(inputDir)
     })
     .then(outputDir => {
-      result.outputDir = outputDir
-      return getFiles(outputDir)
+      result.outputDir = outputDir || null
+      return opts.write && getFiles(outputDir)
     })
     .then(files => {
-      result.files = files
+      result.files = files || null
       return determineBuildId(opts.id, result.inputDir)
     })
     .then(id => {
       result.id = id
-      return updateFiles(id, result.files)
+      return opts.write && updateFiles(id, result.files)
     })
     .then(() => result)
 }
diff --git a/test/fixture-next6/next.config.js b/test/fixture-next6/next.config.js
new file mode 100644
index 0000000..58100a8
--- /dev/null
+++ b/test/fixture-next6/next.config.js
@@ -0,0 +1,8 @@
+const nextBuildId = require('../../index')
+
+module.exports = (phase, nextConfig) => {
+  if (process.env.NBI_TEST_CALL_DEFAULT_GENERATEBUILDID) console.log(nextConfig.defaultConfig.generateBuildId())
+  return {
+    generateBuildId: () => nextBuildId({ dir: __dirname }).then(result => result.id)
+  }
+}
diff --git a/test/fixture-next6/package.json b/test/fixture-next6/package.json
new file mode 100644
index 0000000..069c286
--- /dev/null
+++ b/test/fixture-next6/package.json
@@ -0,0 +1,17 @@
+{
+  "name": "fixture",
+  "version": "1.0.0",
+  "description": "Fixture to test against Next.js 6",
+  "private": true,
+  "main": "index.js",
+  "scripts": {
+    "build": "next build"
+  },
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "next": "^6.0.3",
+    "react": "^16.4.0",
+    "react-dom": "^16.4.0"
+  }
+}
diff --git a/test/fixture-next6/pages/index.js b/test/fixture-next6/pages/index.js
new file mode 100644
index 0000000..e346f85
--- /dev/null
+++ b/test/fixture-next6/pages/index.js
@@ -0,0 +1 @@
+export default () => <div>Welcome to next.js!</div>
diff --git a/test/test-next6.js b/test/test-next6.js
new file mode 100644
index 0000000..95ae8af
--- /dev/null
+++ b/test/test-next6.js
@@ -0,0 +1,58 @@
+const path = require('path')
+
+const tap = require('tap')
+const rimraf = require('rimraf')
+
+const utils = require('./utils')
+
+const exec = utils.exec
+const mockedGitEnv = utils.mockedGitEnv
+const fixturePath = path.resolve(__dirname, 'fixture-next6')
+
+let npmi = false
+tap.beforeEach(() => {
+  if (npmi) return Promise.resolve()
+  npmi = true
+  return exec('npm', 'i', fixturePath)
+})
+
+tap.afterEach(() => {
+  return Promise.all(['.next'].map(dir => {
+    return new Promise((resolve, reject) => {
+      rimraf(path.resolve(fixturePath, dir), err => {
+        if (err) return reject(err)
+        resolve()
+      })
+    })
+  }))
+})
+
+tap.test('with next.config.js that uses a generateBuildId function', t => {
+  return exec('npm', 'run build', fixturePath, mockedGitEnv())
+    .then(io => {
+      t.notOk(io.err)
+      t.notOk(io.stderr)
+      return utils.readTextFile(path.resolve(fixturePath, '.next', 'BUILD_ID'))
+    })
+    .then(buildId => {
+      t.equal(buildId, '0123456789abcdef0123456789abcdef01234567')
+    })
+})
+
+tap.test('call stupid default generateBuildId for code coverage', t => {
+  return exec('npm', 'run build', fixturePath, mockedGitEnv())
+    .then(() => {
+      return exec(path.resolve(__dirname, '..', 'cli.js'), '', fixturePath, Object.assign(mockedGitEnv(), { NBI_TEST_CALL_DEFAULT_GENERATEBUILDID: true }))
+    })
+    .then(io => {
+      t.notOk(io.err)
+      t.notOk(io.stderr)
+      t.match(io.stdout, /9f2a37be-4545-445e-91bd/)
+      t.match(io.stdout, /Build ID: 0123456789abcdef0123456789abcdef01234567/)
+      t.match(io.stdout, /Updated:.*BUILD_ID/)
+      return utils.readTextFile(path.resolve(fixturePath, '.next', 'BUILD_ID'))
+    })
+    .then(buildId => {
+      t.equal(buildId, '0123456789abcdef0123456789abcdef01234567')
+    })
+})
diff --git a/test/utils.js b/test/utils.js
index 1652fe9..bb5453d 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -60,6 +60,7 @@ function writeTextFile (file, data) {
 
 module.exports = {
   exec,
+  mockedGitEnv,
   cli,
   readTextFile,
   readJsonFile,