I work in Canada a lot. What they are looking for is that certain forms of "work" have to pay tax on whatever work is done. If it something where you are performing some sort of labor they will make you pay the tax / fee to perform this type of work. Its better to have an invitation letter stating what you're actually doing. I've been going up every few weeks to do a training class, the agent saw that and was not happy with me. They even yelled at me for taking jobs away from Canadian teachers, but I didn't have to pay the tax :)

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Even the "kids" heading up to Montreal from Vermont for the strip clubs when they're 19 get admitted for 6 months each time. There's no 30 day limit for any sort of visitor to Canada unless specifically limited by an immigration officer on a specific case-by-case basis. Please don't spread incorrect information if you're not learned enough to comment.

Yeah, don't lie. Worth pointing out that CBSA officers (in my experience) tend to be jerks and a PITA compared with their relatively friendly CBP counterparts.

The biggest perk of having NEXUS is getting to interact with a computer screen and not them.

If you are US national, you don't see the jerk CBP counterparts in action.

Going by air you will get an awful lot of passport stamps.

Wow, I've had the polar opposite experience!

It's a matter of perspective. Americans think the CBSA are the worst. Canadians feel the same way about CBP. Both are right from their perspective.

I retired from a position as a training consultant and trainer.

"I'm coming up to train some Canadian employees on cybersecurity issues for XYZ Company." Truthful, minimalist, appeals to patriotism and supporting Canadian jobs. Do have a letter of assignment or invitation.

Some of the border guards at the Blaine crossing (Seattle / Vancouver) can be very tough, unfriendly sticklers.

Terminology seems very important to the Canadian officials. Never say you are there for "work", always say "business". I always say I am going on a business trip with business meetings.

I've never had a letter of invitation to Canada, even when I was traveling there fairly frequently for Canada-based clients.

To be fair, OP to whom you're replying lists his location as Dublin, Ireland. I wouldn't assume the rules are the same for him as they are for Americans.

I've had nice guys and jerks on both sides of the border, so it really is in who you get, time of day, mood, any other incidents which may have occurred that day, etc.

Having traveled through both, as a national of a third country, my experience has always been worse with the US variety - far more questions, far more 'attitude', far fewer smiles, and the distinct impression that it would give them no greater pleasure than to find a reason to not let me in.

Well I travel on a Canadian passport and even then they are often jerks to me. Maybe it's because I say I live in the US now.

I used to work with a guy that did a bunch of consulting projects in Calgary and has similar experiences as me. As always YMMV.

I remember years ago a US national heading to Canada to give a training meeting. She gave them A BS reason for visiting Canada (as in visiting a relative) but when they saw the binders of training materials she had with her, they were not amused.

She was detained for a good number of hours..and came to the conclusion never to BS them again.

As a Canadian who occasionally travels to the US for business, I have learned that the key is to tell it in a way that conveys that the employment is in Canada and the business is in the US. If the CBP agent even sniffs something that might, possibly, maybe, sometime, could sort of be an employment-like activity, you will be turned back. If your employment is in Canada, but doing business in the US, it is permissible and you should get through without too much hassle. Having said that, I have been hassled over the most trivial of technicalities.

I believe more or less the same is true in reverse. Don't say anything that might suggest employment or that you are getting paid across the border or you will get flagged.