1. Your first dry run

You will start the service, tell it where a token is, and ask it to check an account and some image addresses without posting anything.

What you need

An Instagram professional account (Business or Creator).
A long-lived access token for it, from your own Meta app using Instagram API with Instagram Login. The token needs the permissions instagram_business_basic and instagram_business_content_publish. Getting one is Meta’s side of the work and their dashboard changes; their documentation is the place to follow it.
The account’s numeric user id (Meta’s token tools show it, and the service will tell you if the token belongs to a different one).
One or more JPEG images reachable by a public https address. Instagram’s servers fetch them, so a file on your laptop will not do.

Install

Either of these:

go install github.com/javadebadi/instagram-publisher/cmd/instagram-publisher@latest

or download the binary for your system from the releases page and make it executable.

Put the token where the service can read it

For a first try, an env file is simplest. Save this as .env.local:

IG_TOKEN=paste-the-long-lived-token-here

Keep that file out of git. (Page 4 shows where else a token can live.)

Start the service

instagram-publisher serve --addr 127.0.0.1:8080

Check it is up:

curl localhost:8080/health
# ok

Check the account

curl -s localhost:8080/account/check -d '{
  "token_ref": "envfile:.env.local#IG_TOKEN",
  "user_id": "YOUR_NUMERIC_USER_ID"
}'
# {"account":"yourname"}

token_ref is the pointer to the token, not the token: envfile: means “a line of a file”, then the path, then # and the name of the line. The service reads the file, asks Instagram who the token belongs to, and compares it with the user_id you gave. If you gave the wrong account it answers 409 instead of posting to the wrong place later.

Dry-run a carousel

curl -s localhost:8080/publish/carousel -d '{
  "token_ref": "envfile:.env.local#IG_TOKEN",
  "user_id": "YOUR_NUMERIC_USER_ID",
  "images": ["https://example.com/slides/1.jpg", "https://example.com/slides/2.jpg"],
  "caption": "My first post",
  "dry_run": true
}'
# {"dry_run":true,"account":"yourname","message":"dry run: would post 2 slide(s) to @yourname; nothing was posted"}

dry_run does everything except the post: it reads the token, checks the account and fetches every image address the way Instagram will (it must answer 200 and be served as image/jpeg). If a link is private, missing or served as the wrong type you find out here, with the address in the message.

Nothing is posted and nothing is stored. Next: post for real.