diff --git a/lib/middleware/runner.js b/lib/middleware/runner.js
index 7c0d13dbc..465fe54df 100644
--- a/lib/middleware/runner.js
+++ b/lib/middleware/runner.js
@@ -4,6 +4,7 @@
  * It basically triggers a test run and streams stdout back.
  */
 
+var _ = require('lodash')
 var path = require('path')
 var helper = require('../helper')
 var log = require('../logger').create()
@@ -48,7 +49,7 @@ var createRunnerMiddleware = function (emitter, fileList, capturedBrowsers, repo
       })
 
       log.debug('Setting client.args to ', data.args)
-      config.client.args = data.args
+      config = _.merge(config, {client: {args: data.args}})
 
       var fullRefresh = true
 
diff --git a/test/unit/middleware/runner.spec.js b/test/unit/middleware/runner.spec.js
index 8e7228aba..db8eb87ec 100644
--- a/test/unit/middleware/runner.spec.js
+++ b/test/unit/middleware/runner.spec.js
@@ -5,6 +5,7 @@ import {Promise} from 'bluebird'
 import Browser from '../../../lib/browser'
 import BrowserCollection from '../../../lib/browser_collection'
 import MultReporter from '../../../lib/reporters/multi'
+var _ = require('lodash')
 var createRunnerMiddleware = require('../../../lib/middleware/runner').create
 var HttpResponseMock = mocks.http.ServerResponse
 var HttpRequestMock = mocks.http.ServerRequest
@@ -166,6 +167,97 @@ describe('middleware.runner', () => {
     request.emit('end')
   })
 
+  it('should not overwrite existing client.args when run passes an empty array for client.args', (done) => {
+    capturedBrowsers.add(new Browser())
+    sinon.stub(capturedBrowsers, 'areAllReady', () => true)
+    config = _.merge(config, {client: {args: ['clientArg']}})
+
+    emitter.once('run_start', () => {
+      expect(config.client.args).to.deep.equal(['clientArg'])
+      done()
+    })
+
+    var RAW_MESSAGE = '{"args": []}'
+
+    var request = new HttpRequestMock('/__run__', {
+      'content-type': 'application/json',
+      'content-length': RAW_MESSAGE.length
+    })
+
+    handler(request, response, nextSpy)
+
+    request.emit('data', RAW_MESSAGE)
+    request.emit('end')
+  })
+
+  it('should not overwrite existing client.args when run passes no client.args', (done) => {
+    capturedBrowsers.add(new Browser())
+    sinon.stub(capturedBrowsers, 'areAllReady', () => true)
+    config = _.merge(config, {client: {args: ['clientArg']}})
+
+    emitter.once('run_start', () => {
+      expect(config.client.args).to.deep.equal(['clientArg'])
+      done()
+    })
+
+    var RAW_MESSAGE = '{}'
+
+    var request = new HttpRequestMock('/__run__', {
+      'content-type': 'application/json',
+      'content-length': RAW_MESSAGE.length
+    })
+
+    handler(request, response, nextSpy)
+
+    request.emit('data', RAW_MESSAGE)
+    request.emit('end')
+  })
+
+  it('should merge existing client.args with client.args passed by run', (done) => {
+    capturedBrowsers.add(new Browser())
+    sinon.stub(capturedBrowsers, 'areAllReady', () => true)
+    config = _.merge(config, {client: {args: {arg1: 'cherry', arg2: 'mango'}}})
+
+    emitter.once('run_start', () => {
+      expect(config.client.args).to.deep.equal({arg1: 'cherry', arg2: 'fig', arg3: 'chocolate'})
+      done()
+    })
+
+    var RAW_MESSAGE = '{"args": {"arg2": "fig", "arg3": "chocolate"}}'
+
+    var request = new HttpRequestMock('/__run__', {
+      'content-type': 'application/json',
+      'content-length': RAW_MESSAGE.length
+    })
+
+    handler(request, response, nextSpy)
+
+    request.emit('data', RAW_MESSAGE)
+    request.emit('end')
+  })
+
+  it('should set client args passed by run when there are no existing client.args', (done) => {
+    capturedBrowsers.add(new Browser())
+    sinon.stub(capturedBrowsers, 'areAllReady', () => true)
+
+    emitter.once('run_start', () => {
+      expect(config.client.args).to.deep.equal({arg2: 'fig', arg3: 'chocolate'})
+      done()
+    })
+
+    var RAW_MESSAGE = '{"args": {"arg2": "fig", "arg3": "chocolate"}}'
+
+    var request = new HttpRequestMock('/__run__', {
+      'content-type': 'application/json',
+      'content-length': RAW_MESSAGE.length
+    })
+
+    handler(request, response, nextSpy)
+
+    request.emit('data', RAW_MESSAGE)
+    request.emit('end')
+  })
+
   it('should refresh explicit files if specified', (done) => {
     capturedBrowsers.add(new Browser())
     sinon.stub(capturedBrowsers, 'areAllReady', () => true)