[Bugfix] DeepSeek V4: recover tool calls within reasoning content

[procr1337]

Purpose

NOTE: This was developed with the help of Claude Opus 4.8

DeepSeek-V4-Flash (and likely also the Pro variant) occasionally omits the closing </think> before emitting a <｜DSML｜tool_calls> block. This is a model bug: the chat template / encoder explicitly instructs it to output its complete reasoning inside <think>... </think> BEFORE any tool calls or final response, and the model intermittently fails to follow that instruction. Reasoning extraction is gated on </think>, so when it is missing the entire tool-call block stays in the reasoning channel, the tool parser never sees it, and the call is silently lost (finish_reason=stop, empty content). DeepSeek V4's own encoding README notes the model can emit malformed output and that production needs "additional error handling".

Workaround: a narrow, opt-in recovery in DelegatingParser that salvages a tool-call block left in the reasoning stream and promotes it to a real tool call. This is deliberately scoped as recovery from malformed model output: remains the only reasoning terminator; the tool-call marker is not added to the reasoning grammar.

• ToolParser gains tool_call_start_token / tool_call_end_token and an opt-in recovers_tool_calls_in_reasoning flag, so the generic composition code keys off the declared interface instead of sniffing subclass internals. DeepSeekV32ToolParser (and thus V4) opts in.
• Non-streaming: when auto tool-choice parsed zero tool calls and produced no content, and the reasoning tail contains a balanced start/end tool-call block, re-run the tool parser over it and keep the preceding prose as reasoning. The balance requirement avoids promoting half-formed output or a marker the model merely mentioned.
• Streaming: watch the reasoning stream for the start marker, buffering partial markers across chunks so they never leak as reasoning; on a full marker, end reasoning right before it and route the remainder to the tool-call phase.
• Tests exercise the composed reasoning+tool DelegatingParser with a stub tokenizer for both the non-streaming and streaming missing- shapes.

Relation to existing PRs

This is not a duplicate of any open PR:

• Fix Qwen3.5 reasoning tool calls embedded inside think #39055 ("Fix Qwen3 reasoning tool calls embedded inside think") targets the same class of bug — tool calls lost inside <think> reasoning — but for Qwen3/Qwen3.5, implemented in the reasoning parser (qwen3_reasoning_parser.py) and only for the non-streaming path. This PR is for DeepSeek V4/V3.2, lives in the generic DelegatingParser composition layer behind an opt-in recovers_tool_calls_in_reasoning flag, covers both streaming and non-streaming, and deliberately does not turn the tool-call marker into a reasoning terminator (</think> stays the only terminator). The two are complementary, not overlapping; happy to align on a shared abstraction if maintainers prefer.
• [Draft][Rust Frontend] Route top-N tool-call families through the Dynamo parser #45451 ("[Draft] Use Dynamo parser for DeepSeek V4") swaps in the Rust Dynamo parser behind an experimental env flag and does not touch this Python recovery path.
• [Frontend] Add Streaming Parser Engine and new Qwen3 Parser #45413 ("[Frontend] Add Streaming Parser Engine and new Qwen3 Parser") - already merged, mentions future work to port DeepSeek V4 parser to the new engine. This is orthogonal, but future refactors should take into account this model behavior bug.

Test Plan

.venv/bin/python -m pytest tests/parser tests/tool_parsers tests/reasoning \
  -p no:cacheprovider -o addopts="" -n 32 -ra \
  --timeout=60 --timeout-method=thread \
  --junit-xml=/tmp/after.xml

Furthermore, multiple people from the RTX Pro 6000 Discord community have been testing a backport of this patch to pre-#45413 code state in production with DeepSeek V4 Flash: procr1337#1

Test Result

No issues or regressions reported from production testing.

Unit test results:

Run	Tests	Passed	Failed	Errored	Skipped
before (f2beaa80c)	3089	2806	50	198	35
after (this PR)	3091	2890	50	116	35

Per-test before→after diff:

• 0 genuine regressions. The diff flagged 2 pass→fail tests, both in test_mistral_tool_parser.py (untouched by this PR) and both error on setup from huggingface_hub.HfHubHTTPError (HF Hub rate-limiting) / xdist worker crashes. 84 other tests flipped the opposite way (errored before, passed after) — confirming the errors column is nondeterministic network/load flakiness, not code. Re-running the 2 flagged tests in isolation: both pass.
• +2 new tests added by this PR: pass.

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[procr1337]

@bbrowning as mentioned in Slack DM, I went with a temporary workaround to solve a real bug users are experiencing. If DSv4 parser is refactored in the future, this may become obsolete and could potentially be partially reverted

[VickY0E]

Nice approach for the missing-</think> case.

As a complementary angle: the DSML leaks reported in #40801 also happen because xgrammar grammar guidance is currently off for tool_choice="auto" + non-strict tools (since #45600), so the model is free to emit malformed DSML that no parser can extract. Re-enabling the deepseek_v4 structural tag for auto + non-strict (xgrammar returns a TriggeredTagsFormat — constrains DSML only when the model triggers a tool call, doesn't force one) would prevent the malformed output at generation time, complementing this PR's post-hoc recovery.

Full root-cause analysis + minimal proposed fix in my comment on #40801. Prevention (grammar guidance) + recovery (this PR) together would cover both the malformed-DSML and missing-</think> failure modes.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @procr1337.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[procr1337]

@bbrowning IIUC your PR #45877 includes this fix. Feel free to close this one if that's accurate :)

[procr1337]

Closing because #45877 includes this fix