diff --git a/lib/middleware/karma.js b/lib/middleware/karma.js
index c68261e4e..6836af577 100644
--- a/lib/middleware/karma.js
+++ b/lib/middleware/karma.js
@@ -83,7 +83,7 @@ var createKarmaMiddleware = function(filesPromise, serveStaticFile,
               filePath = filePathToUrlPath(filePath, basePath);
 
               if (requestUrl === '/context.html') {
-                filePath += '?' + file.mtime.getTime();
+                filePath += '?' + file.sha;
               }
             }
 
@@ -98,7 +98,7 @@ var createKarmaMiddleware = function(filesPromise, serveStaticFile,
           var mappings = files.served.map(function(file) {
             var filePath = filePathToUrlPath(file.path, basePath);
 
-            return util.format('  \'%s\': \'%d\'', filePath, file.mtime.getTime());
+            return util.format('  \'%s\': \'%s\'', filePath, file.sha);
           });
 
           mappings = 'window.__karma__.files = {\n' + mappings.join(',\n') + '\n};\n';
diff --git a/lib/preprocessor.js b/lib/preprocessor.js
index 3743ad170..76caaa02f 100644
--- a/lib/preprocessor.js
+++ b/lib/preprocessor.js
@@ -53,13 +53,15 @@ var createPreprocessor = function(config, basePath, injector) {
       }
     }
 
-    if (preprocessors.length) {
-      return fs.readFile(file.originalPath, function(err, buffer) {
-        nextPreprocessor(buffer.toString());
-      });
-    }
+    // compute SHA of the content
+    preprocessors.push(function(content, file, done) {
+      file.sha = sha1(content);
+      done(content);
+    });
 
-    return process.nextTick(done);
+    return fs.readFile(file.originalPath, function(err, buffer) {
+      nextPreprocessor(buffer.toString());
+    });
   };
 };
 createPreprocessor.$inject = ['config.preprocessors', 'config.basePath', 'injector'];
diff --git a/test/unit/middleware/karma.spec.coffee b/test/unit/middleware/karma.spec.coffee
index 0efcc4d16..010c0b51d 100644
--- a/test/unit/middleware/karma.spec.coffee
+++ b/test/unit/middleware/karma.spec.coffee
@@ -9,6 +9,10 @@ describe 'middleware.karma', ->
   File = require('../../../lib/file-list').File
   Url = require('../../../lib/file-list').Url
 
+  MockFile = (path, sha) ->
+    File.call @, path
+    @sha = sha or 'sha-default'
+
   fsMock = mocks.fs.create
     karma:
       static:
@@ -101,15 +105,15 @@ describe 'middleware.karma', ->
 
   it 'should serve context.html with replaced script tags', (done) ->
     includedFiles [
-      new File('/first.js', new Date 12345)
-      new File('/second.dart', new Date 67890)
+      new MockFile('/first.js', 'sha123')
+      new MockFile('/second.dart', 'sha456')
     ]
 
     response.once 'end', ->
       expect(nextSpy).not.to.have.been.called
       expect(response).to.beServedAs 200, 'CONTEXT\n' +
-      '<script type="text/javascript" src="/absolute/first.js?12345"></script>\n' +
-      '<script type="application/dart" src="/absolute/second.dart?67890"></script>'
+      '<script type="text/javascript" src="/absolute/first.js?sha123"></script>\n' +
+      '<script type="application/dart" src="/absolute/second.dart?sha456"></script>'
       done()
 
     callHandlerWith '/__karma__/context.html'
@@ -117,13 +121,13 @@ describe 'middleware.karma', ->
 
   it 'should serve context.html with replaced link tags', (done) ->
     includedFiles [
-      new File('/first.css', new Date 12345)
+      new MockFile('/first.css', 'sha007')
     ]
 
     response.once 'end', ->
       expect(nextSpy).not.to.have.been.called
       expect(response).to.beServedAs 200, 'CONTEXT\n' +
-      '<link type="text/css" href="/absolute/first.css?12345" rel="stylesheet">'
+      '<link type="text/css" href="/absolute/first.css?sha007" rel="stylesheet">'
       done()
 
     callHandlerWith '/__karma__/context.html'
@@ -131,15 +135,15 @@ describe 'middleware.karma', ->
 
   it 'should serve context.html with the correct path for the script tags', (done) ->
     includedFiles [
-      new File('/some/abc/a.js', new Date 12345)
-      new File('/base/path/b.js', new Date 67890)
+      new MockFile('/some/abc/a.js', 'sha')
+      new MockFile('/base/path/b.js', 'shaaa')
     ]
 
