diff --git a/lib/chai/assertion.js b/lib/chai/assertion.js
index 491add77f..4d4217fb4 100644
--- a/lib/chai/assertion.js
+++ b/lib/chai/assertion.js
@@ -46,14 +46,22 @@ module.exports = function (_chai, util) {
    *   in environments that support `Error.captureStackTrace`, and only when
    *   `Chai.config.includeStack` hasn't been set to `false`.
    *
+   * - `lockSsfi`: This flag controls whether or not the given `ssfi` flag
+   *   should retain its current value, even as assertions are chained off of
+   *   this object. This is usually set to `true` when creating a new assertion
+   *   from within another assertion. It's also temporarily set to `true` before
+   *   an overwritten assertion gets called by the overwriting assertion.
+   *
    * @param {Mixed} obj target of the assertion
    * @param {String} msg (optional) custom error message
-   * @param {Function} stack (optional) starting point for removing stack frames
+   * @param {Function} ssfi (optional) starting point for removing stack frames
+   * @param {Boolean} lockSsfi (optional) whether or not the ssfi flag is locked
    * @api private
    */
 
-  function Assertion (obj, msg, ssfi) {
+  function Assertion (obj, msg, ssfi, lockSsfi) {
     flag(this, 'ssfi', ssfi || Assertion);
+    flag(this, 'lockSsfi', lockSsfi);
     flag(this, 'object', obj);
     flag(this, 'message', msg);
 
diff --git a/lib/chai/core/assertions.js b/lib/chai/core/assertions.js
index d09e16850..4292c9155 100644
--- a/lib/chai/core/assertions.js
+++ b/lib/chai/core/assertions.js
@@ -299,7 +299,7 @@ module.exports = function (chai, _) {
   }
 
   function include (val, msg) {
-    _.expectTypes(this, ['array', 'object', 'string']);
+    _.expectTypes(this, ['array', 'object', 'string'], flag(this, 'ssfi'));
 
     if (msg) flag(this, 'message', msg);
     var obj = flag(this, 'object')
@@ -316,7 +316,8 @@ module.exports = function (chai, _) {
 
       props.forEach(function (prop) {
         var propAssertion = new Assertion(obj);
-        _.transferFlags(this, propAssertion, false);
+        _.transferFlags(this, propAssertion, true);
+        flag(propAssertion, 'lockSsfi', true);
 
         if (!negate || props.length === 1) {
           propAssertion.property(prop, val[prop]);
@@ -533,6 +534,7 @@ module.exports = function (chai, _) {
 
   Assertion.addProperty('empty', function () {
     var val = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi')
       , itemsCount;
 
     switch (_.type(val).toLowerCase()) {
@@ -546,13 +548,21 @@ module.exports = function (chai, _) {
         break;
       case 'weakmap':
       case 'weakset':
-        throw new TypeError('.empty was passed a weak collection');
+        throw new AssertionError(
+          '.empty was passed a weak collection',
+          undefined,
+          ssfi
+        );
       case 'function':
         var msg = '.empty was passed a function ' + _.getName(val);
-        throw new TypeError(msg.trim());
+        throw new AssertionError(msg.trim(), undefined, ssfi);
       default:
         if (val !== Object(val)) {
-          throw new TypeError('.empty was passed non-string primitive ' + _.inspect(val));
+          throw new AssertionError(
+            '.empty was passed non-string primitive ' + _.inspect(val),
+            undefined,
+            ssfi
+          );
         }
         itemsCount = Object.keys(val).length;
     }
@@ -695,16 +705,21 @@ module.exports = function (chai, _) {
   function assertAbove (n, msg) {
     if (msg) flag(this, 'message', msg);
     var obj = flag(this, 'object')
-      , doLength = flag(this, 'doLength');
+      , doLength = flag(this, 'doLength')
+      , ssfi = flag(this, 'ssfi');
 
     if (doLength) {
-      new Assertion(obj, msg).to.have.property('length');
+      new Assertion(obj, msg, ssfi, true).to.have.property('length');
     } else {
-      new Assertion(obj, msg).is.a('number');
+      new Assertion(obj, msg, ssfi, true).is.a('number');
     }
 
     if (typeof n !== 'number') {
-      throw new Error('the argument to above must be a number');
+      throw new AssertionError(
+        'the argument to above must be a number',
+        undefined,
+        ssfi
+      );
     }
 
     if (doLength) {
@@ -756,16 +771,21 @@ module.exports = function (chai, _) {
   function assertLeast (n, msg) {
     if (msg) flag(this, 'message', msg);
     var obj = flag(this, 'object')
-      , doLength = flag(this, 'doLength');
+      , doLength = flag(this, 'doLength')
+      , ssfi = flag(this, 'ssfi');
 
     if (doLength) {
-      new Assertion(obj, msg).to.have.property('length');
+      new Assertion(obj, msg, ssfi, true).to.have.property('length');
     } else {
-      new Assertion(obj, msg).is.a('number');
+      new Assertion(obj, msg, ssfi, true).is.a('number');
     }
 
     if (typeof n !== 'number') {
-      throw new Error('the argument to least must be a number');
+      throw new AssertionError(
+        'the argument to least must be a number',
+        undefined,
+        ssfi
+      );
     }
 
     if (doLength) {
@@ -817,16 +837,21 @@ module.exports = function (chai, _) {
   function assertBelow (n, msg) {
     if (msg) flag(this, 'message', msg);
     var obj = flag(this, 'object')
-      , doLength = flag(this, 'doLength');
+      , doLength = flag(this, 'doLength')
+      , ssfi = flag(this, 'ssfi');
 
     if (doLength) {
-      new Assertion(obj, msg).to.have.property('length');
+      new Assertion(obj, msg, ssfi, true).to.have.property('length');
     } else {
-      new Assertion(obj, msg).is.a('number');
+      new Assertion(obj, msg, ssfi, true).is.a('number');
     }
 
     if (typeof n !== 'number') {
-      throw new Error('the argument to below must be a number');
+      throw new AssertionError(
+        'the argument to below must be a number',
+        undefined,
+        ssfi
+      );
     }
 
     if (doLength) {
@@ -878,16 +903,21 @@ module.exports = function (chai, _) {
   function assertMost (n, msg) {
     if (msg) flag(this, 'message', msg);
     var obj = flag(this, 'object')
-      , doLength = flag(this, 'doLength');
+      , doLength = flag(this, 'doLength')
+      , ssfi = flag(this, 'ssfi');
 
     if (doLength) {
-      new Assertion(obj, msg).to.have.property('length');
+      new Assertion(obj, msg, ssfi, true).to.have.property('length');
     } else {
-      new Assertion(obj, msg).is.a('number');
+      new Assertion(obj, msg, ssfi, true).is.a('number');
     }
 
     if (typeof n !== 'number') {
-      throw new Error('the argument to most must be a number');
+      throw new AssertionError(
+        'the argument to most must be a number',
+        undefined,
+        ssfi
+      );
     }
 
     if (doLength) {
@@ -939,16 +969,21 @@ module.exports = function (chai, _) {
     if (msg) flag(this, 'message', msg);
     var obj = flag(this, 'object')
       , range  = start + '..' + finish
-      , doLength = flag(this, 'doLength');
+      , doLength = flag(this, 'doLength')
+      , ssfi = flag(this, 'ssfi');
 
     if (doLength) {
-      new Assertion(obj, msg).to.have.property('length');
+      new Assertion(obj, msg, ssfi, true).to.have.property('length');
     } else {
-      new Assertion(obj, msg).is.a('number');
+      new Assertion(obj, msg, ssfi, true).is.a('number');
     }
 
     if (typeof start !== 'number' || typeof finish !== 'number') {
-      throw new Error('the arguments to within must be numbers');
+      throw new AssertionError(
+        'the arguments to within must be numbers',
+        undefined,
+        ssfi
+      );
     }
 
     if (doLength) {
@@ -990,6 +1025,7 @@ module.exports = function (chai, _) {
     if (msg) flag(this, 'message', msg);
 
     var target = flag(this, 'object')
+    var ssfi = flag(this, 'ssfi');
     var validInstanceOfTarget = constructor === Object(constructor) && (
         typeof constructor === 'function' ||
         (typeof Symbol !== 'undefined' &&
@@ -999,7 +1035,11 @@ module.exports = function (chai, _) {
 
     if (!validInstanceOfTarget) {
       var constructorType = constructor === null ? 'null' : typeof constructor;
-      throw new Error('The instanceof assertion needs a constructor but ' + constructorType + ' was given.');
+      throw new AssertionError(
+        'The instanceof assertion needs a constructor but ' + constructorType + ' was given.',
+        undefined,
+        ssfi
+      );
     }
 
     var isInstanceOf = target instanceof constructor
@@ -1124,10 +1164,15 @@ module.exports = function (chai, _) {
     if (msg) flag(this, 'message', msg);
 
     var isNested = flag(this, 'nested')
-      , isOwn = flag(this, 'own');
+      , isOwn = flag(this, 'own')
+      , ssfi = flag(this, 'ssfi');
 
     if (isNested && isOwn) {
-      throw new Error('The "nested" and "own" flags cannot be combined.');
+      throw new AssertionError(
+        'The "nested" and "own" flags cannot be combined.',
+        undefined,
+        ssfi
+      );
     }
 
     var isDeep = flag(this, 'deep')
@@ -1279,8 +1324,9 @@ module.exports = function (chai, _) {
 
   function assertLength (n, msg) {
     if (msg) flag(this, 'message', msg);
-    var obj = flag(this, 'object');
-    new Assertion(obj, msg).to.have.property('length');
+    var obj = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi');
+    new Assertion(obj, msg, ssfi, true).to.have.property('length');
     var len = obj.length;
 
     this.assert(
@@ -1338,8 +1384,9 @@ module.exports = function (chai, _) {
 
   Assertion.addMethod('string', function (str, msg) {
     if (msg) flag(this, 'message', msg);
-    var obj = flag(this, 'object');
-    new Assertion(obj, msg).is.a('string');
+    var obj = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi');
+    new Assertion(obj, msg, ssfi, true).is.a('string');
 
     this.assert(
         ~obj.indexOf(str)
@@ -1406,6 +1453,7 @@ module.exports = function (chai, _) {
     var obj = flag(this, 'object')
       , objType = _.type(obj)
       , keysType = _.type(keys)
+      , ssfi = flag(this, 'ssfi')
       , isDeep = flag(this, 'deep')
       , str
       , deepStr = ''
@@ -1428,10 +1476,14 @@ module.exports = function (chai, _) {
 
       switch (keysType) {
         case 'Array':
-          if (arguments.length > 1) throw new Error(mixedArgsMsg);
+          if (arguments.length > 1) {
+            throw new AssertionError(mixedArgsMsg, undefined, ssfi);
+          }
           break;
         case 'Object':
-          if (arguments.length > 1) throw new Error(mixedArgsMsg);
+          if (arguments.length > 1) {
+            throw new AssertionError(mixedArgsMsg, undefined, ssfi);
+          }
           keys = Object.keys(keys);
           break;
         default:
@@ -1444,7 +1496,9 @@ module.exports = function (chai, _) {
       });
     }
 
