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

Annotations only supply destructiveHint=true; the description goes well beyond that by disclosing the upsert behavior, the destructive clearing of omitted custom fields, the re-subscription side effect on unsubscribed addresses, and a concrete rate limit. These are exactly the side effects an agent needs before invoking a write tool.

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?

Tight, front-loaded sentences: the WRITE marker and the core action lead, side effects follow, and the endpoint/rate limit close it out. No sentence is 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 7-parameter mutation with no output schema, the description covers the critical side effects, the upsert semantics, and throttling. It leaves some ambiguity about interaction with the sibling update/unsubscribe tools, but nothing essential to a correct call is missing.

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 documents every parameter and the custom_fields clearing warning. The description adds no field-level syntax or format guidance beyond what the schema provides, so the 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?

States a specific verb and resource ('WRITE: add one subscriber to a mailing list') and immediately resolves the ambiguity with the sibling moosend_update_subscriber by explaining that an existing email gets UPDATED instead. An agent can distinguish this upsert tool from the dedicated update tool without opening either schema.

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?

Gives clear operational context: upsert-on-existing-email, auto re-subscribe of previously unsubscribed addresses, and the 10 calls / 10 s rate limit. It does not explicitly say when to prefer this over moosend_update_subscriber or moosend_subscribe-style flows, so it falls short of full alternative routing.

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