Fix DeepSeek V4 DSML tool argument parsing

[QwertyJack]

Related to #41240

Summary

• fix the existing deepseek_v4 tool parser to respect DSML string="true|false" parameter metadata, preserving literal strings and coercing non-string values through the request schema or JSON fallback
• unwrap single arguments / input wrapper parameters when the wrapper is not part of the requested tool schema and the wrapped object matches the schema fields
• escape real tool parameters named arguments while rendering DeepSeek V4 tool schemas/history, then unescape them when parsing model output
• flush held plain text at the end of streaming when it only looked like the start of a DSML marker, such as 2 <
• keep upstream DeepSeek V4 structural-tag support and add focused parser, tokenizer, and CLI validation coverage for the DeepSeek V4 serving flag combination

This PR does not add the deepseek_v4 parser from scratch; upstream already has the parser registered. Recent upstream #41198 also added generic DSV3.2/V4 non-streaming type conversion. This PR remains narrower and covers V4 DSML string-attribute handling, wrapper repair, real arguments field escaping, and streaming final flush behavior.

This PR also does not add top-level thinking={...} request support. Per review feedback, DeepSeek V4 thinking toggles should continue to use chat_template_kwargs, matching the vLLM DeepSeek V4 recipe.

Duplicate-work check

• Reviewed issue [Bug]: DeepSeek V4 DSML tool parser mishandles wrapped and reserved arguments #41240 with gh issue view 41240 --repo vllm-project/vllm --comments.
• Checked open PRs with gh pr list --repo vllm-project/vllm --state open --search "41240 in:body". No existing PR references the issue.
• Checked open PRs with gh pr list --repo vllm-project/vllm --state open --search "deepseek_v4 DSML tool parser" and gh pr list --repo vllm-project/vllm --state open --search "DeepSeek V4 tool argument parsing". The relevant matches are this PR and unrelated DSv4 backend/model work such as [DSv4][Nvidia] SM12x DeepSeek V4 support #40991.
• Related upstream [Bugfix] DSV32/V4 add missing type conversion for non-streaming tool calls #41198 partially overlaps by adding generic DSV3.2/V4 non-streaming type conversion, but it does not cover this PR's wrapper repair, real arguments field escaping, or streaming final flush behavior.
• Related issue [Bug]: DeepSeek V3.2 & V4 incorrect structured output when thinking enabled #41132 covers structured output routed into reasoning when thinking is enabled; this PR is narrower and only targets DeepSeek V4 DSML tool-call parsing/tokenizer round-tripping.

Tests

• .venv/bin/python -m pytest tests/tool_parsers/test_deepseekv4_tool_parser.py tests/tokenizers_/test_deepseek_v4.py tests/reasoning/test_deepseekv3_reasoning_parser.py::test_deepseek_v4_reasoning_parser_alias tests/entrypoints/openai/test_cli_args.py::test_deepseek_v4_agentic_flags_pass_validation -q
• .venv/bin/python -m pytest tests/entrypoints/openai/chat_completion/test_chat.py::test_chat_completion_request_accepts_model_specific_reasoning_effort -q
• .venv/bin/ruff check vllm/entrypoints/openai/chat_completion/protocol.py tests/entrypoints/openai/chat_completion/test_chat.py vllm/tool_parsers/deepseekv4_tool_parser.py vllm/tokenizers/deepseek_v4_encoding.py tests/tool_parsers/test_deepseekv4_tool_parser.py tests/tokenizers_/test_deepseek_v4.py tests/entrypoints/openai/test_cli_args.py
• pre-commit hooks run during git commit --amend and passed

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[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[gemini-code-assist]

Code Review

This pull request adds support for DeepSeek V4, including a 'thinking' mode toggle in the chat completion protocol and a specialized DeepSeekV4ToolParser for DSML tool calls. The implementation handles parameter name escaping for 'arguments' and normalizes wrapped tool call inputs. Comprehensive tests verify the new protocol fields, CLI arguments, and tool parsing logic in both streaming and non-streaming modes. I have no feedback to provide.

[QwertyJack]

Thanks @chaunceyjiang, that makes sense. I removed the top-level thinking request-field mapping and kept the PR scoped to the DeepSeek V4 parser/tokenizer compatibility work. The PR body now notes that reasoning toggles should use chat_template_kwargs, matching the recipe.

[UmutAlihan]

looking very forward for this fix to be merged and release in stable releases

[QwertyJack]

@chaunceyjiang Here is a minimal deterministic case for why the PR is still needed after #41198.

A DeepSeek V4 tool call may emit the real arguments object inside a single DSML arguments parameter:

import json
from unittest.mock import MagicMock
from vllm.tool_parsers.deepseekv4_tool_parser import DeepSeekV4ToolParser

mock_tokenizer = MagicMock()
mock_tokenizer.get_vocab.return_value = {}

tool = MagicMock()
tool.function.name = "get_weather"
tool.function.parameters = {
    "type": "object",
    "properties": {"location": {"type": "string"}},
}

parser = DeepSeekV4ToolParser(mock_tokenizer, tools=[tool])
request = MagicMock()
request.tools = [tool]

model_output = (
    '<｜DSML｜tool_calls>'
    '<｜DSML｜invoke name="get_weather">'
    '<｜DSML｜parameter name="arguments" string="false">{"location":"Beijing"}</｜DSML｜parameter>'
    '</｜DSML｜invoke>'
    '</｜DSML｜tool_calls>'
)

result = parser.extract_tool_calls(model_output, request)
print(json.loads(result.tool_calls[0].function.arguments))

On current upstream main (b53c507bc9), this prints:

{"arguments": "{\"location\":\"Beijing\"}"}

With this PR, it prints the OpenAI-compatible arguments object expected by the tool schema:

{"location": "Beijing"}

This is covered by test_extract_tool_calls_repairs_arguments_wrapper_object. The rest of the PR handles the same edge class for input, real schema fields named arguments, and final streaming flush when held text only looked like a partial DSML marker.

[chaunceyjiang]

Thanks @QwertyJack for identifying this issue. I think your fix is a bit of a hack.

I’ve submitted a new implementation here: #41801.

Could you help test it? @QwertyJack @UmutAlihan

[QwertyJack]

LGTM

[chaunceyjiang]

Thanks @QwertyJack for identifying this issue. I think your fix is a bit of a hack.

I’ve submitted a new implementation here: #41801.

Could you help test it? @QwertyJack @UmutAlihan

[QwertyJack]

LGTM

[sfeng33]

Thanks @QwertyJack, I added you as co-author on #41801.
Closing this PR as fixed.