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Flows can pause or suspend execution and automatically resume when they receive type-checked input in Prefect’s UI. Flows can also send and receive type-checked input at any time while running—without pausing or suspending. This guide explains how to use these features to build interactive workflows.

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Pause or suspend a flow until it receives input

You can pause or suspend a flow until it receives input from a user in Prefect’s UI. This is useful when you need to ask for additional information or feedback before resuming a flow. These workflows are often called human-in-the-loop (HITL) systems.
Human-in-the-loop interactivity Approval workflows that pause to ask a human to confirm whether a workflow should continue are very common in the business world. Certain types of machine learning training and artificial intelligence workflows benefit from incorporating HITL design.

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Wait for input

To receive input while paused or suspended use the wait_for_input parameter in the pause_flow_run or suspend_flow_run functions. This parameter accepts one of the following:
  • A built-in type like int or str, or a built-in collection like List[int]
  • A pydantic.BaseModel subclass
  • A subclass of prefect.input.RunInput
When to use a RunModel or BaseModel instead of a built-in type” There are a few reasons to use a RunModel or BaseModel. The first is that when you let Prefect automatically create one of these classes for your input type, the field that users see in Prefect’s UI when they click “Resume” on a flow run is named value and has no help text to suggest what the field is. If you create a RunInput or BaseModel, you can change details like the field name, help text, and default value, and users see those reflected in the “Resume” form.
The simplest way to pause or suspend and wait for input is to pass a built-in type: 
from prefect import flow
from prefect.flow_runs import pause_flow_run
from prefect.logging import get_run_logger

@flow
def greet_user():
    logger = get_run_logger()

    user = pause_flow_run(wait_for_input=str)

    logger.info(f"Hello, {user}!")
In this example, the flow run pauses until a user clicks the Resume button in the Prefect UI, enters a name, and submits the form.
Types can you pass for wait_for_inputWhen you pass a built-in type such as int as an argument for the wait_for_input parameter to pause_flow_run or suspend_flow_run, Prefect automatically creates a Pydantic model containing one field annotated with the type you specified. This means you can use any type annotation that Pydantic accepts for model fields with these functions.
Instead of a built-in type, you can pass in a pydantic.BaseModel class. This is useful if you already have a BaseModel you want to use: 
from prefect import flow
from prefect.flow_runs import pause_flow_run
from prefect.logging import get_run_logger
from pydantic import BaseModel


class User(BaseModel):
    name: str
    age: int


@flow
async def greet_user():
    logger = get_run_logger()

    user = await pause_flow_run(wait_for_input=User)

    logger.info(f"Hello, {user.name}!")
BaseModel classes are upgraded to RunInput classes automaticallyWhen you pass a pydantic.BaseModel class as the wait_for_input argument to pause_flow_run or suspend_flow_run, Prefect automatically creates a RunInput class with the same behavior as your BaseModel and uses that instead.RunInput classes contain extra logic that allows flows to send and receive them at runtime. You shouldn’t notice any difference.
For advanced use cases such as overriding how Prefect stores flow run inputs, create a RunInput class: 
from prefect import flow
from prefect.logging import get_run_logger
from prefect.input import RunInput

class UserInput(RunInput):
    name: str
    age: int

    # Imagine overridden methods here.
    def override_something(self, *args, **kwargs):
        super().override_something(*args, **kwargs)

@flow
async def greet_user():
    logger = get_run_logger()

    user = await pause_flow_run(wait_for_input=UserInput)

    logger.info(f"Hello, {user.name}!")

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Provide initial data

Set default values for fields in your model with the with_initial_data method. This is useful for providing default values for the fields in your own RunInput class. Expanding on the example above, you can make the name field default to “anonymous”: 
from prefect import flow
from prefect.logging import get_run_logger
from prefect.input import RunInput

class UserInput(RunInput):
    name: str
    age: int

@flow
async def greet_user():
    logger = get_run_logger()

    user_input = await pause_flow_run(
        wait_for_input=UserInput.with_initial_data(name="anonymous")
    )

    if user_input.name == "anonymous":
        logger.info("Hello, stranger!")
    else:
        logger.info(f"Hello, {user_input.name}!")
When a user sees the form for this input, the name field contains “anonymous” as the default.

