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

Annotations only declare readOnly=false, destructive=false, openWorld=true and idempotent=false; the description adds substantive behavioral context beyond that: a subscription prerequisite and that the call consumes the daily messages quota. It does not, however, reconcile the idempotency_key parameter with idempotentHint=false, though the schema itself covers that.

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?

Four short sentences, front-loaded with the action and scope, then alternatives, then prerequisites and quota. Every sentence carries distinct information; there is no 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?

An output schema exists, so return values need not be described. For a mutating 5-parameter tool the description covers prerequisites, correct usage vs. siblings, the thread_id origin, and quota cost — sufficient for an agent to invoke it correctly.

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 already 100%, so the baseline is 3, but the description adds real value by naming the source of thread_id (salesnav_list_messaging_threads), which the schema alone does not say. It contributes nothing further about text/account_id/copy_to_crm, which the schema handles.

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 ('Send a message in an existing Sales Navigator thread') and immediately scopes it against the sibling send_message ('everything else ... goes through send_message'). An agent can distinguish it from send_message without inspecting either 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?

Explicitly gives when to use it (reply inside an existing Sales Navigator thread), when not to (new messages, classic inbox), the alternative to use instead (send_message), the prerequisite (Sales Navigator subscription), and where the required thread_id comes from (salesnav_list_messaging_threads). Nothing is left to inference.

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