Sitecore Version: 9.0.2

Environment: Azure PaaS CD

We are trying to call /api/sitecore/controller/actionname with AJAX. It's working on the CM server but not on the CD server.

I disabled custom errors and removed HTTP error configuration in web.config, but it still redirects to the custom 404 page I've created.

It was moved to SPEAK routes in Sitecore 9 and unavailable on CD server. You can resolve it by adding this snippet to your routing configuration (if you really need this API route - best practice is to create your own routes for your API calls):

if (ConfigurationManager.AppSettings["role:define"] == "ContentDelivery")

 RouteTable.Routes.MapRoute("Sitecore.Speak.Commands", "api/sitecore/controller/action");

The /sitecore/api route is a default route added by Sitecore in configs that are specific to the content management aspects of the platform. These configs are not enabled for content delivery. As noted in x3mxray's answer, they can be reenabled, but I wouldn't recommend it.

Instead of reenabling the default routes for other functionality you (probably) don't need on your CD servers, I would recommend mapping a custom route to your method and not relying on the /api/sitecore prefix.

1. Create pipeline processor to register route

using System.Web.Mvc;
using System.Web.Routing;
using Sitecore.Pipelines;

public class RegisterWebApiRoutes

 public virtual void Process(PipelineArgs args)
 
 RouteTable.Routes.MapRoute("CustomApiRoute",
 "api/customaction",
 new controller = "ControllerName", action = "ActionName" ,
 new httpMethod = new HttpMethodConstraint("GET") 
 );
 

2. Create patch config to patch in your pipeline processor

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:role="http://www.sitecore.net/xmlconfig/role/">
 <sitecore role:require="Standalone OR ContentManagement OR ContentDelivery">
 <pipelines>
 <initialize>
 <processor type="Custom.Pipelines.Initialize.RegisterWebApiRoutes,Custom" patch:before="processor[@type='Sitecore.Mvc.Pipelines.Loader.InitializeRoutes, Sitecore.Mvc']"/>
 </initialize>
 </pipelines>
 </sitecore>
</configuration>

3. Update your code to reference this new route

Don't forget to update your code to use the new route!

I have experienced the same issue on a project which had a custom pipeline for 404 redirects. Can you look for a similar code in your .config files from CD server?

<pipelines>
 <httpRequestBegin>
 <processor type="MyCustomDLL.NotFoundHttpRequestProcessor, MyCustomDLL"
 patch:after="processor[@type='Sitecore.Pipelines.HttpRequest.ItemResolver, Sitecore.Kernel']">
 </processor>
 </httpRequestBegin>
</pipelines>

404 response is hard-coded on the pipeline:

HttpContext.Current.Response.StatusCode = 404;
HttpContext.Current.Response.TrySkipIisCustomErrors = true;
HttpContext.Current.Response.StatusDescription = "404 File Not Found";

If that is your case, a possible solution would be checking which URL is being requested in your custom pipeline.
e.g.:

if (args.RequestUrl.LocalPath.StartsWith("/api/sitecore", StringComparison.InvariantCultureIgnoreCase))

 return;