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The _PARTIAL_PARAM_RE regex in vllm/parser/deepseek_v4.py used a greedy
(.*)$ capture that swallowed partial DSML closing tags (e.g.
Paris</｜DSML｜parameter without the final >) into parameter values during
streaming. When _compute_arg_delta called _dsml_arg_converter with
partial=True on slot.args containing such a partial closing tag, the
converter captured the tag text as part of the parameter value. The
_safe_arg_prefix method then streamed the unterminated string content
(including DSML markup) as a tool call argument delta.
Replace the greedy capture with a tempered greedy token using a negative
lookahead that stops at DSML tag prefixes:
((?:(?!</?{_ESCAPED_DSML}).)*)
This preserves the ability to capture literal < in values (the concern
from vllm-project#46047) while preventing partial DSML closing tags from leaking into
parameter values.
The bug was introduced in vllm-project#45877 which ported DeepSeek V4 from the
hand-written DeepSeekV32ToolParser to the StreamingParserEngine framework.
The old parser used a buffer-and-split approach that searched for the
complete closing tag string as a boundary and never ran a regex on partial
args.
Verified on a live 8xH200 deployment: all streaming tool call tests
produce clean JSON arguments with no DSML leakage after the fix.
Co-authored-by: GLM5.2
Signed-off-by: William Schmitt <w.schmitt@criteo.com>
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August 21, 2026 07:22
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…xt DSML tag _PARTIAL_PARAM_RE captures the in-progress parameter value with a greedy `(.*)$` under re.DOTALL, so it absorbs everything left in the buffer. Two consequences seen in production: * the well-formed case leaks the closer itself into the streamed argument (`done</｜DSML｜parameter>` instead of `done`); * when the model mis-spells a closer -- `</｜DSML｜>`, `</+>`, `</percent_placeholder>` were all observed -- the value swallows every following `<｜DSML｜parameter ...>` block, collapsing a multi-argument call into one corrupt string. 33 of 78 leaked responses in a 9034-response sample had this shape. Stop the capture at the next `<｜DSML｜` / `</｜DSML｜` boundary. A bare `<` in a value is unaffected because the lookahead requires the full marker. Same fix as vllm-project#53228, reached independently from production captures. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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4ca34ce bounded only _PARTIAL_PARAM_RE, the streaming path. The complete match _PARAM_RE has the same defect: `(.*?)</｜DSML｜parameter>` under DOTALL is non-greedy but still skips a mis-spelled closer and latches onto the next real one, so a single bad parameter absorbs every parameter after it. Observed live on a patched box: args = {"alpha": "first</｜DSML｜>\\n<｜DSML｜parameter name=\\"beta\\" string=\\"true\\">second"} Both arguments are lost: alpha carries markup, beta never appears. With the value bounded at the DSML tag boundary the malformed parameter is dropped and the rest survive -- {"beta": "second"}. Well-formed input, values containing a bare '<', and multiline values are unaffected. Upstream vllm-project#53228 patches only the partial regex and has the same gap. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Purpose
Fixes streaming tool call argument corruption in the DeepSeek V4 parser where DSML closing tag fragments leak into tool call arguments.
_PARTIAL_PARAM_REused a greedy(.*)$capture that swallowed partial DSML closing tags (e.g.Paris</｜DSML｜parameterwithout the final>) into parameter values during streaming. Replaced with a tempered greedy token using a negative lookahead that stops at DSML tag prefixes:This preserves literal
<in values (the concern from #46047) while preventing partial closing tags from leaking into arguments.Root cause
Introduced in #45877 which ported DeepSeek V4 to the
StreamingParserEngine. The old hand-written parser (DeepSeekV32ToolParser, #42879) used a buffer-and-split approach that never ran a regex on partial args.Verification
Tested on a live 8xH200 deployment (vLLM v0.27.1, dspark speculative decoding):
stream: true, tool_choice: required: DSML markup no longer leaks into arguments (3/3 runs clean)stream: true, tool_choice: auto: tool calls now stream proper incremental argument deltas (previously emitted empty{})stream: false: unchanged (always worked)Test plan
Added two regression tests in
tests/parser/engine/test_deepseek_v4.py:test_partial_does_not_capture_closing_tag_prefix— partial closing tag in a single parametertest_partial_with_complete_param_and_partial_closing_tag— complete parameter followed by a partial closing tagAI assistance was used. Every changed line was reviewed and tested.
Closes #53227