Flows

Comprehensive skill for building CrewAI Flows with structured state management, event-driven control, and production-ready patterns.

When To Use

• Create flow-based AI orchestration
• Manage state across multi-step workflows
• Implement conditional routing with @router
• Build event-driven systems with @listen
• Add persistence for long-running workflows
• Combine Flows with Crews for production systems
• Design complex control logic (loops, branches, joins)

Quick Start Workflow

1. Scaffold New Flow Project

python scripts/scaffold_flow.py my_flow

2. Define State Model

Use structured state with Pydantic for type safety:

from pydantic import BaseModel

class MyState(BaseModel):
    input_data: str = ""
    processed: bool = False
    results: list = []

3. Implement Flow Logic

from crewai.flow.flow import Flow, listen, start

class MyFlow(Flow[MyState]):
    @start()
    def initialize(self):
        self.state.input_data = "sample"
        return "initialized"
    
    @listen(initialize)
    def process(self, _):
        self.state.processed = True
        return "completed"

4. Validate and Run

python scripts/validate_flow.py my_flow/main.py
python my_flow/main.py

Detailed Workflow

Design Phase

1. Model State: Define Pydantic model with all fields needed
2. Map Flow Topology: Plan @start, @listen, @router relationships
3. Identify Control Points: Where do you need branches or loops?
4. Plan Persistence: Which steps need checkpointing?

Implementation Phase

1. Create State Model: Use Pydantic BaseModel
2. Define Flow Class: Inherit from Flow[StateType]
3. Add Decorators: @start, @listen, @router as needed
4. Implement Methods: Each method updates state and returns values
5. Add Persistence: Use @persist decorator for critical steps

Validation Phase

python scripts/validate_flow.py ./my_flow/main.py

Validates:

• Flow class inheritance
• @start decorator presence
• Router labels match listeners
• State model validity
• No circular dependencies

Execution Phase

flow = MyFlow()
result = flow.kickoff()

State Management

Structured State (Recommended)

from pydantic import BaseModel

class AppState(BaseModel):
    user_input: str = ""
    processing_result: str = ""
    completed: bool = False

class MyFlow(Flow[AppState]):
    @start()
    def init(self):
        self.state.user_input = "hello"
        return "ok"

Benefits:

• Type checking at development time
• IDE autocompletion
• Automatic validation
• Self-documenting

Unstructured State

class MyFlow(Flow):
    @start()
    def init(self):
        self.state["key"] = "value"  # Dictionary access
        return "ok"

Use for:

• Quick prototyping
• Dynamic schemas
• Simple flows

Decorator Reference

@start()

Entry point for flow execution.

Unconditional:

@start()
def init(self):
    return "started"

Conditional:

@start("previous_method")
def resume(self):
    return "resumed"

@listen()

Listen to method completion or router labels.

Single source:

@listen(init)
def process(self, result):
    return f"Got: {result}"

Multiple sources (AND):

from crewai.flow.flow import and_

@listen(and_(step_a, step_b))
def merge(self, _):
    return "both completed"

Alternative sources (OR):

from crewai.flow.flow import or_

@listen(or_(option_a, option_b))
def handle_either(self, _):
    return "one completed"

Label-based:

@listen("approved")
def publish(self):
    return "published"

@router()

Conditional routing with labels.

@router(evaluate)
def quality_gate(self, _):
    if self.state.score > 80:
        return "approved"
    elif self.state.score > 60:
        return "revision"
    else:
        return "rejected"

@listen("approved")
def handle_approval(self): ...

@listen("revision")
def handle_revision(self): ...

@persist()

Automatic state persistence.

Class-level:

from crewai.flow.persistence import persist

@persist()
class MyFlow(Flow[MyState]): ...

Method-level:

@persist()
@listen(process)
def critical_step(self, _): ...

Common Patterns

Sequential Pipeline

@start() → @listen → @listen → final

Conditional with Retry Loop

@start() → @router → "revision" → @listen → back to @router
                     → "approved" → @listen → complete

Parallel Execution with Join

          ┌→ branch_a ─┐
@start() ─┤            ├→ @listen(and_(a, b)) → complete
          └→ branch_b ─┘

State Accumulator

@start()
def collect_items(self):
    self.state.items = []
    return "start"

@listen(collect_items)
def add_item(self, _):
    self.state.items.append(new_item)
    if len(self.state.items) < 10:
        return self.add_item("")  # Loop
    return "complete"

Tool Usage

scaffold_flow.py

Create new flow projects:

python scripts/scaffold_flow.py my_flow
python scripts/scaffold_flow.py my_flow --path ./projects --with-crew

validate_flow.py

Validate flow structure:

python scripts/validate_flow.py ./my_flow/main.py

plot_flow.py

Generate visualization:

python scripts/plot_flow.py ./my_flow/main.py --output flow.html

generate_state.py

Interactive state generator:

python scripts/generate_state.py --output state.py

Asset Templates

Starter Project

• assets/starter/main.py - Complete working flow with persistence
• assets/starter/README.md - Project documentation

