feat(laguna): add poolside/Laguna-S-2.1 support via a dedicated renderer

README.md

@@ -40,7 +40,7 @@ next_prompt_ids = r.bridge_to_next_turn(
 )
 ```
 
-Hand-coded renderers ship for `qwen3`, `qwen3-vl`, `qwen3.5`, `qwen3.6`, `glm-5`, `glm-5.1`, `glm-4.5`, `minimax-m2`, `deepseek-v3`, `deepseek-r1`, `kimi-k2`, `kimi-k2.5` / `kimi-k2.6`, `laguna-xs.2`, `laguna-xs-2.1`, `laguna-m.1`, `nemotron-3`, `nemotron-3-ultra`, `llama-3`, `gpt-oss`, `hy3`, and `prime-qwen3`. Anything else falls back to `DefaultRenderer`, a generic `apply_chat_template` wrapper.
+Hand-coded renderers ship for `qwen3`, `qwen3-vl`, `qwen3.5`, `qwen3.6`, `glm-5`, `glm-5.1`, `glm-4.5`, `minimax-m2`, `deepseek-v3`, `deepseek-r1`, `kimi-k2`, `kimi-k2.5` / `kimi-k2.6`, `laguna-xs.2`, `laguna-xs-2.1`, `laguna-s-2.1`, `laguna-m.1`, `nemotron-3`, `nemotron-3-ultra`, `llama-3`, `gpt-oss`, `hy3`, and `prime-qwen3`. Anything else falls back to `DefaultRenderer`, a generic `apply_chat_template` wrapper.
 
 ## API
 

docs/renderer-config.md

@@ -39,6 +39,7 @@ chat-template kwargs. Those fields are covered by parity tests against
 | Laguna XS.2 | `LagunaXS2RendererConfig` | `enable_thinking`, `render_assistant_messages_raw` | - |
 | Laguna M.1 | `LagunaM1RendererConfig` | `enable_thinking`, `render_assistant_messages_raw` | - |
 | Laguna XS-2.1 | `LagunaXS21RendererConfig` | `enable_thinking` | - |
+| Laguna S-2.1 | `LagunaS21RendererConfig` | `enable_thinking`, `preserve_thinking` | - |
 | Llama 3 | `Llama3RendererConfig` | `date_string`, `tools_in_user_message` | - |
 | MiniMax M2 | `MiniMaxM2RendererConfig` | `model_identity` | - |
 | Nemotron-3 Nano / Super | `Nemotron3RendererConfig` | `enable_thinking`, `truncate_history_thinking`, `low_effort` | - |
@@ -137,7 +138,7 @@ the knobs its template actually exposes:
 | Nemotron-3 | `truncate_history_thinking=False -> all`; else `enable_thinking=False -> all`; else `tool_cycle` |
 | DeepSeek R1 | `template` |
 | MiniMax M2 | `tool_cycle` |
-| DeepSeek V3, Qwen3-VL, Kimi K2, Laguna XS.2 / M.1 / XS-2.1, Llama 3 | `all` |
+| DeepSeek V3, Qwen3-VL, Kimi K2, Laguna XS.2 / M.1 / XS-2.1 / S-2.1, Llama 3 | `all` |
 | PrimeIntellect Qwen3 | `all` |
 
 Config construction raises when an explicit template knob directly contradicts

renderers/__init__.py

@@ -55,6 +55,7 @@
     KimiK25RendererConfig,
     KimiK2RendererConfig,
     LagunaM1RendererConfig,
+    LagunaS21RendererConfig,
     LagunaXS2RendererConfig,
     LagunaXS21RendererConfig,
     Llama3RendererConfig,
@@ -92,6 +93,7 @@
     "KimiK25Renderer": "renderers.kimi_k25",
     "KimiK2Renderer": "renderers.kimi_k2",
     "LagunaM1Renderer": "renderers.laguna_xs2",
+    "LagunaS21Renderer": "renderers.laguna_xs2",
     "LagunaXS21Renderer": "renderers.laguna_xs2",
     "LagunaXS2Renderer": "renderers.laguna_xs2",
     "Llama3Renderer": "renderers.llama_3",
@@ -149,6 +151,8 @@ def __dir__() -> list[str]:
     "KimiK2RendererConfig",
     "LagunaM1Renderer",
     "LagunaM1RendererConfig",
+    "LagunaS21Renderer",
+    "LagunaS21RendererConfig",
     "LagunaXS2Renderer",
     "LagunaXS2RendererConfig",
     "LagunaXS21Renderer",

renderers/base.py

@@ -1073,11 +1073,16 @@ def bridge_to_next_turn(self, *args: Any, **kwargs: Any) -> "RenderedTokens | No
