By default, the Client API is not turned on. Go to _services->manage services_ and give it a port to get it started. I recommend you not allow non-local connections (i.e. only requests from the same computer will work) to start with.
The Client API should start immediately. It will only be active while the client is open. To test it is running all correct (and assuming you used the default port of 45869), try loading this:
You should get a welcome page. By default, the Client API is HTTP, which means it is ok for communication on the same computer or across your home network (e.g. your computer's web browser talking to your computer's hydrus), but not secure for transmission across the internet (e.g. your phone to your home computer). You can turn on HTTPS, but due to technical complexities it will give itself a self-signed 'certificate', so the security is good but imperfect, and whatever is talking to it (e.g. your web browser looking at [https://127.0.0.1:45869](https://127.0.0.1:45869)) may need to add an exception.
The Client API is still experimental and sometimes not user friendly. If you want to talk to your home computer across the internet, you will need some networking experience. You'll need a static IP or reverse proxy service or dynamic domain solution like no-ip.org so your device can locate it, and potentially port-forwarding on your router to expose the port. If you have a way of hosting a domain and have a signed certificate (e.g. from [Let's Encrypt](https://letsencrypt.org/)), you can overwrite the client.crt and client.key files in your 'db' directory and HTTPS hydrus should host with those.
Once the API is running, go to its entry in _services->review services_. Each external program trying to access the API will need its own access key, which is the familiar 64-character hexadecimal used in many places in hydrus. You can enter the details manually from the review services panel and then copy/paste the key to your external program, or the program may have the ability to request its own access while a mini-dialog launched from the review services panel waits to catch the request.
* [Hydrus Companion](https://gitgud.io/prkc/hydrus-companion): a Chrome/Firefox extension for hydrus that allows easy download queueing as you browse and advanced login support
* [Hydrus Video Deduplicator](https://github.com/appleappleapplenanner/hydrus-video-deduplicator): Discovers duplicate videos in your client and queues them for the duplicate filter
* [Hydrus Archive Delete](https://gitgud.io/koto/hydrus-archive-delete): Archive/Delete filter in your web browser
* [tagrank](https://github.com/matjojo/tagrank): Shows you comparison images and cleverly ranks your favourite tag.
