diff --git a/atroposlib/envs/teacher_distillation_env.py b/atroposlib/envs/teacher_distillation_env.py
index 521c3762..12c16079 100644
--- a/atroposlib/envs/teacher_distillation_env.py
+++ b/atroposlib/envs/teacher_distillation_env.py
@@ -146,22 +146,48 @@ class TeacherDistillationEnv(BaseEnv, ABC):
             )
 
             # Detect vocabulary mismatch.
+            # Compare by name first; if names differ, load the teacher tokenizer
+            # and do a vocab-size sanity check.  Same-family models (e.g. Qwen3-4B
+            # and Qwen3-30B) share the same vocabulary, so even though the
+            # name_or_path strings differ they should use the fast path.
             student_tok_name = getattr(self.tokenizer, "name_or_path", None) or ""
             if student_tok_name and teacher_tok_name and student_tok_name != teacher_tok_name:
                 try:
                     from transformers import AutoTokenizer
 
-                    self._teacher_tokenizer = AutoTokenizer.from_pretrained(
+                    loaded = AutoTokenizer.from_pretrained(
                         teacher_tok_name, use_fast=True
                     )
-                    self._same_tokenizer = False
-                    logger.warning(
-                        "TeacherDistillationEnv: cross-tokenizer mode active. "
-                        "student=%s  teacher=%s. "
-                        "Token IDs will be decoded → re-tokenized → vocab-remapped.",
-                        student_tok_name,
-                        teacher_tok_name,
-                    )
+                    student_vocab_size = getattr(self.tokenizer, "vocab_size", None)
+                    teacher_vocab_size = getattr(loaded, "vocab_size", None)
+                    if (
+                        student_vocab_size is not None
+                        and teacher_vocab_size is not None
+                        and student_vocab_size == teacher_vocab_size
+                    ):
+                        # Same vocab size — treat as same tokenizer (fast path).
+                        # This covers same-family models (e.g. all Qwen3 variants).
+                        self._same_tokenizer = True
+                        logger.warning(
+                            "TeacherDistillationEnv: names differ but vocab sizes match "
+                            "(%d tokens). Using fast (same-tokenizer) path. "
+                            "student=%s  teacher=%s",
+                            student_vocab_size,
+                            student_tok_name,
+                            teacher_tok_name,
+                        )
+                    else:
+                        self._teacher_tokenizer = loaded
+                        self._same_tokenizer = False
+                        logger.warning(
+                            "TeacherDistillationEnv: cross-tokenizer mode active. "
+                            "student=%s (%s tokens)  teacher=%s (%s tokens). "
+                            "Token IDs will be decoded → re-tokenized → vocab-remapped.",
+                            student_tok_name,
+                            student_vocab_size,
+                            teacher_tok_name,
+                            teacher_vocab_size,
+                        )
                 except Exception as exc:
                     logger.warning(
                         "TeacherDistillationEnv: could not load teacher tokenizer '%s' "