diff --git a/changelog/4401.changed.md b/changelog/4401.changed.md
new file mode 100644
index 000000000..dc413e566
--- /dev/null
+++ b/changelog/4401.changed.md
@@ -0,0 +1 @@
+- Changed the default model for `GrokRealtimeLLMService` to `grok-voice-think-fast-1.0`, xAI's recommended Voice Agent model. The previous default of `grok-voice-fast-1.0` has been deprecated by xAI and is being removed.
diff --git a/changelog/4401.fixed.md b/changelog/4401.fixed.md
new file mode 100644
index 000000000..f427ca42a
--- /dev/null
+++ b/changelog/4401.fixed.md
@@ -0,0 +1 @@
+- Fixed `GrokRealtimeLLMService` ignoring the configured model. The model was stored in `Settings` but never sent to xAI, so every session silently fell back to xAI's server-side default. The model is now passed via the `?model=` query parameter on the WebSocket URL as xAI's Voice Agent API requires.
diff --git a/src/pipecat/services/xai/realtime/llm.py b/src/pipecat/services/xai/realtime/llm.py
index 55a7be4cd..79ea421ed 100644
--- a/src/pipecat/services/xai/realtime/llm.py
+++ b/src/pipecat/services/xai/realtime/llm.py
@@ -17,6 +17,7 @@ from collections.abc import Mapping
 from dataclasses import dataclass, field
 from dataclasses import fields as dataclass_fields
 from typing import Any
+from urllib.parse import quote
 
 from loguru import logger
 
@@ -235,7 +236,7 @@ class GrokRealtimeLLMService(LLMService[GrokRealtimeLLMAdapter]):
         """
         # 1. Initialize default_settings with hardcoded defaults
         default_settings = self.Settings(
-            model=None,
+            model="grok-voice-think-fast-1.0",
             system_instruction=None,
             temperature=None,
             max_tokens=None,
@@ -533,8 +534,13 @@ class GrokRealtimeLLMService(LLMService[GrokRealtimeLLMAdapter]):
             if self._websocket:
                 return
 
+            # Model is selected via query param at connection time; xAI does
+            # not support changing it via session.update.
+            model = assert_given(self._settings.model)
+            uri = f"{self.base_url}?model={quote(model, safe='')}"
+
             self._websocket = await websocket_connect(
-                uri=self.base_url,
+                uri=uri,
                 additional_headers={
                     "Authorization": f"Bearer {self.api_key}",
                 },