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

Annotations already cover safety (readOnlyHint=true, openWorldHint=true), so the bar is lower; the description adds the critical non-obvious trait that each call costs $0.003 USDC via x402, which an agent must know before invoking. It does not mention size limits, partial-header failure behavior, or latency, but the paid-call disclosure is substantial value beyond the annotations.

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?

One dense sentence that front-loads the input form and then the analysis outputs, plus a short cost sentence. Every clause maps to a real capability, though the enumeration is long enough to read more like a feature list than prose.

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?

With no output schema, the description compensates by listing what is returned (auth results, mismatches, relay path, origin IP, attachments, links, spoofing-risk level). The paid-call note covers the transactional aspect. Missing only edge-case behavior such as handling of malformed or header-only input.

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% and the single 'raw' parameter is an undescribed string, so the description carries the burden. It specifies that the value is a raw email and may be a full .eml source or just headers, which meaningfully constrains what to pass; it stops short of stating encoding or whether partial headers degrade results.

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?

States a specific verb and resource (analyse a raw email / .eml source or headers) and enumerates the analysis surface: SPF, DKIM, DMARC, header mismatches, relay path, origin IP, attachments, links, phishing wording, spoofing-risk level. That scope is unmistakably distinct from siblings like email_verify (deliverability of an address) or email_inbox (mailbox access), so an agent can pick it without opening the schema.

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 implies when it applies by specifying accepted inputs (.eml source, or headers alone), which is useful narrowing. However, it never names an alternative (email_verify, email_inbox, http_inspect) or states an exclusion, so routing is inferred rather than guided.

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