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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so the safety profile is covered. The description adds real value beyond them: it discloses that no slot is reserved, that the slot is consumed only when the order lands, and the return shape ('valid=true with discount info'). It stops short of auth or rate-limit context, so a 4 rather than 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?

A single dense sentence, front-loaded with the core action and scope caveat, followed by the downstream usage pointer — only one sentence and no filler. Slightly long/parenthetical-heavy but every clause carries 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?

With no output schema, the description supplies the return semantics (valid=true plus discount info) and the side-effect contract (no slot consumed). Given only 2 params and one being a session identifier conventional across the sibling set, this is nearly complete, with only agent_session_id unaddressed.

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 0%, so the description must carry the parameter burden. It does clarify 'code' semantics by tying it to place_order.promo_code, but says nothing about agent_session_id, leaving one of two required parameters undocumented anywhere. Partial compensation merits a mid score.

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+resource ('Validate a promo code') and explicitly delimits the scope ('eligibility preview only — does NOT reserve a slot'), which resolves the tension between the imperative name apply_promo and the preview title. An agent can distinguish this from place_order without opening either schema.

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?

Names the alternative path explicitly ('pass the same code as place_order.promo_code to actually apply + redeem it') and gives the condition that selects each path (preview vs. actual consumption on order landing). This is exactly the when/when-not/alternative guidance the dimension asks for.

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