Add: constrain DSpark tool calls with XGrammar

[sjduan]

Summary

Support constrained DeepSeek V4 DSpark tool calls for strict auto, required and named tool choice; ordinary auto remains free generation. Compile request-local XGrammar structural tags from the declared schemas, validate constraints before SSE headers, and pass the packed mask through scheduling, IPC and the fused K7 graph. Worker-local matcher state advances only on committed output and is released on completion or cancellation.

The ordinary path reuses device-resident default inputs. Constrained requests reuse Host mask buffers and prepare only the valid draft prefix plus bonus row; the fused graph and three-stage worker pipeline remain unchanged. If constrained recompute preemption would be required and no in-flight step can make progress, the scheduler rejects one stalled request explicitly instead of waiting indefinitely.

Pins merged PyPTO-Lib main 3902122, which includes hw-native-sys/pypto-lib#1376. Requires a fresh compile cache and XGrammar 0.2.7 on the Host.

Performance: default Serving vs final Serving

Real 16-device DSpark K7 service, same W8A8 weights, greedy decoding, DP4/TP4/EP16 and prefix caching off. The fixed-tool workload emits the same 44 tokens and tool call in every arm. Each median covers 12 measured requests after warmup.

HTTP end-to-end workload	Default Serving main	Final constrained Serving
Ordinary tool request	2,005.608 ms	2,059.021 ms
Strict tool request	Not supported	2,078.744 ms

The default and final measurements were made on different test nodes and request sequences. Their numerical difference is descriptive, not a controlled regression estimate; no default-to-final percentage is claimed. Within the final same-node paired run, the median strict-minus-ordinary premium was 9.168 ms. The strict prompt was five tokens longer because its schema serialized strict; that premium includes both the prompt and constraint handling. A separate earlier matched-node comparison of default Serving against the Device-only candidate showed +1.25% ordinary latency, but it does not establish the final version's overhead.

Validation and limits

• On 2026-09-30, Serving fea9672 was checked against merged Lib main 3902122 with PyPTO 5f71449f, pinned Simpler 6e383fc, PTOAS 0.65 and XGrammar 0.2.7. Imports and dependency checks passed; 111 focused Serving unit tests passed; default TP2/EP2 prefill passed all golden outputs; and fused K7 compile-only validation passed. This PR now pins that tested Lib commit. End-to-end HTTP regression for this updated dependency pin remains pending.
• On 2026-09-29, the committed PR heads (Serving 0731e9c, Lib 84e098c) were tested with PyPTO ee49fce, its pinned Simpler 6e383fc, PTOAS 0.65, XGrammar 0.2.7, and the real 16-device DeepSeek V4 Flash DSpark K7 W8A8 model. Prefix caching was disabled for this functional regression.
• PyPTO rebuilt and imported successfully. Lib prefill ragged2 and fused DSpark K7 TP2/EP2 compile-only checks passed. Serving/DSpark unit tests passed (364 tests, including the 111 focused constraint/tool/sampling tests).
• Live HTTP regression passed 19/19 requests: 13 typical and 6 edge cases. Coverage included required/named/strict-auto tool choice; streaming and non-streaming tool calls; an adversarial cmd instruction that still produced the required shell.command; two tool calls in one response; streamed reasoning followed by a tool call; ordinary chat and completions; request/schema rejection with HTTP 400 before SSE; concurrent requests; length truncation; tool-result continuation; and cancellation followed by a healthy strict request. The service remained healthy with no server ERROR/Traceback/HEAP_RING entries observed after the run.
• The shell.command result is established for the tested constrained schemas and tool-choice paths, not as a guarantee for ordinary non-strict auto. The exact external agent-client request was not replayed. Broad schemas, saturated throughput, and all model shapes were not validated.
• This version still uploads the full 40 MiB mask tensor for constrained requests; Host preparation is reduced, but H2D bytes are not. Constrained matcher replay on recompute preemption is not implemented. Ordinary auto is intentionally not forced into structured generation.

Refs #265

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📝 Walkthrough

Walkthrough

This change adds request-local XGrammar constraints for supported DeepSeek V4 tool generation. It validates and preflights constraints, carries them through serving and worker execution, and stages grammar masks in DSpark prefill and decode.

