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

Annotations already declare readOnly, closed-world, idempotent, non-destructive, so 'read-only' is partly redundant. However, the description adds genuinely new behavioral facts: no network access, no stored state, determinism, and concrete limits (64 KB input, 1 s per call, 1 MB result). It does not elaborate on the failure shape, but the output schema covers that.

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 purpose, then when/not-for, then effects and limits — a good structural order with no filler sentences. The only cost is the full ES/EN duplication, which roughly doubles length for the same content.

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?

For a single-parameter validation tool with an output schema and full annotations, the description covers format rules, scope, alternatives, side effects, determinism, and operational limits. Nothing an agent needs 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% with a single required string, so the baseline is 3. The description still earns credit by adding accepted-format semantics the schema lacks: 8 characters, mod-11 check digit, 'X' allowed — an agent knows what input will pass without guessing.

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 and resource plus the defining technical rule: 'Validates an ISSN (8 characters, mod-11 check digit with X) for serial publications.' It explicitly names the confusion-prone sibling (isbn-validate), so an agent can separate it from the other identifier validators in the list.

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?

Provides explicit when ('format and check digit have to be verified offline before it is used') and when-not ('Not for: validating an ISBN-10 (isbn-validate)'), including the alternative tool name. Nothing about tool selection is left to inference.

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