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

Annotations already declare the write/destructive/idempotent profile, so the description is free to add the non-obvious parts: partial-update semantics that clarify what is left untouched, and the hard constraint that segment dates must fall within the parent trip's dates. It does not cover permissions or failure behavior, but it adds real behavioral value beyond the 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 short sentences, front-loaded with the update action, then the partial-update rule, then the constraint. Every sentence carries distinct information with no filler.

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?

For a mutation tool with full schema coverage and no output schema, the description supplies the two things an agent most needs: how to populate the free-form fields object and the trip-date constraint. Nothing essential for a correct call 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 meaningful semantics for the untyped nested `fields` object: only supply fields you intend to change, and fetch valid field names from segment_get_schema. That is genuine guidance the schema does not encode.

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 opens with a specific verb+resource ("Update an existing segment") and the word "existing" implicitly separates it from the create/delete siblings. It is clear and actionable, but there is no explicit naming of sibling tools for writes, so it stops just short of full sibling differentiation.

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 gives partial-update guidance ("Only provide the fields you want to change"), routes the agent to segment_get_schema for field names, and states a precondition with an alternative ("use trip_update first to extend the trip"). No explicit when-not-to-use is given, but the routing to two siblings by condition is strong.

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