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Provide a description with runtime data

You can provide a dynamic, Markdown description that appears in the Prefect UI when the flow run pauses. This feature enables context-specific prompts, enhancing clarity and user interaction. Building on the example above: 
from datetime import datetime
from prefect import flow
from prefect.flow_runs import pause_flow_run
from prefect.logging import get_run_logger
from prefect.input import RunInput


class UserInput(RunInput):
    name: str
    age: int


@flow
async def greet_user():
    logger = get_run_logger()
    current_date = datetime.now().strftime("%B %d, %Y")

    description_md = f"""
**Welcome to the User Greeting Flow!**
Today's Date: {current_date}

Please enter your details below:
- **Name**: What should we call you?
- **Age**: Just a number, nothing more.
"""

    user_input = await pause_flow_run(
        wait_for_input=UserInput.with_initial_data(
            description=description_md, name="anonymous"
        )
    )

    if user_input.name == "anonymous":
        logger.info("Hello, stranger!")
    else:
        logger.info(f"Hello, {user_input.name}!")
When a user sees the form for this input, the given Markdown appears above the input fields.

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Handle custom validation

Prefect uses the fields and type hints on your RunInput or BaseModel class to validate the general structure of input your flow receives. If you require more complex validation, use Pydantic model_validators.
Calling custom validation runs after the flow resumesPrefect transforms the type annotations in your RunInput or BaseModel class to a JSON schema and uses that schema in the UI for client-side validation. However, custom validation requires running Python logic defined in your RunInput class. Because of this, validation happens after the flow resumes, so you should handle it explicitly in your flow. Continue reading for an example best practice.
The following is an example RunInput class that uses a custom model_validator: 
from typing import Literal

import pydantic

from prefect.input import RunInput


class ShirtOrder(RunInput):
    size: Literal["small", "medium", "large", "xlarge"]
    color: Literal["red", "green", "black"]

    @pydantic.model_validator(mode="after")
    def validate_age(self):
        if self.color == "green" and self.size == "small":
            raise ValueError(
                "Green is only in-stock for medium, large, and XL sizes."
            )

        return self
In the example, we use Pydantic’s model_validator decorator to define custom validation for our ShirtOrder class. You can use it in a flow like this: 
from typing import Literal

import pydantic

from prefect import flow, pause_flow_run
from prefect.input import RunInput


class ShirtOrder(RunInput):
    size: Literal["small", "medium", "large", "xlarge"]
    color: Literal["red", "green", "black"]

    @pydantic.model_validator(mode="after")
    def validate_age(self):
        if self.color == "green" and self.size == "small":
            raise ValueError(
                "Green is only in-stock for medium, large, and XL sizes."
            )

        return self


@flow
def get_shirt_order():
    shirt_order = pause_flow_run(wait_for_input=ShirtOrder)
If a user chooses any size and color combination other than small and green, the flow run resumes successfully. However, if the user chooses size small and color green, the flow run will resume, and pause_flow_run raises a ValidationError exception. This causes the flow run to fail and log the error. To avoid a flow run failure, use a while loop and pause again if the ValidationError exception is raised: 
from typing import Literal

import pydantic
from prefect import flow
from prefect.flow_runs import pause_flow_run
from prefect.logging import get_run_logger
from prefect.input import RunInput


class ShirtOrder(RunInput):
    size: Literal["small", "medium", "large", "xlarge"]
    color: Literal["red", "green", "black"]

    @pydantic.model_validator(mode="after")
    def validate_age(self):
        if self.color == "green" and self.size == "small":
            raise ValueError(
                "Green is only in-stock for medium, large, and XL sizes."
            )

        return self


@flow
def get_shirt_order():
    logger = get_run_logger()
    shirt_order = None

    while shirt_order is None:
        try:
            shirt_order = pause_flow_run(wait_for_input=ShirtOrder)
        except pydantic.ValidationError as exc:
            logger.error(f"Invalid size and color combination: {exc}")

    logger.info(
        f"Shirt order: {shirt_order.size}, {shirt_order.color}"
    )
This code causes the flow run to continually pause until the user enters a valid age. As an additional step, you can use an automation to alert the user to the error.

