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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context: returns a date-keyed map, covers every business day, coverage starts from 1999-01-04, and omitting end_date gets rates through the latest available date. It doesn't mention pagination or rate limits, but for a read-only public API this is sufficient.

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 and front-loaded: it states the core function first, then return format, coverage start, optional parameter behavior, and use cases. Every sentence adds value, and there is no redundancy with the schema or annotations.

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?

The description is complete for a read-only time-series tool: it covers return format, date range behavior, optional filtering, use cases, and auth requirements. The output schema exists, so return values don't need elaboration. Minor gap: no mention of pagination or response size limits, but for this tool it's not critical.

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 the schema already documents all 5 parameters. The description adds some context (e.g., 'reduce response size' for symbols, 'defaults to the latest available date' for end_date) but mostly reinforces what the schema already states. Baseline 3 is appropriate since the schema carries the heavy lifting.

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 clearly states the tool fetches a time series of ECB exchange rates between two dates from Frankfurter.dev, with a specific verb ('Fetch'), resource ('time series of ECB exchange rates'), and scope ('between two dates'). It distinguishes itself from siblings like frankfurter.currency.latest and frankfurter.currency.historical by emphasizing the date range and business-day coverage.

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 explicitly states when to use this tool ('Use for trend analysis, backtesting, historical P&L calculations, and charting FX movements') and provides clear guidance on optional parameters (omit end_date for latest, use symbols to narrow results). It also notes no auth required and public domain data, which helps an agent decide to use it without hesitation.

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