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

Annotations already cover read-only, idempotent, closed-world, non-destructive, but the description adds materially: the 17′ window, that the previous cazimi is also returned, the 12-hour 'today's' convention, minute-level UTC precision, and a source/coverage constraint (JPL DE421, 2025–2030) that tells the agent the result is bounded in time. That ephemeris range is exactly the kind of caveat structured fields cannot express.

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 core answer (what is returned) and then the qualifiers, with no filler sentences. It is dense — the list of return components plus the ephemeris parenthesis packs a lot into a few clauses — but essentially every clause carries information an agent needs.

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 no output schema, the description carries the burden of describing return values and does so: UTC minute, degree, 17′ window, and the previous cazimi. Combined with the source and date-range limits, an agent has enough to call it and interpret results without further documentation.

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 the single parameter is fully documented, so the baseline is 3. The description adds beyond that by explaining how the boundary is interpreted — a cazimi within 12 hours counts as today's and is returned as the previous one — which materially changes how an agent should pass and read 'from'.

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 and resource ('Returns the next Mercury cazimi') and immediately defines the term ('exact Sun–Mercury conjunction during a retrograde'), which distinguishes it from the sibling retrograde tools without the agent needing to open a schema. It also disambiguates scope with 'This is astronomical timing, not a horoscope.'

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 'after a date' framing and the 12-hour rule make the selection context reasonably clear, and the definition of cazimi inherently separates it from next_mercury_retrograde and the other siblings. However, it never explicitly says 'use next_mercury_retrograde instead if you want the retrograde station' or states any exclusion, so routing still requires inference.

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