SGLang DeepSeek V3.1 Optimization

Overview

This skill covers the DeepSeek V3.1 support and optimization ladder that is active in SGLang main. V3.1 shares the DeepSeek V3/R1 model backbone, but its tool-call format, chat template, thinking flag, streaming parser, and validation lanes are separate enough to require an independent skill.

Current-main snapshot:

• SGLang origin/main: 929e00eea on 2026-04-21
• sgl-cookbook origin/main: 8ec4d03 on 2026-04-21
• core runtime entry: python/sglang/srt/models/deepseek_v2.py
• V3.1 tool parser: python/sglang/srt/function_call/deepseekv31_detector.py
• V3.1 tool template: examples/chat_template/tool_chat_template_deepseekv31.jinja

The historical evidence lives in:

• references/pr-history.md: chronological PR evidence and code-level notes
• references/playbook.md: investigation order, symptom mapping, validation commands

Before You Change Anything

Record the exact serving shape first:

• model: deepseek-ai/DeepSeek-V3.1, DeepSeek-V3.1-Terminus, or DeepSeek-V3.1-Speciale
• whether thinking mode is enabled with chat_template_kwargs.thinking
• whether tool calling is enabled with --tool-call-parser deepseekv31
• whether the V3.1 tool template is used: examples/chat_template/tool_chat_template_deepseekv31.jinja
• --reasoning-parser deepseek-v3
• TP / DP / EP / PP topology
• MTP enabled or not
• backend and quantization inherited from the DeepSeek V3/R1 backbone
• streaming or non-streaming OpenAI API path
• exact test lane: manual, nightly, parser unit, chat-template unit, or model-backed accuracy/perf

Core Principle

Do not debug V3.1 as ordinary V3.

• The model class is shared with V3/R1, so MLA, MoE, quantization, DeepEP, and MTP bugs usually belong to sglang-deepseek-v3-r1-optimization.
• The user-visible V3.1 delta is parser and template behavior: hybrid thinking, tool calling, streaming deltas, and structural-tag constrained decoding.
• V3.1 tool calls do not use V3's literal function marker or fenced JSON block.
• V3.1 uses chat_template_kwargs: {"thinking": true} with --reasoning-parser deepseek-v3, not R1's parser.
• DeepSeek-V3.1-Speciale should not be treated as a tool-calling target.

The optimization order matters:

1. confirm the parser/template contract
2. confirm streaming and non-streaming parity
3. confirm thinking mode and </think> template behavior
4. confirm model loading and MTP inherited from V3/R1
5. only then tune MoE/backend performance
6. add CPU parser tests for parser-only changes before running model-backed tests

Main Runtime Surfaces

Start from these files before changing behavior:

• python/sglang/srt/function_call/deepseekv31_detector.py
• python/sglang/srt/function_call/function_call_parser.py
• examples/chat_template/tool_chat_template_deepseekv31.jinja
• python/sglang/srt/entrypoints/openai/serving_chat.py
• python/sglang/srt/parser/reasoning_parser.py
• python/sglang/srt/managers/schedule_batch.py
• python/sglang/srt/mem_cache/common.py
• python/sglang/srt/models/deepseek_v2.py
• python/sglang/srt/models/deepseek_common/deepseek_weight_loader.py
• test/manual/test_deepseek_v31.py
• test/manual/nightly/test_deepseek_v31_perf.py
• test/manual/test_deepseek_chat_templates.py

Open PRs to Track

Check these before declaring a V3.1 gap:

• #17761: missing Assistant token after tool output in V3.1/V3.2 chat templates, open.
• #18236: function-call arguments missing in V3.1 streaming mode, open.
• #21739: NPU deployment docs for V3.1/V3.2, open.
• #22433: CPU unit tests for DeepSeekV31Detector, open.
• #22981: broader CPU tests for missing function-call detectors, open.
• #23336: adaptive speculative-num-steps support for spec v2 EAGLE workers, open and relevant when V3.1 MTP runs with spec v2.

