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

Annotations already declare readOnlyHint, idempotentHint, and non-destructive behavior, lowering the burden on the description. The description adds useful behavioral context: it returns a paginated list with count/limit/next_page_token, details the event fields, and clarifies that it reveals post-send activity. This goes beyond annotations without contradicting them.

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?

Four sentences, each earning its place: purpose and scope, return envelope, event fields, and usage guidance with alternative routing. The purpose is front-loaded and no filler or redundant restatement of the tool name exists.

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, the description need not explain return values in detail, but it still does usefully enumerate the event fields and pagination envelope. It also covers the relationship to a sibling. Minor gaps like default ordering or the meaning of openWorldHint are not essential for correct invocation.

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 input schema already documents all parameters. The description only lightly touches pagination with 'page with next_page_token' and does not add extra meaning for limit, ascending, or Authorization. Baseline 3 is appropriate since the schema carries the semantic load.

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?

Description states a specific verb and resource: 'Lists delivery and activity events for one inbox.' It immediately frames the tool as the audit trail behind sends and receipts, and explicitly names the sibling get_agentmail_inbox_metrics as the alternative for aggregate counts, so the agent can distinguish it from related tools.

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?

Provides explicit when-to-use guidance: 'Use this to find out what happened to a message after post_agentmail_inbox_message_send returned, which the send call itself cannot tell you.' It also names the alternative for aggregate counts, giving a clear decision boundary between this tool and get_agentmail_inbox_metrics.

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