Agent States - PiCrust

AgentState Enum

Agents transition through various states during execution:

pub enum AgentState {
    Idle,
    Processing,
    WaitingForPermission,
    ExecutingTool {
        tool_name: String,
        tool_use_id: String,
    },
    WaitingForSubAgent {
        session_id: String,
    },
    WaitingForUserInput {
        request_id: String,
    },
    Done,
    Error {
        message: String,
    },
}

State Descriptions

Idle

Meaning: Agent is waiting for input When:

After spawning
After completing a task
Between turns

Transitions to:

Processing (when input received)
Done (when shutdown)

if handle.is_idle().await {
    // Ready to accept new input
    handle.send_input("New task").await?;
}

Processing

Meaning: Agent is calling the LLM When:

After receiving user input
During multi-turn conversations
While waiting for LLM response

Transitions to:

ExecutingTool (LLM wants to use a tool)
WaitingForPermission (tool needs permission)
WaitingForSubAgent (spawning subagent)
WaitingForUserInput (asking user questions)
Idle (text-only response complete)
Done (task complete)
Error (LLM call failed)

if handle.is_processing().await {
    // Show loading indicator
    ui.show_spinner();
}

WaitingForPermission

Meaning: Tool execution requires user approval When:

LLM wants to use a tool
No matching permission rule exists
Interactive mode enabled

Transitions to:

ExecutingTool (permission granted)
Done (permission denied)
Processing (error in permission handling)

Payload:

Tool name
Action description
Tool input

match handle.state().await {
    AgentState::WaitingForPermission => {
        ui.show_permission_dialog();
    }
    _ => {}
}

ExecutingTool

Meaning: Currently running a tool When:

Permission granted (or auto-allowed)
Tool execution in progress

Transitions to:

Processing (tool complete, continuing)
Done (tool complete, task finished)
Error (tool execution failed)

Payload:

tool_name: Which tool is running
tool_use_id: Unique ID for this execution

match handle.state().await {
    AgentState::ExecutingTool { tool_name, .. } => {
        ui.show_tool_indicator(&tool_name);
    }
    _ => {}
}

WaitingForSubAgent

Meaning: Waiting for a spawned subagent to complete When:

Tool spawned a subagent
Subagent is processing
Parent is blocked

Transitions to:

Processing (subagent completed)
Error (subagent failed)

Payload:

session_id: ID of the child agent

match handle.state().await {
    AgentState::WaitingForSubAgent { session_id } => {
        // Monitor subagent
        if let Some(child) = runtime.get(&session_id).await {
            let child_state = child.state().await;
            ui.show_subagent_status(&session_id, child_state);
        }
    }
    _ => {}
}

WaitingForUserInput

Meaning: Waiting for user to answer questions When:

Agent used AskUserQuestion tool
Questions sent to frontend
Awaiting responses

Transitions to:

Processing (answers received)
Done (interrupted)

Payload:

request_id: ID to match request/response

match handle.state().await {
    AgentState::WaitingForUserInput { request_id } => {
        ui.show_question_modal(&request_id);
    }
    _ => {}
}

Done

Meaning: Agent has completed all work When:

Task finished successfully
User interrupted
Permission denied
Shutdown requested

Transitions to:

Idle (if new input sent)
(Terminal state for this task)

if handle.is_done().await {
    ui.enable_input();
    ui.hide_spinner();
}

Error

Meaning: Agent encountered an unrecoverable error When:

LLM API error
Tool execution failure
Internal error
Invalid state transition

Transitions to:

(Terminal state)
Idle (if new input clears error)

Payload:

message: Error description

match handle.state().await {
    AgentState::Error { message } => {
        ui.show_error(&message);
        log::error!("Agent error: {}", message);
    }
    _ => {}
}

State Transitions

Complete State Machine

Typical Flow Examples

Simple text response:

Idle → Processing → Idle

Tool execution:

Idle → Processing → WaitingForPermission → ExecutingTool → Processing → Idle

Multiple tools:

Idle → Processing →
  ExecutingTool → Processing →
  ExecutingTool → Processing →
  Idle

With subagent:

Idle → Processing →
  ExecutingTool → WaitingForSubAgent →
  Processing → Idle

With questions:

Idle → Processing →
  WaitingForUserInput → Processing →
  Idle

Monitoring State

Polling State

use tokio::time::{interval, Duration};

let mut timer = interval(Duration::from_millis(100));

loop {
    timer.tick().await;

    let state = handle.state().await;

    match state {
        AgentState::Idle => {
            ui.show_status("Ready");
        }
        AgentState::Processing => {
            ui.show_status("Thinking...");
        }
        AgentState::ExecutingTool { tool_name, .. } => {
            ui.show_status(&format!("Using {}...", tool_name));
        }
        AgentState::Done => {
            ui.show_status("Complete");
            break;
        }
        _ => {}
    }
}

Event-Driven Monitoring

let mut rx = handle.subscribe();

while let Ok(chunk) = rx.recv().await {
    match chunk {
        OutputChunk::StateChange(new_state) => {
            println!("State: {:?}", new_state);

            match new_state {
                AgentState::Processing => {
                    ui.show_spinner();
                }
                AgentState::Idle | AgentState::Done => {
                    ui.hide_spinner();
                }
                AgentState::Error { message } => {
                    ui.show_error(&message);
                }
                _ => {}
            }
        }
        _ => {}
    }
}

State-Based UI Updates

React Example (TypeScript)

const [agentState, setAgentState] = useState<AgentState>("idle");

// Listen for state changes
listen<StateChangeEvent>("state-change", (event) => {
  setAgentState(event.payload.state);
});

// Render based on state
function AgentStatus() {
  switch (agentState) {
    case "idle":
      return <Badge color="green">Ready</Badge>;
    case "processing":
      return <Badge color="blue">Thinking...</Badge>;
    case "executing_tool":
      return <Badge color="purple">Using tool</Badge>;
    case "waiting_for_permission":
      return <Badge color="yellow">Permission needed</Badge>;
    case "done":
      return <Badge color="gray">Complete</Badge>;
    case "error":
      return <Badge color="red">Error</Badge>;
    default:
      return <Badge>Unknown</Badge>;
  }
}

Tauri Backend

#[tauri::command]
async fn monitor_agent_state(
    window: tauri::Window,
    session_id: String,
    runtime: tauri::State<'_, Arc<Mutex<AgentRuntime>>>,
) -> Result<(), String> {
    let runtime = runtime.lock().await;

    if let Some(handle) = runtime.get(&session_id).await {
        tokio::spawn(async move {
            let mut rx = handle.subscribe();

            while let Ok(chunk) = rx.recv().await {
                if let OutputChunk::StateChange(state) = chunk {
                    // Emit to frontend
                    window.emit("state-change", serde_json::json!({
                        "session_id": session_id,
                        "state": state,
                    })).ok();
                }
            }
        });
    }

    Ok(())
}

State Persistence

States are not persisted to disk. They are runtime-only:

// After restart, agent is NOT in previous state
let handle = runtime.spawn(session, |i| agent.run(i)).await;
// Always starts in Idle state

// Must reconstruct state from history if needed
let history = session.history();
// Analyze history to determine what happened

Convenience Methods

The AgentHandle provides convenience methods:

// Check specific states
if handle.is_idle().await {
    // Agent is ready
}

if handle.is_processing().await {
    // Agent is busy
}

if handle.is_done().await {
    // Agent finished
}

// Get full state
let state = handle.state().await;
match state {
    AgentState::ExecutingTool { tool_name, .. } => {
        println!("Using tool: {}", tool_name);
    }
    _ => {}
}

Error States

Entering Error State

Agents enter error state when:

LLM API fails:
- Network error
- API key invalid
- Rate limit exceeded
- Timeout
Tool execution fails:
- Tool threw exception
- Invalid tool input
- Tool not found
Internal errors:
- Session save failed
- Invalid state transition
- Channel closed unexpectedly

Recovering from Errors

match handle.state().await {
    AgentState::Error { message } => {
        eprintln!("Error: {}", message);

        // Log error
        log::error!("Agent {} failed: {}", session_id, message);

        // Notify user
        ui.show_error_dialog(&message);

        // Optionally restart
        handle.send_input("Let's try that again").await?;
        // Agent transitions back to Processing
    }
    _ => {}
}

State and Interrupts

Interrupt in Different States

During Processing:

Partial text preserved
Incomplete thinking discarded
All tool calls removed

During ExecutingTool:

Current tool completes
Remaining tools get “Interrupted” error
Agent transitions to Done

During WaitingForPermission:

Tool gets “Interrupted” error
No execution
Agent transitions to Done

See Interrupt Handling for details.

