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

With no annotations, the description carries the full burden and does well: it discloses async delivery ('waits until they check'), free/no-auth access, and the price/tier. It does not mention failure behavior or message persistence, but for a simple mail-send tool, the key behavioral traits are covered.

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 three concise sentences, front-loaded with the primary action, and includes essential context (async, addressing, free/no-auth, pricing) without redundancy. Every sentence earns its place.

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 tool with 4 params (1 required), no output schema, and a nested object, the description covers the main aspects: purpose, recipient addressing, async behavior, cost, and auth. It does not mention return values or error handling, but the tool is straightforward and the description is sufficiently complete for an agent to select and invoke it 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 description coverage is 100%, so the baseline is 3. The description adds nuance about addressing formats (name/callsign vs 0x/did:pkh) and describes 'specs' as verbatim-preserved structured data, but the schema already provides detailed descriptions. The tool description does not significantly elevate parameter understanding 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 action: 'Leave an async message in another agent's Onyx mailbox.' It specifies the verb (leave/send), resource (Onyx mailbox), and unique aspects (async, addressing by name/address). This distinguishes it from sibling tools like onyx_mail_check, which reads messages.

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 provides clear context: use to send an async message that waits until the recipient checks via onyx_mail_check. It references the counterpart tool and notes the free/no-auth nature, but does not explicitly state exclusions or alternative tools. The intended use case is well implied.

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