diff --git a/index.js b/index.js
index 58165ce..1f7dd9d 100644
--- a/index.js
+++ b/index.js
@@ -30,6 +30,6 @@ module.exports = function supports () {
     encodings: manifest.encodings || false,
 
     // Methods that are not part of abstract-leveldown or levelup
-    additionalMethods: manifest.additionalMethods || {}
+    additionalMethods: xtend(manifest.additionalMethods)
   })
 }
diff --git a/test/cloneable.js b/test/cloneable.js
new file mode 100644
index 0000000..2712416
--- /dev/null
+++ b/test/cloneable.js
@@ -0,0 +1,26 @@
+'use strict'
+
+var supports = require('..')
+
+// Every object in a manifest must have a unique identity, to avoid accidental
+// mutation. In supports() we only shallowly clone the manifest object itself
+// and additionalMethods. If in the future we add more objects to manifests,
+// this test will break and we'll know to start performing a deep clone.
+module.exports = function cloneable (t, manifest) {
+  var copy = supports(manifest)
+  verifyUnique(t, 'manifest', manifest, copy)
+}
+
+function verifyUnique (t, path, a, b) {
+  if (isObject(a) && isObject(b)) {
+    t.ok(a !== b, path + ' has unique identity')
+
+    Object.keys(a).forEach(function (key) {
+      verifyUnique(t, path + '.' + key, a[key], b[key])
+    })
+  }
+}
+
+function isObject (o) {
+  return typeof o === 'object' && o !== null
+}
diff --git a/test/index.js b/test/index.js
index 73cdf96..aba38bd 100644
--- a/test/index.js
+++ b/test/index.js
@@ -2,6 +2,7 @@
 
 var xtend = require('xtend')
 var shape = require('./shape')
+var cloneable = require('./cloneable')
 
 module.exports = function suite (test, testCommon) {
   test('db has manifest', function (t) {
@@ -9,6 +10,7 @@ module.exports = function suite (test, testCommon) {
     var manifest = db.supports
 
     shape(t, manifest)
+    cloneable(t, manifest)
 
     var before = xtend(manifest, {
       additionalMethods: xtend(manifest.additionalMethods)
diff --git a/test/self.js b/test/self.js
index fc67ac4..3c7d75b 100644
--- a/test/self.js
+++ b/test/self.js
@@ -3,9 +3,11 @@
 var test = require('tape')
 var supports = require('..')
 var shape = require('./shape')
+var cloneable = require('./cloneable')
 
 test('no options', function (t) {
   shape(t, supports())
+  cloneable(t, supports())
   t.end()
 })
 
@@ -44,13 +46,30 @@ test('truthy options', function (t) {
 
 test('merges input objects without mutating them', function (t) {
   var input1 = { bufferKeys: null, streams: false }
-  var input2 = { streams: true }
+  var input2 = { streams: true, additionalMethods: {} }
   var manifest = supports(input1, input2)
 
+  manifest.foobar = true
+  manifest.additionalMethods.baz = true
+
   t.same(input1, { bufferKeys: null, streams: false })
-  t.same(input2, { streams: true })
+  t.same(input2, { streams: true, additionalMethods: {} })
   t.is(manifest.bufferKeys, false)
   t.is(manifest.streams, true)
   shape(t, manifest)
   t.end()
 })
+
+test('inherits additionalMethods', function (t) {
+  var manifest = supports({ additionalMethods: { foo: true } }, {})
+  t.same(manifest.additionalMethods, { foo: true })
+  t.end()
+})
+
+test('does not merge additionalMethods', function (t) {
+  var input1 = { additionalMethods: { foo: true } }
+  var input2 = { additionalMethods: { bar: true } }
+  var manifest = supports(input1, input2)
+  t.same(manifest.additionalMethods, { bar: true })
+  t.end()
+})