check_fcrdns

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

The description fully discloses the workflow: 'looks up PTR record, then resolves that PTR's hostname back to A/AAAA records to confirm the round-trip.' It additionally enumerates returned values and common warnings, going far beyond the readOnlyHint annotation and providing actionable expectations.

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?

The description is compact yet information-dense: it opens with the core purpose, then explains the resolution flow, then lists return contents and use cases, and closes with a statement on authentication. Every sentence earns its place without repetition or filler.

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?

Without an output schema, the description compensates well by listing return fields and warning types, and by explaining when to use the tool. Minor omissions like behavior on missing MX records or response pagination are not critical, but additional details would push completeness to a 5.

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?

The schema already fully describes the 'domain' parameter with format restrictions (no URL/path/port), so baseline is 3. The description adds context by framing it as 'the domain's MX records,' but it does not introduce new parameter-level semantics beyond the schema.

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 description uses a specific verb+resource combination: 'Read-only FCrDNS audit for every IP that backs the domain's MX records.' It distinguishes itself from sibling DNS tools by focusing uniquely on forward-confirmed reverse DNS for MX IPs, making the purpose unmistakable.

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?

Provides clear usage context: 'Use for mail deliverability audits, SpamExperts-style cluster checks, and debugging rejected mail.' It also names an alternative/complementary tool: 'pair with check_blacklist for reputation signals,' helping the AI decide between related tools.

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