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

Annotations already declare destructiveHint=true, readOnlyHint=false, and openWorldHint=false, so the safety profile is covered. The description adds the useful constraint that only pending messages can be cancelled, but says nothing about idempotency, failure behavior when the message is already sent, or any auth requirements.

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?

A single front-loaded sentence with no filler; the verb and its precondition come first and the identifier requirement closes it out.

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 an output schema present, return values need no explanation, and annotations carry the destructive/safety profile. The description covers the essential scoping, though it omits edge-case behavior such as what happens on an already-delivered message.

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 both the id and context parameters are fully documented in the schema itself. The description only echoes 'by its public id' and adds no syntax or format detail beyond the schema's example ("msg_…").

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 a specific verb (cancel) and resource (message) and narrows scope with 'still-pending (scheduled or queued) by its public id.' That qualifier distinguishes it from read-only siblings like senderkit_messages_get, but no sibling is named explicitly.

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 phrase 'still-pending (scheduled or queued)' serves as an implied precondition, telling the agent this only applies to messages not yet sent. There is no explicit when-to-use/when-not guidance and no comparison to alternatives such as senderkit_messages_get.

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