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DmitriyGolub

threejs-devtools-mcp

by DmitriyGolub

raycast

Read-onlyIdempotent

Cast a ray from the camera through screen coordinates and return hit objects for object picking or interaction in a Three.js scene.

Instructions

Cast a ray from camera through screen coordinates and return hit objects

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
xYesNormalized device X coordinate (-1 to 1, 0 = center)
yYesNormalized device Y coordinate (-1 to 1, 0 = center)
maxHitsNoMaximum hits to return (default: 10)
Behavior3/5

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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is clear. The description adds that it returns hit objects but does not elaborate on edge cases or behavior beyond the schema, which already documents the input coordinates. No contradiction with annotations.

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

Conciseness5/5

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

The description is a single sentence of 12 words, front-loading the core purpose. Every word is necessary and there is no redundancy.

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 tool's simplicity, the description is largely complete. It explains the input coordinate system and the action. The only minor gap is the lack of detail about the return format, but this is partially mitigated by the phrase 'hit objects' and the presence of annotations.

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

Parameters3/5

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

The input schema has 100% description coverage for all three parameters (x, y, maxHits). The description does not add any meaning beyond what the schema already provides, so a baseline score of 3 is appropriate.

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

Purpose5/5

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

The description uses a specific verb ('cast') and resource ('ray') and describes the result ('return hit objects'). It is distinct from sibling tools like 'click_inspect' or 'scene_tree' which serve different purposes.

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

Usage Guidelines4/5

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

The description clearly implies the tool is for hit testing via screen coordinates, providing context for when to use it. However, it does not explicitly state when not to use it or mention alternatives.

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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