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

The description discloses side effects beyond the readOnlyHint=false annotation: a hit 'stamps the plan and status onto the matching contact' and, when create is true, 'a hit also becomes a new contact when nobody holds the address yet.' This gives important behavioral context, although it does not detail return format or error conditions.

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?

The description is compact and front-loaded with the core purpose, then side effects, then a concrete usage example. There is minimal fluff, though the first sentence is slightly awkward grammatically, preventing a perfect score.

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?

There is no output schema, so the description should clarify what the caller receives. It implies a yes/no answer through 'Whether one email address subscribes,' but never explicitly states the return value or what happens on a miss. Given the tool's simplicityhare, this is a moderate gap.

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 both parameters. The description adds that `create` causes a new contact when nobody holds the address, but this closely mirrors the schema description 'Create the contact when the address is new.' No significant new semantic value 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 clearly states the tool checks whether a single email address subscribes to the user's Substack, with the explicit usage case 'is x@y.com a subscriber?'. It differentiates from bulk tools by emphasizing 'one email address' and 'read from Substack right now', though it does not explicitly name sibling alternatives.

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 a clear, concrete use case in quotes: 'Use it for "is x@y.com a subscriber?".' This tells an agent when to invoke the tool. It does not provide exclusions or explicitly mention alternatives like get_substack_subscribers, but the context is unambiguous.

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