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

Annotations already declare readOnlyHint=true, so the agent knows it is a safe read. The description adds the REST endpoint and expand support, providing some context beyond the annotation, but does not disclose error behavior or return format. Given the existing readOnlyHint, this is adequate.

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 core action and scope. Every word earns its place, with no fluff or repetition.

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 get-by-id tool with full schema coverage and readOnlyHint annotation, the description is complete enough. It identifies the endpoint and the expand option. No output schema exists, but the description does not need to detail return values for such a straightforward retrieval.

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 both 'id' and 'expand' fully described. The description mentions 'Supports expand' but adds no new meaning beyond the schema. Baseline 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 clearly states 'Fetch a single subscription by id', using a specific verb and resource. This distinguishes it from sibling list tools like swell_list_subscriptions, and the REST endpoint disambiguates the exact operation.

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?

The description implies usage when a single subscription id is known ('Fetch a single subscription by id') and supports expand. It does not explicitly mention alternatives, but the contrast with list_subscriptions is implicit and clear from the singular focus.

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