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create_blueprint_function_library

Create a Blueprint Function Library in Unreal Engine to provide globally accessible utility functions across all Blueprints, eliminating the need for instantiation.

Instructions

Create a Blueprint Function Library with shared utility functions.

From Ch. 18: Create a Function Library (e.g., BP_DiceLibrary) whose functions are available globally in every Blueprint of the project. No instantiation needed.

Functions are defined as a list of dicts with:

  • name: function name

  • inputs: [{"name": str, "type": str, "default_value": any}]

  • outputs: [{"name": str, "type": str}]

  • description: optional description

Args: name: Library Blueprint name (e.g., "BP_DiceLibrary", "BP_MathUtils") functions: List of function definitions folder_path: Content browser folder

Example - dice roll library from the book: functions=[ {"name": "RollOneDie", "inputs": [{"name": "NumberOfFaces", "type": "Integer", "default_value": 6}], "outputs": [{"name": "Result", "type": "Integer"}]}, ]

KB: see knowledge_base/11_BLUEPRINT_LIBRARIES_AND_COMPONENTS.md#overview Example: create_blueprint_function_library(name="ExampleName")

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
nameYes
functionsNo
folder_pathNo/Game/Blueprints

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
resultYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv1.0.0

TDQS

A3.6/5.0
Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the behavioral disclosure burden. It does disclose useful traits: created functions become globally available, no instantiation is needed, and function definitions use a specific dict structure. However, it does not cover whether existing assets are overwritten, whether compilation is triggered, or what happens on invalid function definitions.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is well-structured with an overview, parameter list, function schema, and examples. It is somewhat long and contains a KB reference and trailing usage example, but all parts contribute useful information. It earns a high score for organization and front-loading the core purpose.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the sparse schema and absence of annotations, the description is fairly complete: it covers the tool's purpose, parameter meaning, function data shape, and includes examples. The existing output schema relieves the need to document return values. It still omits failure modes and behavior around existing assets, but the KB pointer and examples make it adequately usable.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters5/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The input schema has 0% description coverage and only defines functions as an array of generic objects. The description compensates thoroughly by documenting each parameter, the full dict schema for functions, input/output fields, default_value semantics, optional description, and a concrete dice-roll example. This is far more helpful than the schema alone.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the action: create a Blueprint Function Library with shared utility functions. It adds meaningful details about global availability and no instantiation, which helps separate it from similar Blueprint creation tools. It does not explicitly name a sibling alternative, so it stops short of a perfect score.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides context about what a function library is and gives an example, but it never states when to prefer this tool over alternatives like create_blueprint_macro_library or create_blueprint. No exclusions, prerequisites, or comparison routes are provided.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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