diff --git a/app/src/main/java/io/heckel/ntfy/ui/CustomHeaderFragment.kt b/app/src/main/java/io/heckel/ntfy/ui/CustomHeaderFragment.kt
index 68ce533f..b32297d6 100644
--- a/app/src/main/java/io/heckel/ntfy/ui/CustomHeaderFragment.kt
+++ b/app/src/main/java/io/heckel/ntfy/ui/CustomHeaderFragment.kt
@@ -11,6 +11,7 @@ import androidx.appcompat.app.AlertDialog
 import androidx.fragment.app.DialogFragment
 import com.google.android.material.dialog.MaterialAlertDialogBuilder
 import com.google.android.material.textfield.TextInputEditText
+import com.google.android.material.textfield.TextInputLayout
 import io.heckel.ntfy.R
 import io.heckel.ntfy.db.CustomHeader
 import io.heckel.ntfy.util.AfterChangedTextWatcher
@@ -25,6 +26,7 @@ class CustomHeaderFragment : DialogFragment() {
     private lateinit var baseUrlView: TextInputEditText
     private lateinit var headerNameView: TextInputEditText
     private lateinit var headerValueView: TextInputEditText
+    private lateinit var headerNameLayout: TextInputLayout
     private lateinit var positiveButton: Button
 
     interface CustomHeaderDialogListener {
@@ -60,6 +62,7 @@ class CustomHeaderFragment : DialogFragment() {
         baseUrlView = view.findViewById(R.id.custom_header_dialog_base_url)
         headerNameView = view.findViewById(R.id.custom_header_dialog_name)
         headerValueView = view.findViewById(R.id.custom_header_dialog_value)
+        headerNameLayout = view.findViewById(R.id.custom_header_dialog_name_layout)
 
         var title: String
         if (header == null) {
@@ -155,17 +158,32 @@ class CustomHeaderFragment : DialogFragment() {
         val headerName = headerNameView.text?.toString()?.trim() ?: ""
         val headerValue = headerValueView.text?.toString()?.trim() ?: ""
 
+        // Clear previous errors
+        headerNameLayout.error = null
+
+        // Validate header name
+        var isValid = true
+        if (headerName.isNotEmpty()) {
+            if (!validateHeaderName(headerName)) {
+                headerNameLayout.error = getString(R.string.custom_headers_invalid_name)
+                isValid = false
+            } else if (isReservedHeader(headerName)) {
+                headerNameLayout.error = getString(R.string.custom_headers_reserved_name)
+                isValid = false
+            }
+        }
+
         if (header == null) {
             // New header: baseUrl, name, and value required
             positiveButton.isEnabled = validUrl(baseUrl)
                 && headerName.isNotEmpty()
-                && validateHeaderName(headerName)
                 && headerValue.isNotEmpty()
+                && isValid
         } else {
             // Editing header: name and value required
             positiveButton.isEnabled = headerName.isNotEmpty()
-                && validateHeaderName(headerName)
                 && headerValue.isNotEmpty()
+                && isValid
         }
     }
 
@@ -178,6 +196,21 @@ class CustomHeaderFragment : DialogFragment() {
         return regex.matches(name)
     }
 
+    private fun isReservedHeader(name: String): Boolean {
+        // These headers are already set by ntfy and cannot be overridden
+        val nameLower = name.lowercase()
+        val reservedHeaders = setOf(
+            "user-agent",
+            "authorization",
+            "host",
+            "connection",
+            "upgrade",
+            "accept-encoding"
+        )
+        // Also block all WebSocket-related headers
+        return reservedHeaders.contains(nameLower) || nameLower.startsWith("sec-websocket-")
+    }
+
     companion object {
         const val TAG = "NtfyCustomHeaderFragment"
         private const val BUNDLE_BASE_URL = "baseUrl"
diff --git a/app/src/main/res/layout/fragment_custom_header_dialog.xml b/app/src/main/res/layout/fragment_custom_header_dialog.xml
index 8b82662a..dff4696e 100644
--- a/app/src/main/res/layout/fragment_custom_header_dialog.xml
+++ b/app/src/main/res/layout/fragment_custom_header_dialog.xml
@@ -43,6 +43,7 @@
         </com.google.android.material.textfield.TextInputLayout>
 
         <com.google.android.material.textfield.TextInputLayout
+            android:id="@+id/custom_header_dialog_name_layout"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
             android:layout_marginHorizontal="?dialogPreferredPadding"
diff --git a/app/src/main/res/values-de/strings.xml b/app/src/main/res/values-de/strings.xml
index 42b30f73..facaee3f 100644
--- a/app/src/main/res/values-de/strings.xml
+++ b/app/src/main/res/values-de/strings.xml
@@ -371,6 +371,7 @@
     <string name="custom_headers_delete">Löschen</string>
     <string name="custom_headers_error_title">Ungültiger Header</string>
     <string name="custom_headers_invalid_name">Ungültige Zeichen im Header-Namen</string>
+    <string name="custom_headers_reserved_name">Dieser Header ist reserviert und wird von ntfy gesetzt</string>
     <string name="custom_headers_name_hint">Name (z.B. CF-Access-Client-Id)</string>
     <string name="custom_headers_value_hint">Wert</string>
     <string name="custom_header_dialog_description_add">Einen benutzerdefinierten HTTP-Header hinzufügen, der mit jeder Anfrage an den angegebenen Server gesendet wird.</string>
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index ebb67d12..dce126a6 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -424,6 +424,7 @@
     <string name="custom_headers_delete">Delete</string>
     <string name="custom_headers_error_title">Invalid Header</string>
     <string name="custom_headers_invalid_name">Header name contains invalid characters</string>
+    <string name="custom_headers_reserved_name">This header is reserved and set by ntfy</string>
     <string name="custom_headers_name_hint">Name (e.g. CF-Access-Client-Id)</string>
     <string name="custom_headers_value_hint">Value</string>
     <string name="custom_header_dialog_description_add">Add a custom HTTP header that will be sent with every request to the specified server.</string>