diff --git a/docs/components.md b/docs/components.md
index 77c10a9..5583e48 100644
--- a/docs/components.md
+++ b/docs/components.md
@@ -61,3 +61,24 @@ Reconstructs text from tokens.
 - `BpeTrainer(vocab_size, min_frequency, special_tokens=None, ...)` — in-memory
   greedy trainer. For large/distributed training use UbiTok; this is for small
   corpora and tests.
+
+## Unimplemented options
+
+`BPE` and `BpeTrainer` accept the full HuggingFace option set so that a
+`tokenizer.json` round-trips unchanged. Options that are not implemented raise
+`UnsupportedFeatureError` when set to a value that would change tokenization,
+rather than being accepted and ignored:
+
+| Option | Accepted values | Rejected values |
+| --- | --- | --- |
+| `dropout` | `None`, `0.0` | anything in `(0, 1]` — outside `[0, 1]` raises `ModelError` |
+| `continuing_subword_prefix` | `None`, `""` | any non-empty string |
+| `end_of_word_suffix` | `None`, `""` | any non-empty string |
+
+The accepted values are genuine no-ops in HuggingFace as well, so files that
+tokenize correctly today keep loading — GPT-2-lineage exports ship `""` for both
+affix options.
+
+`cache_capacity` is accepted and retained but not yet used. It is not rejected
+because a cache is a pure optimization: its absence cannot change output.
+HuggingFace does not serialize it into `tokenizer.json`, so `to_dict()` omits it.
diff --git a/src/ubi_tokenizer/_validation.py b/src/ubi_tokenizer/_validation.py
new file mode 100644
index 0000000..7ab5629
--- /dev/null
+++ b/src/ubi_tokenizer/_validation.py
@@ -0,0 +1,54 @@
+"""Shared rejection rules for recognized-but-unimplemented options.
+
+The BPE model and the BPE trainer both accept options that they store, and in
+the model's case serialize, without ever acting on them. Silently accepting
+such an option hands the caller wrong tokenization with no signal, so a value
+that would change behaviour is rejected instead.
+
+The rule has to stay identical in both places, so it lives here rather than
+being duplicated.
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+from ubi_tokenizer.errors import ModelError, UnsupportedFeatureError
+
+_HINT = (
+    "recognized but not implemented by ubi_tokenizer. Drop the option, or use "
+    "huggingface/tokenizers if you need it."
+)
+
+
+def reject_unimplemented_affixes(
+    continuing_subword_prefix: Optional[str],
+    end_of_word_suffix: Optional[str],
+) -> None:
+    """Reject affix options that would change tokenization.
+
+    ``None`` and ``""`` are genuine no-ops in HuggingFace and stay accepted --
+    GPT-2-lineage ``tokenizer.json`` files ship ``""`` for both, and rejecting
+    those would break loading files that tokenize correctly today.
+    """
+    for name, value in (
+        ("continuing_subword_prefix", continuing_subword_prefix),
+        ("end_of_word_suffix", end_of_word_suffix),
+    ):
+        if value:
+            raise UnsupportedFeatureError(f"{name}={value!r} is {_HINT}")
+
+
+def reject_unimplemented_dropout(dropout: Optional[float]) -> None:
+    """Reject BPE dropout.
+
+    ``None`` and ``0.0`` both mean "no dropout" and stay accepted. Values
+    outside ``[0, 1]`` are invalid rather than unimplemented, which is why they
+    raise :class:`ModelError`; HuggingFace rejects them at construction too.
+    """
+    if dropout is None:
+        return
+    if not 0.0 <= dropout <= 1.0:
+        raise ModelError(f"dropout={dropout!r} must be between 0 and 1, inclusive")
+    if dropout > 0.0:
+        raise UnsupportedFeatureError(f"dropout={dropout!r} is {_HINT}")
diff --git a/src/ubi_tokenizer/errors.py b/src/ubi_tokenizer/errors.py
index b72daaf..6907d75 100644
--- a/src/ubi_tokenizer/errors.py
+++ b/src/ubi_tokenizer/errors.py
@@ -13,3 +13,12 @@ class DeserializationError(UbiTokenizerError):
 
