open_conversation

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

With no annotations, the description carries the behavioral burden and it delivers: it discloses the fan-out side effect ('Fans the session key out to every identity key registered on the target listing') and the server-side rejection of contact_card from agent tokens. This tells an agent things it could not infer from the schema.

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?

The primary purpose is front-loaded, and the additional sentences contain dense, necessary behavior. The phrasing 'listing listing_456789' is slightly redundant and the fan-out sentence could be smoother, but 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?

For a three-parameter tool with no output schema and no annotations, the description covers purpose, key side effects, and the one non-obvious parameter value. It does not describe the return value or failure modes, but an agent has enough to call the tool 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 0%, so the description must compensate. It clarifies target_listing_id as the listing being contacted, gives the two content_type meanings, and ties message to the 'first message'. It could say a bit more about message formatting, but the key parameter choices are well explained.

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?

The description opens with a specific action and resource: 'Start a new conversation from this profile ... to another listing.' This clearly distinguishes the tool from sibling send_message, which operates on an existing conversation, and from read_conversation.

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 gives explicit when-to guidance for content_type ('Set content_type to contact_card_request if this first message is asking them to confirm their real identity') and a clear prohibition (agent tokens cannot send contact_card). It does not explicitly name send_message as the alternative for continuing an existing thread, but 'new conversation' makes the intended use obvious.

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