-    if (!keys.length) throw new Error('keys required');
+    if (!keys.length) {
+      throw new AssertionError('keys required', undefined, ssfi);
+    }
 
     var len = keys.length
       , any = flag(this, 'any')
@@ -1645,9 +1699,10 @@ module.exports = function (chai, _) {
 
   function assertThrows (errorLike, errMsgMatcher, msg) {
     if (msg) flag(this, 'message', msg);
-    var obj = flag(this, 'object');
-    var negate = flag(this, 'negate') || false;
-    new Assertion(obj, msg).is.a('function');
+    var obj = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi')
+      , negate = flag(this, 'negate') || false;
+    new Assertion(obj, msg, ssfi, true).is.a('function');
 
     if (errorLike instanceof RegExp || typeof errorLike === 'string') {
       errMsgMatcher = errorLike;
@@ -1882,11 +1937,16 @@ module.exports = function (chai, _) {
 
   function closeTo(expected, delta, msg) {
     if (msg) flag(this, 'message', msg);
-    var obj = flag(this, 'object');
+    var obj = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi');
 
-    new Assertion(obj, msg).is.a('number');
+    new Assertion(obj, msg, ssfi, true).is.a('number');
     if (typeof expected !== 'number' || typeof delta !== 'number') {
-      throw new Error('the arguments to closeTo or approximately must be numbers');
+      throw new AssertionError(
+          'the arguments to closeTo or approximately must be numbers',
+          undefined,
+          ssfi
+      );
     }
 
     this.assert(
@@ -1978,10 +2038,11 @@ module.exports = function (chai, _) {
 
   Assertion.addMethod('members', function (subset, msg) {
     if (msg) flag(this, 'message', msg);
-    var obj = flag(this, 'object');
+    var obj = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi');
 
-    new Assertion(obj).to.be.an('array');
-    new Assertion(subset).to.be.an('array');
+    new Assertion(obj, msg, ssfi, true).to.be.an('array');
+    new Assertion(subset, msg, ssfi, true).to.be.an('array');
 
     var contains = flag(this, 'contains');
     var ordered = flag(this, 'ordered');
@@ -2034,8 +2095,9 @@ module.exports = function (chai, _) {
 
   function oneOf (list, msg) {
     if (msg) flag(this, 'message', msg);
-    var expected = flag(this, 'object');
-    new Assertion(list).to.be.an('array');
+    var expected = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi');
+    new Assertion(list, msg, ssfi, true).to.be.an('array');
 
     this.assert(
         list.indexOf(expected) > -1
@@ -2072,15 +2134,16 @@ module.exports = function (chai, _) {
 
   function assertChanges (target, prop, msg) {
     if (msg) flag(this, 'message', msg);
-    var fn = flag(this, 'object');
-    new Assertion(fn).is.a('function');
+    var fn = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi');
+    new Assertion(fn, msg, ssfi, true).is.a('function');
 
     var initial;
     if (!prop) {
-      new Assertion(target).is.a('function');
+      new Assertion(target, msg, ssfi, true).is.a('function');
       initial = target();
     } else {
-      new Assertion(target, msg).to.have.property(prop);
+      new Assertion(target, msg, ssfi, true).to.have.property(prop);
       initial = target[prop];
     }
 
@@ -2134,20 +2197,21 @@ module.exports = function (chai, _) {
 
   function assertIncreases (target, prop, msg) {
     if (msg) flag(this, 'message', msg);
-    var fn = flag(this, 'object');
-    new Assertion(fn).is.a('function');
+    var fn = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi');
+    new Assertion(fn, msg, ssfi, true).is.a('function');
 
     var initial;
     if (!prop) {
-      new Assertion(target).is.a('function');
+      new Assertion(target, msg, ssfi, true).is.a('function');
       initial = target();
     } else {
-      new Assertion(target, msg).to.have.property(prop);
+      new Assertion(target, msg, ssfi, true).to.have.property(prop);
       initial = target[prop];
     }
 
     // Make sure that the target is a number
-    new Assertion(initial).is.a('number');
+    new Assertion(initial, msg, ssfi, true).is.a('number');
 
     fn();
 
@@ -2198,20 +2262,21 @@ module.exports = function (chai, _) {
 
   function assertDecreases (target, prop, msg) {
     if (msg) flag(this, 'message', msg);
-    var fn = flag(this, 'object');
-    new Assertion(fn).is.a('function');
+    var fn = flag(this, 'object')
+      , ssfi = flag(this, 'ssfi');
+    new Assertion(fn, msg, ssfi, true).is.a('function');
 
     var initial;
     if (!prop) {
-      new Assertion(target).is.a('function');
+      new Assertion(target, msg, ssfi, true).is.a('function');
       initial = target();
     } else {
-      new Assertion(target, msg).to.have.property(prop);
+      new Assertion(target, msg, ssfi, true).to.have.property(prop);
       initial = target[prop];
     }
 
     // Make sure that the target is a number
-    new Assertion(initial).is.a('number');
+    new Assertion(initial, msg, ssfi, true).is.a('number');
 
     fn();
 
diff --git a/lib/chai/interface/assert.js b/lib/chai/interface/assert.js
index c9b852ab4..f4abad240 100644
--- a/lib/chai/interface/assert.js
+++ b/lib/chai/interface/assert.js
@@ -34,7 +34,7 @@ module.exports = function (chai, util) {
    */
 
   var assert = chai.assert = function (express, errmsg) {
-    var test = new Assertion(null, null, chai.assert);
+    var test = new Assertion(null, null, chai.assert, true);
     test.assert(
         express
       , errmsg
@@ -82,7 +82,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isOk = function (val, msg) {
-    new Assertion(val, msg).is.ok;
+    new Assertion(val, msg, assert.isOk, true).is.ok;
   };
 
   /**
@@ -102,7 +102,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotOk = function (val, msg) {
-    new Assertion(val, msg).is.not.ok;
+    new Assertion(val, msg, assert.isNotOk, true).is.not.ok;
   };
 
   /**
@@ -121,7 +121,7 @@ module.exports = function (chai, util) {
    */
 
   assert.equal = function (act, exp, msg) {
-    var test = new Assertion(act, msg, assert.equal);
+    var test = new Assertion(act, msg, assert.equal, true);
 
     test.assert(
         exp == flag(test, 'object')
@@ -149,7 +149,7 @@ module.exports = function (chai, util) {
    */
 
   assert.notEqual = function (act, exp, msg) {
-    var test = new Assertion(act, msg, assert.notEqual);
+    var test = new Assertion(act, msg, assert.notEqual, true);
 
     test.assert(
         exp != flag(test, 'object')
@@ -177,7 +177,7 @@ module.exports = function (chai, util) {
    */
 
   assert.strictEqual = function (act, exp, msg) {
-    new Assertion(act, msg).to.equal(exp);
+    new Assertion(act, msg, assert.strictEqual, true).to.equal(exp);
   };
 
   /**
@@ -196,7 +196,7 @@ module.exports = function (chai, util) {
    */
 
   assert.notStrictEqual = function (act, exp, msg) {
-    new Assertion(act, msg).to.not.equal(exp);
+    new Assertion(act, msg, assert.notStrictEqual, true).to.not.equal(exp);
   };
 
   /**
@@ -216,7 +216,7 @@ module.exports = function (chai, util) {
    */
 
   assert.deepEqual = assert.deepStrictEqual = function (act, exp, msg) {
-    new Assertion(act, msg).to.eql(exp);
+    new Assertion(act, msg, assert.deepEqual, true).to.eql(exp);
   };
 
   /**
@@ -235,7 +235,7 @@ module.exports = function (chai, util) {
    */
 
   assert.notDeepEqual = function (act, exp, msg) {
-    new Assertion(act, msg).to.not.eql(exp);
+    new Assertion(act, msg, assert.notDeepEqual, true).to.not.eql(exp);
   };
 
    /**
@@ -254,7 +254,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isAbove = function (val, abv, msg) {
-    new Assertion(val, msg).to.be.above(abv);
+    new Assertion(val, msg, assert.isAbove, true).to.be.above(abv);
   };
 
    /**
@@ -274,7 +274,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isAtLeast = function (val, atlst, msg) {
-    new Assertion(val, msg).to.be.least(atlst);
+    new Assertion(val, msg, assert.isAtLeast, true).to.be.least(atlst);
   };
 
    /**
@@ -293,7 +293,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isBelow = function (val, blw, msg) {
-    new Assertion(val, msg).to.be.below(blw);
+    new Assertion(val, msg, assert.isBelow, true).to.be.below(blw);
   };
 
    /**
@@ -313,7 +313,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isAtMost = function (val, atmst, msg) {
-    new Assertion(val, msg).to.be.most(atmst);
+    new Assertion(val, msg, assert.isAtMost, true).to.be.most(atmst);
   };
 
   /**
@@ -332,7 +332,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isTrue = function (val, msg) {
-    new Assertion(val, msg).is['true'];
+    new Assertion(val, msg, assert.isTrue, true).is['true'];
   };
 
   /**
@@ -351,7 +351,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotTrue = function (val, msg) {
-    new Assertion(val, msg).to.not.equal(true);
+    new Assertion(val, msg, assert.isNotTrue, true).to.not.equal(true);
   };
 
   /**
@@ -370,7 +370,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isFalse = function (val, msg) {
-    new Assertion(val, msg).is['false'];
+    new Assertion(val, msg, assert.isFalse, true).is['false'];
   };
 
   /**
@@ -389,7 +389,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotFalse = function (val, msg) {
-    new Assertion(val, msg).to.not.equal(false);
+    new Assertion(val, msg, assert.isNotFalse, true).to.not.equal(false);
   };
 
   /**
@@ -407,7 +407,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNull = function (val, msg) {
-    new Assertion(val, msg).to.equal(null);
+    new Assertion(val, msg, assert.isNull, true).to.equal(null);
   };
 
   /**
@@ -426,7 +426,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotNull = function (val, msg) {
-    new Assertion(val, msg).to.not.equal(null);
+    new Assertion(val, msg, assert.isNotNull, true).to.not.equal(null);
   };
 
   /**
@@ -444,7 +444,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNaN = function (val, msg) {
-    new Assertion(val, msg).to.be.NaN;
+    new Assertion(val, msg, assert.isNaN, true).to.be.NaN;
   };
 
   /**
@@ -461,7 +461,7 @@ module.exports = function (chai, util) {
    * @api public
    */
   assert.isNotNaN = function (val, msg) {
-    new Assertion(val, msg).not.to.be.NaN;
+    new Assertion(val, msg, assert.isNotNaN, true).not.to.be.NaN;
   };
 
   /**
@@ -481,7 +481,7 @@ module.exports = function (chai, util) {
    */
 
   assert.exists = function (val, msg) {
-    new Assertion(val, msg).to.exist;
+    new Assertion(val, msg, assert.exists, true).to.exist;
   };
 
   /**
@@ -503,7 +503,7 @@ module.exports = function (chai, util) {
    */
 
   assert.notExists = function (val, msg) {
-    new Assertion(val, msg).to.not.exist;
+    new Assertion(val, msg, assert.notExists, true).to.not.exist;
   };
 
   /**
@@ -522,7 +522,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isUndefined = function (val, msg) {
-    new Assertion(val, msg).to.equal(undefined);
+    new Assertion(val, msg, assert.isUndefined, true).to.equal(undefined);
   };
 
   /**
@@ -541,7 +541,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isDefined = function (val, msg) {
-    new Assertion(val, msg).to.not.equal(undefined);
+    new Assertion(val, msg, assert.isDefined, true).to.not.equal(undefined);
   };
 
   /**
@@ -560,7 +560,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isFunction = function (val, msg) {
-    new Assertion(val, msg).to.be.a('function');
+    new Assertion(val, msg, assert.isFunction, true).to.be.a('function');
   };
 
   /**
@@ -579,7 +579,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotFunction = function (val, msg) {
-    new Assertion(val, msg).to.not.be.a('function');
+    new Assertion(val, msg, assert.isNotFunction, true).to.not.be.a('function');
   };
 
