feat: Add --custom-jinja-template argument to pass a custom chat template for vLLM (#2829)

Signed-off-by: Krishnan Prashanth <kprashanth@nvidia.com>

Author: KrishnanPrash

components/backends/vllm/src/dynamo/vllm/args.py

@@ -47,6 +47,7 @@ class Config:
     migration_limit: int = 0
     kv_port: Optional[int] = None
     port_range: DynamoPortRange
+    custom_jinja_template: Optional[str] = None
 
     # mirror vLLM
     model: str
@@ -100,7 +101,7 @@ def parse_args() -> Config:
         help="List of connectors to use in order (e.g., --connector nixl lmcache). "
         "Options: nixl, lmcache, kvbm, null, none. Default: nixl. Order will be preserved in MultiConnector.",
     )
-    # To avoid name conflicts with different backends, adoped prefix "dyn-" for dynamo specific args
+    # To avoid name conflicts with different backends, adopted prefix "dyn-" for dynamo specific args
     parser.add_argument(
         "--dyn-tool-call-parser",
         type=str,
@@ -115,6 +116,12 @@ def parse_args() -> Config:
         choices=get_reasoning_parser_names(),
         help="Reasoning parser name for the model.",
     )
+    parser.add_argument(
+        "--custom-jinja-template",
+        type=str,
+        default=None,
+        help="Path to a custom Jinja template file to override the model's default chat template. This template will take precedence over any template found in the model repository.",
+    )
 
     parser = AsyncEngineArgs.add_cli_args(parser)
     args = parser.parse_args()
@@ -148,6 +155,7 @@ def parse_args() -> Config:
     )
     config.tool_call_parser = args.dyn_tool_call_parser
     config.reasoning_parser = args.dyn_reasoning_parser
+    config.custom_jinja_template = args.custom_jinja_template
     # Check for conflicting flags
     has_kv_transfer_config = (
         hasattr(engine_args, "kv_transfer_config")

components/backends/vllm/src/dynamo/vllm/main.py

@@ -258,6 +258,7 @@ async def init(runtime: DistributedRuntime, config: Config):
             kv_cache_block_size=config.engine_args.block_size,
             migration_limit=config.migration_limit,
             runtime_config=runtime_config,
+            custom_template_path=config.custom_jinja_template,
         )
 
     try: