From 0ad5652a523d7bfe191a08b97ca45ead585b1897 Mon Sep 17 00:00:00 2001
From: Pratham Shirbhate <prathamshirbhate1909@gmail.com>
Date: Tue, 1 Sep 2026 23:47:54 +0530
Subject: [PATCH] [Bugfix][Frontend] Restore the chat template content format
 mismatch warning (#54622)

Signed-off-by: Pratham Shirbhate <prathamshirbhate1909@gmail.com>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>
---
 .../chat_completion/test_serving_chat.py      |  2 +
 vllm/renderers/hf.py                          | 82 +++++++++++--------
 2 files changed, 51 insertions(+), 33 deletions(-)

diff --git a/tests/entrypoints/openai/chat_completion/test_serving_chat.py b/tests/entrypoints/openai/chat_completion/test_serving_chat.py
index b7b02efd12..f750830296 100644
--- a/tests/entrypoints/openai/chat_completion/test_serving_chat.py
+++ b/tests/entrypoints/openai/chat_completion/test_serving_chat.py
@@ -539,6 +539,8 @@ class MockModelConfig:
     trust_remote_code = False
     tokenizer_mode = "auto"
     max_model_len = 100
+    revision = None
+    code_revision = None
     tokenizer_revision = None
     multimodal_config = MultiModalConfig()
     hf_config = MockHFConfig()
diff --git a/vllm/renderers/hf.py b/vllm/renderers/hf.py
index 54625af820..16906bb1da 100644
--- a/vllm/renderers/hf.py
+++ b/vllm/renderers/hf.py
@@ -509,6 +509,22 @@ def _consolidate_system_messages(
     return [merged, *non_system]
 
 
+_CONTENT_FORMATS_MAXSIZE = 32
+_CONTENT_FORMATS = dict[
+    tuple[str | None, bool, str, str | None, str | None, bool],
+    "ChatTemplateContentFormat",
+]()
+"""
+Used in `_resolve_chat_template_content_format` to avoid resolving and parsing
+the chat template on every request.
+
+`lru_cache` cannot be used because `tools` is a list and `ModelConfig` defines
+`__eq__` without `__hash__`, so the key holds the fields `resolve_chat_template`
+selects the template by. Only the presence of `tools` is part of it: it decides
+whether the processor template is tried, and its contents never reach the AST.
+"""
+
+
 def _resolve_chat_template_content_format(
     chat_template: str | None,
     tools: list[dict[str, Any]] | None,
@@ -516,6 +532,17 @@ def _resolve_chat_template_content_format(
     *,
     model_config: ModelConfig,
 ) -> ChatTemplateContentFormat:
+    cache_key = (
+        chat_template,
+        tools is None,
+        tokenizer.name_or_path,
+        model_config.revision,
+        model_config.code_revision,
+        model_config.trust_remote_code,
+    )
+    if (cached_format := _CONTENT_FORMATS.get(cache_key)) is not None:
+        return cached_format
+
     resolved_chat_template = resolve_chat_template(
         tokenizer,
         chat_template=chat_template,
@@ -535,33 +562,14 @@ def _resolve_chat_template_content_format(
         else _detect_content_format(jinja_text, default="string")
     )
 
+    # Requests may carry their own chat template, so bound the cache.
+    if len(_CONTENT_FORMATS) >= _CONTENT_FORMATS_MAXSIZE:
+        _CONTENT_FORMATS.clear()
+    _CONTENT_FORMATS[cache_key] = detected_format
+
     return detected_format
 
 
-@lru_cache
-def _log_chat_template_content_format(
-    chat_template: str | None,  # For caching purposes
-    given_format: ChatTemplateContentFormatOption,
-    detected_format: ChatTemplateContentFormatOption,
-):
-    logger.info(
-        "Detected the chat template content format to be '%s'. "
-        "You can set `--chat-template-content-format` to override this.",
-        detected_format,
-    )
-
-    if given_format != "auto" and given_format != detected_format:
-        logger.warning(
-            "You specified `--chat-template-content-format %s` "
-            "which is different from the detected format '%s'. "
-            "If our automatic detection is incorrect, please consider "
-            "opening a GitHub issue so that we can improve it: "
-            "https://github.com/vllm-project/vllm/issues/new/choose",
-            given_format,
-            detected_format,
-        )
-
-
 def resolve_chat_template_content_format(
     chat_template: str | None,
     tools: list[dict[str, Any]] | None,
@@ -570,9 +578,6 @@ def resolve_chat_template_content_format(
     *,
     model_config: ModelConfig,
 ) -> ChatTemplateContentFormat:
-    if given_format != "auto":
-        return given_format
-
     detected_format = _resolve_chat_template_content_format(
         chat_template,
         tools,
@@ -580,13 +585,24 @@ def resolve_chat_template_content_format(
         model_config=model_config,
     )
 
-    _log_chat_template_content_format(
-        chat_template,
-        given_format=given_format,
-        detected_format=detected_format,
-    )
+    if given_format == "auto":
+        logger.info_once(
+            "Detected the chat template content format to be '%s'. "
+            "You can set `--chat-template-content-format` to override this.",
+            detected_format,
+        )
+    elif given_format != detected_format:
+        logger.warning_once(
+            "You specified `--chat-template-content-format %s` "
+            "which is different from the detected format '%s'. "
+            "If our automatic detection is incorrect, please consider "
+            "opening a GitHub issue so that we can improve it: "
+            "https://github.com/vllm-project/vllm/issues/new/choose",
+            given_format,
+            detected_format,
+        )
 
-    return detected_format
+    return detected_format if given_format == "auto" else given_format
 
 
 # adapted from https://github.com/huggingface/transformers/blob/v4.56.2/src/transformers/utils/chat_template_utils.py#L398-L412