diff --git a/HISTORY.rst b/HISTORY.rst
index 9e5bff4e..be0f00d5 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -8,6 +8,11 @@ PlatformIO Core 5
 
 **A professional collaborative platform for embedded development**
 
+5.0.4 (2020-12-??)
+~~~~~~~~~~~~~~~~~~
+
+* Check for debug server's "ready_pattern" in "stderr"
+
 5.0.4 (2020-12-30)
 ~~~~~~~~~~~~~~~~~~
 
diff --git a/platformio/commands/debug/process/server.py b/platformio/commands/debug/process/server.py
index 7bd5d485..050feac6 100644
--- a/platformio/commands/debug/process/server.py
+++ b/platformio/commands/debug/process/server.py
@@ -134,6 +134,14 @@ class DebugServer(BaseProcess):
                 self._ready = self._last_activity < (time.time() - auto_ready_delay)
             elapsed += delay
 
+    def _check_ready_by_pattern(self, data):
+        if self._ready:
+            return self._ready
+        ready_pattern = self.debug_options.get("server", {}).get("ready_pattern")
+        if ready_pattern:
+            self._ready = ready_pattern.encode() in data
+        return self._ready
+
     @staticmethod
     def async_sleep(secs):
         d = defer.Deferred()
@@ -147,11 +155,11 @@ class DebugServer(BaseProcess):
         super(DebugServer, self).outReceived(
             escape_gdbmi_stream("@", data) if is_gdbmi_mode() else data
         )
-        if self._ready:
-            return
-        ready_pattern = self.debug_options.get("server", {}).get("ready_pattern")
-        if ready_pattern:
-            self._ready = ready_pattern.encode() in data
+        self._check_ready_by_pattern(data)
+
+    def errReceived(self, data):
+        super(DebugServer, self).errReceived(data)
+        self._check_ready_by_pattern(data)
 
     def processEnded(self, reason):
         self._process_ended = True