diff --git a/tests/integration/test_draft_vocab_mapping_contract.py b/tests/integration/test_draft_vocab_mapping_contract.py
new file mode 100644
index 00000000..d06e3b96
--- /dev/null
+++ b/tests/integration/test_draft_vocab_mapping_contract.py
@@ -0,0 +1,128 @@
+# Copyright 2026 Bytedance Ltd. and/or its affiliates
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Contract tests for the draft vocabulary mapping buffers.
+
+``d2t`` holds offsets, not absolute ids: the target id of draft id ``i`` is
+``i + d2t[i]``. That is what ``process_token_dict_to_mappings`` emits and what the
+serving engines apply, so every producer of a ``d2t`` buffer has to agree on it.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+
+def _tiny_eagle3_config(vocab_size: int, draft_vocab_size: int):
+    from transformers import LlamaConfig
+
+    return LlamaConfig(
+        hidden_size=8,
+        intermediate_size=16,
+        num_attention_heads=2,
+        num_key_value_heads=2,
+        num_hidden_layers=1,
+        vocab_size=vocab_size,
+        draft_vocab_size=draft_vocab_size,
+        target_hidden_size=8,
+        num_aux_hidden_states=3,
+        pad_token_id=0,
+        rms_norm_eps=1e-6,
+        max_position_embeddings=64,
+    )
+
+
+def _tiny_peagle_config(vocab_size: int, draft_vocab_size: int):
+    from verl_speco.models.peagle import PeagleConfig
+
+    return PeagleConfig(
+        hidden_size=8,
+        intermediate_size=16,
+        num_attention_heads=2,
+        num_key_value_heads=2,
+        num_hidden_layers=1,
+        num_draft_layers=1,
+        target_hidden_size=8,
+        num_aux_hidden_states=3,
+        vocab_size=vocab_size,
+        draft_vocab_size=draft_vocab_size,
+        mask_token_id=vocab_size - 1,
+        pad_token_id=0,
+        rms_norm_eps=1e-6,
+        max_position_embeddings=64,
+    )
+
+
+def _draft_models(vocab_size: int, draft_vocab_size: int):
+    from verl_speco.models.eagle.llama_eagle import LlamaForCausalLMEagle3
+    from verl_speco.models.peagle import LlamaForCausalLMPeagle
+
+    return {
+        "eagle3": LlamaForCausalLMEagle3(
+            _tiny_eagle3_config(vocab_size, draft_vocab_size)
+        ),
+        "peagle": LlamaForCausalLMPeagle(
+            _tiny_peagle_config(vocab_size, draft_vocab_size)
+        ),
+    }
+
+
+@pytest.mark.parametrize("name", ["eagle3", "peagle"])
+def test_default_d2t_is_the_offset_identity(name) -> None:
+    """The default mapping must be the identity under the offset convention.
+
+    ``torch.arange`` looks like an identity only if ``d2t`` held absolute ids. It
+    does not, so an arange default resolves to ``target_id = 2 * draft_id``.
+    """
+    torch = pytest.importorskip("torch")
+    pytest.importorskip("transformers")
+
+    model = _draft_models(vocab_size=32, draft_vocab_size=32)[name]
+
+    assert torch.equal(model.d2t, torch.zeros(32, dtype=model.d2t.dtype))
+
+    # This is what the serving engine computes.
+    target_ids = torch.arange(model.draft_vocab_size) + model.d2t
+    assert torch.equal(target_ids, torch.arange(32))
+    assert int(target_ids.max()) < model.vocab_size
+
+
+@pytest.mark.parametrize("name", ["eagle3", "peagle"])
+def test_default_d2t_stays_inside_the_target_vocabulary(name) -> None:
+    """A reduced draft vocabulary must still resolve to in-range target ids."""
+    torch = pytest.importorskip("torch")
+    pytest.importorskip("transformers")
+
+    model = _draft_models(vocab_size=32, draft_vocab_size=8)[name]
+
+    target_ids = torch.arange(model.draft_vocab_size) + model.d2t
+    assert int(target_ids.max()) < model.vocab_size
+    # t2d marks the same tokens the offsets resolve to.
+    assert torch.equal(torch.nonzero(model.t2d, as_tuple=False).flatten(), target_ids)
+
+
+def test_generated_mapping_round_trips_through_the_offset_convention() -> None:
+    """The repo's own mapping generator emits offsets, so decoding must match."""
+    torch = pytest.importorskip("torch")
+    from collections import Counter
+
+    from verl_speco.data.preprocessing import process_token_dict_to_mappings
+
+    token_dict = Counter({3: 10, 9: 8, 17: 5, 21: 2})
+    d2t, t2d = process_token_dict_to_mappings(
+        token_dict, draft_vocab_size=4, target_vocab_size=32
+    )
+
+    target_ids = torch.arange(4) + d2t
+    assert target_ids.tolist() == [3, 9, 17, 21]
+    assert torch.nonzero(t2d, as_tuple=False).flatten().tolist() == [3, 9, 17, 21]
diff --git a/verl_speco/models/eagle/llama_eagle.py b/verl_speco/models/eagle/llama_eagle.py
index 932eaf37..7fd5001c 100644
--- a/verl_speco/models/eagle/llama_eagle.py
+++ b/verl_speco/models/eagle/llama_eagle.py
@@ -1493,10 +1493,14 @@ def __init__(self, config, quant_config=None, attention_backend="sdpa") -> None:
 
         self.post_init()
 
-        # create vocab buffers
+        # Create vocab buffers. d2t holds OFFSETS, not absolute ids: the target id
+        # of draft id i is `i + d2t[i]`. That is what
+        # `preprocessing.process_token_dict_to_mappings` emits (`used_tokens[i] - i`)
+        # and what the serving engines apply, so the identity mapping used when no
+        # frequency mapping is supplied is all zeros.
         t2d = torch.zeros(self.vocab_size, dtype=torch.bool)
         t2d[: self.draft_vocab_size] = True
-        d2t = torch.arange(self.draft_vocab_size, dtype=torch.int64)
+        d2t = torch.zeros(self.draft_vocab_size, dtype=torch.int64)
         self.register_buffer("t2d", t2d)
         self.register_buffer("d2t", d2t)
 
diff --git a/verl_speco/models/peagle/modeling_peagle.py b/verl_speco/models/peagle/modeling_peagle.py
index 4bc7dbc7..15e547ac 100644
--- a/verl_speco/models/peagle/modeling_peagle.py
+++ b/verl_speco/models/peagle/modeling_peagle.py
@@ -268,9 +268,12 @@ def __init__(self, config: PeagleConfig):
 
         self.post_init()
 
+        # d2t holds OFFSETS, not absolute ids: the target id of draft id i is
+        # `i + d2t[i]`, so the identity mapping is all zeros. See the EAGLE-3
+        # draft for the full note.
         t2d = torch.zeros(self.vocab_size, dtype=torch.bool)
         t2d[: self.draft_vocab_size] = True
-        d2t = torch.arange(self.draft_vocab_size, dtype=torch.int64)
+        d2t = torch.zeros(self.draft_vocab_size, dtype=torch.int64)
         self.register_buffer("t2d", t2d)
         self.register_buffer("d2t", d2t)
         nn.init.normal_(self.mask_hidden, mean=0.0, std=1.0)