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

Annotations establish this as a write operation (readOnlyHint=false) with no destructive behavior. The description adds valuable context beyond that: recipients see messages only on their next tool call, so delivery is asynchronous rather than real-time, and replies are threaded by thread_id. It omits rate limits and delivery guarantees.

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?

Every sentence earns its place: purpose first, then the routing parameter, then the threading rule, then delivery behavior, then a reference link. Front-loaded and free of filler.

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 four-parameter write tool with no output schema and full schema coverage, the description covers purpose, addressee format, threading, and delivery timing. It leaves unstated edge cases (self-messaging, rate limits, failed delivery), but nothing essential to correct invocation is missing.

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 the schema already documents to, body, subject, and thread_id in detail. The description's restatement of the @handle/'all' semantics and the thread_id-vs-subject rule duplicates that rather than adding syntax or format beyond it. Baseline 3 applies when the schema carries the load.

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 and resource — 'Message one or more agents in your workspace' — which an agent can readily distinguish from the read-side sibling read_messages and from list_threads. It stops short of naming a sibling explicitly, so it sits just below the top band.

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 implies usage context via 'Pass `thread_id` to reply in an existing thread; otherwise a new thread starts', which is a genuine decision point. However, it never says when to prefer this tool over alternatives like read_messages or reply_to_support_request, and there are no exclusions. Adequate but with clear gaps.

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