diff --git a/spec/requests/oauth/token_spec.rb b/spec/requests/oauth/token_spec.rb
index 7be65e7ab3f..c3ca353d54c 100644
--- a/spec/requests/oauth/token_spec.rb
+++ b/spec/requests/oauth/token_spec.rb
@@ -5,103 +5,151 @@ require 'rails_helper'
 RSpec.describe 'Managing OAuth Tokens' do
   describe 'POST /oauth/token' do
     subject do
-      post '/oauth/token', params: params
+      post '/oauth/token', params: params.merge(additional_params), headers: headers
     end
 
+    let(:headers) { nil }
+    let(:additional_params) { {} }
+    let(:params) { {} }
+
     let(:application) do
       Fabricate(:application, scopes: 'read write follow', redirect_uri: 'urn:ietf:wg:oauth:2.0:oob')
     end
-    let(:params) do
-      {
-        grant_type: grant_type,
-        client_id: application.uid,
-        client_secret: application.secret,
-        redirect_uri: 'urn:ietf:wg:oauth:2.0:oob',
-        code: code,
-        scope: scope,
-      }
-    end
 
     context "with grant_type 'authorization_code'" do
-      let(:grant_type) { 'authorization_code' }
-      let(:code) do
-        access_grant = Fabricate(:access_grant, application: application, redirect_uri: 'urn:ietf:wg:oauth:2.0:oob', scopes: 'read write')
-        access_grant.plaintext_token
-      end
+      let(:access_grant) { Fabricate(:access_grant, application: application, redirect_uri: 'urn:ietf:wg:oauth:2.0:oob', scopes: 'read write') }
+      let(:access_grant_scopes) { access_grant.scopes.to_s }
+      let(:code) { access_grant.plaintext_token }
 
-      shared_examples 'original scope request preservation' do
-        it 'returns all scopes requested for the given code' do
+      shared_examples 'returns a correctly scoped access token' do
+        it 'returns the scopes requested by the authorization code' do
           subject
 
           expect(response).to have_http_status(200)
-          expect(response.parsed_body[:scope]).to eq 'read write'
+          expect(response.parsed_body[:scope]).to eq access_grant_scopes
+        end
+
+        context 'with additional parameters not used by the grant type' do
+          # When performing an authorization code grant flow, the `/oauth/token`
+          # endpoint does not accept a `scope` parameter, and should not
+          # override the scopes from the authorization grant.
+          let(:additional_params) do
+            {
+              scope: 'write',
+            }
+          end
+
+          it 'returns the scopes requested by the authorization code' do
+            subject
+
+            expect(response).to have_http_status(200)
+            expect(response.parsed_body[:scope]).to eq access_grant_scopes
+          end
         end
       end
 
-      context 'with no scopes specified' do
-        let(:scope) { nil }
+      context 'with client authentication via params' do
+        let(:headers) { nil }
+        let(:params) do
+          {
+            grant_type: 'authorization_code',
+            redirect_uri: 'urn:ietf:wg:oauth:2.0:oob',
+            client_id: application.uid,
+            client_secret: application.secret,
+            code: code,
+          }
+        end
 
-        it_behaves_like 'original scope request preservation'
+        it_behaves_like 'returns a correctly scoped access token'
       end
 
-      context 'with scopes specified' do
-        context 'when the scopes were requested for this code' do
-          let(:scope) { 'write' }
-
-          it_behaves_like 'original scope request preservation'
+      context 'with client authentication via basic auth' do
+        let(:headers) do
+          {
+            Authorization: ActionController::HttpAuthentication::Basic.encode_credentials(application.uid, application.secret),
+          }
         end
 
-        context 'when the scope was not requested for the code' do
-          let(:scope) { 'follow' }
-
-          it_behaves_like 'original scope request preservation'
+        let(:params) do
+          {
+            grant_type: 'authorization_code',
+            redirect_uri: 'urn:ietf:wg:oauth:2.0:oob',
+            code: code,
+          }
         end
 
-        context 'when the scope does not belong to the application' do
-          let(:scope) { 'push' }
-
-          it_behaves_like 'original scope request preservation'
-        end
+        it_behaves_like 'returns a correctly scoped access token'
       end
     end
 
     context "with grant_type 'client_credentials'" do
-      let(:grant_type) { 'client_credentials' }
-      let(:code) { nil }
+      shared_examples 'returns the correct scopes' do
+        context 'with no scopes specified' do
+          let(:scope) { nil }
 
-      context 'with no scopes specified' do
-        let(:scope) { nil }
-
-        it 'returns only the default scope' do
-          subject
-
-          expect(response).to have_http_status(200)
-          expect(response.parsed_body[:scope]).to eq('read')
-        end
-      end
-
-      context 'with scopes specified' do
-        context 'when the scopes belong to the application' do
-          let(:scope) { 'read write' }
-
-          it 'returns all the requested scopes' do
+          it 'returns only the authorization server default scope (read)' do
             subject
 
             expect(response).to have_http_status(200)
-            expect(response.parsed_body[:scope]).to eq 'read write'
+            expect(response.parsed_body[:scope]).to eq('read')
           end
         end
 
-        context 'when some scopes do not belong to the application' do
-          let(:scope) { 'read write push' }
+        context 'with scopes specified' do
+          context 'when the scopes belong to the application' do
+            let(:scope) { 'read write' }
 
-          it 'returns an error' do
-            subject
+            it 'returns all the requested scopes' do
+              subject
 
-            expect(response).to have_http_status(400)
+              expect(response).to have_http_status(200)
+              expect(response.parsed_body[:scope]).to eq 'read write'
+            end
+          end
+
+          context 'when some scopes do not belong to the application' do
+            let(:scope) { 'read write push' }
+
+            it 'returns an error' do
+              subject
+
+              expect(response).to have_http_status(400)
+              expect(response.parsed_body[:error]).to eq 'invalid_scope'
+            end
           end
         end
       end
+
+      context 'with client authentication via basic auth' do
+        let(:headers) do
+          {
+            Authorization: ActionController::HttpAuthentication::Basic.encode_credentials(application.uid, application.secret),
+          }
+        end
+
+        let(:params) do
+          {
+            grant_type: 'client_credentials',
+            scope: scope,
+          }
+        end
+
+        it_behaves_like 'returns the correct scopes'
+      end
+
+      context 'with client authentication via params' do
+        let(:headers) { nil }
+        let(:params) do
+          {
+            grant_type: 'client_credentials',
+            client_id: application.uid,
+            client_secret: application.secret,
+            scope: scope,
+          }
+        end
+
+        it_behaves_like 'returns the correct scopes'
+      end
     end
   end