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Send and receive input at runtime

Use the send_input and receive_input functions to send input to a flow or receive input from a flow at runtime. You don’t need to pause or suspend the flow to send or receive input.
Reasons to send or receive input without pausing or suspendingYou might want to send or receive input without pausing or suspending in scenarios where the flow run is designed to handle real-time data. For example, in a live monitoring system, you might need to update certain parameters based on the incoming data without interrupting the flow. Another example is having a long-running flow that continually responds to runtime input with low latency. For example, if you’re building a chatbot, you could have a flow that starts a GPT Assistant and manages a conversation thread.
The most important parameter to the send_input and receive_input functions is run_type, which should be one of the following:
  • A built-in type such as int or str
  • A pydantic.BaseModel class
  • A prefect.input.RunInput class
When to use a BaseModel or RunInput instead of a built-in typeMost built-in types and collections of built-in types should work with send_input and receive_input, but there is a caveat with nested collection types, such as lists of tuples. For example, List[Tuple[str, float]]). In this case, validation may happen after your flow receives the data, so calling receive_input may raise a ValidationError. You can plan to catch this exception, and consider placing the field in an explicit BaseModel or RunInput so your flow only receives exact type matches.See examples below of receive_input, send_input, and the two functions working together.

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Receiving input

The following flow uses receive_input to continually receive names and print a personalized greeting for each name it receives: 
from prefect import flow
from prefect.input.run_input import receive_input


@flow
async def greeter_flow():
    async for name_input in receive_input(str, timeout=None):
        # Prints "Hello, andrew!" if another flow sent "andrew"
        print(f"Hello, {name_input}!")
When you pass a type such as str into receive_input, Prefect creates a RunInput class to manage your input automatically. When a flow sends input of this type, Prefect uses the RunInput class to validate the input. If the validation succeeds, your flow receives the input in the type you specified. In this example, if the flow received a valid string as input, the variable name_input contains the string value. If, instead, you pass a BaseModel, Prefect upgrades your BaseModel to a RunInput class, and the variable your flow sees (in this case, name_input), is a RunInput instance that behaves like a BaseModel. If you pass in a RunInput class, no upgrade is needed and you’ll get a RunInput instance. A simpler approach is to pass types such as str into receive_input . If you need access to the generated RunInput that contains the received value, pass with_metadata=True to receive_input: 
from prefect import flow
from prefect.input.run_input import receive_input


@flow
async def greeter_flow():
    async for name_input in receive_input(
        str,
        timeout=None,
        with_metadata=True
    ):
        # Input will always be in the field "value" on this object.
        print(f"Hello, {name_input.value}!")
When to use with_metadata=TrueThe primary uses of accessing the RunInput object for a receive input are to respond to the sender with the RunInput.respond() function, or to access the unique key for an input.
Notice that the printing of name_input.value. When Prefect generates a RunInput for you from a built-in type, the RunInput class has a single field, value, that uses a type annotation matching the type you specified. So if you call receive_input like this: receive_input(str, with_metadata=True), it’s equivalent to manually creating the following RunInput class and receive_input call: 
from prefect import flow
from prefect.input.run_input import RunInput

class GreeterInput(RunInput):
    value: str

@flow
async def greeter_flow():
    async for name_input in receive_input(GreeterInput, timeout=None):
        print(f"Hello, {name_input.value}!")
The type used in receive_input and send_input must matchFor a flow to receive input, the sender must use the same type that the receiver is receiving. This means that if the receiver is receiving GreeterInput, the sender must send GreeterInput. If the receiver is receiving GreeterInput and the sender sends the str input that Prefect automatically upgrades to a RunInput class, the types won’t match; which means the receiving flow run won’t receive the input. However, the input will wait for if the flow ever calls receive_input(str).

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Keep track of inputs you’ve already seen

By default, each time you call receive_input, you get an iterator that iterates over all known inputs to a specific flow run, starting with the first received. The iterator keeps track of your current position as you iterate over it, or you can call next() to explicitly get the next input. If you’re using the iterator in a loop, you should assign it to a variable: 
from prefect import flow, get_client
from prefect.deployments import run_deployment
from prefect.input.run_input import receive_input, send_input

EXIT_SIGNAL = "__EXIT__"


@flow
async def sender():
    greeter_flow_run = await run_deployment(
        "greeter/send-receive", timeout=0, as_subflow=False
    )
    client = get_client()

    # Assigning the `receive_input` iterator to a variable
    # outside of the the `while True` loop allows us to continue
    # iterating over inputs in subsequent passes through the
    # while loop without losing our position.
    receiver = receive_input(
        str,
        with_metadata=True,
        timeout=None,
        poll_interval=0.1
    )

    while True:
        name = input("What is your name? ")
        if not name:
            continue

        if name == "q" or name == "quit":
            await send_input(
                EXIT_SIGNAL,
                flow_run_id=greeter_flow_run.id
            )
            print("Goodbye!")
            break

        await send_input(name, flow_run_id=greeter_flow_run.id)

