Tool Return

How tool plugins return messages (text, links, images, files, JSON), create standard and streaming variables, and define output schemas for workflow references

Overview

A tool returns its results as messages and variables. This page covers the message interfaces, variable creation, and output schema definitions.

Message Types

Return different types of messages such as text, links, images, and JSON

Variables

Create and manipulate variables for workflow integration

Output Schema

Define custom output variables for workflow references

Data Structure

Message Return

Dify supports several message types—text, links, images, file blobs, and JSON—each returned through a dedicated interface.

By default, a tool’s output in a workflow includes three fixed variables: files, text, and json. The methods below populate these variables.

While you can use methods like create_image_message to return an image, tools also support custom output variables, which make it easier to reference specific data in a workflow.

Message Types

Image URL

def create_image_message(self, image: str) -> ToolInvokeMessage:
    """
    Return an image URL message
    
    Dify will automatically download the image from the provided URL
    and display it to the user.
    
    Args:
        image: URL to an image file
        
    Returns:
        ToolInvokeMessage: Message object for the tool response
    """
    pass

Parameters

image · string · required

URL to an image that will be downloaded and displayed.

link · string · required

URL to display as a clickable link.

text · string · required

Text content to display.

blob · bytes · required

Raw file data in bytes.

meta · dict

File metadata, including:

• mime_type: The MIME type of the file, for example image/png.
• Other metadata relevant to the file.

json · dict · required

Python dictionary to serialize as JSON.

When working with file blobs, always specify the mime_type in the meta dictionary to ensure proper handling of the file. For example: {"mime_type": "image/png"}.

Variables

Standard Variable

from typing import Any

def create_variable_message(self, variable_name: str, variable_value: Any) -> ToolInvokeMessage:
    """
    Create a named variable for workflow integration
    
    For non-streaming output variables. If multiple instances with the 
    same name are created, the latest one overrides previous values.
    
    Args:
        variable_name: Name of the variable to create
        variable_value: Value of the variable (any Python data type)
        
    Returns:
        ToolInvokeMessage: Message object for the tool response
    """
    pass

Parameters

variable_name · string · required

Name of the variable to create or update.

variable_value · Any/string · required

Value to assign to the variable:

• Standard variables: any Python data type.
• Streaming variables: string data only.

create_stream_variable_message currently only supports string data. Complex data types cannot be streamed with the typewriter effect.

Custom Output Variables

To reference a tool’s output variables in a workflow application, declare which variables the tool might output using JSON Schema in the tool’s manifest.

Define Output Schema

Tool Manifest with Output Schema

identity:
  author: example_author
  name: example_tool
  label:
    en_US: Example Tool
    zh_Hans: 示例工具
    ja_JP: ツール例
    pt_BR: Ferramenta de exemplo
description:
  human:
    en_US: A simple tool that returns a name
    zh_Hans: 返回名称的简单工具
    ja_JP: 名前を返す簡単なツール
    pt_BR: Uma ferramenta simples que retorna um nome
  llm: A simple tool that returns a name variable
output_schema:
  type: object
  properties:
    name:
      type: string
      description: "The name returned by the tool"
    age:
      type: integer
      description: "The age returned by the tool"
    profile:
      type: object
      properties:
        interests:
          type: array
          items:
            type: string
        location:
          type: string

Schema Structure

output_schema · object · required

Root object defining the tool’s output schema.

type · string · required

Must be object for tool output schemas.

properties · object · required

Dictionary of all possible output variables.

properties.[variable_name] · object

Definition of each output variable, including its type and description.

Defining an output schema is not enough on its own: your implementation must still return each variable with create_variable_message(). Otherwise, the workflow receives None for that variable.

Example Implementation

Basic Variable Example

def run(self, inputs):
    # Process inputs and generate a name
    generated_name = "Alice"
    
    # Return the name as a variable that matches the output_schema
    return self.create_variable_message("name", generated_name)

For complex workflows, you can define multiple output variables and return them all. This gives workflow designers more flexibility when using your tool.

Examples

Complete Tool Implementation

Weather Forecast Tool

import requests
from typing import Any

class WeatherForecastTool:
    def run(self, inputs: dict) -> Any:
        # Get location from inputs
        location = inputs.get("location", "London")
        
        try:
            # Call weather API (example only)
            weather_data = self._get_weather_data(location)
            
            # Create variables for workflow use
            self.create_variable_message("temperature", weather_data["temperature"])
            self.create_variable_message("conditions", weather_data["conditions"])
            self.create_variable_message("forecast", weather_data["forecast"])
            
            # Create a JSON message for data transmission
            self.create_json_message(weather_data)
            
            # Create an image message for the weather map
            self.create_image_message(weather_data["map_url"])
            
            # Return a formatted text response
            return self.create_text_message(
                f"Weather in {location}: {weather_data['temperature']}°C, {weather_data['conditions']}. "
                f"Forecast: {weather_data['forecast']}"
            )
            
        except Exception as e:
            # Handle errors gracefully
            return self.create_text_message(f"Error retrieving weather data: {str(e)}")
    
    def _get_weather_data(self, location: str) -> dict:
        # Mock implementation - in a real tool, this would call a weather API
        return {
            "location": location,
            "temperature": 22,
            "conditions": "Partly Cloudy",
            "forecast": "Sunny with occasional showers tomorrow",
            "map_url": "https://example.com/weather-map.png"
        }

When designing tools, consider both the direct output (what the user sees) and the variable output (what other workflow nodes can use). This separation provides flexibility in how your tool is used.

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