   /**
@@ -599,7 +599,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isObject = function (val, msg) {
-    new Assertion(val, msg).to.be.a('object');
+    new Assertion(val, msg, assert.isObject, true).to.be.a('object');
   };
 
   /**
@@ -619,7 +619,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotObject = function (val, msg) {
-    new Assertion(val, msg).to.not.be.a('object');
+    new Assertion(val, msg, assert.isNotObject, true).to.not.be.a('object');
   };
 
   /**
@@ -638,7 +638,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isArray = function (val, msg) {
-    new Assertion(val, msg).to.be.an('array');
+    new Assertion(val, msg, assert.isArray, true).to.be.an('array');
   };
 
   /**
@@ -657,7 +657,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotArray = function (val, msg) {
-    new Assertion(val, msg).to.not.be.an('array');
+    new Assertion(val, msg, assert.isNotArray, true).to.not.be.an('array');
   };
 
   /**
@@ -676,7 +676,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isString = function (val, msg) {
-    new Assertion(val, msg).to.be.a('string');
+    new Assertion(val, msg, assert.isString, true).to.be.a('string');
   };
 
   /**
@@ -695,7 +695,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotString = function (val, msg) {
-    new Assertion(val, msg).to.not.be.a('string');
+    new Assertion(val, msg, assert.isNotString, true).to.not.be.a('string');
   };
 
   /**
@@ -714,7 +714,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNumber = function (val, msg) {
-    new Assertion(val, msg).to.be.a('number');
+    new Assertion(val, msg, assert.isNumber, true).to.be.a('number');
   };
 
   /**
@@ -733,7 +733,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotNumber = function (val, msg) {
-    new Assertion(val, msg).to.not.be.a('number');
+    new Assertion(val, msg, assert.isNotNumber, true).to.not.be.a('number');
   };
 
    /**
@@ -754,7 +754,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isFinite = function (val, msg) {
-    new Assertion(val, msg).to.be.finite;
+    new Assertion(val, msg, assert.isFinite, true).to.be.finite;
   };
 
   /**
@@ -776,7 +776,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isBoolean = function (val, msg) {
-    new Assertion(val, msg).to.be.a('boolean');
+    new Assertion(val, msg, assert.isBoolean, true).to.be.a('boolean');
   };
 
   /**
@@ -798,7 +798,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotBoolean = function (val, msg) {
-    new Assertion(val, msg).to.not.be.a('boolean');
+    new Assertion(val, msg, assert.isNotBoolean, true).to.not.be.a('boolean');
   };
 
   /**
@@ -823,7 +823,7 @@ module.exports = function (chai, util) {
    */
 
   assert.typeOf = function (val, type, msg) {
-    new Assertion(val, msg).to.be.a(type);
+    new Assertion(val, msg, assert.typeOf, true).to.be.a(type);
   };
 
   /**
@@ -843,7 +843,7 @@ module.exports = function (chai, util) {
    */
 
   assert.notTypeOf = function (val, type, msg) {
-    new Assertion(val, msg).to.not.be.a(type);
+    new Assertion(val, msg, assert.notTypeOf, true).to.not.be.a(type);
   };
 
   /**
@@ -865,7 +865,7 @@ module.exports = function (chai, util) {
    */
 
   assert.instanceOf = function (val, type, msg) {
-    new Assertion(val, msg).to.be.instanceOf(type);
+    new Assertion(val, msg, assert.instanceOf, true).to.be.instanceOf(type);
   };
 
   /**
@@ -887,7 +887,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notInstanceOf = function (val, type, msg) {
-    new Assertion(val, msg).to.not.be.instanceOf(type);
+    new Assertion(val, msg, assert.notInstanceOf, true)
+      .to.not.be.instanceOf(type);
   };
 
   /**
@@ -922,7 +923,7 @@ module.exports = function (chai, util) {
    */
 
   assert.include = function (exp, inc, msg) {
-    new Assertion(exp, msg, assert.include).include(inc);
+    new Assertion(exp, msg, assert.include, true).include(inc);
   };
 
   /**
@@ -958,7 +959,7 @@ module.exports = function (chai, util) {
    */
 
   assert.notInclude = function (exp, inc, msg) {
-    new Assertion(exp, msg, assert.notInclude).not.include(inc);
+    new Assertion(exp, msg, assert.notInclude, true).not.include(inc);
   };
 
   /**
@@ -983,7 +984,7 @@ module.exports = function (chai, util) {
    */
 
   assert.deepInclude = function (exp, inc, msg) {
-    new Assertion(exp, msg, assert.include).deep.include(inc);
+    new Assertion(exp, msg, assert.deepInclude, true).deep.include(inc);
   };
 
   /**
@@ -1008,7 +1009,7 @@ module.exports = function (chai, util) {
    */
 
   assert.notDeepInclude = function (exp, inc, msg) {
-    new Assertion(exp, msg, assert.notInclude).not.deep.include(inc);
+    new Assertion(exp, msg, assert.notDeepInclude, true).not.deep.include(inc);
   };
 
   /**
@@ -1027,7 +1028,7 @@ module.exports = function (chai, util) {
    */
 
   assert.match = function (exp, re, msg) {
-    new Assertion(exp, msg).to.match(re);
+    new Assertion(exp, msg, assert.match, true).to.match(re);
   };
 
   /**
@@ -1046,7 +1047,7 @@ module.exports = function (chai, util) {
    */
 
   assert.notMatch = function (exp, re, msg) {
-    new Assertion(exp, msg).to.not.match(re);
+    new Assertion(exp, msg, assert.notMatch, true).to.not.match(re);
   };
 
   /**
@@ -1067,7 +1068,7 @@ module.exports = function (chai, util) {
    */
 
   assert.property = function (obj, prop, msg) {
-    new Assertion(obj, msg).to.have.property(prop);
+    new Assertion(obj, msg, assert.property, true).to.have.property(prop);
   };
 
   /**
@@ -1087,7 +1088,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notProperty = function (obj, prop, msg) {
-    new Assertion(obj, msg).to.not.have.property(prop);
+    new Assertion(obj, msg, assert.notProperty, true)
+      .to.not.have.property(prop);
   };
 
   /**
@@ -1109,7 +1111,8 @@ module.exports = function (chai, util) {
    */
 
   assert.propertyVal = function (obj, prop, val, msg) {
-    new Assertion(obj, msg).to.have.property(prop, val);
+    new Assertion(obj, msg, assert.propertyVal, true)
+      .to.have.property(prop, val);
   };
 
   /**
@@ -1132,7 +1135,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notPropertyVal = function (obj, prop, val, msg) {
-    new Assertion(obj, msg).to.not.have.property(prop, val);
+    new Assertion(obj, msg, assert.notPropertyVal, true)
+      .to.not.have.property(prop, val);
   };
 
   /**
@@ -1153,7 +1157,8 @@ module.exports = function (chai, util) {
    */
 
   assert.deepPropertyVal = function (obj, prop, val, msg) {
-    new Assertion(obj, msg).to.have.deep.property(prop, val);
+    new Assertion(obj, msg, assert.deepPropertyVal, true)
+      .to.have.deep.property(prop, val);
   };
 
   /**
@@ -1176,7 +1181,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notDeepPropertyVal = function (obj, prop, val, msg) {
-    new Assertion(obj, msg).to.not.have.deep.property(prop, val);
+    new Assertion(obj, msg, assert.notDeepPropertyVal, true)
+      .to.not.have.deep.property(prop, val);
   };
 
   /**
@@ -1195,7 +1201,8 @@ module.exports = function (chai, util) {
    */
 
   assert.ownProperty = function (obj, prop, msg) {
-    new Assertion(obj, msg).to.have.own.property(prop);
+    new Assertion(obj, msg, assert.ownProperty, true)
+      .to.have.own.property(prop);
   };
 
   /**
@@ -1215,7 +1222,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notOwnProperty = function (obj, prop, msg) {
-    new Assertion(obj, msg).to.not.have.own.property(prop);
+    new Assertion(obj, msg, assert.notOwnProperty, true)
+      .to.not.have.own.property(prop);
   };
 
   /**
@@ -1236,7 +1244,8 @@ module.exports = function (chai, util) {
    */
 
   assert.ownPropertyVal = function (obj, prop, value, msg) {
-    new Assertion(obj, msg).to.have.own.property(prop, value);
+    new Assertion(obj, msg, assert.ownPropertyVal, true)
+      .to.have.own.property(prop, value);
   };
 
   /**
@@ -1258,7 +1267,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notOwnPropertyVal = function (obj, prop, value, msg) {
-    new Assertion(obj, msg).to.not.have.own.property(prop, value);
+    new Assertion(obj, msg, assert.notOwnPropertyVal, true)
+      .to.not.have.own.property(prop, value);
   };
 
   /**
@@ -1279,7 +1289,8 @@ module.exports = function (chai, util) {
    */
 
   assert.deepOwnPropertyVal = function (obj, prop, value, msg) {
-    new Assertion(obj, msg).to.have.deep.own.property(prop, value);
+    new Assertion(obj, msg, assert.deepOwnPropertyVal, true)
+      .to.have.deep.own.property(prop, value);
   };
 
   /**
@@ -1303,7 +1314,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notDeepOwnPropertyVal = function (obj, prop, value, msg) {
-    new Assertion(obj, msg).to.not.have.deep.own.property(prop, value);
+    new Assertion(obj, msg, assert.notDeepOwnPropertyVal, true)
+      .to.not.have.deep.own.property(prop, value);
   };
 
   /**
@@ -1324,7 +1336,8 @@ module.exports = function (chai, util) {
    */
 
   assert.nestedProperty = function (obj, prop, msg) {
-    new Assertion(obj, msg).to.have.nested.property(prop);
+    new Assertion(obj, msg, assert.nestedProperty, true)
+      .to.have.nested.property(prop);
   };
 
   /**
@@ -1345,7 +1358,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notNestedProperty = function (obj, prop, msg) {
-    new Assertion(obj, msg).to.not.have.nested.property(prop);
+    new Assertion(obj, msg, assert.notNestedProperty, true)
+      .to.not.have.nested.property(prop);
   };
 
   /**
@@ -1367,7 +1381,8 @@ module.exports = function (chai, util) {
    */
 
   assert.nestedPropertyVal = function (obj, prop, val, msg) {
-    new Assertion(obj, msg).to.have.nested.property(prop, val);
+    new Assertion(obj, msg, assert.nestedPropertyVal, true)
+      .to.have.nested.property(prop, val);
   };
 
   /**
@@ -1390,7 +1405,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notNestedPropertyVal = function (obj, prop, val, msg) {
-    new Assertion(obj, msg).to.not.have.nested.property(prop, val);
+    new Assertion(obj, msg, assert.notNestedPropertyVal, true)
+      .to.not.have.nested.property(prop, val);
   };
 
   /**
@@ -1412,7 +1428,8 @@ module.exports = function (chai, util) {
    */
 
   assert.deepNestedPropertyVal = function (obj, prop, val, msg) {
-    new Assertion(obj, msg).to.have.deep.nested.property(prop, val);
+    new Assertion(obj, msg, assert.deepNestedPropertyVal, true)
+      .to.have.deep.nested.property(prop, val);
   };
 
   /**
@@ -1436,7 +1453,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notDeepNestedPropertyVal = function (obj, prop, val, msg) {
-    new Assertion(obj, msg).to.not.have.deep.nested.property(prop, val);
+    new Assertion(obj, msg, assert.notDeepNestedPropertyVal, true)
+      .to.not.have.deep.nested.property(prop, val);
   }
 
