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

Goes well beyond the annotations: discloses the createReadings scope requirement, that a chart may be saved, the readReadings-scoped return of only owned purchase state, that no payment or model generation starts, and that an explicitly empty preview is possible. It also flags injected names as untrusted content, which is exactly the kind of security 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-loads purpose and the one-of constraint, then layers permissions, defaults, and return behavior. Dense but every clause carries information; a couple of sentences (return shape, readReadings scoping) pack multiple ideas and could be split for readability.

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?

Despite an output schema existing, the description still sketches the return shape (literal civil dates, contact labels, explicit empty preview), and it covers permissions, side effects, and injection safety for a nested-object tool. Nothing an agent needs 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 parameters are already documented, including the oneOf chartId/person constraint and the start default. The description restates the default-start timezone behavior and the exactly-one rule but adds no format or syntax detail beyond the schema, so the baseline 3 applies.

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 ('Calculates a free personal 30-day Transit preview') plus the key constraint: exactly one chartId OR person. An agent can distinguish it from compute_birth_chart or preview_compatibility without opening any schema.

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?

Gives clear context for invocation (chartId when a chart exists, birth details otherwise) and routes saved reading handles to get_reading. It never explicitly says when to prefer a paid/full reading or a sibling over this preview, so no exclusion is stated.

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