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

Annotations already declare non-destructive, idempotent, non-read-only behavior. The description adds genuinely new behavioral context beyond them: this records internal acknowledgement and does NOT surface as seen to the human counterpart. It doesn't cover permissions or failure modes, but the added semantic distinction is substantive.

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 sentence that front-loads the trigger condition and then the effect, with zero redundant restatement of the tool name or 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?

With an output schema present and annotations covering safety/idempotency, the description only needs to convey intent and the seen-vs-acknowledged distinction, which it does. The one remaining gap is not identifying the alternative operation for marking messages seen.

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 only two parameters, and the schema already documents message_id and the agent_processed/explicit_ack kind values. The description echoes the kind concept ('agent-processed or explicit acknowledgement') without adding format, default, or constraint detail, so 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 (record acknowledgement) and resource (message) plus the trigger condition ('when you have processed a message'). It also distinguishes its semantics from a seen-marking operation, but never names the sibling that does mark messages seen, so an agent must still infer which tool to use instead.

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?

Gives a clear triggering context ('when you have processed a message') and an implicit exclusion ('without marking it seen by the human'). It stops short of naming an explicit alternative tool or stating hard preconditions, so nothing forces correct routing against messages.read or messages.reply.

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