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

Annotations cover read-only, idempotent, open-world, and non-destructive hints. The description adds valuable context about authentication and rate-limiting ('free; uses NASA_API_KEY or the shared DEMO_KEY which is heavily rate-limited'), which is not in the annotations. It also mentions return fields, though output schema exists. Missing details like error handling or caching 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?

Two front-loaded sentences that are dense with useful information and contain no wasted words. Authentication, rate limit, return fields, and use case are all stated efficiently.

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?

With only 2 optional parameters, full schema coverage, and an output schema, the description is nearly complete. It covers purpose, auth, rate limit, and return fields. Minor gap: no mention of pagination or date range limits, but these may not apply given the simple parameter set.

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% and descriptions for date and count are clear. The tool description adds no parameter-specific semantics beyond what the schema already provides, so baseline 3 is appropriate.

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?

States a specific verb+resource ('Fetch NASA Astronomy Picture of the Day entries') and clarifies the returned content ('date, title, explanation and media URL'). It clearly distinguishes itself from siblings like arxiv_search or wikimedia_pageviews by naming the exact dataset and use case.

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?

Provides clear context ('Use for NASA APOD imagery and captions') and notes authentication ('free; uses NASA_API_KEY or the shared DEMO_KEY which is heavily rate-limited'). However, it does not explicitly state when to avoid this tool or list alternatives for astronomical imagery, though none are obvious in the sibling list.

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