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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and a closed world, so the safety profile is covered structurally. The description adds one genuinely useful behavioral fact not in the annotations: a cost of 50 credits (Tier 3). It says nothing about what the response contains, though the output schema exists to cover that.

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 short lines, front-loaded with the core action and method, then the group and cost metadata. Every sentence earns its place; no padding or repetition of the tool name.

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 an output schema present, the description need not explain return values, and it correctly states inputs ('two natal charts'). What is missing is disambiguation from the crowded family of composite/Davison/synastry tools, which is the main risk for an agent selecting this 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%, and the nested chart1/chart2 schemas fully document date, time, coordinates, timezone handling and house system, so the description is not required to carry parameter meaning. It adds no parameter detail beyond what the schema already provides, which is the baseline 3 case.

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 names a specific verb and resource ('Calculate a midpoint composite chart') and even states the method ('averaging the planetary positions and house cusps of two natal charts'), so an agent knows exactly what is produced. It does not, however, distinguish itself from sibling relational techniques such as astroway_relational_davison, astroway_relational_coalescent, or astroway_render_composite.

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?

There is no guidance on when to prefer this composite over Davison, coalescent, or synastry tools, and no prerequisites or alternatives are named. The '[Group: Comparisons]' tag is a thin hint at context but does not route the agent between the many sibling comparison and composite endpoints.

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