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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered; the description adds real extra context: no network, no stored state, determinism, and concrete operational limits (64 KB input, 1 s per call, 1 MB result). It is duplicative of the annotations in the 'side effects' sentence, which keeps it from a 5.

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 capability and clearly segmented (When / Not for / Side effects / Deterministic / Limits). The full Spanish and English duplication roughly doubles the length for no informational gain, which is the main deduction.

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?

An output schema exists, so return values need not be explained. With the bound, the method, the exactness guarantee, the alternative tool, the side-effect profile and the resource limits all present, an agent has everything needed to call it correctly.

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 description coverage is 100%, so both parameters are already documented, and the schema already gives min/max, default and examples. The description only notes 'optionally the first terms' without adding the 100-term cap or format detail, so the schema does the heavy lifting and 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 specific verb and resource ('Computes the exact n-th Fibonacci number F(n)') with the base cases F(0)=0, F(1)=1 and the algorithm (fast doubling), plus the hard bound n ≤ 20,000. It also names the sibling it is not (bigint-calc), so an agent can distinguish it without opening any 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 both sides explicitly: 'When: a numeric result has to be exact or reproducible instead of estimated' and 'Not for: computing exact arbitrary-precision integer arithmetic (bigint-calc)'. The alternative is named and the selecting condition is stated, 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.