   /**
@@ -1456,7 +1474,7 @@ module.exports = function (chai, util) {
    */
 
   assert.lengthOf = function (exp, len, msg) {
-    new Assertion(exp, msg).to.have.lengthOf(len);
+    new Assertion(exp, msg, assert.lengthOf, true).to.have.lengthOf(len);
   };
 
   /**
@@ -1480,7 +1498,7 @@ module.exports = function (chai, util) {
    */
 
   assert.hasAnyKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.have.any.keys(keys);
+    new Assertion(obj, msg, assert.hasAnyKeys, true).to.have.any.keys(keys);
   }
 
   /**
@@ -1504,7 +1522,7 @@ module.exports = function (chai, util) {
    */
 
   assert.hasAllKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.have.all.keys(keys);
+    new Assertion(obj, msg, assert.hasAllKeys, true).to.have.all.keys(keys);
   }
 
   /**
@@ -1532,7 +1550,8 @@ module.exports = function (chai, util) {
    */
 
   assert.containsAllKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.contain.all.keys(keys);
+    new Assertion(obj, msg, assert.containsAllKeys, true)
+      .to.contain.all.keys(keys);
   }
 
   /**
@@ -1556,7 +1575,8 @@ module.exports = function (chai, util) {
    */
 
   assert.doesNotHaveAnyKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.not.have.any.keys(keys);
+    new Assertion(obj, msg, assert.doesNotHaveAnyKeys, true)
+      .to.not.have.any.keys(keys);
   }
 
   /**
@@ -1580,7 +1600,8 @@ module.exports = function (chai, util) {
    */
 
   assert.doesNotHaveAllKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.not.have.all.keys(keys);
+    new Assertion(obj, msg, assert.doesNotHaveAllKeys, true)
+      .to.not.have.all.keys(keys);
   }
 
   /**
@@ -1608,7 +1629,8 @@ module.exports = function (chai, util) {
    */
 
   assert.hasAnyDeepKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.have.any.deep.keys(keys);
+    new Assertion(obj, msg, assert.hasAnyDeepKeys, true)
+      .to.have.any.deep.keys(keys);
   }
 
  /**
@@ -1634,7 +1656,8 @@ module.exports = function (chai, util) {
    */
 
   assert.hasAllDeepKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.have.all.deep.keys(keys);
+    new Assertion(obj, msg, assert.hasAllDeepKeys, true)
+      .to.have.all.deep.keys(keys);
   }
 
  /**
@@ -1660,7 +1683,8 @@ module.exports = function (chai, util) {
    */
 
   assert.containsAllDeepKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.contain.all.deep.keys(keys);
+    new Assertion(obj, msg, assert.containsAllDeepKeys, true)
+      .to.contain.all.deep.keys(keys);
   }
 
  /**
@@ -1686,7 +1710,8 @@ module.exports = function (chai, util) {
    */
 
   assert.doesNotHaveAnyDeepKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.not.have.any.deep.keys(keys);
+    new Assertion(obj, msg, assert.doesNotHaveAnyDeepKeys, true)
+      .to.not.have.any.deep.keys(keys);
   }
 
  /**
@@ -1712,7 +1737,8 @@ module.exports = function (chai, util) {
    */
 
   assert.doesNotHaveAllDeepKeys = function (obj, keys, msg) {
-    new Assertion(obj, msg).to.not.have.all.deep.keys(keys);
+    new Assertion(obj, msg, assert.doesNotHaveAllDeepKeys, true)
+      .to.not.have.all.deep.keys(keys);
   }
 
  /**
@@ -1752,7 +1778,8 @@ module.exports = function (chai, util) {
       errorLike = null;
     }
 
-    var assertErr = new Assertion(fn, msg).to.throw(errorLike, errMsgMatcher);
+    var assertErr = new Assertion(fn, msg, assert.throws, true)
+      .to.throw(errorLike, errMsgMatcher);
     return flag(assertErr, 'object');
   };
 
@@ -1791,7 +1818,8 @@ module.exports = function (chai, util) {
       errorLike = null;
     }
 
-    new Assertion(fn, msg).to.not.throw(errorLike, errMsgMatcher);
+    new Assertion(fn, msg, assert.doesNotThrow, true)
+      .to.not.throw(errorLike, errMsgMatcher);
   };
 
   /**
@@ -1839,9 +1867,13 @@ module.exports = function (chai, util) {
         ok = val !== val2;
         break;
       default:
-        throw new Error('Invalid operator "' + operator + '"');
+        throw new chai.AssertionError(
+          'Invalid operator "' + operator + '"',
+          undefined,
+          assert.operator
+        );
     }
-    var test = new Assertion(ok, msg);
+    var test = new Assertion(ok, msg, assert.operator, true);
     test.assert(
         true === flag(test, 'object')
       , 'expected ' + util.inspect(val) + ' to be ' + operator + ' ' + util.inspect(val2)
@@ -1865,7 +1897,7 @@ module.exports = function (chai, util) {
    */
 
   assert.closeTo = function (act, exp, delta, msg) {
-    new Assertion(act, msg).to.be.closeTo(exp, delta);
+    new Assertion(act, msg, assert.closeTo, true).to.be.closeTo(exp, delta);
   };
 
   /**
@@ -1885,7 +1917,8 @@ module.exports = function (chai, util) {
    */
 
   assert.approximately = function (act, exp, delta, msg) {
-    new Assertion(act, msg).to.be.approximately(exp, delta);
+    new Assertion(act, msg, assert.approximately, true)
+      .to.be.approximately(exp, delta);
   };
 
   /**
@@ -1905,7 +1938,8 @@ module.exports = function (chai, util) {
    */
 
   assert.sameMembers = function (set1, set2, msg) {
-    new Assertion(set1, msg).to.have.same.members(set2);
+    new Assertion(set1, msg, assert.sameMembers, true)
+      .to.have.same.members(set2);
   }
 
   /**
@@ -1925,7 +1959,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notSameMembers = function (set1, set2, msg) {
-    new Assertion(set1, msg).to.not.have.same.members(set2);
+    new Assertion(set1, msg, assert.notSameMembers, true)
+      .to.not.have.same.members(set2);
   }
 
   /**
@@ -1945,7 +1980,8 @@ module.exports = function (chai, util) {
    */
 
   assert.sameDeepMembers = function (set1, set2, msg) {
-    new Assertion(set1, msg).to.have.same.deep.members(set2);
+    new Assertion(set1, msg, assert.sameDeepMembers, true)
+      .to.have.same.deep.members(set2);
   }
 
   /**
@@ -1965,7 +2001,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notSameDeepMembers = function (set1, set2, msg) {
-    new Assertion(set1, msg).to.not.have.same.deep.members(set2);
+    new Assertion(set1, msg, assert.notSameDeepMembers, true)
+      .to.not.have.same.deep.members(set2);
   }
 
   /**
@@ -1985,7 +2022,8 @@ module.exports = function (chai, util) {
    */
 
   assert.sameOrderedMembers = function (set1, set2, msg) {
-    new Assertion(set1, msg).to.have.same.ordered.members(set2);
+    new Assertion(set1, msg, assert.sameOrderedMembers, true)
+      .to.have.same.ordered.members(set2);
   }
 
   /**
@@ -2005,7 +2043,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notSameOrderedMembers = function (set1, set2, msg) {
-    new Assertion(set1, msg).to.not.have.same.ordered.members(set2);
+    new Assertion(set1, msg, assert.notSameOrderedMembers, true)
+      .to.not.have.same.ordered.members(set2);
   }
 
   /**
@@ -2025,7 +2064,8 @@ module.exports = function (chai, util) {
    */
 
   assert.sameDeepOrderedMembers = function (set1, set2, msg) {
-    new Assertion(set1, msg).to.have.same.deep.ordered.members(set2);
+    new Assertion(set1, msg, assert.sameDeepOrderedMembers, true)
+      .to.have.same.deep.ordered.members(set2);
   }
 
   /**
@@ -2046,7 +2086,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notSameDeepOrderedMembers = function (set1, set2, msg) {
-    new Assertion(set1, msg).to.not.have.same.deep.ordered.members(set2);
+    new Assertion(set1, msg, assert.notSameDeepOrderedMembers, true)
+      .to.not.have.same.deep.ordered.members(set2);
   }
 
   /**
@@ -2066,7 +2107,8 @@ module.exports = function (chai, util) {
    */
 
   assert.includeMembers = function (superset, subset, msg) {
-    new Assertion(superset, msg).to.include.members(subset);
+    new Assertion(superset, msg, assert.includeMembers, true)
+      .to.include.members(subset);
   }
 
   /**
@@ -2086,7 +2128,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notIncludeMembers = function (superset, subset, msg) {
-    new Assertion(superset, msg).to.not.include.members(subset);
+    new Assertion(superset, msg, assert.notIncludeMembers, true)
+      .to.not.include.members(subset);
   }
 
   /**
@@ -2106,7 +2149,8 @@ module.exports = function (chai, util) {
    */
 
   assert.includeDeepMembers = function (superset, subset, msg) {
-    new Assertion(superset, msg).to.include.deep.members(subset);
+    new Assertion(superset, msg, assert.includeDeepMembers, true)
+      .to.include.deep.members(subset);
   }
 
   /**
@@ -2126,7 +2170,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notIncludeDeepMembers = function (superset, subset, msg) {
-    new Assertion(superset, msg).to.not.include.deep.members(subset);
+    new Assertion(superset, msg, assert.notIncludeDeepMembers, true)
+      .to.not.include.deep.members(subset);
   }
 
   /**
@@ -2147,7 +2192,8 @@ module.exports = function (chai, util) {
    */
 
   assert.includeOrderedMembers = function (superset, subset, msg) {
-    new Assertion(superset, msg).to.include.ordered.members(subset);
+    new Assertion(superset, msg, assert.includeOrderedMembers, true)
+      .to.include.ordered.members(subset);
   }
 
   /**
@@ -2169,7 +2215,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notIncludeOrderedMembers = function (superset, subset, msg) {
-    new Assertion(superset, msg).to.not.include.ordered.members(subset);
+    new Assertion(superset, msg, assert.notIncludeOrderedMembers, true)
+      .to.not.include.ordered.members(subset);
   }
 
   /**
@@ -2190,7 +2237,8 @@ module.exports = function (chai, util) {
    */
 
   assert.includeDeepOrderedMembers = function (superset, subset, msg) {
-    new Assertion(superset, msg).to.include.deep.ordered.members(subset);
+    new Assertion(superset, msg, assert.includeDeepOrderedMembers, true)
+      .to.include.deep.ordered.members(subset);
   }
 
   /**
@@ -2213,7 +2261,8 @@ module.exports = function (chai, util) {
    */
 
   assert.notIncludeDeepOrderedMembers = function (superset, subset, msg) {
-    new Assertion(superset, msg).to.not.include.deep.ordered.members(subset);
+    new Assertion(superset, msg, assert.notIncludeDeepOrderedMembers, true)
+      .to.not.include.deep.ordered.members(subset);
   }
 
   /**
@@ -2232,7 +2281,7 @@ module.exports = function (chai, util) {
    */
 
   assert.oneOf = function (inList, list, msg) {
-    new Assertion(inList, msg).to.be.oneOf(list);
+    new Assertion(inList, msg, assert.oneOf, true).to.be.oneOf(list);
   }
 
   /**
@@ -2259,7 +2308,7 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    new Assertion(fn, msg).to.change(obj, prop);
+    new Assertion(fn, msg, assert.changes, true).to.change(obj, prop);
   }
 
    /**
@@ -2291,7 +2340,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    new Assertion(fn, msg).to.change(obj, prop).by(delta);
+    new Assertion(fn, msg, assert.changesBy, true)
+      .to.change(obj, prop).by(delta);
   }
 
