Eino Agent

Eino ADK Overview

Import: github.com/cloudwego/eino/adk

The Agent Development Kit (ADK) provides a framework for building agents in Go. The ADK is generically parameterized by MessageType to support both classic *schema.Message and the new *schema.AgenticMessage. Prefer *schema.AgenticMessage for new usage.

type MessageType interface {
    *schema.Message | *schema.AgenticMessage
}

type TypedAgent[M MessageType] interface {
    Name(ctx context.Context) string
    Description(ctx context.Context) string
    Run(ctx context.Context, input *TypedAgentInput[M], options ...AgentRunOption) *AsyncIterator[*TypedAgentEvent[M]]
}

// Convenience aliases for classic message type
type Agent = TypedAgent[*schema.Message]

Agent Types

ChatModelAgent Quick Start

import (
    "context"
    "fmt"
    "log"

    "github.com/cloudwego/eino-ext/components/model/openai"
    "github.com/cloudwego/eino/adk"
    "github.com/cloudwego/eino/components/tool"
    "github.com/cloudwego/eino/components/tool/utils"
    "github.com/cloudwego/eino/compose"
)

func main() {
    ctx := context.Background()

    // 1. Create a tool
    searchTool, _ := utils.InferTool("search_book", "Search books by genre",
        func(ctx context.Context, input *struct {
            Genre string `json:"genre" jsonschema_description:"Book genre"`
        }) (string, error) {
            return `{"books": ["The Great Gatsby"]}`, nil
        })

    // 2. Create model (BaseModel[M], not ToolCallingChatModel)
    cm, _ := openai.NewChatModel(ctx, &openai.ChatModelConfig{
        APIKey: "your-key", Model: "gpt-4o",
    })

    // 3. Create agent
    agent, _ := adk.NewChatModelAgent(ctx, &adk.ChatModelAgentConfig{
        Name:        "BookRecommender",
        Description: "Recommends books",
        Instruction: "You recommend books using the search_book tool.",
        Model:       cm,
        ToolsConfig: adk.ToolsConfig{
            ToolsNodeConfig: compose.ToolsNodeConfig{
                Tools: []tool.BaseTool{searchTool},
            },
        },
    })

    // 4. Run with Runner
    runner := adk.NewRunner(ctx, adk.RunnerConfig{Agent: agent})
    iter := runner.Query(ctx, "recommend a fiction book")
    for {
        event, ok := iter.Next()
        if !ok {
            break
        }
        if event.Err != nil {
            log.Fatal(event.Err)
        }
        if event.Output != nil && event.Output.MessageOutput != nil {
            msg, _ := event.Output.MessageOutput.GetMessage()
            fmt.Printf("Agent[%s]: %v\n", event.AgentName, msg)
        }
    }
}

Cancel Mechanism

Cancel provides safe, controllable termination of agent execution.

// Create a cancel function alongside the run
cancelOpt, cancelFn := adk.WithCancel()
iter := runner.Query(ctx, "do something", cancelOpt)

// ... iterate events ...

// Cancel at a safe point (cancelFn is non-blocking, Wait blocks until complete)
handle, ok := cancelFn(adk.WithAgentCancelMode(adk.CancelAfterChatModel))
if ok {
    handle.Wait()
}

CancelMode (bitmask):

Cancel options:

• WithAgentCancelMode(mode) -- set safe point
• WithAgentCancelTimeout(d) -- escalate to immediate if safe point not reached in time
• WithRecursive() -- propagate cancel into nested AgentTool agents

Cancel produces a CancelError on the event stream with checkpoint data for later resumption.

Model Retry

Output-based retry with full control over retry decisions.

agent, _ := adk.NewChatModelAgent(ctx, &adk.ChatModelAgentConfig{
    // ...
    ModelRetryConfig: &adk.ModelRetryConfig{
        MaxRetries: 3,
        ShouldRetry: func(ctx context.Context, retryCtx *adk.RetryContext) *adk.RetryDecision {
            // Retry based on output content (e.g., empty response, bad finish reason)
            if retryCtx.Err != nil {
                return &adk.RetryDecision{Retry: true, Backoff: time.Second}
            }
            if retryCtx.OutputMessage == nil || retryCtx.OutputMessage.Content == "" {
                return &adk.RetryDecision{Retry: true, Backoff: time.Second}
            }
            return &adk.RetryDecision{Retry: false}
        },
    },
})

RetryContext provides: RetryAttempt, InputMessages, OutputMessage (full concatenated response), Err.

RetryDecision controls: Retry, ModifiedInputMessages, AdditionalOptions, Backoff, RejectReason.

When streaming, a WillRetryError is emitted on the stream to signal retry is occurring.

Model Failover

Dynamic model switching when primary model fails or produces unsatisfactory output.

agent, _ := adk.NewChatModelAgent(ctx, &adk.ChatModelAgentConfig{
    // ...
    ModelFailoverConfig: &adk.ModelFailoverConfig[*schema.Message]{
        MaxRetries: 2,
        ShouldFailover: func(ctx context.Context, outputMessage *schema.Message, outputErr error) bool {
            return outputErr != nil // failover on any error
        },
        GetFailoverModel: func(ctx context.Context, failoverCtx *adk.FailoverContext[*schema.Message]) (
            model.BaseModel[*schema.Message], []*schema.Message, error) {
            // Return a different model and optionally modified input
            return backupModel, failoverCtx.InputMessages, nil
        },
    },
})

Failover interacts with Retry: when both are configured, FailoverContext.LastErr will be a *RetryExhaustedError if retry was also exhausted.

