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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint. The description adds meaningful behavior beyond that: it reports every inter-chart aspect at exact orbs, includes house overlays, and embeds both full natal charts in the response. No contradiction with annotations.

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?

Three sentences with the primary trigger and prohibition front-loaded, followed by a compact but complete specification of the output. No filler or repetition of schema content.

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 value, and it does: every inter-chart aspect with orbs, house overlays both directions, and embedded natal charts. The input schema covers all parameters, so an agent has everything needed to select and call the tool correctly.

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%, with detailed descriptions for birth_datetime, coordinates, house_system, and include_minor_aspects. The prose adds no parameter-specific semantics, so the baseline of 3 is appropriate.

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 the exact resource ('compatibility / relationship / synastry question about two people') and enumerates the computed output: every inter-chart aspect with exact orbs plus house overlays. This differentiates it from single-chart tools like calculate_natal_chart or time-based get_transits_and_aspects.

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?

It explicitly says to use the tool for ANY two-person compatibility/synastry question and forbids estimating aspects or web-searching them. It does not name sibling tools or state when not to use this tool (e.g., single chart → calculate_natal_chart), so it stops short of full alternative guidance.

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