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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint. The description adds meaningful behavioral context beyond that: despite openWorldHint=true, this is a pure offline format/checksum check with no external registry call, no registration-status lookup and no identity verification — a real clarification of the tool's actual behavior.

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?

Two sentences, zero waste, with the core action front-loaded and the scope exclusions immediately after. Nothing needs to be trimmed.

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 validator with an output schema and full annotations, the description covers what is checked and what is explicitly out of scope. Only minor gaps remain (nothing on error/return signaling), which the output schema largely absorbs.

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% and the single 'lei' parameter is fully documented in the schema. The description reinforces the 20-character length and MOD-97 algorithm but adds no format or normalization detail beyond that, so the baseline 3 applies.

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 precise verb+resource: validating a 20-character LEI with MOD-97 check digits. The second sentence draws explicit boundaries ('No registry lookup, active-registration check or legal-entity identity verification'), which separates it from registry/identity siblings like sanctions_address_screen or vendor_onboarding_pack.

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?

The negative scoping ('No registry lookup, active-registration check or legal-entity identity verification') tells the agent exactly when this tool is insufficient, which is genuine routing guidance. It stops short of naming the alternative tool for those excluded cases, so it is not a full 5.

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