compose_message

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

With no annotations provided, the description carries the full burden of behavioral disclosure. It thoroughly covers the safety profile: the tool prefills the message, stops, and leaves sending to the user. It also states behavior for empty body and service transport selection, which goes beyond basic schema details.

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 description is front-loaded with the most important fact (does not send), then gives the safety rationale, routing guidance, and compact parameter docs. Every sentence adds value and none are redundant with the schema.

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?

The tool is a simple three-parameter compose action with an output schema, and the description covers all parameters, the core behavior, the user-visible side effect, and the distinction from send_message. Nothing an agent needs to invoke it correctly is missing.

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 is the only source of parameter meaning. It compensates fully: handle gets a concrete example, body explains prefill behavior and emptiness, and service gets explicit valid values and meaning ('which transport Messages preselects').

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 clearly states a specific verb and resource: 'Open Messages with a recipient and text prefilled, WITHOUT sending.' It immediately distinguishes itself from send_message and names exactly what the tool does not do, so an agent can tell it apart from the sibling tool without opening 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?

The description gives explicit routing guidance: 'Prefer it over send_message, and use it whenever the conversation does not exist yet — send_message can only reply to an existing one.' This tells the agent when to choose this tool and when to choose the alternative.

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