     # construction to pin a different date.
     "meta-llama/Llama-3.2-1B-Instruct": "llama-3",
     "meta-llama/Llama-3.2-3B-Instruct": "llama-3",
-    # Poolside Laguna. These checkpoints ship distinct chat templates,
-    # each mirrored by its own renderer class/config discriminator.
+    # Poolside Laguna. These checkpoints ship distinct chat templates, each
+    # mirrored by its own renderer class/config discriminator. S-2.1 is a
+    # larger sibling of XS-2.1 that shares its tokenizer but NOT its chat
+    # template: S-2.1 defaults ``enable_thinking`` to True and adds a
+    # ``preserve_thinking`` gate, so it gets its own renderer/config
+    # (``LagunaS21Renderer``), a thin subclass of the XS-2.1 renderer.
     "poolside/Laguna-XS.2": "laguna-xs.2",
     "poolside/Laguna-M.1": "laguna-m.1",
     "poolside/Laguna-XS-2.1": "laguna-xs-2.1",
+    "poolside/Laguna-S-2.1": "laguna-s-2.1",
     # GPT-OSS.
     "openai/gpt-oss-20b": "gpt-oss",
     "openai/gpt-oss-120b": "gpt-oss",
@@ -1327,6 +1332,7 @@ def _populate_registry():
     from renderers.kimi_k25 import KimiK25Renderer
     from renderers.laguna_xs2 import (
         LagunaM1Renderer,
+        LagunaS21Renderer,
         LagunaXS2Renderer,
         LagunaXS21Renderer,
     )
@@ -1359,6 +1365,7 @@ def _populate_registry():
             "laguna-xs.2": LagunaXS2Renderer,
             "laguna-m.1": LagunaM1Renderer,
             "laguna-xs-2.1": LagunaXS21Renderer,
+            "laguna-s-2.1": LagunaS21Renderer,
             "llama-3": Llama3Renderer,
             "nemotron-3": Nemotron3Renderer,
             "nemotron-3-ultra": Nemotron3UltraRenderer,

renderers/configs.py

@@ -546,6 +546,37 @@ class LagunaXS21RendererConfig(BaseRendererConfig):
     upstream default."""
 
 
+class LagunaS21RendererConfig(BaseRendererConfig):
+    """Laguna S-2.1 renderer config.
+
+    S-2.1 is a larger sibling of XS-2.1 sharing its tokenizer (same vocab,
+    special tokens, and merges), but its chat template is *not* byte-identical:
+    ``enable_thinking`` defaults to ``True`` (XS-2.1 defaults ``False``), and a
+    new ``preserve_thinking`` kwarg widens the reasoning-display gate to
+    ``enable_thinking or preserve_thinking``. The token format is otherwise
+    identical, so this is served by
+    :class:`renderers.laguna_xs2.LagunaS21Renderer`, a thin subclass of
+    ``LagunaXS21Renderer`` that only overrides that gate.
+    """
+
+    name: Literal["laguna-s-2.1"] = "laguna-s-2.1"
+
+    enable_thinking: bool = True
+    """When ``True``, the generation prompt ends with ``<think>`` and every
+    assistant turn renders ``<think>{reasoning}</think>``; when ``False``,
+    turns open with a bare ``</think>``. Mirrors the template's
+    ``enable_thinking`` kwarg — note S-2.1's upstream default is ``True``,
+    unlike XS-2.1's ``False``."""
+
+    preserve_thinking: bool = False
+    """When ``True``, assistant turns keep their ``<think>{reasoning}</think>``
+    block even while ``enable_thinking`` is ``False`` — the template gates
+    reasoning display on ``enable_thinking or preserve_thinking``. With the
+    default ``False`` the gate collapses to ``enable_thinking`` and the
+    renderer matches XS-2.1 turn-for-turn. Mirrors the template's
+    ``preserve_thinking`` kwarg and its upstream default."""
