Trying to make dinner plans with friend, who is in the wrong?

Stop 'implying' things. It's really annoying when people do that.
Be clear and concise about what you want / expect.
People aren't mind readers.

You need to be clear that the voucher can be used for all of you, they might be thinking you want to use it purely for yourself

I would assume if you said you had a voucher for 50% off that you were just offering to use your voucher. I wouldn’t assume you were skint and could only go if you use the voucher.
you need to be more clear in what you are suggesting. Not everyone thinks like you and would understand

I think you were actually pretty clear, OP. I would have understood your intent. I think your friend is being obtuse so she can control plans. Agree that reiterating is how you should resolve though. Good luck - hope you have a nice evening!

I agree with you. My EH communicates in this way where the expectation is to work out what he means with certain statements. It usually isn't clear and the addition of a few extra words would help massively.
It was a source of constant tension between us.

If you can only afford to eat out using a voucher then that needed to be explained.
Is the voucher half price for two meals? Then just use it with someone else who likes that place and might also have financial struggles. If it's just for you to get a discount then the motive for someone else is diminished rather.
If she's that close a friend you'd think it wouldn't be such a big issue. Though she must be very fond of Indian food to volunteer it twice in a row. But you can just order something different. Enjoy the company.

Well actually not this if the voucher is only taking 50%off one meal and she doesn't intend to share the savings...

You need to be a lot more clear. "Cost of living" does not impact everyone the same way. And given you told her you ate out last night, she'll be under the impression you can definitely afford to eat out.

You also need to clarify how you intend to use the voucher. Is it to take money off your meal or both of yours? Because any confusion on that will make everything worse when the bill comes.

This!

The lack of clarity was due to the OP thinking that the reasons behind her suggestions were implied:

"I was annoyed... because the reason I suggested this place is because I can't afford a full-priced meal right now. I didn't directly say that, but I figured with the cost of living it's implied"

and

"I just went to an Indian restaurant yesterday and don't want to eat the same food two days in a row... So I replied saying "I went to an Indian restaurant yesterday" again, thinking that the part that I don't want to have Indian food twice in a row is implied. But she didn't understand what I tried to say and just said "me too""

No, your messages weren't clear at all. People aren't mind readers. Your friend also had Indian yesterday. That didn't mean she didn't want it today so how can you think that you saying you had it yesterday makes it clear you don't want it today? Quite mean of you to allude to her not having common sense.

Hope you start to feel better soon OP - autistic burnout is really rough and it's exhausting trying to navigate all the do's and don'ts in life..
I thought you came across really lovely in your posts and very humble ; genuinely wanting to learn how to react etc. I relate and wish you the best xx

I've had this type of dynamic before.

I eventually worked out that I like cheaper cafe type restaurants where the owner runs it with their family as I feel the food is made with care and love.

And my friend likes places where the owner exploits employees as she thinks they have better food hygiene standards and she finds it easier to complain if things aren't quite as she likes them.

Now we've worked this out, I've decided to just tell her I'm no longer able to afford meals out as I'm prioritising other things- but all the faffing with her saying she wanted to eat or have a coffee out with me then trying to make me change venue last minute drove me potty.

TLDR: Tell her your sorry you have to cancel last minute, and don't make dinner plans with her again.

Did she specifically agree to the original restaurant? It's entirely possile she's forgotten I may have done that in the past or is throwing out an idea.

The answer is to respond, "Oh, can we stick with the orignal place - I'd like to save the money by using the voucher and I like it there, especially as I'm not a fan of Indian food two days in a row."

I don't see where the OP has been unclear.

She said "My friend invited me to dinner and I responded suggesting a restaurant and told her I have a half-price voucher for this restaurant. She agreed."

The friend already agreed to the suggested (voucher) restaurant.

I think it would have been better to be more upfront today though e.g. I would like to stick to [voucher restaurant] as agreed before.

Hi OP, I'm also ND and one of the things I struggle with is people not being absolutely clear in their messages, especially if it involves arrangements to do things with others, By the same token, I can see that I also need to be clear myself in my communication with others. You weren't really clear with your messages - would you be able to just call your friend and explain things? just say "sorry I wasn't clear, I was just hoping we could have a cheaper night, hence suggesting the voucher...". Maybe organise something verbally for another week if you're feeling burnt out too?

How did you plan to use the voucher OP?

Is the voucher for one meal only at half price and therefore only you would benefit? Or

Does the voucher allow all meals at half price so you both benefit ? Or

If the voucher only applies to one meal would you get the total bill, deduct the value of the voucher from overall bill and split the balance

What a muddle. More trouble than it's worth. If she invited you up to her to suggest a place. Or she should have asked for your suggestions. Ring her up

She might just have forgotten your previous conversation.

But your current replies being so unclear mean you have to send more message, not less and now you’re posting here about it, giving yourself even more messages to read and reply to.

Your friend is not a mind reader, get to the point and just be honest.