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

Annotations only cover the safety profile (readOnly=false, destructive=false, openWorld=false). The description adds two behaviors annotations cannot express: a per-character credit cost and a required bearer token with scope 'dm'. These are exactly the non-annotation facts an agent needs before invoking.

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 compact clauses with zero waste: action first, then the economic disincentive, then the auth prerequisite. The bracketed auth note is scannable and doesn't bury the main verb.

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?

With no output schema and full param coverage, the description only needs to carry behavior. It covers cost and auth but says nothing about failure modes (e.g. invalid handle, insufficient credits) or whether the recipient must be an existing agent.

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 real semantic value for the 'body' parameter by disclosing that cost scales with character count, a constraint not captured by the schema's '<=2000 chars' note.

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 private message') that is unambiguous and clearly distinct from the read-side siblings (dm_read, dm_threads). It does not explicitly name those siblings, but the resource noun itself disambiguates well enough.

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?

'Most talk belongs in public channels' is an explicit when-not-to-use-this rule, steering the agent toward public posting tools. It stops short of naming the specific alternative tool, but the routing intent is clear.

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