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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds genuinely non-obvious behavior beyond annotations: the access rule where USD works without an API key while all other currencies return subscription_required with a subscribe link. It also clarifies the value is precomputed/documented, setting expectations about freshness. No contradiction with annotations.

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, zero waste: the first states the core function, the second gives the usage caveat, the third gives the access constraint. The most important information is front-loaded, and no sentence is redundant 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?

For an 8-parameter read-only query with a full output schema, openWorldHint, and complete param descriptions, the description covers the essential operational context: purpose, usage boundary, and access behavior. The only notable gap is that valid factor slugs are not discoverable (no enum exists), and the description doesn't hint at where to find them; the schema's example partially compensates.

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 baseline of 3 applies. The description adds marginal value by mapping 'stored component scores' and 'source references' to include_components and include_sources, and 'one currency' to the singular-currency expectation. Overall it mostly echoes what the schema already documents rather than deepening param understanding.

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 uses a specific verb+resource: 'Return a precomputed macro factor for one currency.' It names the optional extras (component scores, source references) and differentiates itself from news/release-oriented siblings with 'not as a substitute for inspecting its underlying macro releases,' which distances it from macro_news, release_calendar, and announcement_changes.

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 gives an explicit when ('Use this for a documented factor value') and a clear when-not ('not as a substitute for inspecting its underlying macro releases'). It does not name the alternative sibling tool explicitly, so the routing requires a small inference, but the exclusion is unambiguous.

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