Package akka.javasdk.annotations

Annotation Interface FunctionTool

@Target(METHOD)
@Retention(RUNTIME)
@Documented
public @interface FunctionTool

Annotation to expose methods as function tools that can be invoked by AI agents.

Methods annotated with @FunctionTool become available as tools that the AI model can choose to invoke based on the task requirements. The LLM determines which tools to call and with which parameters based on the tool descriptions and the user's request.

Tool Discovery: Function tools can be defined in three ways:

• Agent-defined: Methods within the agent class itself (can be private)
• External tools: Methods in separate classes registered via effects().tools()
• Components as tools: Methods in Workflows, Event Sourced Entities, Key Value Entities, and Views classes registered via effects().tools()

Components as tools rules:

• Only Workflows, Event Sourced Entities, Key Value Entities, and Views can be used as tools
• For Workflows, only public methods returning Workflow.Effect or Workflow.ReadOnlyEffect can be annotated with FunctionTool. Methods returning Workflow.StepEffect are not allowed.
• For Event Sourced Entities, only public methods returning EventSourcedEntity.Effect or EventSourcedEntity.ReadOnlyEffect can be annotated with FunctionTool.
• For Key Value Entities, only public methods returning KeyValueEntity.Effect or KeyValueEntity.ReadOnlyEffect can be annotated with FunctionTool.
• For Views, only public methods returning View.QueryEffect can be annotated with FunctionTool. Methods returning View.QueryStreamEffect are not allowed.

Best Practices:

• Provide clear, descriptive tool descriptions
• Use Description annotations on parameters to help the LLM understand usage
• Use Optional parameters for non-required arguments
• Keep tool functions focused on a single, well-defined task

Tool Execution: The agent automatically handles the tool execution loop: the LLM requests tool calls, the agent executes them, incorporates results into the session context, and continues until the LLM no longer needs to invoke tools.

Required Element Summary

Required Elements

Modifier and Type	Required Element	Description
String	description

Optional Element Summary

Optional Elements

Modifier and Type	Optional Element	Description
String	name

Element Details

name

String name

Default:

""

description

String description