    /**
@@ -2318,7 +2368,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    return new Assertion(fn, msg).to.not.change(obj, prop);
+    return new Assertion(fn, msg, assert.doesNotChange, true)
+      .to.not.change(obj, prop);
   }
 
   /**
@@ -2350,7 +2401,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    new Assertion(fn, msg).to.change(obj, prop).but.not.by(delta);
+    new Assertion(fn, msg, assert.changesButNotBy, true)
+      .to.change(obj, prop).but.not.by(delta);
   }
 
   /**
@@ -2377,7 +2429,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    return new Assertion(fn, msg).to.increase(obj, prop);
+    return new Assertion(fn, msg, assert.increases, true)
+      .to.increase(obj, prop);
   }
 
   /**
@@ -2409,7 +2462,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    new Assertion(fn, msg).to.increase(obj, prop).by(delta);
+    new Assertion(fn, msg, assert.increasesBy, true)
+      .to.increase(obj, prop).by(delta);
   }
 
   /**
@@ -2436,7 +2490,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    return new Assertion(fn, msg).to.not.increase(obj, prop);
+    return new Assertion(fn, msg, assert.doesNotIncrease, true)
+      .to.not.increase(obj, prop);
   }
 
   /**
@@ -2468,7 +2523,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    new Assertion(fn, msg).to.increase(obj, prop).but.not.by(delta);
+    new Assertion(fn, msg, assert.increasesButNotBy, true)
+      .to.increase(obj, prop).but.not.by(delta);
   }
 
   /**
@@ -2495,7 +2551,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    return new Assertion(fn, msg).to.decrease(obj, prop);
+    return new Assertion(fn, msg, assert.decreases, true)
+      .to.decrease(obj, prop);
   }
 
   /**
@@ -2527,7 +2584,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    new Assertion(fn, msg).to.decrease(obj, prop).by(delta);
+    new Assertion(fn, msg, assert.decreasesBy, true)
+      .to.decrease(obj, prop).by(delta);
   }
 
   /**
@@ -2554,7 +2612,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    return new Assertion(fn, msg).to.not.decrease(obj, prop);
+    return new Assertion(fn, msg, assert.doesNotDecrease, true)
+      .to.not.decrease(obj, prop);
   }
 
   /**
@@ -2586,7 +2645,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    return new Assertion(fn, msg).to.not.decrease(obj, prop).by(delta);
+    return new Assertion(fn, msg, assert.doesNotDecreaseBy, true)
+      .to.not.decrease(obj, prop).by(delta);
   }
 
   /**
@@ -2618,7 +2678,8 @@ module.exports = function (chai, util) {
       prop = null;
     }
 
-    new Assertion(fn, msg).to.decrease(obj, prop).but.not.by(delta);
+    new Assertion(fn, msg, assert.decreasesButNotBy, true)
+      .to.decrease(obj, prop).but.not.by(delta);
   }
 
   /*!
@@ -2659,7 +2720,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isExtensible = function (obj, msg) {
-    new Assertion(obj, msg).to.be.extensible;
+    new Assertion(obj, msg, assert.isExtensible, true).to.be.extensible;
   };
 
   /**
@@ -2684,7 +2745,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotExtensible = function (obj, msg) {
-    new Assertion(obj, msg).to.not.be.extensible;
+    new Assertion(obj, msg, assert.isNotExtensible, true).to.not.be.extensible;
   };
 
   /**
@@ -2708,7 +2769,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isSealed = function (obj, msg) {
-    new Assertion(obj, msg).to.be.sealed;
+    new Assertion(obj, msg, assert.isSealed, true).to.be.sealed;
   };
 
   /**
@@ -2727,7 +2788,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotSealed = function (obj, msg) {
-    new Assertion(obj, msg).to.not.be.sealed;
+    new Assertion(obj, msg, assert.isNotSealed, true).to.not.be.sealed;
   };
 
   /**
@@ -2748,7 +2809,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isFrozen = function (obj, msg) {
-    new Assertion(obj, msg).to.be.frozen;
+    new Assertion(obj, msg, assert.isFrozen, true).to.be.frozen;
   };
 
   /**
@@ -2767,7 +2828,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotFrozen = function (obj, msg) {
-    new Assertion(obj, msg).to.not.be.frozen;
+    new Assertion(obj, msg, assert.isNotFrozen, true).to.not.be.frozen;
   };
 
   /**
@@ -2793,7 +2854,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isEmpty = function(val, msg) {
-    new Assertion(val, msg).to.be.empty;
+    new Assertion(val, msg, assert.isEmpty, true).to.be.empty;
   };
 
   /**
@@ -2819,7 +2880,7 @@ module.exports = function (chai, util) {
    */
 
   assert.isNotEmpty = function(val, msg) {
-    new Assertion(val, msg).to.not.be.empty;
+    new Assertion(val, msg, assert.isNotEmpty, true).to.not.be.empty;
   };
 
   /*!
diff --git a/lib/chai/utils/addChainableMethod.js b/lib/chai/utils/addChainableMethod.js
index 7cb4e495b..3e1edcbdc 100644
--- a/lib/chai/utils/addChainableMethod.js
+++ b/lib/chai/utils/addChainableMethod.js
@@ -82,13 +82,24 @@ module.exports = function addChainableMethod(ctx, name, method, chainingBehavior
         chainableBehavior.chainingBehavior.call(this);
 
         var chainableMethodWrapper = function () {
-          // Use this chainable method wrapper as the starting point for
-          // removing implementation frames from the stack trace of a failed
-          // assertion. Note that this is the correct starting point even if
-          // this assertion has been overwritten since overwriting a chainable
-          // method merely replaces the saved methods in `ctx.__methods` instead
-          // of completely replacing the overwritten assertion.
-          flag(this, 'ssfi', chainableMethodWrapper);
+          // Setting the `ssfi` flag to `chainableMethodWrapper` causes this
+          // function to be the starting point for removing implementation
+          // frames from the stack trace of a failed assertion.
+          //
+          // However, we only want to use this function as the starting point if
+          // the `lockSsfi` flag isn't set.
+          //
+          // If the `lockSsfi` flag is set, then this assertion is being
+          // invoked from inside of another assertion. In this case, the `ssfi`
+          // flag has already been set by the outer assertion.
+          //
+          // Note that overwriting a chainable method merely replaces the saved
+          // methods in `ctx.__methods` instead of completely replacing the
+          // overwritten assertion. Therefore, an overwriting assertion won't
+          // set the `ssfi` or `lockSsfi` flags.
+          if (!flag(this, 'lockSsfi')) {
+            flag(this, 'ssfi', chainableMethodWrapper);
+          }
 
           var result = chainableBehavior.method.apply(this, arguments);
           if (result !== undefined) {
diff --git a/lib/chai/utils/addMethod.js b/lib/chai/utils/addMethod.js
index 56bd5b818..021f08042 100644
--- a/lib/chai/utils/addMethod.js
+++ b/lib/chai/utils/addMethod.js
@@ -38,15 +38,19 @@ var transferFlags = require('./transferFlags');
 
 module.exports = function addMethod(ctx, name, method) {
   var methodWrapper = function () {
-    // If this assertion hasn't been overwritten, then use this method wrapper
-    // as the starting point for removing implementation frames from the stack
-    // trace of a failed assertion.
+    // Setting the `ssfi` flag to `methodWrapper` causes this function to be the
+    // starting point for removing implementation frames from the stack trace of
+    // a failed assertion.
     //
-    // Note: If this assertion has been overwritten, and thus the `keep_ssfi`
-    // flag is set, then the overwriting method wrapper is used as the starting
-    // point instead. This prevents the overwriting method wrapper from showing
-    // up in the stack trace since it's invoked before this method wrapper.
-    if (!flag(this, 'keep_ssfi')) {
+    // However, we only want to use this function as the starting point if the
+    // `lockSsfi` flag isn't set.
+    //
+    // If the `lockSsfi` flag is set, then either this assertion has been
+    // overwritten by another assertion, or this assertion is being invoked from
+    // inside of another assertion. In the first case, the `ssfi` flag has
+    // already been set by the overwriting assertion. In the second case, the
+    // `ssfi` flag has already been set by the outer assertion.
+    if (!flag(this, 'lockSsfi')) {
       flag(this, 'ssfi', methodWrapper);
     }
 
diff --git a/lib/chai/utils/addProperty.js b/lib/chai/utils/addProperty.js
index d3757e6c4..7cbc8569f 100644
--- a/lib/chai/utils/addProperty.js
+++ b/lib/chai/utils/addProperty.js
@@ -40,24 +40,22 @@ module.exports = function addProperty(ctx, name, getter) {
 
   Object.defineProperty(ctx, name,
     { get: function propertyGetter() {
-        // If proxy protection is disabled and this assertion hasn't been
-        // overwritten, then use this property getter as the starting point for
-        // removing implementation frames from the stack trace of a failed
-        // assertion.
+        // Setting the `ssfi` flag to `propertyGetter` causes this function to
+        // be the starting point for removing implementation frames from the
+        // stack trace of a failed assertion.
         //
-        // Notes:
+        // However, we only want to use this function as the starting point if
+        // the `lockSsfi` flag isn't set and proxy protection is disabled.
         //
-        //  - If proxy protection is enabled, then the proxy getter is used as
-        //    the starting point instead. This prevents the proxy getter from
-        //    showing up in the stack trace since it's invoked before this
-        //    property getter.
+        // If the `lockSsfi` flag is set, then either this assertion has been
+        // overwritten by another assertion, or this assertion is being invoked
+        // from inside of another assertion. In the first case, the `ssfi` flag
+        // has already been set by the overwriting assertion. In the second
+        // case, the `ssfi` flag has already been set by the outer assertion.
         //
-        //  - If proxy protection is disabled but this assertion has been
-        //    overwritten, and thus the `keep_ssfi` flag is set, then the
-        //    overwriting property getter is used as the starting point instead.
-        //    This prevents the overwriting property getter from showing up in
-        //    the stack trace since it's invoked before this property getter.
-        if (!isProxyEnabled() && !flag(this, 'keep_ssfi')) {
+        // If proxy protection is enabled, then the `ssfi` flag has already been
+        // set by the proxy getter.
+        if (!isProxyEnabled() && !flag(this, 'lockSsfi')) {
           flag(this, 'ssfi', propertyGetter);
         }
 
diff --git a/lib/chai/utils/expectTypes.js b/lib/chai/utils/expectTypes.js
index 48d842f7a..2ffcadd41 100644
--- a/lib/chai/utils/expectTypes.js
+++ b/lib/chai/utils/expectTypes.js
@@ -13,6 +13,8 @@
  *
  * @param {Mixed} obj constructed Assertion
  * @param {Array} type A list of allowed types for this assertion
+ * @param {Function} ssfi starting point for removing implementation frames from
+ *                        stack trace of AssertionError
  * @namespace Utils
  * @name expectTypes
  * @api public
@@ -22,7 +24,7 @@ var AssertionError = require('assertion-error');
 var flag = require('./flag');
 var type = require('type-detect');
 
