diff --git a/tests/renderers/test_chat_utils_prompt_embeds.py b/tests/renderers/test_chat_utils_prompt_embeds.py
index b537d2a13a..bbd4dc5c5f 100644
--- a/tests/renderers/test_chat_utils_prompt_embeds.py
+++ b/tests/renderers/test_chat_utils_prompt_embeds.py
@@ -27,6 +27,7 @@ from vllm.entrypoints.chat_utils import (
     parse_chat_messages_async,
 )
 from vllm.exceptions import VLLMValidationError
+from vllm.renderers import TokenizeParams
 from vllm.renderers.hf import (
     _PROMPT_EMBEDS_PLACEHOLDER_SPAN_MISMATCH_ERROR,
     _build_mixed_prompt_embeds,
@@ -575,3 +576,49 @@ async def test_end_to_end_multi_message_conversation(tokenizer, parse_fn):
     assert mask.count(False) == LEN_SYS + LEN_USR
     assert torch.equal(embeds[positions[0][0] : positions[0][0] + LEN_SYS], t_sys)
     assert torch.equal(embeds[positions[1][0] : positions[1][0] + LEN_USR], t_usr)
+
+
+def test_truncation_keeps_the_mixed_mask_aligned_with_the_prompt():
+    """`prompt_is_token_ids` must be truncated with `prompt_token_ids`.
+
+    The mask is positional: the engine pairs it element-wise with the prompt
+    tokens to decide which positions read a real token id and which read a row
+    of `prompt_embeds`. Truncating the prompt without truncating the mask
+    leaves the two describing different positions, so embed placeholders stop
+    being recognised as such.
+    """
+    hidden_size = 8
+    placeholder_token_id = 0
+    num_head, embed_len, num_tail = 10, 5, 5
+
+    token_ids = (
+        list(range(1, num_head + 1))
+        + [placeholder_token_id] * embed_len
+        + list(range(1, num_tail + 1))
+    )
+    tensors = [torch.randn(embed_len, hidden_size)]
+    positions = [(num_head, embed_len)]
+    embeds, mask = _build_mixed_prompt_embeds(token_ids, tensors, positions)
+
+    prompt = {
+        "prompt_token_ids": token_ids,
+        "prompt_embeds": embeds,
+        "prompt_is_token_ids": mask,
+    }
+    keep = 8
+    tok_params = TokenizeParams(
+        max_total_tokens=100,
+        truncate_prompt_tokens=keep,
+        truncation_side="left",
+    )
+
+    result = tok_params.apply_post_tokenization(None, prompt)
+
+    # Keeping the last 8 of 20 positions starts inside the embed span, so the
+    # first 3 surviving positions are embed rows and the rest are real tokens.
+    # Comparing contents rather than lengths is what pins the alignment: a
+    # mask reduced from the wrong side has the right length and still
+    # describes the wrong positions.
+    assert result["prompt_is_token_ids"] == [False] * 3 + [True] * num_tail
+    assert result["prompt_token_ids"] == token_ids[-keep:]
+    assert torch.equal(result["prompt_embeds"], embeds[-keep:])
diff --git a/vllm/renderers/params.py b/vllm/renderers/params.py
index 048f450ac1..3ecd8f4828 100644
--- a/vllm/renderers/params.py
+++ b/vllm/renderers/params.py
@@ -420,8 +420,8 @@ class TokenizeParams:
         """The slice truncation applies to a sequence of `length` tokens.
 
         `None` means no truncation. Anything parallel to the prompt tokens
-        (see `prompt_token_offsets`) must be reduced with this same slice to
-        stay aligned with them.
+        (see `prompt_token_offsets` and `prompt_is_token_ids`) must be reduced
+        with this same slice to stay aligned with them.
         """
         max_length = self.truncate_prompt_tokens
         if max_length is not None and max_length < 0:
@@ -516,5 +516,10 @@ class TokenizeParams:
             truncation = self._truncation_slice(tokenizer, len(offsets))
             if truncation is not None:
                 prompt["prompt_token_offsets"] = offsets[truncation]  # type: ignore[typeddict-unknown-key]
+        is_token_ids = cast(EmbedsPrompt, prompt).get("prompt_is_token_ids")
+        if is_token_ids is not None:
+            truncation = self._truncation_slice(tokenizer, len(is_token_ids))
+            if truncation is not None:
+                prompt["prompt_is_token_ids"] = is_token_ids[truncation]  # type: ignore[typeddict-unknown-key]
 
         return prompt