 class ModelError(UbiTokenizerError):
     """Raised for invalid model state (missing tokens, bad merges, etc.)."""
+
+
+class UnsupportedFeatureError(UbiTokenizerError):
+    """Raised when a requested option is recognized but not implemented.
+
+    Distinct from :class:`ModelError`: the request is well-formed, it simply
+    cannot be honoured. Raised eagerly so the caller finds out when the
+    tokenizer is built rather than getting silently wrong tokenization.
+    """
diff --git a/src/ubi_tokenizer/models/bpe.py b/src/ubi_tokenizer/models/bpe.py
index 4be8077..16b1a86 100644
--- a/src/ubi_tokenizer/models/bpe.py
+++ b/src/ubi_tokenizer/models/bpe.py
@@ -10,6 +10,11 @@
 
 from typing import Any, Optional
 
+from ubi_tokenizer._validation import (
+    reject_unimplemented_affixes,
+    reject_unimplemented_dropout,
+)
+
 
 class Model:
     """Abstract base for tokenization models."""
@@ -47,8 +52,14 @@ def __init__(
         byte_fallback: bool = False,
         ignore_merges: bool = False,
     ) -> None:
+        reject_unimplemented_dropout(dropout)
+        reject_unimplemented_affixes(continuing_subword_prefix, end_of_word_suffix)
         self.vocab: dict[str, int] = dict(vocab) if vocab else {}
         self.merges: list[tuple[str, str]] = [tuple(m) for m in (merges or [])]
+        # Accepted rather than rejected: a cache is a pure optimization, so its
+        # absence cannot change output. HuggingFace does not serialize it into
+        # tokenizer.json either, so to_dict() stays faithful by omitting it.
+        self.cache_capacity = cache_capacity
         self.dropout = dropout
         self.unk_token = unk_token
         self.continuing_subword_prefix = continuing_subword_prefix
diff --git a/src/ubi_tokenizer/trainers/bpe.py b/src/ubi_tokenizer/trainers/bpe.py
index 3fc3fa6..a021631 100644
--- a/src/ubi_tokenizer/trainers/bpe.py
+++ b/src/ubi_tokenizer/trainers/bpe.py
@@ -14,6 +14,7 @@
 from collections.abc import Iterable, Iterator
 from typing import Optional
 
