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

Goes well beyond annotations: discloses the $7 cost with postage included, that nothing is charged or mailed until the recipient acts, an explicit 'never ask for payment details' rule, a ~1-week link expiry, and the four-question challenge flow including the proof-of-work needing a code tool. Annotations cover only the generic write/open-world profile.

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?

Front-loaded with what the postcard is, then photo/address inputs, then return value and the challenge caveat. Dense but every sentence carries operational weight; slightly long, with the challenge detail extending the tail.

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?

Explains the link the recipient receives and that payment/mailing happens on the website, which supplements the output schema rather than repeating it. Together with complete param docs and the disclosure of the challenge flow, an agent has what it needs to call this 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 coverage is already 100%, but the description adds real meaning: base64 is preferred and must be read from the file (never invented), accepted formats and ~10MB limit, and the semantics of the challenge round-trip (re-send other args unchanged). Adds genuine value over 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?

States a specific verb (start) and resource (a Paper Plane Post postcard) with concrete scope: a photo printed on a 4×6 card, optional illustrated version, mailed first class for $7. An agent immediately knows what the tool produces without a sibling to disambiguate.

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?

Explicitly routes between photo_base64 (preferred, when you have the file) and photo_url (only when the photo is already online), tells the agent to add note/address only if known, and describes when the challenge argument applies. When-to-use for each input is spelled out.

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