diff --git a/web/modules/custom/server_general/server_general.module b/web/modules/custom/server_general/server_general.module
index 795c8c19f..c4be045a7 100644
--- a/web/modules/custom/server_general/server_general.module
+++ b/web/modules/custom/server_general/server_general.module
@@ -5,6 +5,7 @@
  * Module file.
  */
 
+use Drupal\Component\Utility\Bytes;
 use Drupal\Core\Access\AccessResult;
 use Drupal\Core\Entity\ContentEntityFormInterface;
 use Drupal\Core\Entity\EntityInterface;
@@ -373,4 +374,31 @@ function server_general_page_attachments(array &$attachments) {
     ];
     $attachments['#attached']['html_head'][] = [$noindex_tag, 'noindex_error_pages'];
   }
+
+  // Attach file size validation library and settings for client-side
+  // validation of file uploads.
+  $attachments['#attached']['library'][] = 'server_theme/file-size-validation';
+  $attachments['#attached']['drupalSettings']['fileSizeValidation'] = [
+    'maxFileSize' => server_general_get_max_upload_size(),
+  ];
+}
+
+/**
+ * Get the maximum file upload size in bytes.
+ *
+ * Returns the smaller of upload_max_filesize and post_max_size PHP settings.
+ *
+ * @return int
+ *   Maximum upload size in bytes.
+ */
+function server_general_get_max_upload_size(): int {
+  $upload_max = Bytes::toNumber(ini_get('upload_max_filesize'));
+  $post_max = Bytes::toNumber(ini_get('post_max_size'));
+
+  // post_max_size of 0 means unlimited.
+  if ($post_max == 0) {
+    return (int) $upload_max;
+  }
+
+  return (int) min($upload_max, $post_max);
 }
diff --git a/web/themes/custom/server_theme/server_theme.libraries.yml b/web/themes/custom/server_theme/server_theme.libraries.yml
index 6b05401da..2fb3a046f 100644
--- a/web/themes/custom/server_theme/server_theme.libraries.yml
+++ b/web/themes/custom/server_theme/server_theme.libraries.yml
@@ -87,3 +87,12 @@ expanding-text:
     - core/drupal
     - core/jquery
     - core/once
+
+file-size-validation:
+  js:
+    dist/js/file-size-validation.js: {}
+  dependencies:
+    - core/drupal
+    - core/drupalSettings
+    - core/jquery
+    - core/once
diff --git a/web/themes/custom/server_theme/src/js/file-size-validation.js b/web/themes/custom/server_theme/src/js/file-size-validation.js
new file mode 100644
index 000000000..4f41102ab
--- /dev/null
+++ b/web/themes/custom/server_theme/src/js/file-size-validation.js
@@ -0,0 +1,85 @@
+/**
+ * @file
+ * Client-side file size validation to prevent upload errors.
+ *
+ * Validates file size before upload to show a user-friendly error message
+ * instead of cryptic AJAX errors when files exceed PHP limits.
+ */
+(function ($, Drupal, once) {
+
+  /**
+   * Validates file size before upload.
+   *
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.fileSizeValidation = {
+    attach: function (context, settings) {
+      const maxFileSize = settings.fileSizeValidation?.maxFileSize;
+      if (!maxFileSize) {
+        return;
+      }
+
+      const $fileInputs = $(
+        once('file-size-validate', 'input[type="file"]', context)
+      );
+
+      if (!$fileInputs.length) {
+        return;
+      }
+
+      $fileInputs.on('change.fileSizeValidation', function (event) {
+        const input = this;
+        const files = input.files;
+
+        if (!files || !files.length) {
+          return;
+        }
+
+        // Remove any previous file size errors.
+        $(input)
+          .closest('div.js-form-managed-file, .form-item')
+          .find('.file-size-error')
+          .remove();
+
+        // Check each file's size.
+        for (let i = 0; i < files.length; i++) {
+          const file = files[i];
+          if (file.size > maxFileSize) {
+            const fileSizeMB = (file.size / (1024 * 1024)).toFixed(2);
+            const maxSizeMB = (maxFileSize / (1024 * 1024)).toFixed(0);
+
+            const error = Drupal.t(
+              'The file "@filename" (@filesize MB) exceeds the maximum upload size of @maxsize MB. Please choose a smaller file.',
+              {
+                '@filename': file.name,
+                '@filesize': fileSizeMB,
+                '@maxsize': maxSizeMB,
+              }
+            );
+
+            $(input)
+              .closest('div.js-form-managed-file, .form-item')
+              .prepend(
+                '<div class="messages messages--error file-size-error" aria-live="polite">' + error + '</div>'
+              );
+
+            // Clear the file input to prevent upload attempt.
+            input.value = '';
+
+            // Stop processing and prevent upload.
+            event.stopImmediatePropagation();
+            return;
+          }
+        }
+      });
+    },
+    detach: function (context, settings, trigger) {
+      if (trigger === 'unload') {
+        $(once.remove('file-size-validate', 'input[type="file"]', context)).off(
+          '.fileSizeValidation'
+        );
+      }
+    },
+  };
+
+})(jQuery, Drupal, once);