Changes

XGrammar-constrained tool generation

Layer / File(s)	Summary
Constraint and parser contracts pypto_serving/serving/constraints/*, pypto_serving/serving/reasoning/*	Adds the serializable ConstraintSpec and XGrammar provider/state interfaces. Parser validation accepts required tool choice, and the DeepSeek V4 parser publishes tool calls for choices other than none.
Chat validation and constraint handoff pypto_serving/serving/server/server.py, pypto_serving/serving/engine/async_engine.py, pypto_serving/serving/server/ipc.py, pypto_serving/serving/sched/scheduler.py, tests/unit/serving/constraints/*, tests/unit/serving/server/test_tool_calls.py, tests/unit/serving/sched/test_async_scheduler.py	Accepts required and named tool choices, preflights eligible constraints, and forwards constraint specifications through engine registration and worker IPC. The scheduler excludes constrained requests from preemption. Tests cover validation, forwarding, tool parsing, and preemption behavior.
Worker constraint lifecycle pypto_serving/config/types.py, pypto_serving/serving/server/serving_worker.py, tests/unit/serving/engine/test_async_pipeline.py, tests/unit/serving/server/test_worker_step_protocol.py	Stores request-local constraint states, attaches them to prefill and decode batches, advances them on accepted tokens, and closes them during request cleanup. Tests cover errors, aborts, and state cleanup.
DSpark grammar-mask staging pypto_serving/model/deepseek_dspark/npu_runner.py, pypto_serving/model/deepseek_dspark/task_args.py, tests/unit/model/deepseek_dspark/*	Adds grammar-mask buffers and tensor ordering, stages prefill and decode masks, and rejects constrained decode on the non-fused path. Tests cover mask packing, staging cleanup, and fused ABI ordering.

Priority: ⬇️ Low

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant ServingServer
  participant AsyncLLMEngine
  participant WorkerProcess
  participant XGrammarProvider
  participant DSparkModelRunner
  ServingServer->>XGrammarProvider: Preflight-compile ConstraintSpec
  ServingServer->>AsyncLLMEngine: Submit request with ConstraintSpec
  AsyncLLMEngine->>WorkerProcess: Send serialized ConstraintSpec
  WorkerProcess->>XGrammarProvider: Compile request-local constraint state
  WorkerProcess->>DSparkModelRunner: Pass state in PrefillBatch or DecodeBatch
  DSparkModelRunner->>WorkerProcess: Return constrained execution results

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Merge Risk: 🟡 Moderate · up to 7897b

Strict tool-choice requests on DeepSeek V4 DSpark can hang indefinitely when the KV cache fills with constrained requests. They can also fail at mask staging if the tokenizer's vocabulary width differs from the model's fixed 129280-token vocabulary. Both issues should be fixed, or explicitly accepted, before this merges alongside the paired pypto-lib change.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.89% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 112 functions across 20 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the main change: adding XGrammar-constrained DSpark tool calls.
Description check	✅ Passed	The description is detailed and directly explains the constrained DSpark tool-call implementation, scope, validation, dependencies, and limitations.

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_{A rabbit checks the grammar, then hops into the queue.
A mask of bits marks tokens that the request may pursue.
The worker keeps each matcher close while decoded tokens flow.
The runner stages rows for drafts, then clears the ones that go.
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Actionable comments posted: 2

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Inline comments:
In `@pypto_serving/serving/constraints/provider.py`:
- Around line 43-53: Update XGrammarProvider to accept ModelConfig.vocab_size
and use it for XGrammar’s packed vocabulary width, rejecting values smaller than
the tokenizer-derived size. Keep tokenizer contiguity validation based on the
tokenizer-derived size, and pass ModelConfig.vocab_size at both XGrammarProvider
construction sites.

In `@pypto_serving/serving/sched/scheduler.py`:
- Around line 997-1004: Update _preempt_lowest_priority and its caller so that
when block allocation fails and no unconstrained victim is available, a stalled
constrained running request is finished with FINISHED_LENGTH or added to
output.rejected_requests, releasing its resources and allowing the scheduler to
progress.

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Configuration used: Organization UI

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Run ID: 7afe7686-5876-4ba6-86e2-f93008e1b2e3

📥 Commits

Reviewing files that changed from the base of the PR and between bcba8d2 and 7897b8d.

📒 Files selected for processing (20)

• pypto_serving/config/types.py
• pypto_serving/model/deepseek_dspark/npu_runner.py
• pypto_serving/model/deepseek_dspark/task_args.py
• pypto_serving/serving/constraints/__init__.py
• pypto_serving/serving/constraints/provider.py
• pypto_serving/serving/constraints/spec.py
• pypto_serving/serving/engine/async_engine.py
• pypto_serving/serving/reasoning/deepseek_v4_tools.py
• pypto_serving/serving/reasoning/parser.py
• pypto_serving/serving/sched/scheduler.py
• pypto_serving/serving/server/ipc.py
• pypto_serving/serving/server/server.py
• pypto_serving/serving/server/serving_worker.py
• tests/unit/model/deepseek_dspark/test_dspark_model.py
• tests/unit/model/deepseek_dspark/test_grammar_staging.py
• tests/unit/serving/constraints/test_spec.py
• tests/unit/serving/engine/test_async_pipeline.py
• tests/unit/serving/sched/test_async_scheduler.py
• tests/unit/serving/server/test_tool_calls.py
• tests/unit/serving/server/test_worker_step_protocol.py

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