+
+
 class Llama3RendererConfig(BaseRendererConfig):
     """Llama-3.x Instruct renderer config.
 
@@ -704,6 +735,7 @@ class DeepSeekR1RendererConfig(BaseRendererConfig):
         LagunaXS2RendererConfig,
         LagunaM1RendererConfig,
         LagunaXS21RendererConfig,
+        LagunaS21RendererConfig,
         Llama3RendererConfig,
         MiniMaxM2RendererConfig,
         Nemotron3RendererConfig,
@@ -745,6 +777,7 @@ class DeepSeekR1RendererConfig(BaseRendererConfig):
     "laguna-xs.2": LagunaXS2RendererConfig,
     "laguna-m.1": LagunaM1RendererConfig,
     "laguna-xs-2.1": LagunaXS21RendererConfig,
+    "laguna-s-2.1": LagunaS21RendererConfig,
     "llama-3": Llama3RendererConfig,
     "minimax-m2": MiniMaxM2RendererConfig,
     "nemotron-3": Nemotron3RendererConfig,
@@ -792,6 +825,7 @@ def config_from_name(name: str) -> BaseRendererConfig | None:
     "KimiK25RendererConfig",
     "KimiK2RendererConfig",
     "LagunaM1RendererConfig",
+    "LagunaS21RendererConfig",
     "LagunaXS2RendererConfig",
     "LagunaXS21RendererConfig",
     "Llama3RendererConfig",

renderers/laguna_xs2.py

@@ -41,6 +41,13 @@
 :class:`LagunaM1Renderer`. It shares XS.2's byte layout, tool syntax, and
 generation prompt, but has no fallback system prompt and reads assistant
 reasoning from ``reasoning`` before falling back to ``reasoning_content``.
+
+S-2.1 is a larger sibling of XS-2.1 served by :class:`LagunaS21Renderer`. It
+shares XS-2.1's tokenizer and token format, so the two differ only in the chat
+template's thinking knobs: ``enable_thinking`` defaults to ``True`` (XS-2.1
+defaults ``False``), and a new ``preserve_thinking`` kwarg widens the
+reasoning-display gate to ``enable_thinking or preserve_thinking``. The
+subclass overrides only that gate; everything else is inherited.
 """
 
 from __future__ import annotations
@@ -63,6 +70,7 @@
 )
 from renderers.configs import (
     LagunaM1RendererConfig,
+    LagunaS21RendererConfig,
     LagunaXS2RendererConfig,
     LagunaXS21RendererConfig,
 )
@@ -120,6 +128,7 @@ def __init__(
             LagunaXS2RendererConfig
             | LagunaM1RendererConfig
             | LagunaXS21RendererConfig
+            | LagunaS21RendererConfig
             | None
         ) = None,
     ):
@@ -688,10 +697,17 @@ class LagunaXS21Renderer(LagunaXS2Renderer):
     def __init__(
         self,
         tokenizer: PreTrainedTokenizer,
-        config: LagunaXS21RendererConfig | None = None,
+        config: LagunaXS21RendererConfig | LagunaS21RendererConfig | None = None,
     ):
         super().__init__(tokenizer, config or LagunaXS21RendererConfig())
 
+    def _render_history_reasoning(self) -> bool:
+        """Whether an assistant turn renders its ``<think>{reasoning}</think>``
+        block (vs opening with a bare ``</think>``). Mirrors the template's
+        reasoning-display gate; XS-2.1 gates this on ``enable_thinking`` alone.
+        """
+        return self.config.enable_thinking
+
     def render(
         self,
         messages: list[Message],
@@ -975,7 +991,7 @@ def _render_assistant(
         # scaffolding the model never samples.
         emit_special(self._assistant, msg_idx, is_sampled=False, is_content=False)
 
-        if self.config.enable_thinking:
+        if self._render_history_reasoning():
             # ``<think>{reasoning}</think>`` renders verbatim, even when
             # the reasoning is empty; the opener is the gen-prompt
             # prefill, the rest the model sampled.
@@ -1017,3 +1033,36 @@ def _render_assistant(
 
         emit_special(self._assistant_end, msg_idx, is_sampled=True, is_content=True)
         emit_text("\n", msg_idx, is_sampled=False, is_content=False)
+
+
+class LagunaS21Renderer(LagunaXS21Renderer):
+    """Laguna-S-2.1 — a larger sibling of XS-2.1 that shares its tokenizer
+    and token format but ships a slightly different chat template.
