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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so safety is covered. The description adds genuinely useful behavioral context beyond that: the exact astrological conventions applied (tropical zodiac, Placidus houses) and that exact input precision is mandatory, which shapes how the agent must gather data. It doesn't discuss failure modes for invalid timezones or ambiguous times, which the dst_fold parameter implies exist.

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 sentences, front-loaded with the calculation and its conventions, followed by prerequisites. Every clause carries information an agent needs, and nothing is repeated or padded.

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 usefully enumerates what comes back (signs, planets in houses, tightest aspects), which is what an agent needs to report results. The gap is the absence of guidance on precision tolerance, ambiguous DST times, or what happens with unknown birth times — relevant for a 7-parameter, 5-required precision tool.

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 every parameter is already documented, including the IANA format hint and the instruction not to guess birth time. The description restates those same requirements ("exact birth date, local birth time, IANA time zone and birthplace coordinates") without adding format, range, or edge-case meaning. Baseline 3 is appropriate since the schema carries the load.

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?

Uses a specific verb and resource ("Calculate a Western natal birth chart") and names the underlying system (tropical zodiac, Placidus houses) plus the concrete outputs: Sun/Moon/Rising, planets in signs and houses, tightest major aspects. This lets an agent distinguish it from siblings like bazi_chart or tuvi_chart, which are different astrological systems, without opening any schema.

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 clearly establishes the invocation context: it requires exact birth date, local birth time, IANA timezone and coordinates, which tells an agent this is a precision calculation, not a rough or date-only reading. It stops short of naming alternatives (e.g. get_transits for current sky positions) or stating when not to use it, so it falls short of a 5.

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