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

With no annotations, the description carries the full burden of disclosing behavior. It clearly indicates this is a mutating operation (setting a rate), explains the series semantics and that trips earn the rate in force on the day driven, and notes that no rates ship with the server (data must be user-provided). It also discloses free-tier limits. This goes beyond the schema and gives the agent a solid mental model, though it does not detail error conditions or overwrite 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?

The description is compact but not skeletal. It front-loads the core action in the first sentence, then adds necessary context (series, earning logic, data verification, free tier). Every sentence contributes value and the structure flows logically. It is slightly longer than minimal but earns its length with material context.

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 mutation tool with no output schema and no annotations, the description covers the essential behavioral contract: what is set, how the series works, how rates apply to trips, the effective date flexibility, and free-tier constraints. It does not mention response format or error handling, but those are often inferred from the tool's purpose. The description is strong enough for an agent to call it correctly in most contexts.

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 is 3. The description adds high-level context (e.g., 'each trip earns the rate in force on the day it was driven' clarifies effective_from, and 'the figures are yours to verify' hints at data ownership), but it does not add per-parameter syntax beyond what the schema already states. It supplements the schema meaningfully without being essential, so 3 is correct.

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 opens with a precise action ('Set what one mile or km is worth') and specifies the exact scope: one category, one jurisdiction, from a date forward. It clearly distinguishes the write operation from siblings like rate_list by framing it as an act of setting, and adds context about the rate series and earning logic. This is specific and unambiguous.

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 description implies when the tool is used (to define or update a rate) and provides useful context about the rate series and effective dates, but it does not explicitly direct the agent away from alternatives or name sibling tools. For example, it never says 'to view rates, use rate_list'. The guidance is implicit rather than explicit, so a 3 is appropriate.

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