+
+    The only template differences are the ``enable_thinking`` default (``True``
+    here, ``False`` for XS-2.1) and a new ``preserve_thinking`` kwarg that
+    widens the reasoning-display gate to ``enable_thinking or
+    preserve_thinking``. Everything else — role tags, tool-call packing,
+    system-block gate, generation prompt, and the whole parse skeleton — is
+    inherited unchanged from :class:`LagunaXS21Renderer`.
+    """
+
+    # Narrow the inherited ``config`` attribute so ``preserve_thinking`` (an
+    # S-2.1-only field) resolves; ``__init__`` always stores an S-2.1 config.
+    config: LagunaS21RendererConfig
+
+    def __init__(
+        self,
+        tokenizer: PreTrainedTokenizer,
+        config: LagunaS21RendererConfig | None = None,
+    ):
+        # ``LagunaXS21Renderer.__init__`` forwards any config straight through;
+        # pass an S-2.1 config so ``enable_thinking`` defaults to ``True`` and
+        # ``preserve_thinking`` is available.
+        super().__init__(tokenizer, config or LagunaS21RendererConfig())
+
+    def _render_history_reasoning(self) -> bool:
+        # S-2.1's template gates reasoning display on
+        # ``enable_thinking or preserve_thinking`` — the latter keeps
+        # historical ``<think>`` blocks even while thinking is off.
+        return self.config.enable_thinking or self.config.preserve_thinking

tests/conftest.py

@@ -41,6 +41,10 @@
     # XS-2.1 resolves to `LagunaXS21Renderer` via the model name — its
     # chat template differs from XS.2's (see renderers/laguna_xs2.py).
     ("poolside/Laguna-XS-2.1", "auto"),
+    # S-2.1 is a larger sibling of XS-2.1 sharing its tokenizer but NOT its
+    # chat template (S-2.1 defaults enable_thinking=True and adds a
+    # preserve_thinking gate), so it resolves to LagunaS21Renderer.
+    ("poolside/Laguna-S-2.1", "auto"),
     # DeepSeek-V3/R1 are intentionally NOT in this shared barrage: their
     # chat templates can't render the barrage's tool-call fixtures (the
     # templates require ``tool['type']`` and a string-serialized

tests/test_laguna_s21.py

@@ -0,0 +1,128 @@
+"""Laguna-S-2.1 focused tests.
+
+S-2.1 shares XS-2.1's tokenizer and token format, so the shared matrices
+(conftest / config-parity) already assert byte parity on the common shapes
+via :class:`LagunaS21Renderer`. This file pins the two behaviours that make
+S-2.1 *differ* from XS-2.1 — and that the shared barrage can't reach because
+it never sets the relevant kwargs:
+
+- ``enable_thinking`` defaults to ``True`` (XS-2.1 defaults ``False``), so the
+  auto-resolved renderer renders reasoning by default, and
+- the new ``preserve_thinking`` kwarg keeps historical ``<think>`` blocks even
+  while ``enable_thinking`` is ``False`` — the template gates reasoning display
+  on ``enable_thinking or preserve_thinking``.
+"""
+
+from __future__ import annotations
+
+from functools import lru_cache
+from itertools import product
+
+from renderers import create_renderer
+from renderers.base import load_tokenizer
+from renderers.configs import LagunaS21RendererConfig
+from renderers.laguna_xs2 import LagunaS21Renderer
+
+_MODEL = "poolside/Laguna-S-2.1"
+
+
+@lru_cache(maxsize=None)
+def _tok():
+    return load_tokenizer(_MODEL)
+
+
+def _renderer(**config_kwargs) -> LagunaS21Renderer:
+    renderer = create_renderer(_tok(), LagunaS21RendererConfig(**config_kwargs))
+    assert isinstance(renderer, LagunaS21Renderer)
+    return renderer
+
+
+def _expected(msgs, *, add_generation_prompt=False, **template_kwargs):
+    return list(
+        _tok().apply_chat_template(
+            msgs,
+            add_generation_prompt=add_generation_prompt,
+            tokenize=True,
+            return_dict=False,
+            **template_kwargs,
+        )
+    )
+
+
+def test_auto_resolves_to_s21_renderer_with_thinking_on_by_default():
+    """``poolside/Laguna-S-2.1`` resolves to :class:`LagunaS21Renderer`, and
+    its config defaults ``enable_thinking=True`` / ``preserve_thinking=False``
+    — matching S-2.1's template defaults (unlike XS-2.1's thinking-off)."""
