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

The readOnlyHint annotation already communicates that this is a safe read operation. The description adds useful context about what a subscription means for list-addressed sends, but does not describe output shape or pagination beyond what the schema's cursor parameter already covers. This is adequate but not rich.

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 short sentences carry the purpose and the primary use case with no filler. The main action is front-loaded, and every clause earns its place.

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-only tool with a single required parameter, a fully documented cursor, and a clear usage scenario, nothing essential is missing. The absence of an output schema is acceptable here because the tool's purpose and semantics are already clear from the description and annotations.

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%, so the schema already fully documents both parameters. The description adds only a slight clarification that user_id identifies the 'one user' whose subscriptions are listed, but does not otherwise extend parameter meaning beyond the schema.

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 starts with a specific verb and resource: 'List the lists one user is subscribed to.' It clearly distinguishes this from the sibling tool get_list_subscribers by making the direction explicit (subscriptions belonging to a user, not subscribers of a list).

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 gives an explicit use case: 'Use this when explaining why a user received something — a subscription here is the reason a list-addressed send reached them.' This gives the agent a clear decision rule, though it does not name alternatives or explicitly state when not to use it.

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