-module.exports = function expectTypes(obj, types) {
+module.exports = function expectTypes(obj, types, ssfi) {
   obj = flag(obj, 'object');
   types = types.map(function (t) { return t.toLowerCase(); });
   types.sort();
@@ -38,7 +40,9 @@ module.exports = function expectTypes(obj, types) {
 
   if (!types.some(function (expected) { return objType === expected; })) {
     throw new AssertionError(
-      'object tested must be ' + str + ', but ' + objType + ' given'
+      'object tested must be ' + str + ', but ' + objType + ' given',
+      undefined,
+      ssfi
     );
   }
 };
diff --git a/lib/chai/utils/overwriteMethod.js b/lib/chai/utils/overwriteMethod.js
index 004b85cdd..f77a4f493 100644
--- a/lib/chai/utils/overwriteMethod.js
+++ b/lib/chai/utils/overwriteMethod.js
@@ -54,26 +54,29 @@ module.exports = function overwriteMethod(ctx, name, method) {
     _super = _method;
 
   var overwritingMethodWrapper = function () {
-    // If proxy protection is disabled and this overwriting assertion hasn't
-    // been overwritten again by yet another assertion, then use this method
-    // wrapper as the starting point for removing implementation frames from the
-    // stack trace of a failed assertion.
+    // Setting the `ssfi` flag to `overwritingMethodWrapper` causes this
+    // function to be the starting point for removing implementation frames from
+    // the stack trace of a failed assertion.
     //
-    // Note: If this assertion has been overwritten, and thus the `keep_ssfi`
-    // flag is set, then the overwriting method wrapper is used as the starting
-    // point instead. This prevents the overwriting method wrapper from showing
-    // up in the stack trace since it's invoked before this method wrapper.
-    if (!flag(this, 'keep_ssfi')) {
+    // However, we only want to use this function as the starting point if the
+    // `lockSsfi` flag isn't set.
+    //
+    // If the `lockSsfi` flag is set, then either this assertion has been
+    // overwritten by another assertion, or this assertion is being invoked from
+    // inside of another assertion. In the first case, the `ssfi` flag has
+    // already been set by the overwriting assertion. In the second case, the
+    // `ssfi` flag has already been set by the outer assertion.
+    if (!flag(this, 'lockSsfi')) {
       flag(this, 'ssfi', overwritingMethodWrapper);
     }
 
-    // The `keep_ssfi` flag is set so that if this assertion ends up calling
-    // the overwritten assertion, then the overwritten assertion doesn't attempt
-    // to use itself as the starting point for removing implementation frames
-    // from the stack trace of a failed assertion.
-    flag(this, 'keep_ssfi', true);
+    // Setting the `lockSsfi` flag to `true` prevents the overwritten assertion
+    // from changing the `ssfi` flag. By this point, the `ssfi` flag is already
+    // set to the correct starting point for this assertion.
+    var origLockSsfi = flag(this, 'lockSsfi');
+    flag(this, 'lockSsfi', true);
     var result = method(_super).apply(this, arguments);
-    flag(this, 'keep_ssfi', false);
+    flag(this, 'lockSsfi', origLockSsfi);
 
     if (result !== undefined) {
       return result;
diff --git a/lib/chai/utils/overwriteProperty.js b/lib/chai/utils/overwriteProperty.js
index 73e62180a..82ed3c876 100644
--- a/lib/chai/utils/overwriteProperty.js
+++ b/lib/chai/utils/overwriteProperty.js
@@ -52,34 +52,32 @@ module.exports = function overwriteProperty(ctx, name, getter) {
 
   Object.defineProperty(ctx, name,
     { get: function overwritingPropertyGetter() {
-        // If proxy protection is disabled and this overwriting assertion hasn't
-        // been overwritten again by yet another assertion, then use this
-        // property getter as the starting point for removing implementation
-        // frames from the stack trace of a failed assertion.
+        // Setting the `ssfi` flag to `overwritingPropertyGetter` causes this
+        // function to be the starting point for removing implementation frames
+        // from the stack trace of a failed assertion.
         //
-        // Notes:
+        // However, we only want to use this function as the starting point if
+        // the `lockSsfi` flag isn't set and proxy protection is disabled.
         //
-        //  - If proxy protection is enabled, then the proxy getter is used as
-        //    the starting point instead. This prevents the proxy getter from
-        //    showing up in the stack trace since it's invoked before this
-        //    property getter.
+        // If the `lockSsfi` flag is set, then either this assertion has been
+        // overwritten by another assertion, or this assertion is being invoked
+        // from inside of another assertion. In the first case, the `ssfi` flag
+        // has already been set by the overwriting assertion. In the second
+        // case, the `ssfi` flag has already been set by the outer assertion.
         //
-        //  - If proxy protection is disabled but this assertion has been
-        //    overwritten, and thus the `keep_ssfi` flag is set, then the
-        //    overwriting property getter is used as the starting point instead.
-        //    This prevents the overwriting property getter from showing up in
-        //    the stack trace since it's invoked before this property getter.
-        if (!isProxyEnabled() && !flag(this, 'keep_ssfi')) {
+        // If proxy protection is enabled, then the `ssfi` flag has already been
+        // set by the proxy getter.
+        if (!isProxyEnabled() && !flag(this, 'lockSsfi')) {
           flag(this, 'ssfi', overwritingPropertyGetter);
         }
 
-        // The `keep_ssfi` flag is set so that if this assertion ends up calling
-        // the overwritten assertion, then the overwritten assertion doesn't
-        // attempt to use itself as the starting point for removing
-        // implementation frames from the stack trace of a failed assertion.
-        flag(this, 'keep_ssfi', true);
+        // Setting the `lockSsfi` flag to `true` prevents the overwritten
+        // assertion from changing the `ssfi` flag. By this point, the `ssfi`
+        // flag is already set to the correct starting point for this assertion.
+        var origLockSsfi = flag(this, 'lockSsfi');
+        flag(this, 'lockSsfi', true);
         var result = getter(_super).call(this);
-        flag(this, 'keep_ssfi', false);
+        flag(this, 'lockSsfi', origLockSsfi);
 
         if (result !== undefined) {
           return result;
diff --git a/lib/chai/utils/proxify.js b/lib/chai/utils/proxify.js
index 9ea8b7f49..2b3b0203d 100644
--- a/lib/chai/utils/proxify.js
+++ b/lib/chai/utils/proxify.js
@@ -75,8 +75,11 @@ module.exports = function proxify(obj, nonChainableMethodName) {
       // the method wrapper, which is good since this frame will no longer be in
       // the stack once the method is invoked. Note that Chai builtin assertion
       // properties such as `__flags` are skipped since this is only meant to
-      // capture the starting point of an assertion.
-      if (builtins.indexOf(property) === -1) {
+      // capture the starting point of an assertion. This step is also skipped
+      // if the `lockSsfi` flag is set, thus indicating that this assertion is
+      // being called from within another assertion. In that case, the `ssfi`
+      // flag is already set to the outer assertion's starting point.
+      if (builtins.indexOf(property) === -1 && !flag(target, 'lockSsfi')) {
         flag(target, 'ssfi', proxyGetter);
       }
 
diff --git a/lib/chai/utils/transferFlags.js b/lib/chai/utils/transferFlags.js
index 16684047d..634e0537a 100644
--- a/lib/chai/utils/transferFlags.js
+++ b/lib/chai/utils/transferFlags.js
@@ -9,8 +9,8 @@
  *
  * Transfer all the flags for `assertion` to `object`. If
  * `includeAll` is set to `false`, then the base Chai
- * assertion flags (namely `object`, `ssfi`, and `message`)
- * will not be transferred.
+ * assertion flags (namely `object`, `ssfi`, `lockSsfi`,
+ * and `message`) will not be transferred.
  *
  *
  *     var newAssertion = new Assertion();
@@ -38,7 +38,7 @@ module.exports = function transferFlags(assertion, object, includeAll) {
 
   for (var flag in flags) {
     if (includeAll ||
-        (flag !== 'object' && flag !== 'ssfi' && flag != 'message')) {
+        (flag !== 'object' && flag !== 'ssfi' && flag !== 'lockSsfi' && flag != 'message')) {
       object.__flags[flag] = flags[flag];
     }
   }
diff --git a/test/assert.js b/test/assert.js
index 5ca21c383..a3f14a9f7 100644
--- a/test/assert.js
+++ b/test/assert.js
@@ -177,7 +177,7 @@ describe('assert', function () {
       var t = new Thing();
       Thing.prototype = 1337;
       assert.instanceOf(t, Thing);
-    }, expectedError);
+    }, expectedError, true);
 
     if (typeof Symbol !== 'undefined' && typeof Symbol.hasInstance !== 'undefined') {
         err(function(){
@@ -1981,7 +1981,7 @@ describe('assert', function () {
         err(function() {
           // isExtensible should not suppress errors, thrown in proxy traps
           assert[isExtensible](proxy);
-        }, { name: 'TypeError' });
+        }, { name: 'TypeError' }, true);
       }
     });
   });
@@ -2018,7 +2018,7 @@ describe('assert', function () {
         err(function() {
           // isNotExtensible should not suppress errors, thrown in proxy traps
           assert[isNotExtensible](proxy);
-        }, { name: 'TypeError' });
+        }, { name: 'TypeError' }, true);
       }
     });
   });
@@ -2058,7 +2058,7 @@ describe('assert', function () {
         err(function() {
           // isSealed should not suppress errors, thrown in proxy traps
           assert[isSealed](proxy);
-        }, { name: 'TypeError' });
+        }, { name: 'TypeError' }, true);
       }
     });
   });
@@ -2108,7 +2108,7 @@ describe('assert', function () {
         err(function() {
           // isNotSealed should not suppress errors, thrown in proxy traps
           assert[isNotSealed](proxy);
-        }, { name: 'TypeError' });
+        }, { name: 'TypeError' }, true);
       }
     });
   });
@@ -2148,7 +2148,7 @@ describe('assert', function () {
         err(function() {
           // isFrozen should not suppress errors, thrown in proxy traps
           assert[isFrozen](proxy);
-        }, { name: 'TypeError' });
+        }, { name: 'TypeError' }, true);
       }
     });
   });
@@ -2161,7 +2161,7 @@ describe('assert', function () {
 
       err(function() {
         assert[isNotFrozen](frozenObject);
-      }, 'expected {} to not be frozen');
+      }, 'expected {} to not be frozen', true);
 
       // Making sure ES6-like Object.isFrozen response is respected for all primitive types
 
@@ -2198,7 +2198,7 @@ describe('assert', function () {
         err(function() {
           // isNotFrozen should not suppress errors, thrown in proxy traps
           assert[isNotFrozen](proxy);
-        }, { name: 'TypeError' });
+        }, { name: 'TypeError' }, true);
       }
     });
   });
diff --git a/test/bootstrap/index.js b/test/bootstrap/index.js
index aa0c7ccf6..6e4cf5a91 100644
--- a/test/bootstrap/index.js
+++ b/test/bootstrap/index.js
@@ -4,9 +4,23 @@ if (typeof window === 'object') {
   global.chai = require('../..');
 }
 
+var isStackSupported = false;
+if (typeof Error.captureStackTrace !== 'undefined') {
+  try {
+    throw Error();
+  } catch (err) {
+    if (typeof err.stack !== 'undefined') isStackSupported = true;
+  }
+}
+
 /**
- * Validate that the given function throws an error. Optionally validate some
- * additional properties of the error:
+ * Validate that the given function throws an error.
+ *
+ * By default, also validate that the thrown error's stack trace doesn't contain
+ * Chai implementation frames. Stack trace validation can be disabled by
+ * providing a truthy `skipStackTest` argument.
+ *
+ * Optionally validate some additional properties of the error:
  *
  * If val is a string, validate val equals the error's .message
  * If val is a regex, validate val matches the error's .message
@@ -14,15 +28,25 @@ if (typeof window === 'object') {
  *
  * @param {Function} function that's expected to throw an error
  * @param {Mixed} expected properties of the expected error
+ * @param {Boolean} skipStackTest if truthy, don't validate stack trace
  */
 
