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Symmetrically decrypt data with crypt.decrypt

crypt-decrypt
Read-onlyIdempotent

Decrypt ciphertext with the executor's crypt.decrypt function by supplying the original Base64 key, IV, and optional cipher mode; returns plaintext or an error.

Instructions

Decrypt a ciphertext with the executor's crypt.decrypt(data, key, iv, mode?). Supply the same Base64 key and iv that were used to encrypt, plus the optional cipher mode if a non-default mode was used. The function returns the recovered plaintext. This is a pure, side-effect-free compute that runs entirely in-game. Requires crypt.decrypt; on an executor without it, it returns { error } instead of throwing. The call is pcall-guarded so a wrong key/iv/mode degrades to a clean error. Returns { plaintext } or { error }. Signature: { data: string, key: string, iv: string, mode: string?, threadContext: number?, timeoutMs: number? }. Phase: observe; cost=medium; idempotency=read-only. Requires: active-client. Produces: structured-result. Safety: read-only. On failure: inspect tool-schema for exact fields, defaults, constraints, and an invocation example.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
ivYesThe Base64-encoded initialization vector that was used to encrypt.
keyYesThe Base64-encoded symmetric key that was used to encrypt.
dataYesThe ciphertext to decrypt.
modeNoOptional cipher mode; must match the mode used at encrypt time if it was non-default.
timeoutMsNoOptional per-call deadline in milliseconds; omit it to use the tool or server default.
threadContextNoOptional Roblox thread identity for this call; omit it to use the server default.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv2.0.0-spies.2

TDQS

A4/5.0
Behavior4/5

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

Annotations already declare readOnly/idempotent/destructive, but the description adds real value beyond them: it discloses the pure side-effect-free nature, the executor requirement for crypt.decrypt, and the pcall-guarded failure contract where wrong key/iv/mode yields { error } rather than a throw.

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

Conciseness3/5

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

The core is front-loaded well, but the trailing metadata string ('Phase: observe; cost=medium; idempotency=read-only... Safety: read-only') repeats what the annotations and earlier sentences already convey, adding bulk without new meaning.

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?

With no output schema, the description correctly explains return values ({ plaintext } or { error }) and failure handling. It covers the key facts an agent needs, though it does not detail output format nuances or timeout/threadContext usage.

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

Parameters4/5

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

Schema coverage is 100%, so baseline is 3, but the description adds a genuine cross-parameter constraint absent from the schema: key and iv must be the SAME Base64 values used at encrypt time, and mode must match if non-default.

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 names a specific verb (decrypt), resource (ciphertext), and even the underlying executor function crypt.decrypt(data, key, iv, mode?). It is clearly distinguishable from crypt-encrypt and other crypt-* siblings by stating it recovers plaintext.

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

Usage Guidelines3/5

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

It gives prerequisite guidance ('supply the same Base64 key and iv that were used to encrypt', plus the mode if non-default), but never names alternatives or when-not-to-use conditions. Usage is implied rather than spelled out.

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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