diff --git a/app.coffee b/app.coffee
index 44d3157..dd2bae5 100644
--- a/app.coffee
+++ b/app.coffee
@@ -36,7 +36,24 @@ app.delete "/project/:project_id", CompileController.clearCache
 app.get  "/project/:project_id/sync/code", CompileController.syncFromCode
 app.get  "/project/:project_id/sync/pdf", CompileController.syncFromPdf
 
-staticServer = express.static Settings.path.compilesDir, setHeaders: (res, path, stat) ->
+url = require "url"
+
+staticForbidSymLinks = (root, options) ->
+	expressStatic = express.static root, options
+	basePath = Path.resolve(root)
+	return (req, res, next) ->
+		path = url.parse(req.url).pathname
+		requestedFsPath = Path.normalize("#{basePath}/#{path}")
+		fs.realpath requestedFsPath, (err, realFsPath)->
+			if err?
+				return res.send(500)
+			else if requestedFsPath != realFsPath
+				logger.warn requestedFsPath:requestedFsPath, realFsPath:realFsPath, path: req.params[0], project_id: req.params.project_id, "trying to access a different file (symlink), aborting"
+				return res.send(404)
+			else
+				expressStatic(req, res, next)
+
+staticServer = staticForbidSymLinks Settings.path.compilesDir, setHeaders: (res, path, stat) ->
 	if Path.basename(path) == "output.pdf"
 		res.set("Content-Type", "application/pdf")
 		# Calculate an etag in the same way as nginx
@@ -51,7 +68,10 @@ staticServer = express.static Settings.path.compilesDir, setHeaders: (res, path,
 		res.set("Content-Type", "text/plain")
 
 app.get "/project/:project_id/output/*", require("./app/js/SymlinkCheckerMiddlewear"), (req, res, next) ->
-	req.url = "/#{req.params.project_id}/#{req.params[0]}"
+	if req.query?.build? && req.query.build.match(/^[0-9]+$/)
+		req.url = "/#{req.params.project_id}/.cache/clsi/#{req.query.build}/#{req.params[0]}"
+	else
+		req.url = "/#{req.params.project_id}/#{req.params[0]}"
 	staticServer(req, res, next)
 
 app.get "/status", (req, res, next) ->
diff --git a/app/coffee/CompileController.coffee b/app/coffee/CompileController.coffee
index c738202..29373c3 100644
--- a/app/coffee/CompileController.coffee
+++ b/app/coffee/CompileController.coffee
@@ -35,6 +35,7 @@ module.exports = CompileController =
 							outputFiles: outputFiles.map (file) ->
 								url: "#{Settings.apis.clsi.url}/project/#{request.project_id}/output/#{file.path}"
 								type: file.type
+								build: file.build
 					}
 		
 	clearCache: (req, res, next = (error) ->) ->
diff --git a/app/coffee/CompileManager.coffee b/app/coffee/CompileManager.coffee
index dbdb39f..ac0c241 100644
--- a/app/coffee/CompileManager.coffee
+++ b/app/coffee/CompileManager.coffee
@@ -33,7 +33,7 @@ module.exports = CompileManager =
 
 				OutputFileFinder.findOutputFiles request.resources, compileDir, (error, outputFiles) ->
 					return callback(error) if error?
-					OutputCacheManager.saveOutputFiles outputFiles, compileDir,  (error, newOutputFiles) -> 
+					OutputCacheManager.saveOutputFiles outputFiles, compileDir,  (error, newOutputFiles) ->
 						callback null, newOutputFiles
 	
 	clearProject: (project_id, _callback = (error) ->) ->
diff --git a/app/coffee/OutputCacheManager.coffee b/app/coffee/OutputCacheManager.coffee
index 479637d..c34a662 100644
--- a/app/coffee/OutputCacheManager.coffee
+++ b/app/coffee/OutputCacheManager.coffee
@@ -15,7 +15,7 @@ module.exports = OutputCacheManager =
 		# copy all the output files into it
 		# 
 		# TODO: use Path module
-		buildId = 'build-' + Date.now()
+		buildId = Date.now()
 		relDir = OutputCacheManager.CACHE_DIR + '/' + buildId
 		newDir = target + '/' + relDir
 		OutputCacheManager.expireOutputFiles target
@@ -25,9 +25,8 @@ module.exports = OutputCacheManager =
 			else
 				async.mapSeries outputFiles, (file, cb) ->
 					newFile = _.clone(file)
-					newFile.path = relDir + '/' + file.path
 					src = target + '/' + file.path
-					dst = target + '/' + newFile.path
+					dst = target + '/' + relDir + '/' + file.path
 					#console.log 'src', src, 'dst', dst
 					fs.stat src, (err, stats) ->
 						if err?
@@ -36,6 +35,8 @@ module.exports = OutputCacheManager =
 							#console.log 'isFile: copying'
 							fse.copy src, dst, (err) ->
 								OutputFileOptimiser.optimiseFile src, dst, (err, result) ->
+									console.log 'setting buildId on', newFile, 'to', buildId
+									newFile.build = buildId
 									cb(err, newFile)
 						else
 							# other filetype - shouldn't happen