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

Annotations already indicate that this is a non-read-only, idempotent, non-destructive operation. The description adds the requirement for an Authorization: Bearer header, which is a behavioral detail not captured in annotations. However, it does not describe side effects, return behavior, or any other operational nuances, so the added transparency is minimal but present.

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 very short, with only two sentences, but the first sentence is largely redundant with the tool name and title. The second sentence about the auth header is useful. There is minimal verbosity, but the description is under-specified, making it less effective than a concise, information-rich version could be.

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 in a complex domain with many sibling operations, the description is insufficient. It lacks critical context about what a direct message is, whether the session must be active, how it relates to session lifecycle, and potential error conditions. The output schema and annotations provide some structure, but the description is too thin to confidently invoke the tool 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?

The schema has 100% description coverage for all three parameters, though the descriptions are formulaic ('Canonical ... argument'). The tool description itself does not add any parameter-level meaning beyond what the schema provides. Baseline for high schema coverage is 3, and the description fails to elevate it further.

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 states 'Execute the canonical Direct Session send_direct_message operation,' which essentially restates the tool name and title. It does not explain what sending a direct message does or how it differs from sibling operations like open_direct_session or close_direct_session. This is a tautological description that fails to convey the tool's actual function.

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 description provides no guidance on when to use this tool versus alternatives. It does not mention any prerequisites, such as having an active direct session, or when not to use it. The only note about requiring an Authorization: Bearer header is a general authentication requirement, not specific usage context.

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