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

Annotations declare destructiveHint=true and idempotentHint=true, so the safety profile is partly covered. The description adds genuinely useful context beyond that: queued messages are free to cancel, already-started messages incur a credit charge, and message.credits reflects the amount. It doesn't detail permissions or reversibility, but the cost disclosure is valuable.

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 tight sentences with the core action front-loaded, followed by cost implications. No filler, though the final clause about message.credits is slightly dense.

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?

No output schema exists, and the description compensates by referencing message.credits and its 0 value, hinting at the return shape. For a two-param cancel tool with annotations covering safety, this is nearly complete, missing only permission or post-cancel state details.

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 parameters (message_id, template_id) are already documented with their ID patterns. The description adds no additional parameter meaning, so the 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 (cancel) and resource (pending template chat work), which is clearly distinct in action from siblings like retry_batch or create_batch. However, it never explicitly differentiates itself from the other cancel_* siblings (cancel_ai_image, cancel_batch), so it stops short of a 5.

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 purpose implies when to use it (to cancel a pending message), but there is no explicit when-to-use guidance, no prerequisites, and no comparison against alternative tools. Usage is inferable but not spelled out.

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