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

Annotations already declare readOnlyHint/openWorldHint=false/idempotent/no-destructive, but the description adds genuinely new operational facts: no network, no stored state, determinism, and concrete limits (64 KB input, 1 s per call, 1 MB result). These are the kinds of constraints an agent needs to avoid timeouts or oversized payloads, and they go beyond the annotation set.

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 action and then cleanly segmented into When / Not for / Side effects / Determinism / Limits, so an agent can skim the labels. The only cost is full Spanish-then-English duplication, which roughly doubles the token footprint for a single-language consumer.

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?

Output schema exists, so return values need not be spelled out, yet the description still hints at the LOU-prefix split. With the algorithm, offline nature, limits, and exclusion all present, nothing an agent needs to call this 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 baseline would be 3, but the description adds meaning the schema lacks: the identifier must be exactly 20 characters while the schema allows maxLength 24, and it explains that the check digit is verified via mod 97-10. That resolves a real ambiguity an agent would otherwise hit on 21–24 character input.

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 concrete verb and resource (validates a 20-character LEI per ISO 17442), names the exact algorithm (ISO 7064 mod 97-10) and an extra output behavior (LOU prefix split). It explicitly distinguishes itself from the nearest sibling, isin-validate, so an agent can route without opening either 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?

Uses explicit 'When:' / 'Not for:' labels with a named alternative (isin-validate) and the condition selecting it (12-character ISIN). Covers both the use case and the exclusion, leaving nothing to inference.

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