WebSocket Events

Event Categories

ComfyUI WebSocket events are divided into two categories:

JSON Events

Text-based messages sent as JSON objects for workflow state changes

Binary Events

Binary messages containing preview images and other binary data

JSON Events

JSON events follow this structure:

{
  "type": "event_name",
  "data": { /* event-specific data */ }
}

status

Sent when the queue status changes or on initial connection. Contains information about the current execution queue.

data.status

object

required

Show properties

exec_info

object

Show properties

queue_remaining

integer

Number of prompts remaining in the queue

data.sid

string

Session ID (only included in the initial connection message)

Example

{
  "type": "status",
  "data": {
    "status": {
      "exec_info": {
        "queue_remaining": 3
      }
    },
    "sid": "a1b2c3d4e5f6789012345678"
  }
}

executing

Sent when a node begins execution or when execution completes.

data.node

string | null

required

The ID of the node currently executing. null indicates that execution has completed.

data.display_node

string

The ID of the node to display in the UI (may differ from the executing node for optimization)

data.prompt_id

string

required

The unique identifier for the workflow execution

Example: Node Execution

{
  "type": "executing",
  "data": {
    "node": "3",
    "display_node": "3",
    "prompt_id": "550e8400-e29b-41d4-a716-446655440000"
  }
}

Example: Execution Complete

{
  "type": "executing",
  "data": {
    "node": null,
    "prompt_id": "550e8400-e29b-41d4-a716-446655440000"
  }
}

When node is null, the workflow execution is complete. This is the signal to retrieve final outputs from the /history endpoint.

executed

Sent when a node completes execution successfully.

data.node

string

required

The ID of the node that finished executing

data.display_node

string

The ID of the node displayed in the UI

data.output

object

The UI output from the node (e.g., generated images, text)

data.prompt_id

string

required

The unique identifier for the workflow execution

Example

{
  "type": "executed",
  "data": {
    "node": "9",
    "display_node": "9",
    "output": {
      "images": [
        {
          "filename": "ComfyUI_00001_.png",
          "subfolder": "",
          "type": "output"
        }
      ]
    },
    "prompt_id": "550e8400-e29b-41d4-a716-446655440000"
  }
}

progress

Sent during node execution to report progress (e.g., sampling steps).

data.value

integer

required

Current progress value (e.g., current step)

data.max

integer

required

Maximum progress value (e.g., total steps)

data.prompt_id

string

The unique identifier for the workflow execution

data.node

string

The ID of the node reporting progress

Example

{
  "type": "progress",
  "data": {
    "value": 15,
    "max": 20,
    "prompt_id": "550e8400-e29b-41d4-a716-446655440000",
    "node": "3"
  }
}

execution_error

Sent when an error occurs during workflow execution.

data.prompt_id

string

required

The unique identifier for the failed workflow execution

data.node_id

string

The ID of the node that caused the error

data.exception_message

string

Human-readable error message

data.exception_type

string

The type of exception that occurred

data.traceback

string

Full Python traceback for debugging

Example

{
  "type": "execution_error",
  "data": {
    "prompt_id": "550e8400-e29b-41d4-a716-446655440000",
    "node_id": "3",
    "exception_message": "Error: invalid seed value",
    "exception_type": "ValueError",
    "traceback": "Traceback (most recent call last):\n..."
  }
}

feature_flags

Exchanged between client and server to negotiate supported features.

data

object

required

Key-value pairs of feature flags and their support status (boolean)

Example: Server Features

{
  "type": "feature_flags",
  "data": {
    "binary_preview_images": true,
    "image_metadata": true,
    "workflow_execution_api": true
  }
}

logs

Sent when server logs are available (if log streaming is enabled).

data.entries

array

required

Array of log entry objects

data.size

integer

Total size of log data

Example

{
  "type": "logs",
  "data": {
    "entries": [
      {"level": "info", "message": "Model loaded successfully"},
      {"level": "warning", "message": "GPU memory low"}
    ],
    "size": 1024
  }
}

Binary Events

Binary messages use a structured format:

[4 bytes: event type ID (big-endian)][remaining bytes: binary data]

Event Type IDs

Defined in protocol.py:

class BinaryEventTypes:
    PREVIEW_IMAGE = 1
    UNENCODED_PREVIEW_IMAGE = 2
    TEXT = 3
    PREVIEW_IMAGE_WITH_METADATA = 4

PREVIEW_IMAGE (Type 1)

Contains an encoded preview image generated during workflow execution.

