diff --git a/WorkflowEngine.php b/WorkflowEngine.php
index 16f74e5..256de23 100644
--- a/WorkflowEngine.php
+++ b/WorkflowEngine.php
@@ -404,13 +404,20 @@ class WorkflowEngine {
         $stmt->execute([$instanceId, $eventType, $message, $nodeId, json_encode($payload), $userId]);
     }
 
-        public function getOrCreateInstanceByDefId(int $personId, int $processDefinitionId, int $userId): ?array {
+    public function getOrCreateInstanceByDefId(int $personId, int $processDefinitionId, int $userId): ?array {
+        if (!is_int($processDefinitionId) || $processDefinitionId <= 0) {
+            throw new InvalidArgumentException("processDefinitionId must be a positive integer.");
+        }
+        if (!is_int($personId) || $personId <= 0) {
+            throw new InvalidArgumentException("personId must be a positive integer.");
+        }
+
         $stmt = $this->pdo->prepare("SELECT * FROM process_instances WHERE `person_id` = ? AND `process_definition_id` = ?");
         $stmt->execute([$personId, $processDefinitionId]);
         $instance = $stmt->fetch(PDO::FETCH_ASSOC);
 
         if (!$instance) {
-            $stmt_def = $this->pdo->prepare("SELECT definition_json, code FROM process_definitions WHERE id = ?");
+            $stmt_def = $this->pdo->prepare("SELECT definition_json, code, is_active FROM process_definitions WHERE id = ?");
             $stmt_def->execute([$processDefinitionId]);
             $definition = $stmt_def->fetch(PDO::FETCH_ASSOC);
 
@@ -418,17 +425,24 @@ class WorkflowEngine {
                 throw new WorkflowNotFoundException("Process definition #$processDefinitionId not found.");
             }
 
-            $this->checkEligibility($personId, $definition);
+            if (empty($definition['is_active'])) {
+                throw new WorkflowNotAllowedException("Process is not active and cannot be started.");
+            }
+
+            $eligibility = $this->checkEligibility($personId, $processDefinitionId);
+            if (!$eligibility['is_eligible']) {
+                throw new WorkflowEligibilityException("Person is not eligible to start this process.", $eligibility['reasons']);
+            }
 
             $definition_json = !empty($definition['definition_json']) ? json_decode($definition['definition_json'], true) : [];
 
-            $processCode = ($definition_json && isset($definition_json['type']) && $definition_json['type'] === 'checklist') 
-                           ? (string) $processDefinitionId 
+            $processCode = ($definition_json && isset($definition_json['type']) && $definition_json['type'] === 'checklist')
+                           ? (string) $processDefinitionId
                            : $definition['code'];
 
-            if($processCode) {
+            if ($processCode) {
                 $instanceId = $this->startProcess($processCode, $personId, $userId);
-                if($instanceId) {
+                if ($instanceId) {
                     $stmt->execute([$personId, $processDefinitionId]);
                     $instance = $stmt->fetch(PDO::FETCH_ASSOC);
                 }
diff --git a/_init_single_instance.php b/_init_single_instance.php
index 60b5c26..9141324 100644
--- a/_init_single_instance.php
+++ b/_init_single_instance.php
@@ -1,29 +1,44 @@
 <?php
-session_start();
-require_once 'WorkflowEngine.php';
 require_once 'lib/ErrorHandler.php';
-require_once 'lib/WorkflowExceptions.php';
+register_error_handler();
+
+require_once 'db/config.php';
+require_once 'WorkflowEngine.php';
+
+if (session_status() == PHP_SESSION_NONE) {
+    session_start();
+}
+
+header('Content-Type: application/json');
 
 if (!isset($_SESSION['user_id'])) {
-    throw new WorkflowNotAllowedException('Authentication required.');
+    http_response_code(401);
+    echo json_encode(['error' => ['message' => 'Authentication required.']]);
+    exit;
 }
-$userId = $_SESSION['user_id']; 
 
-$personId = $_GET['personId'] ?? null;
-$processDefinitionId = $_GET['processId'] ?? null;
+$userId = $_SESSION['user_id'];
+$personId = filter_input(INPUT_POST, 'person_id', FILTER_VALIDATE_INT);
+$processDefinitionId = filter_input(INPUT_POST, 'process_id', FILTER_VALIDATE_INT);
 
 if (!$personId || !$processDefinitionId) {
-    throw new WorkflowRuleFailedException('Missing parameters for process initialization.');
+    // InvalidArgumentException will be caught by the handler and result in a 400 Bad Request
+    throw new InvalidArgumentException('Invalid or missing person_id or process_id.');
 }
 
 $engine = new WorkflowEngine();
+
+// The getOrCreateInstanceByDefId method is now responsible for all checks:
+// 1. Validating the process definition exists.
+// 2. Checking if the process is active.
+// 3. Checking if the person is eligible.
+// 4. Creating the instance if it doesn't exist.
+// It will throw specific exceptions (WorkflowNotFoundException, WorkflowNotAllowedException, WorkflowEligibilityException) which our ErrorHandler will turn into 404, 409, and 422 responses.
 $instance = $engine->getOrCreateInstanceByDefId($personId, $processDefinitionId, $userId);
 
 if ($instance) {
-    $_SESSION['success_message'] = "Process initialized successfully.";
+    echo json_encode(['success' => true, 'message' => 'Process initialized successfully.', 'instance_id' => $instance['id']]);
 } else {
-    $_SESSION['error_message'] = "Failed to initialize process.";
+    // This case should not be reached if the engine works as expected, as failures should throw exceptions.
+    throw new Exception("Failed to initialize process for an unknown reason.");
 }
-
-header('Location: index.php');
-exit();
diff --git a/index.php b/index.php
index 6fc68cb..5223afc 100644
--- a/index.php
+++ b/index.php
@@ -607,6 +607,12 @@ document.addEventListener('DOMContentLoaded', function () {
                 handleAddNote(noteBtn);
                 return;
             }
+            
+            const startBtn = event.target.closest('#startProcessBtn');
+            if (startBtn) {
+                handleStartProcess(startBtn);
+                return;
+            }
         });
 
         instanceModal.addEventListener('change', function(event) {
@@ -708,24 +714,76 @@ document.addEventListener('DOMContentLoaded', function () {
         submitRequestAndReloadModal('_apply_transition.php', formData);
     }
 
