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

Annotations already indicate destructiveHint=true. The description adds that the operation is a partial update and applies only to scheduled messages before send time, providing useful behavioral context beyond the annotations. It does not detail error handling or side effects after send time, but the added clarity justifies a 4.

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 two sentences: the first defines the action and resource, the second clarifies the partial update behavior. No extraneous words, front-loaded with the key verb and object.

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?

Given the complexity (10 parameters, no output schema), the description conveys the essential purpose and constraint. It omits error conditions (e.g., what happens if the message was already sent) but is otherwise complete for a patch operation with well-documented schema and annotations.

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 coverage is 100% with each parameter described. The description adds the crucial semantic that only provided fields are changed, which reinforces the patch behavior and is not evident from the individual parameter descriptions. This extra meaning elevates the score above the baseline 3.

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 clearly states the tool partially updates a scheduled message before send time. The verb-pair 'patch message' and resource 'scheduled message' are explicit, and the qualifier 'only provided fields are changed' distinguishes it from a full update, effectively differentiating it from siblings like hires_update_message.

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 specifies the use case 'before send time', which gives clear context. It does not explicitly list when not to use (e.g., after send time) or name alternatives, so the score is high but not maximal. The condition implies proper usage.

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