diff --git a/.semversioner/next-release/patch-2026053119245844167500.json b/.semversioner/next-release/patch-2026053119245844167500.json
new file mode 100644
index 0000000000..a3849c8fc5
--- /dev/null
+++ b/.semversioner/next-release/patch-2026053119245844167500.json
@@ -0,0 +1,4 @@
+{
+  "type": "patch",
+  "description": "Allow non-OpenAI service_tier values in LLMCompletionResponse so indexing does not fail with LiteLLM-routed providers (e.g. Gemini via OpenRouter)."
+}
\ No newline at end of file
diff --git a/packages/graphrag-llm/graphrag_llm/types/types.py b/packages/graphrag-llm/graphrag_llm/types/types.py
index 0980cba3aa..e2b8c2213d 100644
--- a/packages/graphrag-llm/graphrag_llm/types/types.py
+++ b/packages/graphrag-llm/graphrag_llm/types/types.py
@@ -91,6 +91,12 @@ class LLMCompletionResponse(ChatCompletion, Generic[ResponseFormat]):
     provided ResponseFormat model.
     """
 
+    # Widen OpenAI's strict `service_tier` literal. Non-OpenAI providers routed
+    # through LiteLLM (e.g. Gemini via OpenRouter) return values outside OpenAI's
+    # enum, which would otherwise fail pydantic validation when constructing this
+    # model. graphrag-llm does not use this field. See issue #2389.
+    service_tier: str | None = None  # type: ignore
+
     formatted_response: ResponseFormat | None = None  # type: ignore
     """Formatted response according to the specified response_format json schema."""