+    function handleStartProcess(button) {
+        const personId = button.dataset.personId;
+        const processId = button.dataset.processId;
+
+        if (!personId || !processId) {
+            alert('Missing data for starting process. Please close the modal and try again.');
+            return;
+        }
+
+        const formData = new FormData();
+        formData.append('person_id', personId);
+        formData.append('process_id', processId);
+        
+        submitRequestAndReloadModal('_init_single_instance.php', formData);
+    }
+
     function submitRequestAndReloadModal(url, formData) {
         const modalBody = instanceModal.querySelector('.modal-body');
         showLoading(modalBody);
-    
+
         fetch(url, {
             method: 'POST',
             body: formData
         })
         .then(response => {
             if (!response.ok) {
-                throw new Error('Network response was not ok');
+                // If response is not OK, it's an error. Clone the response to read it twice.
+                const clone = response.clone();
+                return response.json()
+                    .then(json => {
+                        // We have a JSON error body, throw a custom error with its details
+                        const error = new Error(json.error?.message || 'An unkown error occurred.');
+                        error.correlation_id = json.correlation_id;
+                        error.response = response; // Attach full response
+                        throw error;
+                    })
+                    .catch(() => {
+                        // If JSON parsing fails, fall back to the text body
+                        return clone.text().then(text => {
+                           const error = new Error(text || 'Network response was not ok and could not parse error body.');
+                           error.response = response;
+                           throw error;
+                        });
+                    });
+            }
+            return response.json(); // On success, just parse the JSON
+        })
+        .then(data => {
+            if (data.success) {
+                // Reload modal content for the same person/process after successful submission
+                fetchAndRenderModalContent(currentPersonId, currentProcessId);
+            } else {
+                // Handle cases where the server returns 200 OK but with success: false
+                const error = new Error(data.message || 'An unknown error occurred.');
+                if (data.correlation_id) {
+                    error.correlation_id = data.correlation_id;
+                }
+                throw error;
             }
-            // Reload modal content after successful submission
-            fetchAndRenderModalContent(currentPersonId, currentProcessId);
         })
         .catch(error => {
             console.error('Error submitting request:', error);
-            modalBody.innerHTML = `<div class="alert alert-danger">Wystąpił błąd sieciowy.</div>`;
+
+            let errorMessage = `<div class="alert alert-danger">`;
+            errorMessage += `<strong>Error:</strong> ${error.message}`;
+            if (error.correlation_id) {
+                errorMessage += `<br><small class="text-muted">Correlation ID: ${error.correlation_id}</small>`;
+            }
+            errorMessage += `</div>`;
+            modalBody.innerHTML = errorMessage;
         });
     }
 
diff --git a/lib/WorkflowExceptions.php b/lib/WorkflowExceptions.php
index 1d3a5af..f17534b 100644
--- a/lib/WorkflowExceptions.php
+++ b/lib/WorkflowExceptions.php
@@ -1,44 +1,18 @@
 <?php
 
-class WorkflowException extends Exception {
-    protected $httpCode;
-    protected $details;
+class WorkflowNotFoundException extends Exception {}
+class WorkflowNotAllowedException extends Exception {}
+class WorkflowRuleFailedException extends Exception {}
 
-    public function __construct($message = "", $code = 0, $httpCode = 500, $details = [], Throwable $previous = null) {
+class WorkflowEligibilityException extends Exception {
+    private $reasons;
+
+    public function __construct($message = "", $reasons = [], $code = 0, Throwable $previous = null) {
         parent::__construct($message, $code, $previous);
-        $this->httpCode = $httpCode;
-        $this->details = $details;
+        $this->reasons = $reasons;
     }
 
-    public function getHttpCode() {
-        return $this->httpCode;
+    public function getReasons() {
+        return $this->reasons;
     }
-
-    public function getDetails() {
-        return $this->details;
-    }
-}
-
-class WorkflowNotFoundException extends WorkflowException {
-    public function __construct($message = "Not Found", $details = [], Throwable $previous = null) {
-        parent::__construct($message, 404, 404, $details, $previous);
-    }
-}
-
-class WorkflowNotAllowedException extends WorkflowException {
-    public function __construct($message = "Bad Request", $details = [], Throwable $previous = null) {
-        parent::__construct($message, 400, 400, $details, $previous);
-    }
-}
-
-class WorkflowRuleFailedException extends WorkflowException {
-    public function __construct($message = "Unprocessable Entity", $details = [], Throwable $previous = null) {
-        parent::__construct($message, 422, 422, $details, $previous);
-    }
-}
-
-class WorkflowConflictException extends WorkflowException {
-    public function __construct($message = "Conflict", $details = [], Throwable $previous = null) {
-        parent::__construct($message, 409, 409, $details, $previous);
-    }
-}
+}
\ No newline at end of file