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

Annotations already establish that this writes but is non-destructive. The description adds real behavior beyond them: append semantics ('Existing notes are KEPT'), and that output is staff-facing. It doesn't cover permissions or failure modes, but for a low-risk note append this is solid added context.

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?

Three sentences, all load-bearing: purpose, append-vs-replace behavior, prerequisite source, and authoring guideline. Front-loaded with the action and zero 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 2-parameter append tool with 100% schema coverage, full annotations and no output schema, nothing an agent needs to invoke it correctly is missing. Return-value detail is unnecessary here.

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%, so baseline is 3. The description goes slightly beyond it by framing what the message should contain ('write what the merchant said, not your own summary'), which is genuine content-level guidance the schema's 'The note text' does not supply.

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+resource ('Add a note to a customer') and immediately disambiguates the mutation flavor: existing notes are KEPT, this appends. That separates it from hub_update_customer and any replace-style sibling without needing the 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?

Gives a clear precondition ('Takes the customer ID from get_customers'), routing the agent to the lookup tool first. It doesn't name an explicit when-not case (e.g., when to edit the customer record instead of annotating), but the usage context is unambiguous.

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