What is the Mozilla Compatible Agent?

Written by Jamie Courtnell on Tuesday, 30 April 2013. Posted in SEO

What is the Mozilla Compatible Agent?


If you use Google analytics there is a chance your may have come across the Mozilla Compatible Agent, which may cause a large over inflation in certain GA metrics and skew your SEO decisions.

After carrying out a analysis of my website analytics, I noticed a huge spike in visitors on a random week day, nothing happening in the news or industry that should have caused this variation so I decided to dissect the visits and discovered a problem. I only discovered the Mozilla Compatible Agent bug last week, and after looking into it I found it is quite a wide spread problem.

How it effects your analytics?

The bug doesn't actually 'do' anything negative to your site or analytics account that can't be solved (more on that later) but if you fail to notice it, it can ruin some key analytics data, bounce rate, direct visitor numbers and time on page etc.

What is the MCA?

As the famous Sun Tzu misquote goes “In order to beat your enemy, you must first know your enemy”. Although, the problem in this case is its quite a tricky one. Some webmasters are seeing the Mozilla Agent as legitimate traffic while others are seeing it as quite questionable behaviour.

The general consensus seems to be that it is likely that this may be a form of crawl bot that is not recognised correctly by GA. However, before you apply a full fix to the problem, use the tactics below to identify whether or not it represents relevant traffic or a bug/bot.

How to find if the MCA is legitimate?

I first noticed it as a large spike in visitor numbers, dwarfing my 'non-promotional' visits (daily visits when no emails campaigns/PPC/Social pushes are happening).

Google analytics spikeAlthough it doesn't look like too much of a leap, it represents around 50 extra visitors on the middle day. Usually I may not have looked any further into this except it represented the record number of visits so I was obliged to discover what caused it.


An anomaly is obvious as soon as you look at the visits on the hourly basis. A huge spike at 4pm which is unheard of in my industry (Visits usually taper out from 4 onwards). So now it was time to look at the sources to see where this traffic is coming from.

Direct traffic is much higher than normal, but more worryingly, looking close you can see that average visit duration is low and bounce rates are inflated.

It took me a minute or two to realise that sorting the dimensions by 'browser' would show the cause of the skewed metrics.

I also wanted to check that if this was an isolated problem and whether or not Mozilla Compatible Agent was a browser from an other traffic source.

As you can see, the MCA bug has been effecting my site for the past 8 months and throughout this time the it has been skewing the bounce, time on page and pages/visit metrics (albeit minimally). Yet I am still confused as to why there was such a massive spike in February.

However, this confirms to me, that unlike other users, the Mozilla Compatible Agent represents only a bug and not legitimate traffic. Enabling us to make a fix and remove its adverse effects on our GA metrics.

How to remove the bug

Now we have identified that it is not a legit source, we can add a filter to remove the MCA results from our analytics account.


To build the filter, follow: Admin > Profile > Filters > New Filter

Name your filter as you wish, choose 'custom filter', select exclude and from the Filter Field drop down menu choose 'browser' and add 'mozilla compatible agent' to the Filter Pattern.



The filter will not be effective straight away and may take a couple of days to show results, but should remove the Mozilla Compatible Agent from your analytics and enable you to make much more accurate decisions based on your visitor metrics.

Have you had any encounters with the Mozilla Compatible Agent? If so what are your thoughts on what it is?

About the Author

Jamie Courtnell

Jamie Courtnell

My name is Jamie Courtnell and I am one of the co-founders of Marketing My Journey. I am a digital marketing and SEO enthusiast and blogger who loves all things digital and branding. I first learned SEO while on my marketing degree placement and since graduating I have worked full time within the industry in an in-house role. Follow me on twitter and Google+..

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