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

With no annotations, the description carries the full load and does so well: it discloses the output shape (headline, recommendation enum, failed checks, lookup links), privacy behavior ('processed in memory and never stored'), determinism ('no AI'), and billing ('counts as 1 check with a key; free tier without one'). Genuinely rich beyond any structured field.

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?

Front-loaded with the action (paste headers) and the return contract, then the checks performed, then privacy/billing. Dense and mostly waste-free, though the parenthetical UI hints and the billing clause make it a touch crowded.

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 one-param, no-output-schema, no-annotation tool, the description covers invocation, output shape, privacy, determinism, and billing – everything an agent needs to call it correctly and interpret results.

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% and the description adds practical operational detail beyond the schema: that headers should be copied from 'Show original / View source' and that the body is not needed. This meaningfully helps the agent collect the right input, though it doesn't add format specifics beyond that.

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 title and description together state a specific verb (check) and resource (an email's authenticity via SPF/DKIM/DMARC and lookalike detection), and the input format is spelled out ('Paste the raw headers... Show original / View source'). This distinguishes it clearly from siblings like screen or verify_business, which concern businesses rather than individual messages.

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 clear operational context – paste raw headers, not the body – and instructs how to surface results ('show the user the read first'). It's clear when to use it, but there are no explicit exclusions or named alternatives for cases like verifying a business rather than a message.

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