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

While annotations already declare readOnlyHint=true and destructiveHint=false, the description adds valuable behavioral context beyond that: it recursively traces the entire SPF lookup graph, counts DNS lookups against the RFC-7208 limit, and presents flattening guidance when over the limit. This enriches the agent's understanding of the tool's computational behavior and outputs, though it stops short of detailing exact return structure or pagination. Since annotations already cover safety, a high score is justified.

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 a single, dense paragraph that front-loads the core purpose ('Read-only SPF parse and validation'), then packs behavioral details (recursive walking, DNS count, RFC limit) and usage guidance in an efficient manner. Every sentence adds value, with no fluff or redundancy. It is appropriately sized for the tool's complexity.

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?

For a tool with only one parameter, a clear schema, and safety annotations, the description covers the essential behavioral aspects: what it does (parsing/validation), what it returns (mechanisms, graph, count, qualifier, warnings), and edge-case behavior (close to/over limit). It does not explicitly mention error cases (e.g., invalid domain syntax), but given the tool's read-only nature and the presence of an output schema-less environment, this is a minor gap. The description is sufficiently complete for an agent to make a confident selection.

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 provides a full description of the single 'domain' parameter, including format constraints (no URL/path/port) and a maximum length. The description adds no new parameter-level information beyond what the schema already captures, but it does contextually imply the parameter is a domain needed for SPF lookup. With 100% schema coverage, the baseline is 3, and the description does not detract from it.

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 clearly identifies the tool as a read-only SPF parser and validator for a domain, with a specific verb ('parse and validate') and resource ('SPF records of a domain'). It distinguishes itself from sibling tools like check_email_security and flatten_spf by detailing its unique recursive include/redirect walk and DNS lookup counting, making its purpose unambiguous.

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 explicitly states when to use this tool ('for SPF auditing or before adding new include: senders') and provides a clear alternative ('use check_email_security for the broader SPF+DKIM+DMARC overview'). This gives the agent concrete decision-making guidance, distinguishing it from a generic 'use for SPF' statement.

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