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

The description usefully discloses a side effect beyond the annotations: it persists both charts into the connected Celestio account, which explains why readOnlyHint=false on an operation that reads as a query. It also flags that the call is free, giving cost/quota context the annotations cannot express. It does not address what happens on repeat calls or how much of the analysis the preview truncates.

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?

A single sentence that front-loads the free-preview framing, then the write side effect, then the return value. No filler, no repetition of the title.

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?

Return values are covered by the output schema and the deeply nested birth-detail params are fully documented, so the remaining burden is orchestration guidance. The gap is that an agent cannot tell from this definition whether to call preview_compatibility or compare_birth_charts first, which matters given both take the same two-person input.

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 per-field docs covering date format, optional time semantics, place syntax, and coordinate overrides, so the baseline is 3. The description adds nothing about the person-vs-chartId choice or which form is preferred when a chart already exists.

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 ("compatibility preview for two people") plus the scope of what comes back ("strongest connections between them"). The word "preview" plus "free" signals this is a lighter-weight version of a full comparison, but the sibling compare_birth_charts is never named, so the distinction is inferential rather than explicit.

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?

Usage is only implied: "Free compatibility preview" suggests a teaser/trial call ahead of a fuller comparison, and "saves both charts" hints that this is a reasonable first step. There is no explicit when-to-use, when-not-to-use, or named alternative among the six siblings.

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