diff --git a/src/Psalm/Internal/Fork/ForkProcessErrorMessage.php b/src/Psalm/Internal/Fork/ForkProcessErrorMessage.php
new file mode 100644
index 00000000000..395bdbbc1ff
--- /dev/null
+++ b/src/Psalm/Internal/Fork/ForkProcessErrorMessage.php
@@ -0,0 +1,13 @@
+<?php
+namespace Psalm\Internal\Fork;
+
+class ForkProcessErrorMessage implements ForkMessage
+{
+    /** @var string */
+    public $message;
+
+    public function __construct(string $message)
+    {
+        $this->message = $message;
+    }
+}
diff --git a/src/Psalm/Internal/Fork/Pool.php b/src/Psalm/Internal/Fork/Pool.php
index d6aad182bfa..5e013abd7cb 100644
--- a/src/Psalm/Internal/Fork/Pool.php
+++ b/src/Psalm/Internal/Fork/Pool.php
@@ -177,30 +177,36 @@ public function __construct(
 
         $task_done_buffer = '';
 
-        foreach ($task_data_iterator as $i => $task_data) {
-            $task_result = $task_closure($i, $task_data);
-            $task_done_message = new ForkTaskDoneMessage($task_result);
-            $serialized_message = $task_done_buffer . base64_encode(serialize($task_done_message)) . "\n";
+        try {
+            foreach ($task_data_iterator as $i => $task_data) {
+                $task_result = $task_closure($i, $task_data);
 
-            if (strlen($serialized_message) > 200) {
-                $bytes_written = @fwrite($write_stream, $serialized_message);
+                $task_done_message = new ForkTaskDoneMessage($task_result);
+                $serialized_message = $task_done_buffer . base64_encode(serialize($task_done_message)) . "\n";
 
-                if (strlen($serialized_message) !== $bytes_written) {
-                    $task_done_buffer = substr($serialized_message, $bytes_written);
+                if (strlen($serialized_message) > 200) {
+                    $bytes_written = @fwrite($write_stream, $serialized_message);
+
+                    if (strlen($serialized_message) !== $bytes_written) {
+                        $task_done_buffer = substr($serialized_message, $bytes_written);
+                    } else {
+                        $task_done_buffer = '';
+                    }
                 } else {
-                    $task_done_buffer = '';
+                    $task_done_buffer = $serialized_message;
                 }
-            } else {
-                $task_done_buffer = $serialized_message;
             }
-        }
 
-        // Execute each child's shutdown closure before
-        // exiting the process
-        $results = $shutdown_closure();
+            // Execute each child's shutdown closure before
+            // exiting the process
+            $results = $shutdown_closure();
+
+            // Serialize this child's produced results and send them to the parent.
+            $process_done_message = new ForkProcessDoneMessage($results ?: []);
+        } catch (\Throwable $t) {
+            $process_done_message = new ForkProcessErrorMessage($t->getMessage());
+        }
 
-        // Serialize this child's produced results and send them to the parent.
-        $process_done_message = new ForkProcessDoneMessage($results ?: []);
         $serialized_message = $task_done_buffer . base64_encode(serialize($process_done_message)) . "\n";
 
         $bytes_to_write = strlen($serialized_message);
@@ -324,6 +330,8 @@ private function readResultsFromChildren()
                             if ($this->task_done_closure !== null) {
                                 ($this->task_done_closure)($message->data);
                             }
+                        } elseif ($message instanceof ForkProcessErrorMessage) {
+                            throw new \Exception($message->message);
                         } else {
                             error_log('Child should return ForkMessage - response type=' . gettype($message));
                             $this->did_have_error = true;
diff --git a/src/psalm.php b/src/psalm.php
index 991aa04876f..1aee4c13c67 100644
--- a/src/psalm.php
+++ b/src/psalm.php
@@ -294,11 +294,30 @@ function ($arg) {
     chdir($current_dir);
 }
 
-$threads = isset($options['threads'])
-    ? (int)$options['threads']
-    : max(1, ProjectAnalyzer::getCpuCount() - 2);
+$in_ci = isset($_SERVER['TRAVIS'])
+    || isset($_SERVER['CIRCLECI'])
+    || isset($_SERVER['APPVEYOR'])
+    || isset($_SERVER['JENKINS_URL'])
+    || isset($_SERVER['SCRUTINIZER'])
+    || isset($_SERVER['GITLAB_CI'])
+    || isset($_SERVER['GITHUB_WORKFLOW']);
+
+// disable progressbar on CI
+if ($in_ci) {
+    $options['long-progress'] = true;
+}
+
+if (isset($options['threads'])) {
+    $threads = (int)$options['threads'];
+} elseif (isset($options['debug']) || $in_ci) {
+    $threads = 1;
+} else {
+    $threads = max(1, ProjectAnalyzer::getCpuCount() - 2);
+}
 
-if ($threads === 1
+if (!isset($options['threads'])
+    && !isset($options['debug'])
+    && $threads === 1
     && ini_get('pcre.jit') === '1'
     && PHP_OS === 'Darwin'
     && version_compare(PHP_VERSION, '7.3.0') >= 0
@@ -433,18 +452,6 @@ function ($arg) {
     exit;
 }
 
-// disable progressbar on CI
-if (isset($_SERVER['TRAVIS'])
-    || isset($_SERVER['CIRCLECI'])
-    || isset($_SERVER['APPVEYOR'])
-    || isset($_SERVER['JENKINS_URL'])
-    || isset($_SERVER['SCRUTINIZER'])
-    || isset($_SERVER['GITLAB_CI'])
-    || isset($_SERVER['GITHUB_WORKFLOW'])
-) {
-    $options['long-progress'] = true;
-}
-
 $debug = array_key_exists('debug', $options) || array_key_exists('debug-by-line', $options);
 
 if ($debug) {