        # Saving the iterator outside of the while loop and
        # calling next() on each iteration of the loop ensures
        # that we're always getting the newest greeting. If we
        # had instead called `receive_input` here, we would
        # always get the _first_ greeting this flow received,
        # print it, and then ask for a new name.
        greeting = await receiver.next()
        print(greeting)
An iterator helps keep track of the inputs your flow has already received. If you want your flow to suspend and then resume later, save the keys of the inputs you’ve seen so the flow can read them back out when it resumes. Consider using a block, such as a JSONBlock. The following flow receives input for 30 seconds then suspends itself, which exits the flow and tears down infrastructure: 
from prefect import flow
from prefect.logging import get_run_logger
from prefect.flow_runs import suspend_flow_run
from prefect.blocks.system import JSON
from prefect.context import get_run_context
from prefect.input.run_input import receive_input


EXIT_SIGNAL = "__EXIT__"


@flow
async def greeter():
    logger = get_run_logger()
    run_context = get_run_context()
    assert run_context.flow_run, "Could not see my flow run ID"

    block_name = f"{run_context.flow_run.id}-seen-ids"

    try:
        seen_keys_block = await JSON.load(block_name)
    except ValueError:
        seen_keys_block = JSON(
            value=[],
        )

    try:
        async for name_input in receive_input(
            str,
            with_metadata=True,
            poll_interval=0.1,
            timeout=30,
            exclude_keys=seen_keys_block.value
        ):
            if name_input.value == EXIT_SIGNAL:
                print("Goodbye!")
                return
            await name_input.respond(f"Hello, {name_input.value}!")

            seen_keys_block.value.append(name_input.metadata.key)
            await seen_keys_block.save(
                name=block_name,
                overwrite=True
            )
    except TimeoutError:
        logger.info("Suspending greeter after 30 seconds of idle time")
        await suspend_flow_run(timeout=10000)
As this flow processes name input, it adds the key of the flow run input to the seen_keys_block. When the flow later suspends and then resumes, it reads the keys it has already seen out of the JSON block and passes them as the exlude_keys parameter to receive_input.

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Respond to the input’s sender

When your flow receives input from another flow, Prefect knows the sending flow run ID, so the receiving flow can respond by calling the respond method on the RunInput instance the flow received. There are a couple of requirements:
  • Pass in a BaseModel or RunInput, or use with_metadata=True.
  • The flow you are responding to must receive the same type of input you send to see it.
The respond method is equivalent to calling send_input(..., flow_run_id=sending_flow_run.id). But with respond, your flow doesn’t need to know the sending flow run’s ID. Next, make the greeter_flow respond to name inputs instead of printing them: 
from prefect import flow
from prefect.input.run_input import receive_input


@flow
async def greeter():
    async for name_input in receive_input(
        str,
        with_metadata=True,
        timeout=None
    ):
        await name_input.respond(f"Hello, {name_input.value}!")
However, this flow runs forever unless there’s a signal that it should exit. Here’s how to make it to look for a special string: 
from prefect import flow
from prefect.input.run_input import receive_input



EXIT_SIGNAL = "__EXIT__"


@flow
async def greeter():
    async for name_input in receive_input(
        str,
        with_metadata=True,
        poll_interval=0.1,
        timeout=None
    ):
        if name_input.value == EXIT_SIGNAL:
            print("Goodbye!")
            return
        await name_input.respond(f"Hello, {name_input.value}!")
With a greeter flow in place, create the flow that sends greeter names.

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Send input

Send input to a flow with the send_input function. This works similarly to receive_input and, like that function, accepts the same run_input argument. This can be a built-in type such as str, or else a BaseModel or RunInput subclass.
When to send input to a flow runSend input to a flow run as soon as you have the flow run’s ID. The flow does not have to be receiving input for you to send input. If you send a flow input before it is receiving, it will see your input when it calls receive_input (as long as the types in the send_input and receive_input calls match).
Next, create a sender flow that starts a greeter flow run and then enters a loop—continuously getting input from the terminal and sending it to the greeter flow: 
from prefect import flow
from prefect.deployments import run_deployment

