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

With no annotations, the description carries the burden and does well: it explicitly states 'no network' (offline, safe, no side effects) and lists the outputs it produces, including warnings. It doesn't mention error behavior for malformed headers, but it gives a solid behavioral picture for a read-only parser.

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?

Two sentences, front-loaded with the core action and output list, followed by a critical constraint ('Pure parsing — no network'). Every phrase 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?

For a single-parameter, no-annotation tool, the description covers what it does, what it returns (types of verdicts and warnings), and a key behavioral constraint. It lacks detail on output shape or handling of malformed input, but no output schema exists and the description is sufficient for an agent to invoke 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 coverage is 100%, so the baseline is 3. The description adds that the input is 'pasted raw email headers' which reinforces the required format, but doesn't add syntax or format guidance beyond the schema's 'copied verbatim from the message source'. A baseline 4 is slightly generous; 3.5 would be apt. However, the description does tell the agent the tool expects raw header text specifically, which complements 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 ('Parse') and resource ('pasted raw email headers'), and enumerates exactly what it extracts: Received path with per-hop delays, SPF/DKIM/DMARC verdicts, originating IP, and spoofing warnings. This clearly distinguishes it from siblings like spf_check or dkim_lookup, which perform live lookups rather than parse pasted headers.

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 phrase 'Pure parsing — no network' clearly signals the context: use this when you already have raw headers and want offline analysis, not live DNS checks. It doesn't explicitly name alternatives like dkim_lookup or spf_check, which would be the live-lookup siblings, but the distinction is strongly implied.

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