fix(agent): render OpenAI tool-call arguments as a mapping for chat templates

slime/agent/adapters/common.py

@@ -95,6 +95,28 @@ def json_arguments(value: Any) -> str:
     return json.dumps(value, ensure_ascii=False)
 
 
+def dict_arguments(value: Any) -> dict:
+    """Tool-call arguments for chat-template rendering.
+
+    OpenAI wire responses carry ``function.arguments`` as a JSON string; Qwen-family
+    chat templates expect a mapping and iterate ``arguments.items()``. Decode echoed
+    wire strings back to dicts at the render boundary.
+    """
+    if isinstance(value, dict):
+        return value
+    if isinstance(value, str):
+        try:
+            decoded = json.loads(value or "{}")
+        except json.JSONDecodeError:
+            decoded = value
+        if isinstance(decoded, dict):
+            return decoded
+        return {"_raw_arguments": decoded}
+    if value is None:
+        return {}
+    return {"_raw_arguments": value}
+
+
 def render_token_ids(chain: AdapterChain, tokenizer) -> list[int]:
     enc = tokenizer.apply_chat_template(
         chain.chat_messages,

slime/agent/adapters/openai.py

@@ -23,6 +23,7 @@
 from slime.agent.adapters.common import ADAPTER_KEY, REASONING_PARSER_KEY, TOKENIZER_KEY, TOOL_PARSER_KEY
 from slime.agent.adapters.common import AdapterChain as Chain
 from slime.agent.adapters.common import BaseAdapter, call_sglang_generate
+from slime.agent.adapters.common import dict_arguments as _dict_arguments
 from slime.agent.adapters.common import json_arguments as _json_arguments
 from slime.agent.adapters.common import ok_response, render_token_ids, request_session_id
 from slime.agent.adapters.common import stable_hash as _hash
@@ -105,7 +106,7 @@ def _normalize_tool_call(call: dict[str, Any]) -> dict[str, Any]:
         "type": "function",
         "function": {
             "name": name,
-            "arguments": _json_arguments(arguments),
+            "arguments": _dict_arguments(arguments),
         },
     }
     if call.get("id"):

tests/test_agent_adapters.py

@@ -12,7 +12,7 @@
     sys.path.insert(0, str(REPO_ROOT))
 
 from slime.agent.adapters import anthropic, openai
-from slime.agent.adapters.common import SGLANG_URL_KEY
+from slime.agent.adapters.common import SGLANG_URL_KEY, dict_arguments
 from slime.agent.trajectory import TurnRecord
 
 
@@ -167,6 +167,20 @@ def test_anthropic_translation_keeps_tool_results_and_tool_schema():
     ]
 
 
+@pytest.mark.unit
+def test_openai_render_tool_call_arguments_are_dicts():
+    # Echoed wire JSON string is decoded back to a mapping for chat-template rendering.
+    normalized = openai._normalize_tool_call({"function": {"name": "lookup", "arguments": '{"q": "slime"}'}})
+    assert normalized["function"]["arguments"] == {"q": "slime"}
+    # Valid-JSON-but-non-dict arguments are wrapped so ``arguments.items()`` stays safe.
+    wrapped = openai._normalize_tool_call({"function": {"name": "lookup", "arguments": "[1, 2]"}})
+    assert wrapped["function"]["arguments"] == {"_raw_arguments": [1, 2]}
+    # Falsy native non-dicts are still real argument values; only None means "no arguments".
+    assert dict_arguments(0) == {"_raw_arguments": 0}
+    assert dict_arguments([]) == {"_raw_arguments": []}
+    assert dict_arguments(None) == {}
+
+
 @pytest.mark.unit
 def test_openai_translation_and_responses_input_shapes():
     chat_messages = openai._translate_chat_messages(
@@ -205,7 +219,7 @@ def test_openai_translation_and_responses_input_shapes():
                 {
                     "id": "call_1",
                     "type": "function",
-                    "function": {"name": "lookup", "arguments": '{"q": "slime"}'},
+                    "function": {"name": "lookup", "arguments": {"q": "slime"}},
                 }
             ],
         },