diff --git a/rdagent/oai/llm_utils.py b/rdagent/oai/llm_utils.py
index b27268ab..d0cb8abe 100644
--- a/rdagent/oai/llm_utils.py
+++ b/rdagent/oai/llm_utils.py
@@ -327,7 +327,7 @@ class APIBackend:
 
             self.chat_model = LLM_SETTINGS.chat_model if chat_model is None else chat_model
             self.chat_model_map = json.loads(LLM_SETTINGS.chat_model_map)
-            self.encoder = tiktoken.encoding_for_model(self.chat_model)
+            self.encoder = self._get_encoder()
             self.chat_api_base = LLM_SETTINGS.chat_azure_api_base if chat_api_base is None else chat_api_base
             self.chat_api_version = (
                 LLM_SETTINGS.chat_azure_api_version if chat_api_version is None else chat_api_version
@@ -399,6 +399,33 @@ class APIBackend:
         self.use_gcr_endpoint = LLM_SETTINGS.use_gcr_endpoint
         self.retry_wait_seconds = LLM_SETTINGS.retry_wait_seconds
 
+    def _get_encoder(self):
+        """
+        tiktoken.encoding_for_model(self.chat_model) does not cover all cases it should consider.
+
+        This function attempts to handle several edge cases.
+        """
+
+        # 1) cases
+        def _azure_patch(model: str) -> str:
+            """
+            When using Azure API, self.chat_model is the deployment name that can be any string.
+            For example, it may be `gpt-4o_2024-08-06`. But tiktoken.encoding_for_model can't handle this.
+            """
+            return model.replace("_", "-")
+
+        model = self.chat_model
+        try:
+            return tiktoken.encoding_for_model(model)
+        except KeyError:
+            logger.warning(f"Failed to get encoder. Trying to patch the model name")
+            for patch_func in [_azure_patch]:
+                try:
+                    return tiktoken.encoding_for_model(patch_func(model))
+                except KeyError:
+                    logger.error(f"Failed to get encoder even after patching with {patch_func.__name__}")
+                    raise
+
     def build_chat_session(
         self,
         conversation_id: str | None = None,