fix(rag): avoid empty system messages that crash Vertex AI#426
Merged
basnijholt merged 1 commit intomainfrom Feb 12, 2026
Merged
fix(rag): avoid empty system messages that crash Vertex AI#426basnijholt merged 1 commit intomainfrom
basnijholt merged 1 commit intomainfrom
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When no RAG context is found, the system_prompt was set to an empty
string (""), which pydantic-ai sends as {'role': 'system', 'content': ''}
to the LLM provider. Vertex AI (via litellm) rejects empty system
messages with a 400 error.
Fix by using an empty tuple () instead, which tells pydantic-ai to omit
the system message entirely. Also skip empty system messages from the
incoming request history, as some frontends (e.g., LibreChat title
generation) send {'role': 'system', 'content': ''}.
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Summary
system_promptwas set to""(empty string), causing pydantic-ai to send{'role': 'system', 'content': ''}to the LLM provider. Vertex AI rejects empty system messages with a 400 error.()so pydantic-ai omits the system message entirely when there's no RAG context.{'role': 'system', 'content': ''}.Test plan
test_convert_messages_skips_empty_system— verifies empty system messages are filteredtest_convert_messages_keeps_nonempty_system— verifies non-empty system messages are preservedtest_process_chat_request_no_rag— assertssystem_prompt == ()when no RAG contexttests/rag/test_engine.pypass