Symptom
An agent served through AGUIAdapter, VercelAIAdapter or to_web runs without its system_prompt, and functions decorated with @agent.system_prompt never run.
Evidence what you'll see
- The system prompt function is never called
- The instructions parameter is still applied
- Affects AG-UI, the Vercel AI adapter and to_web
Cause
UI adapters pass the frontend's messages in as message_history, and system_prompt is only added when a run starts without history, so it was left out.
Fix steps
0 of 2 done
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Verify the fix worked
- Send a message through the adapter and confirm the system prompt function runs and the model's reply follows it.
Sources and credit
Summarized in our own words from these public reports. Credit goes to their authors.
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