[Bug]: Gemma4 tool parser raises RecursionError on arguments nested about 1000 levels deep #20995

System Info

• TensorRT-LLM main at 687359e (tensorrt_llm/serve/tool_parser/gemma4_parser.py), pure Python. A CPU is enough to reproduce.
• Python 3.12
• Serving path: trtllm-serve <Gemma 4 model> --tool_parser gemma4, chat completions with tools

Who can help?

@NVIDIA/trt-llm-runtime-devs (CODEOWNERS for tensorrt_llm/serve)

Reproduction

from tensorrt_llm.serve.openai_protocol import ChatCompletionToolsParam
from tensorrt_llm.serve.tool_parser.gemma4_parser import Gemma4ToolParser

tools = [ChatCompletionToolsParam(function={"name": "get_weather", "parameters": {"type": "object"}})]
text = "<|tool_call>call:get_weather{opts:" + "{a:" * 1000 + "1" + "}" * 1000 + "}<tool_call|>"
Gemma4ToolParser().detect_and_parse(text, tools)
# RecursionError: maximum recursion depth exceeded

parser = Gemma4ToolParser()
for i in range(0, len(text), 4):
    parser.parse_streaming_increment(text[i:i + 4], tools)
# RecursionError when the call closes

Arrays fail the same way (tags: followed by 1000 [ and 1000 ]).

Expected behavior

The parser returns a result for any model output. Arguments it cannot parse are handled like other malformed arguments, which the parser already logs and skips.

actual behavior

RecursionError leaves detect_and_parse (first parser frame gemma4_parser.py:334) and parse_streaming_increment (first parser frame gemma4_parser.py:476). The non-streaming chat request then fails with HTTP 400 and the generated text is lost. The streaming request ends without its last chunks and without data: [DONE].

additional notes

_parse_gemma4_args calls itself once per nested object (gemma4_parser.py:206), and _parse_gemma4_value and _parse_gemma4_array call each other once per nested array. The handlers at gemma4_parser.py:335 and :478 catch (ValueError, IndexError, TypeError, KeyError). Adding RecursionError there is enough. glm4_parser.py already catches RecursionError for the same reason. I have a fix with a test ready and will open a PR that references this issue.