State Debugging

Enable State Logging

use tracing::{info, debug};

let mut rx = handle.subscribe();

while let Ok(chunk) = rx.recv().await {
    match chunk {
        OutputChunk::StateChange(state) => {
            info!("State transition: {:?}", state);
        }
        _ => {}
    }
}

State History

Track state history for debugging:

struct StateTracker {
    states: Vec<(DateTime<Utc>, AgentState)>,
}

impl StateTracker {
    fn record(&mut self, state: AgentState) {
        self.states.push((Utc::now(), state));
    }

    fn print_history(&self) {
        for (time, state) in &self.states {
            println!("{}: {:?}", time, state);
        }
    }
}

// Usage
let mut tracker = StateTracker { states: vec![] };
let mut rx = handle.subscribe();

while let Ok(chunk) = rx.recv().await {
    if let OutputChunk::StateChange(state) = chunk {
        tracker.record(state);
    }
}

tracker.print_history();

Best Practices

1. Always Handle Error State

match handle.state().await {
    AgentState::Error { message } => {
        // Don't ignore errors!
        handle_error(&message);
    }
    _ => {}
}

2. Show State to Users

// Visual feedback improves UX
match handle.state().await {
    AgentState::Processing => ui.show_spinner(),
    AgentState::ExecutingTool { tool_name, .. } => {
        ui.show_tool_badge(&tool_name);
    }
    AgentState::WaitingForPermission => {
        ui.pulse_permission_button();
    }
    _ => ui.hide_indicators(),
}

3. Don’t Poll Too Frequently

// Good: 100ms is fine
let mut timer = interval(Duration::from_millis(100));

// Bad: Too frequent, wastes CPU
let mut timer = interval(Duration::from_millis(10));

4. Use Event-Driven When Possible

// Better than polling
let mut rx = handle.subscribe();

while let Ok(chunk) = rx.recv().await {
    if let OutputChunk::StateChange(state) = chunk {
        update_ui(state);
    }
}

5. Handle All States

match handle.state().await {
    AgentState::Idle => { /* ... */ }
    AgentState::Processing => { /* ... */ }
    AgentState::WaitingForPermission => { /* ... */ }
    AgentState::ExecutingTool { .. } => { /* ... */ }
    AgentState::WaitingForSubAgent { .. } => { /* ... */ }
    AgentState::WaitingForUserInput { .. } => { /* ... */ }
    AgentState::Done => { /* ... */ }
    AgentState::Error { .. } => { /* ... */ }
}

Next Steps

Streaming & History

Understand the dual-channel pattern

Interrupt Handling

Learn about graceful cancellation

AgentState API

Complete state API reference

Error Handling

Advanced error handling patterns

​AgentState Enum

​State Descriptions

​Idle

​Processing

​WaitingForPermission

​ExecutingTool

​WaitingForSubAgent

​WaitingForUserInput

​Done

​Error

​State Transitions

​Complete State Machine

​Typical Flow Examples

​Monitoring State

​Polling State

​Event-Driven Monitoring

​State-Based UI Updates

​React Example (TypeScript)

​Tauri Backend

​State Persistence

​Convenience Methods

​Error States

​Entering Error State

​Recovering from Errors

​State and Interrupts

​Interrupt in Different States

​State Debugging

​Enable State Logging

​State History

​Best Practices

​1. Always Handle Error State

​2. Show State to Users

​3. Don’t Poll Too Frequently

​4. Use Event-Driven When Possible

​5. Handle All States

​Next Steps

Streaming & History

Interrupt Handling

AgentState API

Error Handling

AgentState Enum

State Descriptions

Idle

Processing

WaitingForPermission

ExecutingTool

WaitingForSubAgent

WaitingForUserInput

Done

Error

State Transitions

Complete State Machine

Typical Flow Examples

Monitoring State

Polling State

Event-Driven Monitoring

State-Based UI Updates

React Example (TypeScript)

Tauri Backend

State Persistence

Convenience Methods

Error States

Entering Error State

Recovering from Errors

State and Interrupts

Interrupt in Different States

State Debugging

Enable State Logging

State History

Best Practices

1. Always Handle Error State

2. Show State to Users

3. Don’t Poll Too Frequently

4. Use Event-Driven When Possible

5. Handle All States

Next Steps