Add glm4.6 to FORCE_STRING_SERIALIZER (#994)

Author: simonrosenberg

openhands-sdk/openhands/sdk/llm/llm.py

@@ -848,10 +848,9 @@ def format_messages_for_llm(self, messages: list[Message]) -> list[dict]:
             message.cache_enabled = self.is_caching_prompt_active()
             message.vision_enabled = self.vision_is_active()
             message.function_calling_enabled = self.native_tool_calling
-            if "deepseek" in self.model or (
-                "kimi-k2-instruct" in self.model and "groq" in self.model
-            ):
-                message.force_string_serializer = True
+            message.force_string_serializer = get_features(
+                self.model
+            ).force_string_serializer
 
         formatted_messages = [message.to_chat_dict() for message in messages]
 

openhands-sdk/openhands/sdk/llm/utils/model_features.py

@@ -22,6 +22,7 @@ class ModelFeatures:
     supports_prompt_cache: bool
     supports_stop_words: bool
     supports_responses_api: bool
+    force_string_serializer: bool
 
 
 # Pattern tables capturing current behavior. Keep patterns lowercase.
@@ -85,6 +86,18 @@ class ModelFeatures:
     "codex-mini-latest",
 ]
 
+# Models that require string serializer for tool messages
+# These models don't support structured content format [{"type":"text","text":"..."}]
+# and need plain strings instead
+FORCE_STRING_SERIALIZER_PATTERNS: list[str] = [
+    "*deepseek*",
+    "glm*",
+    # kimi-k2-instruct on groq provider requires string serialization
+    # Pattern contains '/' so it matches against full model string
+    # (e.g., "groq/kimi-k2-instruct")
+    "*groq*/kimi-k2-instruct*",
+]
+
 
 def get_features(model: str) -> ModelFeatures:
     """Get model features."""
@@ -96,4 +109,5 @@ def get_features(model: str) -> ModelFeatures:
             model, SUPPORTS_STOP_WORDS_FALSE_PATTERNS
         ),
         supports_responses_api=model_matches(model, RESPONSES_API_PATTERNS),
+        force_string_serializer=model_matches(model, FORCE_STRING_SERIALIZER_PATTERNS),
     )