Format

[4 bytes: type = 1][4 bytes: image type][image data]

Image Type

integer

1: JPEG format
2: PNG format

Decoding Example

import struct
from PIL import Image
from io import BytesIO

def decode_preview_image(binary_data):
    # Read event type (first 4 bytes)
    event_type = struct.unpack(">I", binary_data[:4])[0]
    
    if event_type == 1:  # PREVIEW_IMAGE
        # Read image type (next 4 bytes)
        image_type = struct.unpack(">I", binary_data[4:8])[0]
        
        # Extract image data
        image_data = binary_data[8:]
        
        # Open with PIL
        image = Image.open(BytesIO(image_data))
        return image

PREVIEW_IMAGE_WITH_METADATA (Type 4)

Contains a preview image along with metadata (requires feature flag negotiation).

Format

[4 bytes: type = 4][4 bytes: metadata length][metadata JSON][image data]

Metadata Length

integer

Length of the JSON metadata in bytes (big-endian)

Metadata JSON

object

JSON object containing metadata about the image

Show properties

image_type

string

MIME type: "image/png" or "image/jpeg"

node_id

string

ID of the node that generated the preview

prompt_id

string

The workflow execution ID

Decoding Example

import struct
import json
from PIL import Image
from io import BytesIO

def decode_preview_with_metadata(binary_data):
    # Read event type (first 4 bytes)
    event_type = struct.unpack(">I", binary_data[:4])[0]
    
    if event_type == 4:  # PREVIEW_IMAGE_WITH_METADATA
        # Read metadata length (next 4 bytes)
        metadata_length = struct.unpack(">I", binary_data[4:8])[0]
        
        # Extract and parse metadata
        metadata_json = binary_data[8:8+metadata_length].decode('utf-8')
        metadata = json.loads(metadata_json)
        
        # Extract image data
        image_data = binary_data[8+metadata_length:]
        
        # Open with PIL
        image = Image.open(BytesIO(image_data))
        
        return image, metadata

# Example usage
image, metadata = decode_preview_with_metadata(binary_message)
print(f"Image type: {metadata['image_type']}")
print(f"From node: {metadata.get('node_id')}")
image.show()

UNENCODED_PREVIEW_IMAGE (Type 2)

Raw unencoded preview image data (rarely used in practice).

TEXT (Type 3)

Binary text data (rarely used; most text is sent as JSON).

Event Flow Example

Here’s a typical sequence of events when executing a workflow:

Complete Integration Example

Here’s a production-ready example that handles all event types:

import websocket
import uuid
import json
import struct
from PIL import Image
from io import BytesIO

class ComfyUIClient:
    def __init__(self, server_address="127.0.0.1:8188"):
        self.server_address = server_address
        self.client_id = str(uuid.uuid4())
        self.ws = None
        
    def connect(self):
        self.ws = websocket.WebSocket()
        self.ws.connect(f"ws://{self.server_address}/ws?clientId={self.client_id}")
        print(f"Connected with client ID: {self.client_id}")
        
    def handle_message(self, message):
        if isinstance(message, str):
            # JSON message
            data = json.loads(message)
            event_type = data['type']
            
            if event_type == 'status':
                queue_remaining = data['data']['status']['exec_info']['queue_remaining']
                print(f"Queue remaining: {queue_remaining}")
                
            elif event_type == 'executing':
                node = data['data'].get('node')
                prompt_id = data['data'].get('prompt_id')
                if node is None:
                    print(f"Execution complete: {prompt_id}")
                else:
                    print(f"Executing node: {node}")
                    
            elif event_type == 'executed':
                node = data['data']['node']
                output = data['data'].get('output', {})
                print(f"Node {node} executed. Output: {output}")
                
