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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description adds genuine value beyond that by naming what the response contains (MRR, contract terms, recurring total), which matters because there is no output schema. It stops short of noting auth needs or error behavior for a missing id.

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 compact sentences, front-loaded with the resource and its returned fields, with the endpoint appended as a useful identifier. 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 simple read-by-id tool with full schema coverage, complete annotations, and a description that enumerates the returned fields in place of an output schema, an agent has everything needed to select and invoke it correctly.

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% and the single parameter carries its own description ('Invoiced subscription id.'). The description's 'by id' adds no syntax or format detail beyond the schema, so the baseline of 3 is appropriate.

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 gives a specific resource ('One subscription by id') and enumerates the returned surface (plan, add-ons, billing period, contract terms, recurring total, MRR), plus the underlying endpoint GET /subscriptions/{id}. It is clearly a single-record fetch and distinguishable from invoiced_list_subscriptions, though it never names that sibling explicitly.

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?

There is no when-to-use or when-not-to-use guidance. An agent must infer from the phrase 'by id' that this is the single-record counterpart to invoiced_list_subscriptions, and no prerequisite (e.g. needing an id from a list call) is stated.

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