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

Annotations already declare readOnly, idempotent, and closed-world, so the safety profile is covered. The description adds the default behavior ('full by default') and the contents of each part, but says nothing about computation cost, failure modes, or how unknown birth times affect output (that detail lives only in the schema).

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 purpose, then a single long but information-dense sentence covering the parts and the default. Every clause carries meaning, though the enumeration is heavy enough that it reads as a list rather than terse prose.

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 explaining return contents and does so thoroughly for each part value. It leaves error behavior and the birth sub-field semantics to the schema, which is reasonable given those fields are individually documented there.

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 0% and the 'part' enum has no per-value description in the schema, yet the description enumerates every enum value and spells out exactly what each returns (bodies with sign/degree/speed/retrograde/house, angles, twelve Placidus cusps, aspects with orbs, balances, fixed stars). This is precisely the compensation a low-coverage schema needs.

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?

Opens with a concrete verb+resource: computes a natal chart from birth data, and the 'part' enumeration makes the scope of what is returned unambiguous. It does not differentiate itself from siblings such as solar_return, synastry, or transits, so a 5 is not warranted.

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 helps the agent choose an output shape ('full by default', or one specific part), which is a form of usage guidance. It offers no guidance on when to choose natal_chart over the sibling chart tools (synastry, solar_return, transits), and no exclusions or prerequisites are stated.

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