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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is fully covered by structured data. The description adds the useful behavioral fact that the log is unified across transactional and campaign sources, but it says nothing about pagination behavior, result ordering, or how the limit/offset caps apply. Reasonable value-add on top of annotations, no contradiction.

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?

Two sentences, zero filler, with the resource and its scope front-loaded before the filter list. Every clause earns its place and nothing is repeated from the schema.

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 zero-required-parameter read-only list tool with a fully documented schema and no output schema, the description covers what the tool is and what can be filtered. The only gap is the absence of pagination guidance (default page size of 50, caps at 100), which the schema does carry but which an agent might benefit from seeing up front.

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% across all 9 parameters, including enum values for source and status and the ISO-8601 format for date bounds, so the schema does the heavy lifting. The description only groups the filterable fields at a high level (recipient, sender, status, date range, source) and adds no syntax or semantics beyond the schema. 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?

The description states a specific verb and resource ("List messages from the unified send log") and adds the scope that matters: it merges transactional and campaign sources. That scope helps separate it from list_campaigns and get_message, though it does not name a sibling explicitly. Minor tension with the title's "sent messages," since the status enum includes queued/failed/bounced.

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?

Usage is implied by the enumeration of filter dimensions (recipient, sender, status, date range, source), which tells the agent when this tool is applicable in a general sense. However, it names no alternative (e.g., get_message for a single record) and states no when-not condition. Adequate but leaves routing to inference.

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