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NVIDIA-Nemotron-3-Nano-30B-A3B-BF16

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Summary

Nemotron 3 Nano's chat_template.jinja crashes with:

TypeError: Can only get item pairs from a mapping

when rendering assistant messages containing tool_calls where tool_calls[].function.arguments is provided as a JSON-encoded string (a common format used by OpenAI-style tool calling and supported by transformers).
The template assumes arguments is a dict/mapping and applies |items directly (line 150).

Reproduction

from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained(
    "nvidia/NVIDIA-Nemotron-3-Nano-30B-A3B-BF16",
    trust_remote_code=True,
)

tools = [{
    "type": "function",
    "function": {
        "name": "get_weather",
        "description": "Get the current weather in a city",
        "parameters": {
            "type": "object",
            "properties": {
                "city": {"type": "string", "description": "City name"}
            },
            "required": ["city"]
        }
    }
}]

# ❌ FAILS — arguments as JSON string
messages_string = [
    {"role": "user", "content": "What's the weather in Seoul?"},
    {
        "role": "assistant",
        "content": "",
        "tool_calls": [{
            "id": "call_1",
            "type": "function",
            "function": {
                "name": "get_weather",
                "arguments": '{"city": "Seoul"}'   # JSON string
            }
        }]
    },
    {"role": "tool", "tool_call_id": "call_1", "content": '{"temperature": "5°C"}'},
]

tokenizer.apply_chat_template(
    messages_string, tokenize=False, add_generation_prompt=True, tools=tools
)
# TypeError: Can only get item pairs from a mapping.

# ✅ PASSES — arguments as dict
messages_dict = [
    {"role": "user", "content": "What's the weather in Seoul?"},
    {
        "role": "assistant",
        "content": "",
        "tool_calls": [{
            "id": "call_1",
            "type": "function",
            "function": {
                "name": "get_weather",
                "arguments": {"city": "Seoul"}     # dict
            }
        }]
    },
    {"role": "tool", "tool_call_id": "call_1", "content": '{"temperature": "5°C"}'},
]

tokenizer.apply_chat_template(
    messages_dict, tokenize=False, add_generation_prompt=True, tools=tools
)
# Works without error

Environment

• transformers: 5.0.0
• Python: 3.13
• OS: macOS (Apple Silicon)

Error Traceback (key part)

File "<template>", line 150, in top-level template code
File ".../jinja2/filters.py", line 249, in do_items
    raise TypeError("Can only get item pairs from a mapping.")
TypeError: Can only get item pairs from a mapping.

Cause

In chat_template.jinja (line 150):

{%- for args_name, args_value in tool_call.arguments|items %}

The Jinja |items filter requires a dict/mapping, but tool_call.arguments may be a JSON string in tool-calling message formats. When it is a string, |items raises the TypeError above.

Suggested Fix

Parse JSON strings before iterating, while keeping dict support:

{%- if tool_call.arguments is defined %}
    {%- if tool_call.arguments is string %}
        {%- set parsed_args = tool_call.arguments | fromjson %}
    {%- else %}
        {%- set parsed_args = tool_call.arguments %}
    {%- endif %}
    {%- for args_name, args_value in parsed_args | items %}
        ...
    {%- endfor %}
{%- endif %}

Additional Note

The template appears to render tool outputs using <tool_response> rather than <tool_result>. Some external stacks may label tool output differently, so it might be worth confirming whether multiple tool-output tags should be handled.

Notes

• The model README indicates tool calling is supported
• Tested on the BF16 variant; the same chat_template.jinja also appears in other variants
• Happy to submit a PR with this change if it would be welcome