diff --git a/Jakefile.js b/Jakefile.js
index ea13cce6685..3ac2e8fad13 100644
--- a/Jakefile.js
+++ b/Jakefile.js
@@ -588,9 +588,10 @@ function deleteTemporaryProjectOutput() {
 }
 
 var testTimeout = 20000;
-desc("Runs the tests using the built run.js file. Syntax is jake runtests. Optional parameters 'host=', 'tests=[regex], reporter=[list|spec|json|<more>]'.");
+desc("Runs the tests using the built run.js file. Syntax is jake runtests. Optional parameters 'host=', 'tests=[regex], reporter=[list|spec|json|<more>]', debug=true.");
 task("runtests", ["tests", builtLocalDirectory], function() {
     cleanTestDirs();
+    var debug = process.env.debug || process.env.d;
     host = "mocha"
     tests = process.env.test || process.env.tests || process.env.t;
     var light = process.env.light || false;
@@ -613,7 +614,7 @@ task("runtests", ["tests", builtLocalDirectory], function() {
     reporter = process.env.reporter || process.env.r || 'mocha-fivemat-progress-reporter';
     // timeout normally isn't necessary but Travis-CI has been timing out on compiler baselines occasionally
     // default timeout is 2sec which really should be enough, but maybe we just need a small amount longer
-    var cmd = host + " -R " + reporter + tests + colors + ' -t ' + testTimeout + ' ' + run;
+    var cmd = host + (debug ? " --debug-brk" : "") + " -R " + reporter + tests + colors + ' -t ' + testTimeout + ' ' + run;
     console.log(cmd);
     exec(cmd, deleteTemporaryProjectOutput);
 }, {async: true});