            elif event_type == 'progress':
                value = data['data']['value']
                max_val = data['data']['max']
                print(f"Progress: {value}/{max_val}")
                
            elif event_type == 'execution_error':
                error = data['data']
                print(f"Error in node {error.get('node_id')}: {error.get('exception_message')}")
                
        else:
            # Binary message
            event_type = struct.unpack(">I", message[:4])[0]
            
            if event_type == 1:  # PREVIEW_IMAGE
                image_type = struct.unpack(">I", message[4:8])[0]
                image_data = message[8:]
                image = Image.open(BytesIO(image_data))
                print(f"Received preview image: {image.size}")
                # image.save(f"preview_{uuid.uuid4()}.{'jpg' if image_type == 1 else 'png'}")
                
            elif event_type == 4:  # PREVIEW_IMAGE_WITH_METADATA
                metadata_length = struct.unpack(">I", message[4:8])[0]
                metadata_json = message[8:8+metadata_length].decode('utf-8')
                metadata = json.loads(metadata_json)
                image_data = message[8+metadata_length:]
                image = Image.open(BytesIO(image_data))
                print(f"Received preview with metadata: {metadata}")
                
    def listen(self):
        while True:
            try:
                message = self.ws.recv()
                self.handle_message(message)
            except Exception as e:
                print(f"Error: {e}")
                break
                
    def close(self):
        if self.ws:
            self.ws.close()

# Usage
client = ComfyUIClient()
client.connect()
client.listen()

Best Practices

Check Event Types

Always check the event type before accessing data properties, as different events have different data structures.

Handle Binary Messages

Use struct.unpack(">I", data[:4])[0] to read the big-endian event type from binary messages.

Match Prompt IDs

When tracking multiple concurrent workflows, always check prompt_id to match events with the correct execution.

Detect Completion

Workflow execution is complete when you receive an executing event with node: null.

Error Handling

Listen for execution_error events and implement retry logic or user notifications.

REST API

WebSocket API

Node Reference

WebSocket Events

Event Categories

JSON Events

Binary Events

JSON Events

status

Example

executing

Example: Node Execution

Example: Execution Complete

executed

Example

progress

Example

execution_error

Example

feature_flags

Example: Server Features

logs

Example

Binary Events

Event Type IDs

PREVIEW_IMAGE (Type 1)

Format

Decoding Example

PREVIEW_IMAGE_WITH_METADATA (Type 4)

Format

Decoding Example

UNENCODED_PREVIEW_IMAGE (Type 2)

TEXT (Type 3)

Event Flow Example

Complete Integration Example

Best Practices

Next Steps

WebSocket Overview

History Endpoint

Build docs developers (and LLMs) love

REST API

WebSocket API

Node Reference

​Event Categories

JSON Events

Binary Events

​JSON Events

​status

​Example

​executing

​Example: Node Execution

​Example: Execution Complete

​executed

​Example

​progress

​Example

​execution_error

​Example

​feature_flags

​Example: Server Features

​logs

​Example

​Binary Events

​Event Type IDs

​PREVIEW_IMAGE (Type 1)

​Format

​Decoding Example

​PREVIEW_IMAGE_WITH_METADATA (Type 4)

​Format

​Decoding Example

​UNENCODED_PREVIEW_IMAGE (Type 2)

​TEXT (Type 3)

​Event Flow Example

​Complete Integration Example

​Best Practices

​Next Steps

WebSocket Overview

History Endpoint

Build docs developers (and LLMs) love

Event Categories

JSON Events

status

Example

executing

Example: Node Execution

Example: Execution Complete

executed

Example

progress

Example

execution_error

Example

feature_flags

Example: Server Features

logs

Example

Binary Events

Event Type IDs

PREVIEW_IMAGE (Type 1)

Format

Decoding Example

PREVIEW_IMAGE_WITH_METADATA (Type 4)

Format

Decoding Example

UNENCODED_PREVIEW_IMAGE (Type 2)

TEXT (Type 3)

Event Flow Example

Complete Integration Example

Best Practices

Next Steps