Additional PR Coverage

Additional all-state PR coverage includes parser/template PRs that are relevant to V3.1 even though they are not the core bring-up PRs:

• #9468, #10875, and #11189 refine reasoning/thinking docs, request handling, and eval flags.
• #9895 and #14837 update tool_chat_template_deepseekv31.jinja.
• #10550, #11223, #11589, and #21593 are general tool-choice / constrained-decoding / parser fixes that affect V3.1 serving behavior.
• #17141, #17320, and #17558 are closed attempts around the missing-Assistant-token-after-tool-output issue that remains tracked by open #17761.
• #22950 closed the first parser-gated reasoning radix-cache stripping attempt, while merged #23315 adds the current opt-in strip of thinking tokens from radix cache. For V3.1 thinking mode, this is cache-behavior work rather than a new tool parser.
• #21599 and #22128 are inherited MTP/speculative-decoding infrastructure updates: adaptive EAGLE draft steps and piecewise CUDA graph with speculative decoding.

Evolution Path

Stage V31-0: Split V3.1 tool calling from V3

DeepSeek V3.1 has a distinct tool-call wire format:

<｜tool▁calls▁begin｜><｜tool▁call▁begin｜>{name}<｜tool▁sep｜>{json_args}<｜tool▁call▁end｜><｜tool▁calls▁end｜>

It does not use V3's function literal or fenced json block.

Key PR:

• #9446

Success check:

• --tool-call-parser deepseekv31 resolves to DeepSeekV31Detector
• the template emits V3.1 markers, not V3 markers
• multiple tool calls can be chained without separators

Stage V31-1: Enable hybrid thinking correctly

V3.1 thinking mode is toggled through the chat template, not through the R1 parser.

Key PR:

• #9464

Rules:

• launch with --reasoning-parser deepseek-v3
• send extra_body: {"chat_template_kwargs": {"thinking": true}}
• the template emits <think> when thinking is enabled and </think> when non-thinking is desired

Stage V31-2: Keep chat template argument types stable

V3.1 multi-turn tool calls can pass tool["function"]["arguments"] as either a dict or an already serialized JSON string. The template must not double-escape JSON strings.

Key PR:

• #12123

Success check:

• dict arguments are rendered through tojson
• string arguments are used as-is
• mixed multi-tool calls keep both forms intact

Stage V31-3: Harden structural tags and streaming deltas

The structural trigger should be the generic per-call begin token, not the full name-specific begin string. Streaming must also preserve arguments that arrive in the first chunk and normal text before the tool marker.

Key PRs:

• #13394
• #18236, open
• #22433, open

Success check:

• structure_info().trigger is <｜tool▁call▁begin｜>
• streaming emits the function name and the first argument diff when they appear together
• normal text before the first tool marker is not dropped
• CPU parser tests cover invalid JSON, unknown tools, unicode args, multiple calls, and streaming chunks

Stage V31-4: Keep inherited model loading and MTP healthy

V3.1 still depends on DeepSeek V3/R1 loader, MLA, MoE, and MTP surfaces.

Key PRs:

• #13954
• #16660
• #13190

Success check:

• test/manual/test_deepseek_v31.py covers TP8 and TP8+MTP
• nightly perf no longer carries stale enable_dp_attention flags
• loading fixes are checked in the shared DeepSeek V2/V3 loader and in parser code

Stage V31-5: Tune MoE configs as a DeepSeek-family shape

V3.1 shares the DeepSeek MoE shape with V3/V3.2-style fused MoE config work.

Key PR:

• #17133

Success check:

• H20 and H20-3E configs exist for E=257,N=256,fp8_w8a8
• config selection is validated on the same hardware lane that motivated the tuning

Validation Surface

Use the narrowest lane that matches the change:

• parser-only: open #22433 test pattern, or equivalent DeepSeekV31Detector CPU unit tests
• template-only: test/manual/test_deepseek_chat_templates.py
• V3.1 base/MTP: test/manual/test_deepseek_v31.py
• nightly performance: test/manual/nightly/test_deepseek_v31_perf.py
• inherited MLA/MoE/quant bug: switch to sglang-deepseek-v3-r1-optimization