Evaluate a single Luau expression

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

Annotations cover safety (readOnly/destructive/idempotent/openWorld), but the description adds substantial context beyond them: pcall guarding, the exact failure representation ({ ok = false, error }), purity (PURE Luau), the active-client requirement, and a stated cost/phase. This is materially more than the annotations convey.

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

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

Front-loaded with the core purpose and a concrete example before the constraints. It trail-loads some metadata (Phase/cost/idempotency/Safety) that partly duplicates the annotations, but it is well organized and most sentences earn their place.

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

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

With no output schema, the description carries the return-value burden and does so: typeof plus encoded value, and the error envelope on failure. It also names the active-client prerequisite and the fallback tool, so nothing needed to call it correctly is missing.

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

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

Schema coverage is 100%, so the schema already documents the single `expression` param and the baseline would be 3. The description adds real semantic value the schema lacks: it must be an expression, not statements, and no 'return' or ';' are permitted. That constraint meaningfully raises it above baseline.

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

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

States a specific verb (Evaluate) and resource (ONE Luau expression) with the return shape ('typeof plus an encoded value'). It explicitly positions itself against siblings, calling out run-luau and the `script` tool by name, so an agent can distinguish it without opening a schema.

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

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

Gives explicit when-to-use ('quick reads like workspace.Gravity'), when-not with alternatives ('to compose with other server tools ... use the `script` tool instead'), and a hard input constraint ('pass an expression, not statements'). This is the full when/when-not/alternatives triad.

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