Configuration Templates

• assets/templates/flow-basic.py - Simple @start/@listen
• assets/templates/flow-advanced.py - Router, persistence, and_/or_
• assets/templates/state-structured.py - Pydantic state models
• assets/templates/state-unstructured.py - Dictionary state

Reference Documentation

API Reference

• references/api/decorators.md - Complete decorator reference (@start, @listen, @router, @persist, and_, or_)
• references/api/state-management.md - State approaches, lifecycle, persistence
• references/api/flow-attributes.md - Flow class attributes and methods

Guides

• references/guides/state-patterns.md - Accumulator, Pipeline, Branching, Retry, Progress, Error Recovery, Crew Integration
• references/guides/control-primitives.md - Sequential, Conditional Routing, Parallel, Alternative Paths, Human-in-the-Loop, Multi-Start

External Resources

• references/external.md - Official documentation links

Troubleshooting

Common Issues

"No @start decorator found"

Error: Flow must have at least one @start method

Solution: Add @start() decorator to entry point method

"Router label has no listener"

Error: Router returns label 'approved' but no @listen('approved') found

Solution: Add matching listener for each router label

"Circular dependency detected"

Error: Circular dependency in decorator chain

Solution: Remove circular @listen references

"State field not found"

Error: 'MyState' object has no attribute 'unknown_field'

Solution: Add field to Pydantic model or check field name spelling

"Invalid state model"

Error: State model must inherit from pydantic.BaseModel

Solution: Ensure state class inherits from BaseModel

Debugging Tips

1. Enable verbose logging:

import logging
logging.basicConfig(level=logging.INFO)

2. Log state at each step:

@listen(step)
def next_step(self, _):
    print(f"State: {self.state}")
    ...

3. Visualize the flow:

python scripts/plot_flow.py main.py

4. Check state persistence:

print(f"Flow ID: {flow.state.id}")

Mastery Steps

1. Start Simple: Run assets/templates/flow-basic.py
2. Add State: Define Pydantic model with your fields
3. Build Chain: Connect 3-4 steps with @listen
4. Add Routing: Implement @router for conditional logic
5. Try Persistence: Add @persist decorator
6. Study Patterns: Review references/guides/state-patterns.md
7. Master Control: Study references/guides/control-primitives.md
8. Integrate Crews: Use core-build skill to create crew, call from Flow

File Structure

flows/
├── SKILL.md                          # This file
├── assets/
│   ├── starter/                      # Complete working flow
│   │   ├── main.py                  # Production-ready example
│   │   └── README.md                # Project documentation
│   └── templates/                    # Progressive examples
│       ├── flow-basic.py            # Simple decorators
│       ├── flow-advanced.py         # Router, persistence
│       ├── state-structured.py      # Pydantic models
│       └── state-unstructured.py    # Dictionary state
├── references/
│   ├── api/                          # API documentation
│   │   ├── decorators.md            # @start, @listen, @router, @persist
│   │   ├── state-management.md      # State approaches & lifecycle
│   │   └── flow-attributes.md       # Flow class reference
│   ├── guides/                       # How-to guides
│   │   ├── state-patterns.md        # Common state patterns
│   │   └── control-primitives.md    # Control flow patterns
│   └── external.md                  # Official docs links
└── scripts/                          # Executable tools
    ├── scaffold_flow.py             # Project scaffolding
    ├── validate_flow.py             # Structure validation
    ├── plot_flow.py                 # Visualization
    └── generate_state.py            # State model generator

Coding-Agent Guidelines

• Start with state model before implementing decorators
• Use structured state (Pydantic) for all non-trivial flows
• Keep router labels deterministic and document them
• Reserve Flow code for orchestration, use core-build for crews
• Make state updates idempotent where retry is possible
• Add persistence for long-running or critical workflows
• Log flow ID at key transitions for traceability
• Use plotting as a release gate to verify flow topology
• Document state transitions in method docstrings
• Validate before execution with validate_flow.py

Production Architecture

Flow-First Approach

Always start with Flow when building production AI applications:

• Flows provide state management across steps
• Flows enable precise execution control (loops, conditionals)
• Flows offer observability and debugging capabilities

State Design Principles

• Keep state minimal - only persist what you need
• Use structured data - avoid unstructured dictionaries
• Make fields descriptive and self-documenting

Crew Integration

• Create crews using core-build skill patterns
• Use Flows to orchestrate crew execution order
• Pass typed payloads between Flow state and crew inputs/outputs

Persistence Strategy

• Use @persist() class-level for full workflow checkpointing
• Use @persist() method-level for specific critical steps
• Resume automatically from last checkpoint on restart

Observability

• Log flow state.id at start and key transitions
• Track router decisions with input evidence
• Monitor branch duration and failure counts

Next Steps

1. Review assets/starter/ for a complete working example
2. Study references/guides/ for patterns and best practices
3. Use scripts/scaffold_flow.py to create your first flow
4. Validate with scripts/validate_flow.py
5. Visualize with scripts/plot_flow.py
6. Deploy with CrewAI Enterprise for production hosting