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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered; the description usefully reinforces the closed-world claim with 'reads no DNS records' and discloses output types an agent can anticipate. It does not cover failure behavior (e.g., unparseable or truncated input) or rate limits, which keeps it short of a 5.

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?

Three sentences, each doing distinct work: trigger, output shape, scope boundary. Front-loaded with the usage condition and zero filler; every clause earns its place.

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?

A one-parameter, read-only, closed-world tool with an output schema present, so return values need not be spelled out (and are summarized anyway). Input expectations, scope limits and safety posture are all covered for a tool of this complexity.

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 100% and the single 'text' parameter is already documented, including the 4,000-character overflow guidance, so the baseline is 3. The description adds only the implicit point that the whole input is the pasted text and nothing else is fetched, which is context rather than new parameter semantics.

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?

Specific verb+resource (explain a bounce / SMTP reply / Authentication-Results header) with the exact input shapes an agent will encounter, plus a concrete account of the output (what the server refused, what to change, when it clears, the explaining page). It is clearly distinguishable from analyze_report, build_spf and check_domain, which operate on different resources.

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 trigger is stated explicitly and richly: 'when someone pastes a bounce message, an SMTP reply such as 550 5.7.26, or an Authentication-Results header and asks what it means or how to fix it.' 'Works from the pasted text alone and reads no DNS records' implicitly excludes the DNS-lookup sibling (check_domain), but no alternative is named explicitly, which is what a 5 would require.

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