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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds behavior the annotations cannot convey: the tool returns an RFC 6068 payload string that must be piped into generate_qr_code, clarifying that generation is a separate step. It omits any warning about the resulting payload length versus QR capacity, which would be a useful addition.

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, zero filler, with the output identity front-loaded and the downstream instruction second. Nothing could be removed without losing information.

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 simple, fully-documented parameter builder with no output schema, the description supplies the two things an agent actually needs: what the returned string is and where it goes next. Minor gap is the implied-only return type and no mention of encoding or length constraints on the produced payload.

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% with five well-typed, format-constrained parameters, so the baseline applies. The description adds no semantics beyond the schema — no note on how to/subject/body map into the mailto field ordering or how arrays are joined.

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 and resource ('Build a mailto: payload (RFC 6068)') and adds the standard it conforms to, which pins down the output far more precisely than the name alone. The 'mailto:'/'prefilled email' framing cleanly separates it from qr_payload_sms, qr_payload_wifi, qr_payload_vcard and qr_payload_geo without needing to name them.

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?

Explicitly states the follow-up step: pass the result as `data` to generate_qr_code, so the agent knows this tool does not itself emit a QR image. It stops short of when-not guidance (e.g., when to prefer a plain text payload), so it is clear context rather than full routing.

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