Reinject System Prompt

[ReinjectSystemPrompt][pydantic_ai.capabilities.ReinjectSystemPrompt] is a capability that ensures the agent's configured system_prompt is at the head of the first [ModelRequest][pydantic_ai.messages.ModelRequest] on every model request. By default, if any [SystemPromptPart][pydantic_ai.messages.SystemPromptPart] is already present in the history, the capability is a no-op (so multi-agent handoff and user-managed system prompts remain authoritative). Set replace_existing=True to instead strip any existing SystemPromptParts before prepending the agent's configured prompt — useful when the history comes from an untrusted source and the server's prompt must win.

Useful when message_history comes from a source that doesn't round-trip system prompts — UI frontends, database persistence layers, conversation compaction pipelines. Without this capability, an agent configured with a system_prompt will silently run without it if the history doesn't already include one.

from pydantic_ai import Agent
from pydantic_ai.capabilities import ReinjectSystemPrompt
from pydantic_ai.messages import ModelRequest, ModelResponse, TextPart, UserPromptPart

agent = Agent('test', system_prompt='You are a helpful assistant.', capabilities=[ReinjectSystemPrompt()])

# History that's missing the system prompt (e.g. reconstructed from a UI frontend).
history = [
    ModelRequest(parts=[UserPromptPart(content='Hi')]),
    ModelResponse(parts=[TextPart(content='Hello!')]),
]

# Without the capability, the agent would run without its configured system prompt.
# With the capability, the system prompt is reinjected at the head of the first request.
result = agent.run_sync('Follow up', message_history=history)
first_request = result.all_messages()[0]
assert isinstance(first_request, ModelRequest)
assert first_request.parts[0].content == 'You are a helpful assistant.'

(This example is complete, it can be run "as is")

The UI adapters (AG-UI, Vercel AI) automatically add this capability with replace_existing=True in their manage_system_prompt='server' mode.