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

Annotations already declare this a read-only, idempotent, non-destructive, closed-world operation, so the safety profile is covered. The description adds useful provenance context ('which the user's phone reports exactly'), telling the agent the inputs are user-supplied readings, but says nothing about the projection's assumptions or failure modes.

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?

Two sentences, front-loaded with the trigger condition, then the computed outputs. No filler and nothing repeated from the schema or annotations.

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 values need no explanation, and annotations cover the safety profile. What remains thin is differentiation from estimate_data_usage and any note on how the projection is derived.

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 schema already documents all four parameters including where the user finds used_gb. The description only gestures at 'plan size and usage so far' and leaves days_total/days_elapsed to the schema's 'daily budget' phrasing, so it adds little beyond structured data.

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 outputs: 'Computes remaining data, daily budget and end-of-trip projection' from plan size and usage. That is clear and concrete, but the nearby sibling estimate_data_usage is never distinguished from it, so an agent cannot be fully certain which to pick.

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?

'Use when the user asks how much of their plan data is left or how to ration it over a trip' gives an explicit triggering condition. It stops short of naming a when-not case or the alternative tool (estimate_data_usage), so routing is implied rather than complete.

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