How do I effectively explain that I can use any stack?

Do companies bring freelancers in for interviews without there being some communication about the tech/stack for which they are looking for expertise? I get the OP's point that all of the tech/stacks are just tools in the box: you use the right tool for the job without having any sentimental or emotional attachment to it.

That said, if interviewers are skeptical about someone with multi-stack development you should be able to leverage that by emphasizing that seeing how problems are solved in different stacks gives you unique insight into the hidden gotchas that aren't apparent to devs wedded to a single stack. I did several years of contracting (all in C#) and was able to explain that I had seen Entity Framework implemented in five different ways and could speak the advantages/disadvantages of each set of implementation decisions (and would be happy to mentor the team on such matters).

Ultimately the business needs a problem solved and you can solve the problem. To build on the analogy of another commenter: you can bake the bread they need whether they have an electric oven, gas oven, wood-fired pizza oven or just a bonfire in an open field: you've done it all. Not only can you help them solve their immediate problem but you can advise them on how best to position them to avoid future problems they don't realize they are about to run into because in 20 years you've pretty much seen it all.

Yeah, I doubt you can still use tech you used 5 years ago, never mind 10 or 20.

> But when I explain this in interviews I feel like hardly anyone buys it.

If the interviewer wants to talk about one stack, that's likely the one you'll be using for the job, or the one they have experience with. And please don't tell them "but I can get up to speed in no time in any stack", nobody wants to pay a freelancer to get up to speed.

Show, don’t tell.

If you are getting interviewed like this for freelance gigs someone is doing it wrong. I have freelanced for 15 years leveraging 40 years programming experience. I mostly do web dev. I never do tech interviews or prove I “know” some stack or framework. I focus on the business problems, figure out how to fix or address those.

As a freelancer my job is to immediately solve problems and add business value, not to do jerk-off tech interviews. If I don’t deliver I don’t get paid, pretty simple incentive structure.

You should be driving the process to get hired, not coming in as a temp.

Take an informed guess at the stack they want and use that example to make the general case.

Interviewer: What method do you use to bake an apple pie?

Chef: For a traditional pie, I'll go with Granny Smith apples and cut ice cold shortening into the flour for a light flaky crust. That said, some of my customers like the fuller flavor of Macintosh apples, which need a bit less cooking time and more lemon to balance the sweetness. One of my regulars orders their pies parbaked on graham cracker crust, which required X Y and Z changes from the traditional method.

don't sell that story, it isn't that believable regardless of whether or not it's true.

instead just talk about the thing that they're trying to corner you into talking about and get over the feeling of silliness. they're prompting you with that stuff because that's their desired focus of discussion, it's no use struggling against that tide.

don’t. speak to the audience and focus on relevant topics, which is likely their current stack and your engineering expertise.