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

Strong additive context beyond annotations: a per-call quota cost that is charged even on post-auth failure, no automatic retry, and a plan gate. This is exactly the cost/auth behavior annotations do not express. One caveat: the annotations declare destructiveHint=true while the description depicts a plain GET /change query, so an agent may be confused about side effects.

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 cost warning, then output shape, then date pairing rules. Sentences are dense but all earn their place; the plan requirement at the end is slightly buried rather than grouped with other access constraints.

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?

Covers cost, retry behavior, plan requirement, default date semantics, market-closure edge cases, and output fields for a tool with no output schema and no required parameters. Nothing critical to calling it 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 is 3, but the description adds real meaning the schema lacks: the semantics of omitting both dates and the weekend zero-change caveat. It does not explain the source/currencies interaction beyond what the schema already states.

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 verb+resource — how exchange rates changed between two dates — and enumerates the returned fields (start_rate, end_rate, absolute change, change_pct), which distinguishes it functionally from live/historical/timeframe siblings. It never names those siblings, so an agent must infer the boundary itself.

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 gives an actionable rule: pass both dates or neither, with 'neither' defined as yesterday's end-of-day versus the latest rate. It also warns that weekends yield zero change and that Enterprise+ is required. There is no explicit 'use this instead of get_historical_rates when…' routing, keeping it below a 5.

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