-global.err = function (fn, val) {
+global.err = function globalErr (fn, val, skipStackTest) {
   if (chai.util.type(fn) !== 'function')
     throw new chai.AssertionError('Invalid fn');
 
   try {
     fn();
   } catch (err) {
+    if (isStackSupported && !skipStackTest) {
+      chai.expect(err).to.have.property('stack')
+        .that.has.string('globalErr')
+        .but.does.not.match(
+          /at [a-zA-Z]*(Getter|Wrapper|(\.)*assert)/,
+          'implementation frames not properly filtered from stack trace'
+        );
+    }
+
     switch (chai.util.type(val).toLowerCase()) {
       case 'undefined': return;
       case 'string': return chai.expect(err.message).to.equal(val);
diff --git a/test/expect.js b/test/expect.js
index a4ca2588c..f8a040375 100644
--- a/test/expect.js
+++ b/test/expect.js
@@ -58,73 +58,73 @@ describe('expect', function () {
       it('throws when invalid property follows expect', function () {
         err(function () {
           expect(42).pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows language chain', function () {
         err(function () {
           expect(42).to.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows property assertion', function () {
         err(function () {
           expect(42).ok.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows overwritten property assertion', function () {
         err(function () {
           expect(42).tmpProperty.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows uncalled method assertion', function () {
         err(function () {
           expect(42).equal.pizza;
-        }, 'Invalid Chai property: equal.pizza. See docs for proper usage of "equal".');
+        }, 'Invalid Chai property: equal.pizza. See docs for proper usage of "equal".', true);
       });
 
       it('throws when invalid property follows called method assertion', function () {
         err(function () {
           expect(42).equal(42).pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows uncalled overwritten method assertion', function () {
         err(function () {
           expect(42).tmpMethod.pizza;
-        }, 'Invalid Chai property: tmpMethod.pizza. See docs for proper usage of "tmpMethod".');
+        }, 'Invalid Chai property: tmpMethod.pizza. See docs for proper usage of "tmpMethod".', true);
       });
 
       it('throws when invalid property follows called overwritten method assertion', function () {
         err(function () {
           expect(42).tmpMethod().pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows uncalled chainable method assertion', function () {
         err(function () {
           expect(42).a.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows called chainable method assertion', function () {
         err(function () {
           expect(42).a('number').pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows uncalled overwritten chainable method assertion', function () {
        err(function () {
           expect(42).tmpChainableMethod.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows called overwritten chainable method assertion', function () {
         err(function () {
           expect(42).tmpChainableMethod().pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('doesn\'t throw if invalid property is excluded via config', function () {
@@ -165,7 +165,7 @@ describe('expect', function () {
       it('throws when `.length` follows uncalled method assertion', function () {
         err(function () {
           expect('foo').equal.length;
-        }, 'Invalid Chai property: equal.length. See docs for proper usage of "equal".');
+        }, 'Invalid Chai property: equal.length. See docs for proper usage of "equal".', true);
       });
 
       it('doesn\'t throw when `.length` follows called method assertion', function () {
@@ -177,7 +177,7 @@ describe('expect', function () {
       it('throws when `.length` follows uncalled overwritten method assertion', function () {
         err(function () {
           expect('foo').tmpMethod.length;
-        }, 'Invalid Chai property: tmpMethod.length. See docs for proper usage of "tmpMethod".');
+        }, 'Invalid Chai property: tmpMethod.length. See docs for proper usage of "tmpMethod".', true);
       });
 
       it('doesn\'t throw when `.length` follows called overwritten method assertion', function () {
@@ -189,7 +189,7 @@ describe('expect', function () {
       it('throws when `.length` follows uncalled chainable method assertion', function () {
         err(function () {
           expect('foo').a.length;
-        }, 'Invalid Chai property: a.length. Due to a compatibility issue, "length" cannot directly follow "a". Use "a.lengthOf" instead.');
+        }, 'Invalid Chai property: a.length. Due to a compatibility issue, "length" cannot directly follow "a". Use "a.lengthOf" instead.', true);
       });
 
       it('doesn\'t throw when `.length` follows called chainable method assertion', function () {
@@ -201,7 +201,7 @@ describe('expect', function () {
       it('throws when `.length` follows uncalled overwritten chainable method assertion', function () {
         err(function () {
           expect('foo').tmpChainableMethod.length;
-        }, 'Invalid Chai property: tmpChainableMethod.length. Due to a compatibility issue, "length" cannot directly follow "tmpChainableMethod". Use "tmpChainableMethod.lengthOf" instead.');
+        }, 'Invalid Chai property: tmpChainableMethod.length. Due to a compatibility issue, "length" cannot directly follow "tmpChainableMethod". Use "tmpChainableMethod.lengthOf" instead.', true);
       });
 
       it('doesn\'t throw when `.length` follows called overwritten chainable method assertion', function () {
@@ -403,7 +403,7 @@ describe('expect', function () {
       var t = new Thing();
       Thing.prototype = 1337;
       expect(t).to.an.instanceof(Thing);
-    }, expectedError)
+    }, expectedError, true)
 
     if (typeof Symbol !== 'undefined' && typeof Symbol.hasInstance !== 'undefined') {
         err(function(){
@@ -2701,7 +2701,7 @@ describe('expect', function () {
       err(function() {
         // .extensible should not suppress errors, thrown in proxy traps
         expect(proxy).to.be.extensible;
-      }, { name: 'TypeError' });
+      }, { name: 'TypeError' }, true);
     }
   });
 
@@ -2764,7 +2764,7 @@ describe('expect', function () {
       err(function() {
         // .sealed should not suppress errors, thrown in proxy traps
         expect(proxy).to.be.sealed;
-      }, { name: 'TypeError' });
+      }, { name: 'TypeError' }, true);
     }
   });
 
@@ -2827,7 +2827,7 @@ describe('expect', function () {
       err(function() {
         // .frozen should not suppress errors, thrown in proxy traps
         expect(proxy).to.be.frozen;
-      }, { name: 'TypeError' });
+      }, { name: 'TypeError' }, true);
     }
   });
 });
diff --git a/test/globalErr.js b/test/globalErr.js
index 0858fa38a..83d66f4fc 100644
--- a/test/globalErr.js
+++ b/test/globalErr.js
@@ -56,7 +56,7 @@ describe('globalErr', function () {
 
     err(function () {
       err(function () { throw new Err('cat') }, {message: 'dog'});
-    }, 'expected \'cat\' to deeply equal \'dog\'');
+    }, 'expected \'cat\' to deeply equal \'dog\'', true);
   });
 
   it('should throw if fn does not throw', function () {
@@ -108,4 +108,78 @@ describe('globalErr', function () {
       }, 'Invalid val')
     });
   });
+
+  describe('skipStackTest', function () {
+    // Skip tests if `Error.captureStackTrace` is unsupported
+    if (typeof Error.captureStackTrace === 'undefined') return;
+
+    try {
+      throw Error();
+    } catch (err) {
+      // Skip tests if `err.stack` is unsupported
+      if (typeof err.stack === 'undefined') return;
+    }
+
+    // Note: `.to.not.throw` isn't used for the assertions that aren't expected
+    // to throw an error because it'll pollute the very same stack trace which
+    // is being asserted on. Instead, if `err` throws an error, then Mocha will
+    // use that error as the reason the test failed.
+    describe('falsey', function () {
+      it('should throw if "Getter" is in the stack trace', function () {
+        err(function () {
+          err(function fakeGetter () {
+            throw Error('my stack trace contains a fake implementation frame');
+          });
+        }, /implementation frames not properly filtered from stack trace/, true);
+      });
+
+      it('should throw if "Wrapper" is in the stack trace', function () {
+        err(function () {
+          err(function fakeWrapper () {
+            throw Error('my stack trace contains a fake implementation frame');
+          });
+        }, /implementation frames not properly filtered from stack trace/, true);
+      });
+
+      it('should throw if "assert" is in the stack trace', function () {
+        err(function () {
+          err(function assertFake () {
+            throw Error('my stack trace contains a fake implementation frame');
+          });
+        }, /implementation frames not properly filtered from stack trace/, true);
+      });
+
+      it('shouldn\'t throw if "Getter", "Wrapper", "assert" aren\'t in the stack trace', function () {
+        err(function safeFnName () {
+          throw Error('my stack trace doesn\'t contain implementation frames');
+        });
+      });
+    });
+
+    describe('truthy', function () {
+      it('shouldn\'t throw if "Getter" is in the stack trace', function () {
+        err(function fakeGetter () {
+          throw Error('my stack trace contains a fake implementation frame');
+        }, undefined, true);
+      });
+
+      it('shouldn\'t throw if "Wrapper" is in the stack trace', function () {
+        err(function fakeWrapper () {
+          throw Error('my stack trace contains a fake implementation frame');
+        }, undefined, true);
+      });
+
+      it('shouldn\'t throw if "assert" is in the stack trace', function () {
+        err(function assertFake () {
+          throw Error('my stack trace contains a fake implementation frame');
+        }, undefined, true);
+      });
+
+      it('shouldn\'t throw if "Getter", "Wrapper", "assert" aren\'t in the stack trace', function () {
+        err(function safeFnName () {
+          throw Error('my stack trace doesn\'t contain implementation frames');
+        }, undefined, true);
+      });
+    });
+  });
 });
diff --git a/test/should.js b/test/should.js
index 52d2c1cbb..1ac2e6d4c 100644
--- a/test/should.js
+++ b/test/should.js
@@ -55,73 +55,73 @@ describe('should', function() {
       it('throws when invalid property follows should', function () {
         err(function () {
           (42).should.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows language chain', function () {
         err(function () {
           (42).should.to.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows property assertion', function () {
         err(function () {
           (42).should.ok.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows overwritten property assertion', function () {
         err(function () {
           (42).should.tmpProperty.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows uncalled method assertion', function () {
         err(function () {
           (42).should.equal.pizza;
-        }, 'Invalid Chai property: equal.pizza. See docs for proper usage of "equal".');
+        }, 'Invalid Chai property: equal.pizza. See docs for proper usage of "equal".', true);
       });
 
       it('throws when invalid property follows called method assertion', function () {
         err(function () {
           (42).should.equal(42).pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows uncalled overwritten method assertion', function () {
         err(function () {
           (42).should.tmpMethod.pizza;
-        }, 'Invalid Chai property: tmpMethod.pizza. See docs for proper usage of "tmpMethod".');
+        }, 'Invalid Chai property: tmpMethod.pizza. See docs for proper usage of "tmpMethod".', true);
       });
 
       it('throws when invalid property follows called overwritten method assertion', function () {
         err(function () {
           (42).should.tmpMethod().pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows uncalled chainable method assertion', function () {
         err(function () {
           (42).should.a.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows called chainable method assertion', function () {
         err(function () {
           (42).should.a('number').pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows uncalled overwritten chainable method assertion', function () {
         err(function () {
           (42).should.tmpChainableMethod.pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('throws when invalid property follows called overwritten chainable method assertion', function () {
         err(function () {
           (42).should.tmpChainableMethod().pizza;
-        }, 'Invalid Chai property: pizza');
+        }, 'Invalid Chai property: pizza', true);
       });
 
       it('doesn\'t throw if invalid property is excluded via config', function () {
@@ -162,7 +162,7 @@ describe('should', function() {
       it('throws when `.length` follows uncalled method assertion', function () {
         err(function () {
           ('foo').should.equal.length;
-        }, 'Invalid Chai property: equal.length. See docs for proper usage of "equal".');
+        }, 'Invalid Chai property: equal.length. See docs for proper usage of "equal".', true);
       });
 
