scratch-unified-mcp

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Sb3 Vm Poke

sb3_vm_poke

Modify live Scratch VM state mid-wave for fault injection. Set variables, lists, and sprite properties (position, direction, size, visibility, costume) to simulate failures.

Instructions

Set live VM state (fault injection mid-wave). (proxied)

Each arg is a JSON array string: variables entries {target?, name, value}, lists entries {target?, name, items}, sprites entries {target, x?, y?, direction?, size?, visible?, costume?}. Unknown target/variable/list/costume is an error.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "sb3_vm_poke"
  }
}

Input Schema

Name	Required	Description	Default
`lists`	No
`sprites`	No
`variables`	No

Output Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedv1.0.0Sep 19, 2026

TDQS

A4.2/5.0

Behavior4/5

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

No annotations are provided, so the description carries the behavioral burden. It discloses the mutating nature, the live-VM context, and the error condition for unknown targets/variables/lists/costumes. It does not address permissions, reversibility, or explain 'proxied', but the core side-effect profile is clear.

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?

Two efficient sentences: purpose is front-loaded and the parameter format is dense but necessary. The unexplained 'proxied' parenthetical is minor noise, but there is no substantial waste or repetition.

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?

For a three-parameter VM mutation tool, the description covers the input grammar and failure mode, and an output schema exists so return-value explanation is not required. It omits explicit guidance on how this tool relates to single-varible setters or whether changes apply immediately, but the 'live state' framing implies immediacy.

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 schema has 0% description coverage, so the description must compensate—and it does thoroughly. It defines each parameter's JSON-array-string format and the exact entry shapes, including optional fields and error behavior. This is far more than the bare input schema provides.

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?

Description states a specific action ('Set live VM state') and resource with a clear use-case ('fault injection mid-wave'). It does not explicitly name sibling tools or contrast itself with related tools like sb3_vm_state or sb3_set_varible, so it falls just short of full sibling differentiation.

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?

Gives a clear context for when to use the tool: live fault injection mid-wave. It does not state exclusions or alternatives, but the use-case is unambiguous enough to guide an agent toward this tool among the many VM-related siblings.

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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