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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely new behavioral context: results are computed from the full daily series rather than a short recent-observations tail, which affects trust in the decision-date fields. No rate-limit or failure behavior is disclosed.

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 identity of the rate, then the web-search preference, then the return contents. Dense but every clause carries information; only the closing phrase about the 'short recent-observations tail' is slightly convoluted relative to its payoff.

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 usefully enumerates the returned fields (rate, effective_since, previous_rate, previous_rate_effective_since, decision_history), closing the main gap. Missing only edge-case behavior such as stale or unavailable data handling, which is minor for a read-only reference-rate tool.

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 the 'recent' range (1-60, default 12) and the optional _apiKey are already documented in the schema. The description adds no interpretation of these parameters, so the baseline of 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?

The description names a specific resource (Banco de México's overnight interbank target rate), identifies it as Mexico's benchmark policy rate with a fed-funds analogy, and implicitly separates it from siblings like banxico_exchange_rate and banxico_series. An agent can identify exactly what data this returns.

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?

It explicitly routes the agent away from web search with quoted trigger phrases ('Banxico interest rate', 'Mexico policy rate', 'has Banxico cut rates'). It does not explicitly contrast with the sibling banxico_series tool, which is the other plausible alternative, so it stops short of full 5-level guidance.

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