feat(multiagent): add FunctionNode to improve DX for deterministic cases

[dbschmigelski]

Description

In our docs we call out a Custom Node Type which can be used to implement deterministic nodes. But this requires customers to know about stop_reason and construct the DEEPLY nested return type of MultiAgentResult which uses NodeResult which uses AgentResult - this is too in the weeds in my opinion.

To improve developer experience, we will vend a FunctionNode which handles the hard part.

new DX

def deterministic_function(task, invocation_state=None, **kwargs):
    ...



# Create nodes
agent = Agent()
function_node = FunctionNode(deterministic_function, "deterministic_function")
# Build graph
builder = GraphBuilder()
builder.add_node(agent, "agent")
builder.add_node(function_node, "deterministic_function")

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The alternative here is to vend an abstract class. I am on the fence about this. An abstract class feels more "correct" but the DX of simply providing the method feels easiest to me. Very open to discussing.

Related Issues

N/A

Documentation PR

pending

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[Unshure]

Agree with the devex of this approach, it is very clean. As for a abstract class, I think we can migrate to that if customers want it in the future, but good with this for now.

[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/multiagent/function_node.py	97.82%	0 Missing and 1 partial ⚠️

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