     response.once 'end', ->
       expect(nextSpy).not.to.have.been.called
       expect(response).to.beServedAs 200, 'CONTEXT\n' +
-      '<script type="text/javascript" src="/absolute/some/abc/a.js?12345"></script>\n' +
-      '<script type="text/javascript" src="/base/b.js?67890"></script>'
+      '<script type="text/javascript" src="/absolute/some/abc/a.js?sha"></script>\n' +
+      '<script type="text/javascript" src="/base/b.js?shaaa"></script>'
       done()
 
     callHandlerWith '/__karma__/context.html'
@@ -147,15 +151,15 @@ describe 'middleware.karma', ->
 
   it 'should serve context.html with the correct path for link tags', (done) ->
     includedFiles [
-      new File('/some/abc/a.css', new Date 12345)
-      new File('/base/path/b.css', new Date 67890)
+      new MockFile('/some/abc/a.css', 'sha1')
+      new MockFile('/base/path/b.css', 'sha2')
     ]
 
     response.once 'end', ->
       expect(nextSpy).not.to.have.been.called
       expect(response).to.beServedAs 200, 'CONTEXT\n' +
-      '<link type="text/css" href="/absolute/some/abc/a.css?12345" rel="stylesheet">\n' +
-      '<link type="text/css" href="/base/b.css?67890" rel="stylesheet">'
+      '<link type="text/css" href="/absolute/some/abc/a.css?sha1" rel="stylesheet">\n' +
+      '<link type="text/css" href="/base/b.css?sha2" rel="stylesheet">'
       done()
 
     callHandlerWith '/__karma__/context.html'
@@ -193,14 +197,14 @@ describe 'middleware.karma', ->
   it 'should inline mappings with all served files', (done) ->
     fsMock._touchFile '/karma/static/context.html', 0, '%MAPPINGS%'
     servedFiles [
-      new File('/some/abc/a.js', new Date 12345)
-      new File('/base/path/b.js', new Date 67890)
+      new MockFile('/some/abc/a.js', 'sha_a')
+      new MockFile('/base/path/b.js', 'sha_b')
     ]
 
     response.once 'end', ->
       expect(response).to.beServedAs 200, 'window.__karma__.files = {\n' +
-      "  '/absolute/some/abc/a.js': '12345',\n" +
-      "  '/base/b.js': '67890'\n" +
+      "  '/absolute/some/abc/a.js': 'sha_a',\n" +
+      "  '/base/b.js': 'sha_b'\n" +
       "};\n"
       done()
 
@@ -209,8 +213,8 @@ describe 'middleware.karma', ->
 
   it 'should serve debug.html with replaced script tags without timestamps', (done) ->
     includedFiles [
-      new File('/first.js', new Date 12345)
-      new File('/base/path/b.js', new Date 67890)
+      new MockFile('/first.js')
+      new MockFile('/base/path/b.js')
     ]
 
     response.once 'end', ->
@@ -225,8 +229,8 @@ describe 'middleware.karma', ->
 
   it 'should serve debug.html with replaced link tags without timestamps', (done) ->
     includedFiles [
-      new File('/first.css', new Date 12345)
-      new File('/base/path/b.css', new Date 67890)
+      new MockFile('/first.css')
+      new MockFile('/base/path/b.css')
     ]
 
     response.once 'end', ->
diff --git a/test/unit/preprocessor.spec.coffee b/test/unit/preprocessor.spec.coffee
index cb9742be7..f9d040afc 100644
--- a/test/unit/preprocessor.spec.coffee
+++ b/test/unit/preprocessor.spec.coffee
@@ -11,6 +11,7 @@ describe 'preprocessor', ->
     mockFs = mocks.fs.create
       some:
         'a.js': mocks.fs.file 0, 'content'
+        'a.txt': mocks.fs.file 0, 'some-text'
 
     mocks_ =
       'graceful-fs': mockFs
@@ -74,3 +75,22 @@ describe 'preprocessor', ->
       expect(file.path).to.equal 'path-p1-p2'
       expect(file.content).to.equal 'content-c1-c2'
       done()
+
+
+  it 'should compute SHA', (done) ->
+    pp = m.createPreprocessor {}, null, new di.Injector([])
+    file = {originalPath: '/some/a.js', path: 'path'}
+
+    pp file, ->
+      expect(file.sha).to.exist
+      expect(file.sha.length).to.equal 40
+      previousSHA = file.sha
+
+      pp file, ->
+        expect(file.sha).to.equal previousSHA
+        mockFs._touchFile '/some/a.js', null, 'new-content'
+
+        pp file, ->
+          expect(file.sha.length).to.equal 40
+          expect(file.sha).not.to.equal previousSHA
+          done()