jQuery Ajax returning 404 Error, but correct Response

When you include wp-blog-header.php, you end up bootstrapping the whole WordPress setup routine. The function wp() is called, which calls $wp->main(), which in turn calls various setup functions.

One of these is $wp->query_posts(), which calls $wp_the_query->query(), which in turn calls WP_Query's parse_query() function. I suspect that the 404 indication is generated in there (your AJAX page isn't a WP post, or anything like that), and is later transformed into an actual 404 response header by $wp->handle_404(), the function called after query_posts() in main().

I'm not 100% sure that parse_query() is the definite culprit, but I would suggest seeing if you can just include wp-load.php instead, since I believe it does the actual work of creating the objects that you want to access.

Again, I don't actually use WordPress, so I can't be sure, but looking at the source code this seems to be the most likely case, from what I can tell.

I've added an ajax.php file in a WordPress template once, and had this problem.

I solved it simply by adding at the top of ajax.php

header('Response: HTTP/1.1 200 OK');

Kind of a hack, but it worked.

No one else posted this as an answer, so it's worth noting. You should be including wp-load.php instead of wp-blog-header.php.

If you open up wp-blog-header.php you'll see why:

if ( !isset($wp_did_header) ) {    $wp_did_header = true;    require_once( dirname(__FILE__) . '/wp-load.php' );    wp();    require_once( ABSPATH . WPINC . '/template-loader.php' );}

If you are only outputting json for an AJAX operation, you do not need to include template-loader.php. This will create unnecessary overhead, and then of course provide the 404 error.

This 'workaround' is necessary for current and future versions of WordPress. I'm assuming anything past 3.0 should include wp-load.php as stated.