WordPress REST API: Large Image Uploads Not Reaching Backend Despite Proper PHP Settings

Problem Description:

I'm developing a chat application with image upload functionality using WordPress REST API. When users attach multiple large images (3 images), the request doesn't reach the backend API endpoint, but smaller images work fine.

Key Symptoms:

• Small 3 images work without errors
• Large images don't trigger any backend logs or errors
• No WordPress debug logs generated
• Server provider confirms no server-side issues found

Environment

• WordPress: 6.0+
• PHP: 8.2
• Server: Apache (shared hosting)

PHP Settings:

• upload_max_filesize = 600M
• post_max_size = 650M
• max_execution_time = 500
• max_input_time = 500
• memory_limit = 300M
• max_input_vars = 3000

Frontend JavaScript Code

function sendImageRequest(question, imagesArray, country, conversationHistory) {
    console.log('🖼️ Starting image analysis request with', imagesArray.length, 'images...');
    
    // Prepare image data array with pure base64
    const imageDataArray = imagesArray.map(img => {
      let pureBase64 = img.data;
      if (img.data.startsWith('data:')) {
        pureBase64 = img.data.split(',')[1];
      }
      return {
        id: img.id,
        data: pureBase64,  // Base64 string
        name: img.name
      };
    });
    
    const requestBody = {
      question: question,
      image_data: imageDataArray, // Array of base64 images
      country: country,
      conversation_history: conversationHistory
    };
    
    console.log('📤 Image request payload with', imageDataArray.length, 'images, total size:', JSON.stringify(requestBody).length, 'bytes');
    
    $.ajax({
      url: '/wp-json/chat2find/v1/image',
      method: 'POST',
      contentType: 'application/json',
      data: JSON.stringify(requestBody),
      success: function(response) {
        console.log('✅ Image analysis successful');
      },
      error: function(xhr, status, error) {
        console.error('❌ Image analysis error:', error);
      }
    });
}

Backend WordPress PHP Code

class API_Handler {
public function handle_image_request(WP_REST_Request $request) {
        try {
            error_log('🎯 Image endpoint reached'); // This never fires for large images
            
            $params = $request->get_params();
            $question = sanitize_text_field($params['question'] ?? '');
            $image_data = $params['image_data'] ?? [];
            $country = sanitize_text_field($params['country'] ?? '');
            $conversation_history = $params['conversation_history'] ?? [];
            
            if (empty($question)) {
                return new WP_Error('invalid_input', 'Question is required', array('status' => 400));
            }
            
            if (empty($image_data) || !is_array($image_data)) {
                return new WP_Error('invalid_input', 'Image data array is required', array('status' => 400));
            }
            
            // Validate each image data
            foreach ($image_data as $img) {
                $img_data = is_array($img) ? ($img['data'] ?? '') : $img;
                if (!preg_match('/^[a-zA-Z0-9+\/]+={0,2}$/', $img_data)) {
                    return new WP_Error('invalid_input', 'Invalid image data format', array('status' => 400));
                }
            }
            
            // Limit to maximum 3 images
            if (count($image_data) > 3) {
                return new WP_Error('invalid_input', 'Maximum 3 images allowed', array('status' => 400));
            }
            
            // External API call
            $api_endpoint = get_option('chat2find_api_endpoint', 'https://example.com/api/');
            $image_url = rtrim($api_endpoint, '/') . '/image';
            
            $response = $this->make_api_call($image_url, [
                'question' => $question,
                'image_data' => $image_data,
                'country' => $country,
                'conversation_history' => $conversation_history
            ], false);
            
            if (is_wp_error($response)) {
                error_log('Chat2Find Image API Error: ' . $response->get_error_message());
                return new WP_Error('api_error', 'Image analysis service unavailable', array('status' => 503));
            }
            
            $response_data = json_decode($response, true);
            return new WP_REST_Response($response_data, 200);
            
        } catch (Exception $e) {
            error_log('Chat2Find Image Exception: ' . $e->getMessage());
            return new WP_Error('internal_error', 'Internal server error', array('status' => 500));
        }
    }
    
    public function register_routes() {
        register_rest_route('chat2find/v1', '/image', array(
            'methods' => 'POST',
            'callback' => array($this, 'handle_image_request'),
            'permission_callback' => array($this, 'check_permission')
        ));
    }
    
    public function check_permission() {
        return true;
    }

Debugging Attempts

WordPress Debug Logging

// In wp-config.php
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

Observations

Small images (~100 kb each): works

Large images (~3MB each): Request size ~12-15MB, never reaches backend

Network tab: Shows request being sent but no response received

No server errors in Apache/WordPress logs

No PHP errors even with debugging enabled

Question

What could be silently blocking large JSON requests before they reach WordPress? How do I fix this?