Empty dynamic system prompt should not add a new message

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Initial Checks

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• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

Having an empty string as a system_prompt will result in a new message added.
This behavior is not consistent with instruction, for which an empty string do not add a new message (last line in instruction documentation).

Example Code

from pydantic_ai import Agent, RunContext

agent = Agent(
    'openai:gpt-4o', 
    system_prompt="something",  
)


@agent.system_prompt  
def add_nothing() -> str:
    return ""


result = agent.run_sync('foo')
print(result.all_messages())

Python, Pydantic AI & LLM client version

Python 3.13.3
pydantic-ai 0.6.0

[DouweM]

@Kludex Any concern with making this consistent with dynamic instructions? I can't imagine anyone was relying on this behavior, or that it was causing the model to do anything useful anyway.

[essamgouda97]

with bedrock if system prompt is empty I get this error:

botocore.exceptions.ParamValidationError: Parameter validation failed: Invalid length for parameter system[1].text, value: 0, valid min length: 1

would be nice to have a way to dynamically insert system prompts or not based on a flag. Right now a workaround is to add an empty space which is a bit ugly.

[DouweM]

@essamgouda97 We can fix this just for Bedrock by adding an if part.content: check here:

pydantic-ai/pydantic_ai_slim/pydantic_ai/models/bedrock.py

Lines 426 to 427 in 057c0bc

	if isinstance(part, SystemPromptPart):
	system_prompt.append({'text': part.content})

Feel free to submit a PR for that!

To exclude all empty (dynamic) system parts, we'd need a check here:

pydantic-ai/pydantic_ai_slim/pydantic_ai/_agent_graph.py

Lines 239 to 243 in 057c0bc

	prompt = await sys_prompt_runner.run(run_context)
	if sys_prompt_runner.dynamic:
	messages.append(_messages.SystemPromptPart(prompt, dynamic_ref=sys_prompt_runner.function.__qualname__))
	else:
	messages.append(_messages.SystemPromptPart(prompt))

And here:

pydantic-ai/pydantic_ai_slim/pydantic_ai/_agent_graph.py

Lines 225 to 233 in 057c0bc

	if isinstance(part, _messages.SystemPromptPart) and part.dynamic_ref:
	# Look up the runner by its ref
	if runner := self.system_prompt_dynamic_functions.get( # pragma: lax no cover
	part.dynamic_ref
	):
	updated_part_content = await runner.run(run_context)
	msg.parts[i] = _messages.SystemPromptPart(
	updated_part_content, dynamic_ref=part.dynamic_ref
	)

But we couldn't just exclude system prompt parts if their dynamic text is initially empty/None, as on a next run they could have a value. So we'd likely need to include the part anyway, and decide not to include it only once in the model classes' methods that handle the SystemPromptPart.