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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, but the description adds rich behavioral context: the async nature of bounces, default folder inclusion of Junk, the PERMANENT vs TEMPORARY distinction and its resend implications, and the warning that no bounce is weak evidence of delivery. This goes well beyond what annotations provide.

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 core purpose, then adds necessary behavioral details in a logical flow. It is somewhat long but every sentence carries critical information for correct invocation or interpretation. Minor redundancy exists (e.g., 'which is what this does'), but overall it is efficient.

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 read-only check tool with no output schema, the description covers all an agent needs: what it finds, where it scans by default, how to interpret results (permanent vs temporary), and when to combine with sibling tools. Nothing essential 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 coverage is 100%, so the schema already documents all three parameters with descriptions and constraints. The description adds context about the default folder scanning ('INBOX and the Junk folder by default') and the recipient-based filtering, which complements but doesn't significantly exceed the schema's own parameter descriptions. Baseline 3 is correct when schema does the heavy lifting, but the extra behavioral context about scanning raises it to 4.

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 states a precise verb+resource ('Find messages that came back undelivered') and clearly distinguishes the tool from check_receipts by contrasting failure detection with arrival detection. An agent can tell it apart from siblings like check_deliverability and check_receipts without opening their schemas.

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 explicitly names the alternative check_receipts and states exactly when to call it ('if the user is asking whether a message ARRIVED rather than whether it failed'). It also specifies default folder scanning behavior and conditions for resending. No inference is required.

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