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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, and openWorldHint=true, so safety and idempotency are covered. The description adds meaningful behavioral value by enumerating the derived artifacts (per-hop delays, authentication verdicts, alignment, bulk-sender requirements), though it does not explain the openWorldHint implication that verdict data may be looked up externally, nor any input-completeness caveats.

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?

A single front-loaded sentence that leads with the verb and resource, then lists outputs. It is somewhat list-heavy and run-on, but every enumerated item conveys distinct extracted information, so little is wasted.

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?

There is no output schema, and the description compensates well by naming the concrete results an agent will receive. For a one-parameter, read-only analysis tool whose annotations already cover safety, the remaining gap (no mention of behavior when headers are partial or malformed) is minor.

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?

Only one parameter, and the schema already documents it fully at 100% coverage ('the raw header block (everything above the message body)'). The description restates 'raw email headers' without adding format or edge-case detail, so the baseline of 3 applies when the schema does the heavy lifting.

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 ('Parse') and resource ('raw email headers'), then enumerates exactly what is extracted: Received delivery path with per-hop delays, SPF/DKIM/DMARC verdicts, DMARC alignment, DKIM signatures, sending IP, and bulk-sender requirements. This clearly separates it from the sibling check_* tools that test domains, records, or blacklists rather than analyzing a supplied header block.

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?

Usage is implied: the agent supplies a raw header block it already possesses, and the description makes the input nature evident. However, it never explicitly says when to prefer this over siblings like check_domain or check_record, nor does it state exclusions or prerequisites. No name-checked alternatives, so the agent must infer routing.

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