Return 406 Not Acceptable for unsupported Accept headers

README.md

@@ -181,6 +181,16 @@ add_filter('post_content_to_markdown/post_allowed', function ($allowed, $post) {
 
 **Default:** `true`
 
+#### `post_content_to_markdown/strict_accept`
+
+Controls whether the plugin returns `406 Not Acceptable` when the client's `Accept` header rules out every representation the site can serve (i.e. neither `text/html` nor `text/markdown` is acceptable). Enabled by default for spec-correct content negotiation. Disable if you'd rather always fall back to HTML for any `Accept` value.
+
+```php
+add_filter('post_content_to_markdown/strict_accept', '__return_false');
+```
+
+**Default:** `true`
+
 #### `post_content_to_markdown/emit_vary`
 
 Controls whether the plugin appends `Vary: Accept` to every front-end response. Enabled by default so that downstream caches (browsers, proxies, CDNs) key on the Accept header and don't cross-serve an HTML response to a Markdown request (or vice versa). Disable if you don't want the plugin touching HTML response headers — Markdown responses will still include `Vary: Accept` regardless, so content negotiation stays correct for direct Markdown hits.

post-content-to-markdown.php

@@ -91,6 +91,22 @@ function isMarkdownRequested()
     return $markdown > 0.0 && $markdown > $html;
 }
 
+/**
+ * Whether the client's Accept header allows any representation we can serve
+ * (text/html or text/markdown). Missing Accept counts as acceptable per
+ * RFC 9110 (absence means any type is acceptable).
+ */
+function isAcceptable()
+{
+    $accept = $_SERVER['HTTP_ACCEPT'] ?? '';
+    if ($accept === '') {
+        return true;
+    }
+
+    return acceptQuality($accept, 'text', 'html') > 0.0
+        || acceptQuality($accept, 'text', 'markdown') > 0.0;
+}
+
 /**
  * Tell caching plugins (WP Super Cache, etc.) to skip caching for markdown requests.
  * This runs early to ensure the constant is set before caching plugins check it.
@@ -118,6 +134,31 @@ function isMarkdownRequested()
     header('Vary: Accept', false);
 });
 
+/**
+ * Return 406 Not Acceptable when the client's Accept header rules out every
+ * representation we can serve. Disable via the strict_accept filter to fall
+ * back to HTML instead.
+ */
+add_action('template_redirect', function () {
+    if (is_feed() || is_admin()) {
+        return;
+    }
+
+    if (! apply_filters('post_content_to_markdown/strict_accept', true)) {
+        return;
+    }
+
+    if (isAcceptable()) {
+        return;
+    }
+
+    status_header(406);
+    header('Content-Type: text/plain; charset=utf-8');
+    header('Vary: Accept', false);
+    echo "406 Not Acceptable\n\nAvailable representations: text/html, text/markdown\n";
+    exit;
+}, 1);
+
 /**
  * Serve content as Markdown based on Accept header or query parameter
  */