agent-base

A lightweight Agent Runtime Kernel for building AI agents in Rust.

agent-base provides the minimal orchestration layer needed to build custom AI agents — LLM integration, tool dispatch, multi-turn conversation, approval flows, event streaming, and error recovery — all with zero business assumptions.

Design Principles

• Clear semantics — RunOutcome explicitly distinguishes Completed from Failed; events capture the process, the return value captures the final result.
• Simple state model — Runtime memory is the source of truth for live sessions; SessionStore is an optional persistence adapter.
• Conservative by default — On tool failure, the runtime stops by default (StopOnError) rather than guessing how to recover.
• Strategy injection — All variable behaviors are injected via traits (ToolErrorRecovery, ToolPolicy, ApprovalHandler, Middleware), not hardcoded.

Features

• LLM Abstraction — LlmClient trait with built-in OpenAI and Anthropic implementations
• Tool System — Tool trait + ToolRegistry for registration and dispatch
• Approval Flow — ApprovalHandler trait with AllowOnce / AllowAlways / Deny decisions
• Error Recovery — ToolErrorRecovery trait; defaults to StopOnError, opt-in RetryOnError
• Event Streaming — Structured AgentEvent stream for UI, logging, auditing, and debugging
• Multi-turn Sessions — AgentSession manages message history; SessionStore for optional persistence
• Sub-Agents — SubAgentTool with Ephemeral (default) or Persistent session policies
• Context Management — configurable ContextWindowManager for token budget control
• Middleware — hooks at on_user_message, on_pre_llm, and on_post_llm for extensions
• Checkpoints — structured Checkpoint events enable future replay, debugging, and resume
• MCP Support — built-in McpClient for the Model Context Protocol
• Skills — composable capability units with auto-registered tools and on-demand detailed prompts

Quick Start

1. Define a Tool

Any capability you want your agent to have is expressed as a Tool:

use agent_base::{Tool, ToolContext, ToolOutput, ToolControlFlow, AgentResult};
use async_trait::async_trait;
use serde_json::{json, Value};

struct WeatherTool;

#[async_trait]
impl Tool for WeatherTool {
    fn name(&self) -> &'static str { "get_weather" }

    fn definition(&self) -> Value {
        json!({
            "type": "function",
            "function": {
                "name": "get_weather",
                "description": "Get current weather for a city",
                "parameters": {
                    "type": "object",
                    "properties": {
                        "city": { "type": "string", "description": "City name" }
                    },
                    "required": ["city"]
                }
            }
        })
    }

    async fn call(&self, args: &Value, _ctx: &ToolContext) -> AgentResult<ToolOutput> {
        let city = args["city"].as_str().unwrap_or("unknown");
        Ok(ToolOutput {
            summary: format!("Weather in {}: 22°C, sunny", city),
            raw: None,
            control_flow: ToolControlFlow::Continue,
            truncated: false,
        })
    }
}

2. Build the Agent

use std::sync::Arc;
use agent_base::{
    AgentBuilder, AgentEvent, AgentResult, RunOutcome,
    OpenAiClient, StopOnError,
};

#[tokio::main]
async fn main() -> AgentResult<()> {
    let llm = Arc::new(OpenAiClient::new(
        std::env::var("OPENAI_API_KEY").unwrap(),
        "gpt-4o".into(),
        None,
    ));

    let mut runtime = AgentBuilder::new(llm)
        .system_prompt("You are a helpful weather assistant.")
        .register_tool(WeatherTool)
        .build();

    let session_id = runtime.create_session();
    let (events, outcome) = runtime.run_turn_stream(
        session_id,
        "What's the weather in Tokyo?",
    ).await?;

    for event in &events {
        match event {
            AgentEvent::TextDelta { text, .. } => print!("{}", text),
            AgentEvent::ToolCallStarted { tool_name, .. } => {
                println!("\n[Calling tool: {}]", tool_name);
            }
            AgentEvent::ToolCallFinished { summary, .. } => {
                println!("[Tool result: {}]", summary);
            }
            AgentEvent::RunFinished { .. } => println!("\n[Done]"),
            _ => {}
        }
    }

    assert_eq!(outcome, RunOutcome::Completed);
    Ok(())
}

3. Handle Tool Errors

By default, tool failures stop the run. For self-healing agents (e.g. code agents that retry compilation), inject RetryOnError:

use agent_base::RetryOnError;

let mut runtime = AgentBuilder::new(llm)
    .register_tool(MyTool)
    .error_recovery(Arc::new(RetryOnError))  // ← retry on failure
    .build();

4. Add Approval for Sensitive Tools

use agent_base::{
    ApprovalHandler, ApprovalRequest, ApprovalDecision,
    ToolPolicy, RiskLevel,
};

struct MyApprovalHandler;
#[async_trait::async_trait]
impl ApprovalHandler for MyApprovalHandler {
    async fn approve(&self, _req: ApprovalRequest) -> AgentResult<ApprovalDecision> {
        // Ask user via UI, CLI, etc.
        Ok(ApprovalDecision::AllowOnce)
    }
}

struct MyToolPolicy;
impl ToolPolicy for MyToolPolicy {
    fn evaluate_approval(&self, tool_name: &str, _args: &Value, _json: &str)
        -> Option<ApprovalRequest>
    {
        if tool_name == "dangerous_tool" {
            Some(ApprovalRequest {
                title: "Confirm action".into(),
                message: format!("Execute `{}`?", tool_name),
                risk_level: RiskLevel::Sensitive,
                ..Default::default()
            })
        } else {
            None  // auto-allow
        }
    }
}

let mut runtime = AgentBuilder::new(llm)
    .register_tool(DangerousTool)
    .tool_policy(Arc::new(MyToolPolicy))
    .approval_handler(Arc::new(MyApprovalHandler))
    .build();

5. Use a Sub-Agent

use agent_base::SubAgentTool;

// Build a sub-agent runtime
let sub_llm = Arc::new(OpenAiClient::new(key, model, None));
let sub_runtime = AgentBuilder::new(sub_llm)
    .system_prompt("You are a math expert.")
    .build();

// Wrap it as a tool
let math_tool = SubAgentTool::new(
    "calculate",
    "Delegate math problems to a math expert",
    sub_runtime,
);

// Register in the parent agent
let mut parent = AgentBuilder::new(parent_llm)
    .register_tool(math_tool)
    .build();

Each sub-agent call creates a fresh session by default. Use SubAgentTool::with_persistent() to share context across calls.

Examples

# Configure API key
cp .env.example .env
# Edit .env with your OPENAI_API_KEY or ANTHROPIC_API_KEY

# Run the REPL example
cargo run --example repl

# Run the SubAgent demo
cargo run --example subagent_demo

# Run the MCP demo
cargo run --example mcp_demo

# Run the Skill demo
cargo run --example skill_demo

What agent-base Does NOT Do

• ❌ No SSH, filesystem, or database tools
• ❌ No workflow DAG / multi-agent orchestration engine
• ❌ No memory / RAG framework
• ❌ No terminal UI or approval dialogs
• ❌ No heavy persistence or transaction system

Business-specific tools and strategies belong in upper layers (ops-agent, db-agent, etc.).

Typical Layering

ops-agent / db-agent / browser-agent    ← Business agents
    └── agent-base                       ← Lightweight Runtime Kernel

v1 Semantics

Stability

This project is in early development (v0.1.0). The core abstractions are settling but not yet frozen. Expect minor API changes as the ecosystem evolves.

License

MIT