+from ubi_tokenizer._validation import reject_unimplemented_affixes
 from ubi_tokenizer.byte_level import BYTE_ENCODER, encode_token_bytes
 from ubi_tokenizer.models.bpe import BPE
 from ubi_tokenizer.pre_tokenizers.byte_level import ByteLevel
@@ -40,6 +41,7 @@ def __init__(
         continuing_subword_prefix: Optional[str] = None,
         end_of_word_suffix: Optional[str] = None,
     ) -> None:
+        reject_unimplemented_affixes(continuing_subword_prefix, end_of_word_suffix)
         self.vocab_size = vocab_size
         self.min_frequency = min_frequency
         self.special_tokens = list(special_tokens or [])
diff --git a/tests/test_unsupported_flags.py b/tests/test_unsupported_flags.py
new file mode 100644
index 0000000..89879d7
--- /dev/null
+++ b/tests/test_unsupported_flags.py
@@ -0,0 +1,123 @@
+"""Unimplemented BPE / trainer options must fail loudly instead of silently.
+
+``BPE`` and ``BpeTrainer`` accept several HuggingFace-compatible options that are
+stored and serialized but never acted on. Accepting them silently means a caller
+gets wrong tokenization with no signal, so a non-default value now raises.
+Default values stay silent: they are true no-ops in HuggingFace too, and
+GPT-2-lineage ``tokenizer.json`` files ship ``""`` for the affix options.
+"""
+
+import pytest
+
+from ubi_tokenizer.errors import ModelError, UbiTokenizerError, UnsupportedFeatureError
+from ubi_tokenizer.models.bpe import BPE
+from ubi_tokenizer.trainers.bpe import BpeTrainer
+
+
+def test_unsupported_feature_error_is_a_ubi_tokenizer_error():
+    assert issubclass(UnsupportedFeatureError, UbiTokenizerError)
+
+
+# -- dropout ---------------------------------------------------------------
+
+
+@pytest.mark.parametrize("value", [None, 0.0])
+def test_no_op_dropout_is_accepted(value):
+    # HuggingFace treats both as "no dropout", so they must not raise.
+    assert BPE(dropout=value).dropout == value
+
+
+@pytest.mark.parametrize("value", [0.1, 0.5, 1.0])
+def test_active_dropout_is_rejected(value):
+    with pytest.raises(UnsupportedFeatureError, match="dropout"):
+        BPE(dropout=value)
+
+
+@pytest.mark.parametrize("value", [-0.1, 1.5])
+def test_out_of_range_dropout_is_rejected_as_invalid_model_state(value):
+    # HuggingFace rejects these at construction as invalid rather than
+    # unimplemented, so they are ModelError and not UnsupportedFeatureError.
+    with pytest.raises(ModelError, match="dropout"):
+        BPE(dropout=value)
+
+
+# -- affix options --------------------------------------------------------
+
+
+@pytest.mark.parametrize("option", ["continuing_subword_prefix", "end_of_word_suffix"])
+@pytest.mark.parametrize("value", [None, ""])
+def test_no_op_affix_options_are_accepted(option, value):
+    assert getattr(BPE(**{option: value}), option) == value
+
+
+@pytest.mark.parametrize(
+    ("option", "value"),
+    [("continuing_subword_prefix", "##"), ("end_of_word_suffix", "</w>")],
+)
+def test_active_affix_options_are_rejected(option, value):
+    with pytest.raises(UnsupportedFeatureError, match=option):
+        BPE(**{option: value})
+
+
+@pytest.mark.parametrize(
+    ("option", "value"),
+    [("continuing_subword_prefix", "##"), ("end_of_word_suffix", "</w>")],
+)
+def test_trainer_rejects_active_affix_options(option, value):
+    # The trainer stores these two and never reads them either; fixing only the
+    # model would leave half of the same defect in place.
+    with pytest.raises(UnsupportedFeatureError, match=option):
+        BpeTrainer(**{option: value})
+
+
+@pytest.mark.parametrize("option", ["continuing_subword_prefix", "end_of_word_suffix"])
+@pytest.mark.parametrize("value", [None, ""])
+def test_trainer_accepts_no_op_affix_options(option, value):
+    assert getattr(BpeTrainer(**{option: value}), option) == value
+
+
+def test_trainer_defaults_are_accepted():
+    trainer = BpeTrainer(vocab_size=64, min_frequency=1)
+    assert trainer.continuing_subword_prefix is None
+    assert trainer.end_of_word_suffix is None
+
+
+# -- message quality -------------------------------------------------------
+
+
+def test_rejection_message_names_the_option_and_says_it_is_unimplemented():
+    with pytest.raises(UnsupportedFeatureError) as excinfo:
+        BPE(end_of_word_suffix="</w>")
+    message = str(excinfo.value)
+    assert "end_of_word_suffix" in message
+    assert "not implemented" in message.lower()
+
+
+# -- cache_capacity is accepted, not rejected ------------------------------
+
+
+def test_cache_capacity_is_retained():
+    # A cache is a pure optimization: its absence cannot change output, so this
+    # is accepted rather than rejected. It was previously discarded entirely.
+    assert BPE(cache_capacity=1234).cache_capacity == 1234
+
+
+def test_cache_capacity_stays_out_of_serialization():
+    # HuggingFace does not serialize cache_capacity into tokenizer.json, so
+    # omitting it keeps to_dict() faithful rather than lossy.
+    assert "cache_capacity" not in BPE(cache_capacity=1234).to_dict()
+
+
+# -- deserialization path --------------------------------------------------
+
+
+def test_unsupported_option_in_tokenizer_json_is_rejected_on_load():
+    from ubi_tokenizer.tokenizer import Tokenizer
+
+    payload = {
+        "version": "1.0",
+        "added_tokens": [],
+        "model": {"type": "BPE", "vocab": {"a": 0}, "merges": [], "dropout": 0.5},
+    }
+    with pytest.raises(UnsupportedFeatureError, match="dropout"):
+        Tokenizer.from_dict(payload)