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

Annotations only supply destructiveHint=true; the description goes well beyond by disclosing that this sends physical mail, charges the Lob account, that a test_ key avoids both, and the exact cancellation cutoff (send_date). These are the financial and irreversibility facts an agent most needs.

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?

Three dense sentences with the money warning front-loaded in bold/emoji, followed by the two least-obvious parameter behaviors and the endpoint. No 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 9-parameter nested-object mutation with no output schema, the description covers the highest-risk facts and the trickiest inputs. Secondary params (size, mail_type, merge_variables, description) are left entirely to the schema, which is acceptable given full coverage but leaves no extra operational guidance.

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 including the anyOf address forms. The description restates key semantics (`to`/`from` accept adr_ ids or inline objects; `front`/`back` accept HTML, URL, or tmpl_) in condensed form but adds no syntax details the schema lacks.

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+resource ('SENDS A REAL POSTCARD', POST /v1/postcards) and immediately flags the real-money consequence. It is clearly distinguishable from sibling creation tools like lob_create_letter and lob_create_address.

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 actionable conditional guidance: use a test_ key to avoid real mail/charges, and the send is cancelable only before send_date. It does not, however, route the agent to alternatives (e.g. lob_create_letter for letters, lob_cancel_postcard for cancellation), so context is clear but not fully comparative.

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