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

Annotations already declare readOnly=false, idempotent=false and destructive=true, so the agent knows this is a mutating, non-idempotent write. The description adds a real behavioral rule not present in structured fields: the recipient must satisfy a role-dependent eligibility constraint. It still does not say what happens on rejection or whether a message can be retracted.

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 sentences, zero filler, with the core action front-loaded and the eligibility rules following in the order an agent would need them.

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 3-parameter mutation tool with full schema coverage, annotations and no output schema, the description supplies the key eligibility rules an agent needs to call it correctly. Only failure behavior and any content constraints are unaddressed.

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 baseline is 3, but the description genuinely adds meaning to the 'to' parameter by defining who is an allowed recipient depending on the caller's role. That constraint is absent from the schema, which only says 'the recipient's user id'.

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: send a message on a job to a specific other participant. This clearly separates it from the read-side sibling get_messages. It does not explicitly name alternatives, but the purpose is unambiguous.

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 concrete eligibility context: a buyer may only message someone who has bid, and a bidder may only message the buyer. This tells the agent when a call is valid, though it does not name alternatives or state exclusions for other scenarios.

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