windmill/system_prompts/base/flow-base.md

Windmill Flow Building Guide

The OpenFlow schema (openflow.openapi.yaml) is the source of truth for flow structure. Refer to OPENFLOW_SCHEMA for the complete type definitions.

Reserved Module IDs

• failure - Reserved for failure handler module
• preprocessor - Reserved for preprocessor module
• Input - Reserved for flow input reference

Module ID Rules

• Must be unique across the entire flow
• Use underscores, not spaces (e.g., fetch_data not fetch data)
• Use descriptive names that reflect the step's purpose

Common Mistakes to Avoid

• Missing input_transforms - Rawscript parameters won't receive values without them
• Referencing future steps - results.step_id only works for steps that execute before the current one
• Duplicate module IDs - Each module ID must be unique in the flow

Data Flow Between Steps

• flow_input.property - Access flow input parameters
• results.step_id - Access output from a previous step
• results.step_id.property - Access specific property from previous step output
• flow_input.iter.value - Current item when inside a for-loop
• flow_input.iter.index - Current index when inside a for-loop

Input Transforms

Every rawscript module needs input_transforms to map function parameters to values:

Static transform (fixed value): {"param_name": {"type": "static", "value": "fixed_string"}}

JavaScript transform (dynamic expression): {"param_name": {"type": "javascript", "expr": "results.previous_step.data"}}

Resource References

• For flow inputs: Use type "object" with format "resource-{type}" (e.g., "resource-postgresql")
• For step inputs: Use static value "$res:path/to/resource"

Failure Handler

Executes when any step fails. Has access to error details:

• error.message - Error message
• error.step_id - ID of failed step
• error.name - Error name
• error.stack - Stack trace

S3 Object Operations

Windmill provides built-in support for S3-compatible storage operations.

To accept an S3 object as flow input:

{
  "type": "object",
  "properties": {
    "file": {
      "type": "object",
      "format": "resource-s3_object",
      "description": "File to process"
    }
  }
}

Using Resources in Flows

On Windmill, credentials and configuration are stored in resources. Resource types define the format of the resource.

As Flow Input

In the flow schema, set the property type to "object" with format "resource-{type}":

{
  "type": "object",
  "properties": {
    "database": {
      "type": "object",
      "format": "resource-postgresql",
      "description": "Database connection"
    }
  }
}

As Step Input (Static Reference)

Reference a specific resource using $res: prefix:

{
  "database": {
    "type": "static",
    "value": "$res:f/folder/my_database"
  }
}