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

Annotations already declare readOnlyHint, idempotentHint and non-destructive, so safety is covered. The description adds genuinely useful operational context beyond that: the unit (percent per annum), the upstream series code SF43783, and the data source (Banco de México SIE). It does not mention publication lag or freshness for the "latest" value, which is the one remaining behavioral gap.

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?

Three sentences, front-loaded with the core action and the series code. The mortgage/business-loan/consumer-lending aside is slightly expendable, but it aids relevance matching for finance queries and does not obscure the essential facts.

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-shape detail is unnecessary; the description covers what the tool is, what it returns, and in what unit. Adequate for a two-optional-parameter, read-only series fetch, with only marginal gaps around freshness and sibling routing.

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% and both parameters are fully documented with format examples, so the baseline is 3. The description's date-range sentence restates rather than extends what the schema already says about start_date and end_date.

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 gives a specific verb and resource (get the TIIE 28-day interbank offered rate), names the exact series identifier SF43783, and explains what the instrument is and who uses it. It implicitly separates itself from sibling banxico.rates.target by defining TIIE as an interbank offered rate rather than a policy rate, but it never names that sibling explicitly.

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?

"Returns the latest rate or a historical date range" implies the two usage modes, but the condition for choosing between them (omit start_date for latest) is really stated only in the schema. There is no guidance on when to prefer this tool over banxico.rates.target or banxico.fx.rates.

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