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

Annotations already establish the safety profile (readOnlyHint, idempotentHint, destructiveHint=false), so the bar is lower. The description nonetheless adds real behavioral context the annotations cannot carry: with timeUnknown: true the cross-aspects are still returned while that partner's houses, angles and sect come back null. Cost (50 credits, Tier 3) is also surfaced, though that is arguably structured metadata rather than prose.

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: the core action is in the first clause and the exception case follows immediately. The bracketed metadata block (group, endpoint, required, cost) is scannable and non-redundant. Slightly dense but every sentence earns its place.

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-value explanation is not the description's burden, and annotations cover the safety profile. Two required charts are identified and the timeUnknown degradation is disclosed. The one gap is routing against the many relational siblings, which is a usage-guideline issue rather than missing call mechanics.

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 nested chart1/chart2 objects and the fields/precision parameters are already fully documented in structured form; baseline 3 applies. The description reinforces timeUnknown semantics but adds little about chart1/chart2 payload content that the schema does not already state.

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?

States a specific verb and resource: 'Calculate cross-chart aspects between two natal charts.' That is unambiguous and a step beyond the bare title 'Synastry'. However, it never distinguishes itself from near-name siblings such as astroway_relational_synastry_aspect_grid, astroway_relational_synastry_house_overlay or astroway_relational_composite, so an agent must open the schemas to route correctly.

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 phrase 'for relationship analysis' implies the context of use, but there is no explicit when-to-use vs when-to-use-an-alternative guidance, no note that layered-render tools or the composite/davison siblings exist, and no prerequisites beyond the required-params metadata line. Usage is inferred rather than stated.

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