Fix garbage output for bare-tekken Mistral checkpoints (e.g. Leanstral) (#30396)

python/sglang/srt/utils/hf_transformers/common.py

@@ -185,6 +185,33 @@ def _resolve_local_or_cached_file(model_name_or_path, filename, revision=None):
     )
 
 
+def _cached_file_exists(model_name_or_path, filename, revision=None) -> bool:
+    """Whether *filename* is available locally or in the HF cache (no network)."""
+    try:
+        _resolve_local_or_cached_file(model_name_or_path, filename, revision)
+        return True
+    except Exception:
+        return False
+
+
+def _remote_file_exists(repo_id, filename, revision=None) -> bool:
+    """Whether *filename* exists on the HF hub (HEAD request only, no download).
+
+    Returns False on any error (offline, gated, network, invalid id) so callers
+    fall back to their default path instead of crashing.
+    """
+    from huggingface_hub.constants import HF_HUB_OFFLINE
+
+    if HF_HUB_OFFLINE:
+        return False
+    try:
+        from huggingface_hub import HfApi
+
+        return HfApi().file_exists(repo_id, filename, revision=revision)
+    except Exception:
+        return False
+
+
 def check_gguf_file(model: Union[str, os.PathLike]) -> bool:
     model = Path(model)
     if not model.is_file():

python/sglang/srt/utils/hf_transformers/mistral_utils.py

@@ -11,7 +11,12 @@ from transformers import AutoConfig, PretrainedConfig, WhisperConfig
 
 from sglang.srt.utils import logger
 
-from .common import _ensure_sub_configs, download_from_hf
+from .common import (
+    _cached_file_exists,
+    _ensure_sub_configs,
+    _remote_file_exists,
+    download_from_hf,
+)
 
 
 def adapt_config_dict(
@@ -430,6 +435,34 @@ _MISTRAL_TOKENIZER_REDIRECTS = {
 }
 
 
+def is_bare_tekken_checkpoint(tokenizer_name, revision=None) -> bool:
+    """True iff the checkpoint ships tekken.json but no tokenizer.json.
+
+    AutoTokenizer converts tekken.json on the fly, but the converter assigns
+    BPE ids from rank 0, dropping the 1000 special-token slots that precede
+    the BPE vocab in tekken's id space — every encoded id is shifted and
+    generation produces garbage. Such checkpoints must load through the
+    mistral-common backed tokenizer instead.
+    """
+
+    local_dir = Path(tokenizer_name)
+    if local_dir.is_dir():
+        return (local_dir / "tekken.json").is_file() and not (
+            local_dir / "tokenizer.json"
+        ).is_file()
+
+    if _cached_file_exists(tokenizer_name, "tokenizer.json", revision):
+        return False
+    if _cached_file_exists(tokenizer_name, "tekken.json", revision):
+        return True
+
+    # Cold cache: the tokenizer loads before weights, so tekken.json isn't
+    # cached yet on a first launch — HEAD-probe the hub to still detect it.
+    if not _remote_file_exists(tokenizer_name, "tekken.json", revision):
+        return False
+    return not _remote_file_exists(tokenizer_name, "tokenizer.json", revision)
+
+
 def retry_without_mistral_common_kwargs(tokenizer_name, *args, **common_kwargs):
     """Retry ``AutoTokenizer.from_pretrained`` without kwargs that MistralCommon rejects.
 

python/sglang/srt/utils/hf_transformers/tokenizer.py

@@ -38,6 +38,7 @@ from .common import (
 )
 from .mistral_utils import (
     _MISTRAL_TOKENIZER_REDIRECTS,
+    is_bare_tekken_checkpoint,
     patch_mistral_common_tokenizer,
     retry_without_mistral_common_kwargs,
 )
@@ -496,6 +497,23 @@ def get_tokenizer(
     )
 
     try:
+        if is_bare_tekken_checkpoint(tokenizer_name, tokenizer_revision):
+            from transformers.tokenization_mistral_common import (
+                MistralCommonTokenizer,
+            )
+
+            logger.info(
+                "Detected bare-tekken checkpoint %s (tekken.json, no "
+                "tokenizer.json); loading via mistral-common MistralCommonTokenizer, "
+                "ignoring tokenizer_backend=%r.",
+                tokenizer_name,
+                tokenizer_backend,
+            )
+
+            tokenizer = MistralCommonTokenizer.from_pretrained(
+                tokenizer_name, revision=tokenizer_revision
+            )
+        else:
             tokenizer = _auto_tokenizer_from_pretrained(
                 tokenizer_name, *args, **common_kwargs
             )

test/registered/unit/tokenizer/test_tekken_tokenizer_routing.py

@@ -0,0 +1,48 @@
+"""Unit tests for bare-tekken checkpoint tokenizer routing — no server, no model loading."""
+
+import os
+import shutil
+import tempfile
+import unittest
+
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=60, suite="base-a-test-cpu")
+
+from sglang.srt.utils.hf_transformers.mistral_utils import is_bare_tekken_checkpoint
+from sglang.srt.utils.hf_transformers.tokenizer import get_tokenizer
+
+TEKKEN_REPO = "mistralai/Leanstral-1.5-119B-A6B"
+PROMPT = "The capital of France is"
+# Reference ids from mistral-common, which owns the tekken id space
+# (1000 special-token slots precede the BPE vocab).
+EXPECTED_IDS = [1784, 8961, 1307, 5498, 1395]
+
+
+class TestBareTekkenDetection(CustomTestCase):
+    def test_detects_bare_tekken_dir(self):
+        with tempfile.TemporaryDirectory() as d:
+            self.assertFalse(is_bare_tekken_checkpoint(d))
+            with open(os.path.join(d, "tekken.json"), "w") as f:
+                f.write("{}")
+            self.assertTrue(is_bare_tekken_checkpoint(d))
+            with open(os.path.join(d, "tokenizer.json"), "w") as f:
+                f.write("{}")
+            self.assertFalse(is_bare_tekken_checkpoint(d))
+
+
+class TestTekkenRouting(CustomTestCase):
+    def test_get_tokenizer_matches_mistral_common(self):
+        from huggingface_hub import hf_hub_download
+
+        tekken = hf_hub_download(TEKKEN_REPO, "tekken.json")
+        with tempfile.TemporaryDirectory() as d:
+            shutil.copy(tekken, os.path.join(d, "tekken.json"))
+            tokenizer = get_tokenizer(d)
+            ids = tokenizer.encode(PROMPT, add_special_tokens=False)
+            self.assertEqual(ids, EXPECTED_IDS)
+
+
+if __name__ == "__main__":
+    unittest.main()