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Joined the developer Slack and posted in #pr-reviews for this fix (#54337). DCO and format checks are green. Once a maintainer adds the |
Agent clients send assistant messages with content=null and tool_calls. Chat templates that render content emit a literal None into the prompt, which degrades multi-turn agent sessions over many tool turns. Coerce null content to an empty string in _postprocess_messages, next to the existing tool-call arguments normalization. Fixes vllm-project#54337 Signed-off-by: lllakshit <llakshitmathur239@gmail.com>
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| # Agent clients send assistant messages with content=null and | ||
| # tool_calls. Templates that render {{ content }} emit a literal | ||
| # "None" into the context, which degrades multi-turn generation. | ||
| message["content"] = "" |
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To be honest, I'm not entirely sure whether this fix is correct or whether it should be a general change. I feel that setting content to None would be more appropriate than setting it to "".
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Thanks for taking a look - leaving content as None is what triggers the bug.
Jinja chat templates that render {{ content }} stringify Python None to the literal text None. That is the prompt pollution reported in #54337 (especially over many tool-calling turns).
Empty string is intentional here because:
- OpenAI-style clients already send assistant turns as
content: null+tool_calls; the model-facing prompt should treat that as "no text", not the token/Nonestring. - This stays scoped to assistant messages that also have
tool_calls, so we do not change general null-content handling elsewhere.
Happy to adjust if you prefer deleting the key instead of an empty string, but keeping None would not fix the reported template behavior.
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This pull request has merge conflicts that must be resolved before it can be |
Summary
content: nullandtool_callsto use an empty string before chat templating.{{ content }}from injecting the literal stringNoneinto the prompt.test_chat_utils.py.Problem
OpenAI agent clients (OpenAI SDK, pi, etc.) commonly send assistant turns as
content: nullplustool_calls. When those messages reachapply_chat_template, templates that unconditionally render assistant content emitNoneas text. Over many tool-calling turns in an agent session, this silently degrades generation into repetition loops and looks like a model quality issue rather than a preprocessing bug.Reported in #54337 with reproduction on GLM chat templates; the fix is protocol-layer and model-agnostic.
Fix
Extend
_postprocess_messagesinvllm/entrypoints/chat_utils.pyto coercecontent=Noneto""for assistant messages that includetool_calls, alongside the existing tool-call argument normalization.Test plan
_postprocess_messageslocally (null content becomes"", arguments still parsed)pytest tests/entrypoints/unit_tests/test_chat_utils.py::test_postprocess_messages_null_content_with_tool_calls -qpytest tests/entrypoints/unit_tests/test_chat_utils.py::test_postprocess_messages_null_arguments_string -qFixes #54337