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

Beyond the readOnly/idempotent/non-destructive annotations, the description discloses the cost model (0.1 credit per call, billed per started block of 10, only successful calls charged, free sandbox key), determinism, and an error behavior (422 houses_undefined_at_latitude for Placidus beyond the polar circles). This is exactly the operational context annotations cannot carry.

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 and tight overall, but the first sentence ("Use this when you need the ascendant, midheaven or house cusps...") and the second ("Ascendant, midheaven and twelve cusps...") restate the same fact, so one sentence is partly redundant.

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?

An output schema exists, so return values need no explanation, and the description covers the remaining agent-relevant facts: cost, determinism, the Placidus failure mode, and a worked example. Nothing needed to call it correctly is missing.

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%, so the baseline is 3, but the description adds real semantic value: the Placidus polar-circle limitation constrains how houseSystem can be used, and the example arguments object demonstrates a valid datetime-with-offset combination. That goes beyond what the schema states.

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 output set (ascendant, midheaven, twelve house cusps) for a given moment and place, which cleanly separates it from sibling ephemeris tools that return positions, ayanamsa or sunrise. It does not explicitly name a sibling to contrast with, but the resource is unambiguous.

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?

"Use this when you need the ascendant, midheaven or house cusps for a moment and place" is an explicit trigger condition, so the agent knows when this applies. There is no when-not guidance and no named alternative among the ephemeris siblings, so it stops 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.