When a marketing agency outsources some work to a design company, what are the main points to clarify in a collaboration agreement? Would it be fair to ask the designer not to post work he did on his public portfolio?

Maybe instead of looking at it from a viewpoint of "fair", look at it from a viewpoint of "necessary". Your clients come to you for a solution.  It is assumed that if they are willing to hire you, you must be offering some real value.  This real value may be in customer service, knowledge, creative strategy, work ethic, network power, etc..  Why exactly do they care who actually provides YOU assistance in their ultimate desire for solutions? From this viewpoint, you have a necessity to protect yourself from a designer approaching your clients (but this might mean you need to find more professional designers)... and you have a necessity to protect service offerings.  But Im not sure there is any necessity to keep a designer or design agency from showing work that they do for you.  I would assume that your service offerings are not identical to the firm you would contract to provide one small part of the complete service bundle.  This would be like Ford going to GM to have them manufacture a Ford vehicle.... this doesn't make much sense. Remember, the relationship that exist between your entity and the designer is one of client and provider.  Your client does not belong to the designer.  YOU are the designers client.  What it sounds like is you are thinking about ensuring that a design agency doesn't post work they do for you because your clientele might find out that you dont actually provide any of the final work product, but instead just mark up the work product of others.  If this is truly the case, meaning, you just fwd emails and messages to someone else and arent actually involved in servicing your client or being apart of the solutions, then yes... its necessary that you hide the fact that there isnt much of anything that your marketing agency does other than mark-up service fees. Approaching the agreements from this standpoint is just building a business on the predication that you're providing services for clients that you actually aren't, and you want to make sure no one else finds out about it... which might also mean youre telling potential clients that youre doing this inhouse and a necessity exists to ensure lies arent uncovered. <-- I know this seems a bit harsh, its not intended to be that way, its just an honest assessment. However... I wouldnt look at it this way.... for no other reason than its not necessary IF this isnt actually true. What might be a more value driven approach is to honestly tell your clients that you have built an extensive creative network and your strength is in your creative direction, project management, research, experience, strategic planning, budgeting and excellent customer service...  like I said.. why exactly would they care if this involves contract freelancers in-house, full time employees, or contract design work external to your firm?  The creative production of solutions is one small part of a large stable of valuable services..  And honestly, why wouldnt you tell them "We have a powerful roster of designers, if we cant do it in-house, we'll bring in someone who can". This is something that demonstrates wisdom, experience, and a dedication to client solutions.  AND... it keeps you from having to lie to everyone. If the client wanted to do the research themselves they would, if they wanted or were capable of doing the creative strategy they would, if they wanted to manage a design team they would....  instead they want you to handle it for all of the reasons listed above.. In this scenario, is it necessary to try and keep anyone from showcasing any work that was done for you on behalf of your clients?  Im not sure it is.  I think its probably wise that if any designer wanted to put this work on their portfolio that it include your branding on the examples, and clearly demonstrate that the work was done for you, under your lead and direction.  Most professional designers or design firms have no problem ensuring THEIR clients are taken care of, and you as their client would be no different. Requesting control over how work is presented, requesting the consideration of being asked before posting work to a portfolio, and if permission is granted how that work should be displayed are all reasonable and fairly normal in any collaborative agreement.  Asking if it would be fair to ask a designer NOT to post work completed for you on their public portfolio might be less important if you first considered if that would even be necessary. :)

Make sure that you have a NDA (Non-disclosure Agreement) and that it includes that one is not allowed to post work as part of his portfolio whether or not this design/service was approved by the company.