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

With no annotations, the description carries the behavioral burden and does a good job: it discloses idempotent retry behavior ('Reuse idempotencyKey on retries to prevent duplicates') and a cross-parameter constraint ('replyToId must be in this thread'). It doesn't cover response format or side effects, but the key safety-relevant behavior is disclosed.

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 two tight sentences: the action is front-loaded, followed by the two constraints that are most likely to be missed. 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 four-parameter tool with no output schema and no annotations, this is adequate but not rich. It tells the agent how to handle retries and replies, but does not mention expected response, failure behavior, or required permissions. Works for a simple invocation, but leaves open questions.

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 adds real meaning for idempotencyKey (retry duplicate prevention) and replyToId (thread-membership constraint), but body and threadId are still only defined by their names and types. Partial compensation, not complete.

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 'Send a message,' a specific verb and resource that clearly identifies the operation. It does not explicitly compare itself to siblings like read_messages or create_thread, but the send/read distinction is implicit from the tool name and action.

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?

There is no guidance about when to choose send_message over sibling tools, nor any when-not-to-use note. It does give operational advice about idempotency on retries, but that applies after the tool has been selected, not to tool selection.

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