       it('doesn\'t throw when `.length` follows called method assertion', function () {
@@ -174,7 +174,7 @@ describe('should', function() {
       it('throws when `.length` follows uncalled overwritten method assertion', function () {
         err(function () {
           ('foo').should.tmpMethod.length;
-        }, 'Invalid Chai property: tmpMethod.length. See docs for proper usage of "tmpMethod".');
+        }, 'Invalid Chai property: tmpMethod.length. See docs for proper usage of "tmpMethod".', true);
       });
 
       it('doesn\'t throw when `.length` follows called overwritten method assertion', function () {
@@ -186,7 +186,7 @@ describe('should', function() {
       it('throws when `.length` follows uncalled chainable method assertion', function () {
         err(function () {
           ('foo').should.a.length;
-        }, 'Invalid Chai property: a.length. Due to a compatibility issue, "length" cannot directly follow "a". Use "a.lengthOf" instead.');
+        }, 'Invalid Chai property: a.length. Due to a compatibility issue, "length" cannot directly follow "a". Use "a.lengthOf" instead.', true);
       });
 
       it('doesn\'t throw when `.length` follows called chainable method assertion', function () {
@@ -198,7 +198,7 @@ describe('should', function() {
       it('throws when `.length` follows uncalled overwritten chainable method assertion', function () {
         err(function () {
           ('foo').should.tmpChainableMethod.length;
-        }, 'Invalid Chai property: tmpChainableMethod.length. Due to a compatibility issue, "length" cannot directly follow "tmpChainableMethod". Use "tmpChainableMethod.lengthOf" instead.');
+        }, 'Invalid Chai property: tmpChainableMethod.length. Due to a compatibility issue, "length" cannot directly follow "tmpChainableMethod". Use "tmpChainableMethod.lengthOf" instead.', true);
       });
 
       it('doesn\'t throw when `.length` follows called overwritten chainable method assertion', function () {
@@ -453,7 +453,7 @@ describe('should', function() {
       var t = new Thing();
       Thing.prototype = 1337;
       t.should.be.an.instanceof(Thing);
-    }, expectedError);
+    }, expectedError, true);
 
     if (typeof Symbol !== 'undefined' && typeof Symbol.hasInstance !== 'undefined') {
         err(function(){
diff --git a/test/utilities.js b/test/utilities.js
index c2399c8e7..58b43c48e 100644
--- a/test/utilities.js
+++ b/test/utilities.js
@@ -37,28 +37,54 @@ describe('utilities', function () {
     var foo = 'bar';
 
     chai.use(function (_chai, utils) {
-      var obj = {};
+      var target = {};
       var test = function() {};
 
-      var assertion = new chai.Assertion({}, "message", test);
+      var assertion = new chai.Assertion(target, "message", test, true);
       var flag = {};
-      utils.flag(obj, 'flagMe', flag);
-      utils.flag(obj, 'negate', true);
-      utils.transferFlags(test, obj, false);
+      utils.flag(assertion, 'flagMe', flag);
+      utils.flag(assertion, 'negate', true);
+      var obj = {};
+      utils.transferFlags(assertion, obj, false);
 
       expect(utils.flag(obj, 'object')).to.equal(undefined);
       expect(utils.flag(obj, 'message')).to.equal(undefined);
       expect(utils.flag(obj, 'ssfi')).to.equal(undefined);
+      expect(utils.flag(obj, 'lockSsfi')).to.equal(undefined);
+      expect(utils.flag(obj, 'negate')).to.equal(true);
+      expect(utils.flag(obj, 'flagMe')).to.equal(flag);
+    });
+  });
+
+  it('transferFlags, includeAll = true', function () {
+    var foo = 'bar';
+
+    chai.use(function (_chai, utils) {
+      var target = {};
+      var test = function() {};
+
+      var assertion = new chai.Assertion(target, "message", test, true);
+      var flag = {};
+      utils.flag(assertion, 'flagMe', flag);
+      utils.flag(assertion, 'negate', true);
+      var obj = {};
+      utils.transferFlags(assertion, obj, true);
+
+      expect(utils.flag(obj, 'object')).to.equal(target);
+      expect(utils.flag(obj, 'message')).to.equal("message");
+      expect(utils.flag(obj, 'ssfi')).to.equal(test);
+      expect(utils.flag(obj, 'lockSsfi')).to.equal(true);
       expect(utils.flag(obj, 'negate')).to.equal(true);
       expect(utils.flag(obj, 'flagMe')).to.equal(flag);
     });
   });
 
   describe('addMethod', function() {
-    var assertionConstructor;
+    var assertionConstructor, utils;
 
     before(function() {
-      chai.use(function(_chai, utils) {
+      chai.use(function(_chai, _utils) {
+        utils = _utils;
         assertionConstructor = _chai.Assertion;
 
         expect(_chai.Assertion).to.not.respondTo('eqqqual');
@@ -128,16 +154,39 @@ describe('utilities', function () {
       var anotherAssertion = expect([1, 2, 3]).to.have.a.lengthOf(3).and.to.be.ok;
       expect(anotherAssertion.length.constructor).to.equal(assertionConstructor);
     });
+
+    it('addMethod sets `ssfi` when `lockSsfi` isn\'t set', function () {
+      var origAssertion = expect(1);
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      var newAssertion = origAssertion.eqqqual(1);
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.not.equal(newSsfi);
+    }); 
+
+    it('addMethod doesn\'t set `ssfi` when `lockSsfi` is set', function () {
+      var origAssertion = expect(1);
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      utils.flag(origAssertion, 'lockSsfi', true);
+
+      var newAssertion = origAssertion.eqqqual(1);
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.equal(newSsfi);
+    });
   });
 
   describe('overwriteMethod', function () {
-    var assertionConstructor;
+    var assertionConstructor, utils;
 
     before(function() {
       chai.config.includeStack = false;
 
-      chai.use(function(_chai, utils) {
+      chai.use(function(_chai, _utils) {
         assertionConstructor = _chai.Assertion;
+        utils = _utils;
 
         _chai.Assertion.addMethod('four', function() {
           this.assert(this._obj === 4, 'expected #{this} to be 4', 'expected #{this} to not be 4', 4);
@@ -290,13 +339,37 @@ describe('utilities', function () {
       // Ensure that foo returns an Assertion (not a function)
       expect(expect('four').four()).to.be.an.instanceOf(assertionConstructor);
     });
+
+    it('overwriteMethod sets `ssfi` when `lockSsfi` isn\'t set', function () {
+      var origAssertion = expect(4);
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      var newAssertion = origAssertion.four();
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.not.equal(newSsfi);
+    }); 
+
+    it('overwriteMethod doesn\'t set `ssfi` when `lockSsfi` is set', function () {
+      var origAssertion = expect(4);
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      utils.flag(origAssertion, 'lockSsfi', true);
+
+      var newAssertion = origAssertion.four();
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.equal(newSsfi);
+    });
   });
 
   describe('addProperty', function() {
     var assertionConstructor = chai.Assertion;
+    var utils;
 
     before(function() {
-      chai.use(function (_chai, utils) {
+      chai.use(function (_chai, _utils) {
+        utils = _utils;
         assertionConstructor = _chai.Assertion;
 
         _chai.Assertion.addProperty('tea', function () {
@@ -362,16 +435,39 @@ describe('utilities', function () {
       expect(expect([1, 2, 3]).be).to.be.an.instanceOf(assertionConstructor);
       expect(expect([1, 2, 3]).thing).to.be.an.instanceOf(assertionConstructor);
     });
+
+    it('addProperty sets `ssfi` when `lockSsfi` isn\'t set', function () {
+      var origAssertion = expect(1);
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      var newAssertion = origAssertion.to.be.tea;
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.not.equal(newSsfi);
+    }); 
+
+    it('addProperty doesn\'t set `ssfi` when `lockSsfi` is set', function () {
+      var origAssertion = expect(1);
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      utils.flag(origAssertion, 'lockSsfi', true);
+
+      var newAssertion = origAssertion.to.be.tea;
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.equal(newSsfi);
+    });
   });
 
   describe('overwriteProperty', function () {
-    var assertionConstructor;
+    var assertionConstructor, utils;
 
     before(function() {
       chai.config.includeStack = false;
 
-      chai.use(function(_chai, utils) {
+      chai.use(function(_chai, _utils) {
         assertionConstructor = _chai.Assertion;
+        utils = _utils;
 
         _chai.Assertion.addProperty('tea', function () {
           utils.flag(this, 'tea', 'chai');
@@ -499,6 +595,28 @@ describe('utilities', function () {
       expect(expect([1, 2, 3]).be).to.be.an.instanceOf(assertionConstructor);
       expect(expect([1, 2, 3]).foo).to.be.an.instanceOf(assertionConstructor);
     });
+
+    it('overwriteProperty sets `ssfi` when `lockSsfi` isn\'t set', function () {
+      var origAssertion = expect(4);
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      var newAssertion = origAssertion.to.be.four;
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.not.equal(newSsfi);
+    }); 
+
+    it('overwriteProperty doesn\'t set `ssfi` when `lockSsfi` is set', function () {
+      var origAssertion = expect(4);
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      utils.flag(origAssertion, 'lockSsfi', true);
+
+      var newAssertion = origAssertion.to.be.four;
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.equal(newSsfi);
+    });
   });
 
   it('getMessage', function () {
@@ -699,11 +817,12 @@ describe('utilities', function () {
   });
 
   describe('addChainableMethod', function() {
-    var assertionConstructor;
+    var assertionConstructor, utils;
 
     before(function() {
-      chai.use(function (_chai, utils) {
+      chai.use(function (_chai, _utils) {
         assertionConstructor = _chai.Assertion;
+        utils = _utils;
 
         _chai.Assertion.addChainableMethod('x',
           function () {
@@ -782,6 +901,28 @@ describe('utilities', function () {
       // Ensure that foo returns an Assertion (not a function)
       expect(expect('bar').foo('bar')).to.be.an.instanceOf(assertionConstructor);
     });
+
+    it('addChainableMethod sets `ssfi` when `lockSsfi` isn\'t set', function () {
+      var origAssertion = expect('x');
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      var newAssertion = origAssertion.to.be.x();
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.not.equal(newSsfi);
+    }); 
+
+    it('addChainableMethod doesn\'t set `ssfi` when `lockSsfi` is set', function () {
+      var origAssertion = expect('x');
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      utils.flag(origAssertion, 'lockSsfi', true);
+
+      var newAssertion = origAssertion.to.be.x();
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.equal(newSsfi);
+    });
   });
 
   describe('overwriteChainableMethod', function() {
@@ -887,6 +1028,28 @@ describe('utilities', function () {
         expect(expect('x').x).to.be.an.instanceOf(assertionConstructor);
       }
     });
+
+    it('overwriteChainableMethod sets `ssfi` when `lockSsfi` isn\'t set', function () {
+      var origAssertion = expect('x');
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      var newAssertion = origAssertion.to.be.x();
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.not.equal(newSsfi);
+    }); 
+
+    it('overwriteChainableMethod doesn\'t set `ssfi` when `lockSsfi` is set', function () {
+      var origAssertion = expect('x');
+      var origSsfi = utils.flag(origAssertion, 'ssfi');
+
+      utils.flag(origAssertion, 'lockSsfi', true);
+
+      var newAssertion = origAssertion.to.be.x();
+      var newSsfi = utils.flag(newAssertion, 'ssfi');
+
+      expect(origSsfi).to.equal(newSsfi);
+    });
   });
 
   it('compareByInspect', function () {