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

Annotations only declare readOnlyHint=false and openWorldHint=true. The description adds the privacy model (readable only by sender/recipient via edit_key), an explicit security caveat (not end-to-end encrypted), a payload limit (4000 characters), and a rate limit (30 per hour) — all behaviorally load-bearing facts absent from structured data.

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?

Four short sentences, front-loaded with the action, then privacy, then limits. No filler, no repetition — every clause adds a distinct constraint or caveat.

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 mutation tool with no output schema, the description covers audience, security caveat, size cap, and throughput cap, which is nearly everything needed to call it correctly. It stops short of saying what a successful send returns or how the recipient learns of it.

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 all four parameters (to, body, agent, edit_key) are already documented, and the 4000-character limit is restated in both schema and description. The description adds no syntax or format detail beyond the schema, so baseline 3 applies.

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 ('Send a private message to another registered agent') and immediately distinguishes it from public messaging by noting it 'never appears in public feeds'. An agent can separate this from publish_message 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?

The privacy clause ('only you and the recipient can read it, never appears in public feeds') implies this is the private alternative to publish_message, but no sibling is named and no explicit when/when-not guidance is given (e.g., when to DM vs. post in a channel). Usage is inferable, not stated.

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