+    r = create_renderer(_tok())  # auto-resolve via MODEL_RENDERER_MAP
+    assert isinstance(r, LagunaS21Renderer)
+    assert r.config.name == "laguna-s-2.1"
+    assert r.config.enable_thinking is True
+    assert r.config.preserve_thinking is False
+
+
+def test_default_renders_empty_think_wrapper():
+    """With thinking on by default, a reasoning-free assistant turn still
+    opens with the empty ``<think></think>`` wrapper (this is exactly the
+    render_ids divergence from the XS-2.1 renderer)."""
+    msgs = [
+        {"role": "user", "content": "What is 2+2?"},
+        {"role": "assistant", "content": "4"},
+    ]
+    r = _renderer()  # defaults: enable_thinking=True
+    ours = r.render_ids(msgs)
+    assert ours == _expected(msgs)
+    assert "<assistant><think></think>4</assistant>" in _tok().decode(ours)
+
+
+def test_preserve_thinking_keeps_history_when_thinking_off():
+    """``enable_thinking=False`` alone drops reasoning, but with
+    ``preserve_thinking=True`` the historical ``<think>{reasoning}</think>``
+    survives — matching the template's ``enable_thinking or preserve_thinking``
+    gate."""
+    msgs = [
+        {"role": "user", "content": "What is 2+2?"},
+        {"role": "assistant", "reasoning_content": "Simple arithmetic", "content": "4"},
+    ]
+    r = _renderer(enable_thinking=False, preserve_thinking=True)
+    ours = r.render_ids(msgs)
+    assert ours == _expected(msgs, enable_thinking=False, preserve_thinking=True)
+    assert "<think>Simple arithmetic</think>" in _tok().decode(ours)
+
+
+def test_no_preserve_thinking_drops_history_when_thinking_off():
+    """With both flags off the gate collapses to ``enable_thinking`` and the
+    turn opens with a bare ``</think>``, dropping the reasoning — identical to
+    the XS-2.1 renderer's default."""
+    msgs = [
+        {"role": "user", "content": "What is 2+2?"},
+        {"role": "assistant", "reasoning_content": "Simple arithmetic", "content": "4"},
+    ]
+    r = _renderer(enable_thinking=False, preserve_thinking=False)
+    ours = r.render_ids(msgs)
+    assert ours == _expected(msgs, enable_thinking=False, preserve_thinking=False)
+    assert "Simple arithmetic" not in _tok().decode(ours)
+
+
+def test_all_thinking_flag_combos_match_template():
+    """Byte parity against ``apply_chat_template`` for every
+    ``enable_thinking`` × ``preserve_thinking`` combination, over a multi-turn
+    conversation carrying historical reasoning."""
+    msgs = [
+        {"role": "user", "content": "Q1"},
+        {"role": "assistant", "reasoning_content": "deduce", "content": "A1"},
+        {"role": "user", "content": "Q2"},
+        {"role": "assistant", "reasoning_content": "more", "content": "A2"},
+    ]
+    for enable_thinking, preserve_thinking in product((False, True), repeat=2):
+        r = _renderer(
+            enable_thinking=enable_thinking, preserve_thinking=preserve_thinking
+        )
+        for add_generation_prompt in (False, True):
+            assert r.render_ids(
+                msgs, add_generation_prompt=add_generation_prompt
+            ) == _expected(
+                msgs,
+                add_generation_prompt=add_generation_prompt,
+                enable_thinking=enable_thinking,
+                preserve_thinking=preserve_thinking,
+            ), (enable_thinking, preserve_thinking, add_generation_prompt)

tests/test_renderer_config_parity.py

@@ -66,6 +66,7 @@
     ("poolside/Laguna-XS.2", "auto"),
     ("poolside/Laguna-M.1", "auto"),
     ("poolside/Laguna-XS-2.1", "auto"),
+    ("poolside/Laguna-S-2.1", "auto"),
     ("tencent/Hy3", "auto"),
     ("openai/gpt-oss-20b", "gpt-oss"),
 ]