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

Goes well beyond the annotations (which only give readOnly=false, destructive=false, openWorld=true). It discloses that the call reaches other people, that confirmation is required by default and can be toggled off, and that a Cotlet API key with app access is needed - all behavioral facts an agent cannot infer from structured fields.

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?

Front-loaded with the core action, but it repeats the conversation-persistence idea twice ('the conversation then lives in Messages' and 'The conversation then appears in list_conversations'), which is redundant filler in an otherwise short block.

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 write tool with no output schema, it covers the side effect, the resulting conversation destination, the confirmation policy, and auth requirements. Little is missing; only explicit failure or rate-limit behavior is absent, which is minor.

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 body, ad_id, and phone including the ad_id URL format. The description adds no parameter-level meaning, so the 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 ('Send a first message') and resource ('whoever posted a listing' / advertiser of one public listing). It explicitly frames this as the FIRST message versus following ones, which separates it from reply_to_conversation without opening 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?

Clear context: use it to initiate contact with the poster of a listing, contrasted with the ongoing thread that then lives in Messages/list_conversations. No explicit when-not statement, but the first-message framing gives strong routing guidance.

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