[Bug]: DeepSeek V4 DSML tool parser mishandles wrapped and reserved arguments

[QwertyJack]

Current behavior

vLLM already has registered deepseek_v4 tokenizer, tool-call parser, and reasoning parser support. Recent upstream work also added structural-tag support and generic DSV3.2/V4 non-streaming type conversion.

There are still DeepSeek V4 DSML tool-call edge cases that do not round-trip OpenAI-compatible tool arguments correctly with the serving flag combination:

--tokenizer-mode deepseek_v4 \
--tool-call-parser deepseek_v4 \
--enable-auto-tool-choice \
--reasoning-parser deepseek_v4

Problems

The remaining parser/tokenizer gaps are:

• DSML parameter parsing should respect the string="true|false" attribute and use the request tool schema / JSON fallback for non-string values
• model-emitted single arguments or input wrapper parameters should be unwrapped when those names are not actual fields in the requested tool schema
• real tool schema parameters named arguments should not be confused with OpenAI tool-call wrapper semantics
• streaming responses that end while plain text is being held because it looks like the start of a DSML marker, for example 2 <, should flush that text

Expected behavior

The DeepSeek V4 tokenizer/parser path should:

• preserve literal strings when DSML emits string="true", and coerce string="false" values through the relevant schema or JSON parsing
• unwrap single arguments / input wrapper objects only when the wrapper is not a real schema field and the wrapped object matches the schema fields
• safely render and parse tool schemas/history containing a real parameter named arguments
• flush held plain text at the end of a stream when no DSML tool call actually follows

Notes

This issue is about DeepSeek V4 DSML tool-call parser/tokenizer correctness. It does not require adding the deepseek_v4 parser from scratch, since that parser is already present upstream.

DeepSeek's official top-level thinking={"type": "enabled"|"disabled"} request shape is a related API-compatibility topic, but per review feedback it is not part of the current fix. vLLM users should continue to use chat_template_kwargs for DeepSeek V4 thinking toggles, matching the vLLM DeepSeek V4 recipe.

Upstream #41198 partially overlaps by adding generic DSV3.2/V4 type conversion for non-streaming tool calls. The remaining issue is narrower: V4 DSML string-attribute handling, wrapper repair, real arguments field escaping, and streaming final flush behavior.

Related PR

Proposed fix: #41241

[robertgshaw2-redhat]

cc @sfeng33 @bbrowning

[chaunceyjiang]

see #41801

[sfeng33]

Closing as fixed in #41801