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

Beyond the annotations, the description adds real behavioral context: self-hosted targets may return a reply inline, relay-hosted targets require saving a task ID and polling a2a_get_task, the credential determines the sender, and the operation is free. These are non-obvious behaviors that materially affect how an agent should handle the response and next actions.

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 compact and every sentence earns its place. It leads with the core action, then covers target-type differences, sibling misuses, sender identification, and cost semantics without redundancy. It is dense but highly informative.

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?

Given the output schema exists and the input schema is fully documented, the description covers the remaining critical operational context: what to do after sending depending on hosting type, which tools are not appropriate for replies, and when to switch to paid direct sessions. Nothing essential is missing for an agent to invoke this 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?

The input schema already covers 100% of the parameters with clear descriptions, so the baseline is 3. The tool description does not add new parameter-level meaning; its extra guidance is about follow-up behavior rather than the meaning of text, tenant, context_id, or message_id. This is acceptable because the schema is already strong.

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 verb and resource: 'Send an A2A message to another agent.' It explicitly distinguishes itself from a2a_get_messages, a2a_get_task, and open_direct_session, making it clear this is the tool for sending free A2A communication rather than retrieving replies or doing paid work.

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 provides explicit when-to-use and when-not-to-use guidance: for relay-hosted targets, save the task ID and use a2a_get_task; a2a_get_messages is only for claiming incoming work and will not retrieve replies; paid work should use open_direct_session. This gives an agent clear routing logic among closely related tools.

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