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

Annotations already cover readOnly/idempotent/non-destructive/closed-world, and the description adds substantial extra context: hard input caps (256 KiB, 1,000 combined lines, 50 hops/statuses, 20 auth headers), that the body is ignored, no inbox-delivery guarantee, and a prompt-injection warning that returned evidence is untrusted data. That is material behavior 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?

Four tight sentences, front-loaded with the action and return contents, followed by limits, exclusions, and the safety caveat. Every clause carries decision-relevant information; nothing is padding.

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 takes on the burden of describing returns and does so (Received hops, timezone delays, unverified auth results, SMTP status classifications, source lines, admin next steps). Combined with limits, exclusions, and the untrusted-data warning, an agent has everything needed to call it correctly.

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 only 33% (only trustedAuthservIds is documented in the schema), so the description must compensate. It adds useful limits (256 KiB, 1,000 combined lines, 20 auth headers) and flags Authentication-Results as unverified, but never explains the expected format of 'headers' vs 'deliveryLog' or how they interact, leaving half the parameter surface undocumented.

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 ('Analyze supplied email headers and optional SMTP delivery log offline') plus the scope (offline, header-only). It implicitly distinguishes itself from check_email_dns/lookup_dns by declaring 'No mailbox access, DNS or inbox-delivery guarantee', so an agent can route correctly.

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?

Gives clear usage context: offline header analysis, body ignored, no DNS. This steers agents away from expecting live DNS resolution (covered by check_email_dns/lookup_dns), but it never names a sibling or states an explicit when-to-use-this-vs-that rule.

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