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

Annotations already declare destructiveHint=true, idempotentHint=true, and readOnlyHint=false, so the safety profile is covered structurally. The description adds meaningful behavioral context beyond that: pinning promotes the memory to a permanent tier that is always loaded and never decays. It does not explain the cost/irreversibility angle that 'destructive' implies, which keeps it from a 5.

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?

One tight sentence with the action front-loaded and the consequence trailing. No filler, no redundancy, nothing that could be cut without losing information.

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 single-parameter tool with annotations covering idempotency and destructiveness and no output schema, the description supplies the essential effect an agent needs. The only omission is explicit guidance that brain_unpin reverses this, which would complete the lifecycle story.

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?

Only one parameter (id) and schema coverage is 100%, including an example format in the schema itself. The description's 'by `id`' adds nothing beyond the schema, so the baseline 3 for a fully documented single parameter is appropriate.

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?

Specific verb (pin) plus resource (memory) plus the concrete mechanism: moving it to the 'always-present tier' so it 'loads every session and never decays.' That effect statement cleanly separates it from siblings like brain_forget, brain_recall, and brain_unpin without the agent needing to open any schema.

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?

It conveys when this tool is appropriate ('so it loads every session and never decays'), which is real usage context rather than mere restatement. It stops short of naming the inverse operation (brain_unpin) as the way to undo a pin, so the agent gets clear context but no explicit alternative routing.

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