TurnLoop

Push-based event loop for multi-turn agent execution with preemption, idle timeout, and graceful shutdown.

import "github.com/cloudwego/eino/adk"

loop := adk.NewTurnLoop(adk.TurnLoopConfig[string, *schema.Message]{
    GenInput: func(ctx context.Context, loop *adk.TurnLoop[string, *schema.Message], items []string) (*adk.GenInputResult[string, *schema.Message], error) {
        // Convert pushed items into agent input
        combined := strings.Join(items, "\n")
        return &adk.GenInputResult[string, *schema.Message]{
            RunCtx:   ctx,
            Input:    &adk.TypedAgentInput[*schema.Message]{Messages: []*schema.Message{schema.UserMessage(combined)}},
            Consumed: items,
        }, nil
    },
    PrepareAgent: func(ctx context.Context, loop *adk.TurnLoop[string, *schema.Message], consumed []string) (adk.Agent, error) {
        return myAgent, nil
    },
    OnAgentEvents: func(ctx context.Context, tc *adk.TurnContext[string, *schema.Message], events *adk.AsyncIterator[*adk.AgentEvent]) error {
        for {
            event, ok := events.Next()
            if !ok {
                break
            }
            if event.Err != nil {
                return event.Err
            }
            // Process events (e.g., send to client)
        }
        return nil
    },
})

// Start the loop
loop.Run(ctx)

// Push items (non-blocking, returns (ok bool, resolved <-chan struct{}))
loop.Push("user message 1")

// Preempt current turn with new input
loop.Push("urgent message", adk.WithPreempt[string, *schema.Message](adk.AfterChatModel))

// Stop gracefully
loop.Stop(adk.WithGraceful())

// Wait for exit and get final state
exitState := loop.Wait()

Key concepts:

• Push() queues items; the loop batches and processes them via GenInput
• Preemption: cancel current turn at a safe point and start new turn with pending items
• Idle timeout: UntilIdleFor(d) auto-stops after no items for duration d
• Graceful shutdown: WithGraceful(), WithGracefulTimeout(d), WithImmediate()
• Exit state: TurnLoopExitState contains ExitReason, UnhandledItems, InterruptedItems

Runner

The Runner manages agent lifecycle, context passing, and interrupt/resume:

runner := adk.NewRunner(ctx, adk.RunnerConfig{
    Agent:           myAgent,
    EnableStreaming:  true,
    CheckPointStore: myStore,  // for interrupt/resume
})

// Query (convenience for single user message)
iter := runner.Query(ctx, "hello")

// Run (full control over input messages)
iter := runner.Run(ctx, []*schema.Message{schema.UserMessage("hello")})

Middleware System

Middleware extends ChatModelAgent behavior. Configure via Handlers field:

agent, _ := adk.NewChatModelAgent(ctx, &adk.ChatModelAgentConfig{
    // ...
    Handlers: []adk.ChatModelAgentMiddleware{fsMiddleware, summarizationMW},
})

Eight built-in middleware types (see reference/middleware.md for details):

AgentAsTool

Wrap any Agent as a Tool for use by another agent:

subAgent := createMySubAgent()
agentTool := adk.NewAgentTool(ctx, subAgent)

parentAgent, _ := adk.NewChatModelAgent(ctx, &adk.ChatModelAgentConfig{
    ToolsConfig: adk.ToolsConfig{
        ToolsNodeConfig: compose.ToolsNodeConfig{
            Tools: []tool.BaseTool{agentTool},
        },
    },
})

Human-in-the-Loop

ChatModelAgent supports interrupt and resume for human approval, clarification, and feedback. See reference/human-in-the-loop.md for details.

Key pattern: tool returns compose.Interrupt(ctx, info) to pause, then runner.ResumeWithParams(ctx, checkpointID, params) to continue.

Instructions to Agent

• Default to ChatModelAgent for most use cases (single agent with tools)
• Use Runner to execute agents -- never call agent.Run() directly in production
• Middleware order matters: PatchToolCalls first, then Reduction, then Summarization
• Use DeepAgent (adk/prebuilt/deep) when you need built-in planning + filesystem + sub-agents
• Use AgentAsTool or DeepAgents' SubAgents when a sub-agent needs isolated context (no shared history)
• Use TurnLoop when building interactive applications that need preemption, idle management, or push-based input
• Model field accepts model.BaseModel[M] -- for classic path use any BaseChatModel, for agentic path use AgenticModel
• Cancel, Retry, and Failover can all be combined; failover wraps retry
• WithCancel() is a run option, not a config option -- create fresh per-run

Reference Files

Read these files on-demand for detailed API, examples, and advanced usage:

• reference/chat-model-agent.md -- ChatModelAgentConfig reference, ReAct pattern, ToolsConfig, streaming, Cancel/Retry/Failover details
• reference/deep-agents.md -- DeepAgent concept, config, architecture, comparison with ChatModelAgent
• reference/middleware.md -- All 8 middleware types with interface, config, and examples
• reference/runner-and-events.md -- Runner creation, AgentEvent/AgentOutput, event iteration patterns
• reference/agent-as-tool.md -- Wrapping an Agent as a Tool for use by another agent
• reference/human-in-the-loop.md -- Interrupt APIs, ResumableAgent, CheckPointStore, resume patterns
• reference/filesystem.md -- Filesystem Backend interface, Local and AgentKit implementations, usage with DeepAgent