@flow
async def sender():
    greeter_flow_run = await run_deployment(
        "greeter/send-receive", timeout=0, as_subflow=False
    )
    receiver = receive_input(str, timeout=None, poll_interval=0.1)
    client = get_client()

    while True:
        flow_run = await client.read_flow_run(greeter_flow_run.id)

        if not flow_run.state or not flow_run.state.is_running():
            continue

        name = input("What is your name? ")
        if not name:
            continue

        if name == "q" or name == "quit":
            await send_input(
                EXIT_SIGNAL,
                flow_run_id=greeter_flow_run.id
            )
            print("Goodbye!")
            break

        await send_input(name, flow_run_id=greeter_flow_run.id)
        greeting = await receiver.next()
        print(greeting)
First, run_deployment starts a greeter flow run. This requires a deployed flow running in a process. That process begins running greeter while sender continues to execute. Calling run_deployment(..., timeout=0) ensures that sender won’t wait for the greeter flow run to complete, because it’s running a loop and only exits when sending EXIT_SIGNAL. Next, the iterator returned by receive_input as receiver is captured. This flow works by entering a loop. On each iteration of the loop, the flow asks for terminal input, sends that to the greeter flow, and then runs receiver.next() to wait until it receives the response from greeter. Next, the terminal user who ran this flow is allowed to exit by entering the string q or quit. When that happens, the greeter flow is sent an exit signal to shut down, too. Finally, the new name is sent to greeter. greeter sends back a greeting as a string. When you receive the greeting, print it and continue the loop that gets terminal input.

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A complete example

For a complete example of using send_input and receive_input, here is what the greeter and sender flows look like together: 
import asyncio
import sys
from prefect import flow, get_client
from prefect.blocks.system import JSON
from prefect.context import get_run_context
from prefect.deployments import run_deployment
from prefect.input.run_input import receive_input, send_input


EXIT_SIGNAL = "__EXIT__"


@flow
async def greeter():
    run_context = get_run_context()
    assert run_context.flow_run, "Could not see my flow run ID"

    block_name = f"{run_context.flow_run.id}-seen-ids"

    try:
        seen_keys_block = await JSON.load(block_name)
    except ValueError:
        seen_keys_block = JSON(
            value=[],
        )

    async for name_input in receive_input(
        str,
        with_metadata=True,
        poll_interval=0.1,
        timeout=None
    ):
        if name_input.value == EXIT_SIGNAL:
            print("Goodbye!")
            return
        await name_input.respond(f"Hello, {name_input.value}!")

        seen_keys_block.value.append(name_input.metadata.key)
        await seen_keys_block.save(
            name=block_name,
            overwrite=True
        )


@flow
async def sender():
    greeter_flow_run = await run_deployment(
        "greeter/send-receive", timeout=0, as_subflow=False
    )
    receiver = receive_input(str, timeout=None, poll_interval=0.1)
    client = get_client()

    while True:
        flow_run = await client.read_flow_run(greeter_flow_run.id)

        if not flow_run.state or not flow_run.state.is_running():
            continue

        name = input("What is your name? ")
        if not name:
            continue

        if name == "q" or name == "quit":
            await send_input(
                EXIT_SIGNAL,
                flow_run_id=greeter_flow_run.id
            )
            print("Goodbye!")
            break

        await send_input(name, flow_run_id=greeter_flow_run.id)
        greeting = await receiver.next()
        print(greeting)


if __name__ == "__main__":
    if sys.argv[1] == "greeter":
        asyncio.run(greeter.serve(name="send-receive"))
    elif sys.argv[1] == "sender":
        asyncio.run(sender())
To run the example, you need a Python environment with Prefect installed, pointed at either a Prefect Cloud account or a self-hosted Prefect server instance. With your environment set up, start a flow runner in one terminal with the following command: 
python my_file_name greeter
For example, with Prefect Cloud, you should see output like this: 
______________________________________________________________________
| Your flow 'greeter' is being served and polling for scheduled runs |
|                                                                    |
| To trigger a run for this flow, use the following command:         |
|                                                                    |
|         $ prefect deployment run 'greeter/send-receive'            |
|                                                                    |
| You can also run your flow via the Prefect UI:                     |
| https://app.prefect.cloud/account/...(a URL for your account)      |
|                                                                    |
______________________________________________________________________
Then start the greeter process in another terminal: 
python my_file_name sender
You should see output like this: 
11:38:41.800 | INFO    | prefect.engine - Created flow run 'gregarious-owl' for flow 'sender'
11:38:41.802 | INFO    | Flow run 'gregarious-owl' - View at https://app.prefect.cloud/account/...
What is your name?
Type a name and press the enter key to see a greeting to see sending and receiving in action: 
What is your name? andrew
Hello, andrew!